RELIABILITY OF AN ELECTRIC MOTOR SYSTEM

By

Chao ran Tang

Dissertation

Submitted in partial fulfillment of the requirements

for the degree:

MAGISTER INGENERIAE

IN ENGINEERING MANAGEMENT

in the

FACULTY OF ENGINEERING AND THE

BUILT ENVIRONMENT

at the

UNIVERSITY OF JOHANNESBURG

SUPERVISOR: PROF. L PRETORIUS

CO- SUPERVISOR: PROF. JHC PRETORIUS July 2005

!UNIVERSITY

JOHANNESBURG

©2005 University of Johannesburg

ALL RIGHTS RESERVED

"I hereby declare that the dissertation submitted for the

MAGIS TER INGENERIAE

degree to the UJ, apart from help recognized, is my own work and has not

been formerly submitted to another university for a degree."

Chao ran, Tang

Thanks PROF. L PRETORIUS and PROF. JHC PRETORIUS for helping

me during the duration of my project

ABSTRACT

The design of electric motor systems as we know it today, is very important and has a

direct influence on the reliability of the system. In this dissertation, recommendations

in design are given to obtain a reliable electric motor system.

This dissertation covers a literature review of reliability engineering, and this is then

applied to an electric motor system in order to determine the reliability of the system.

This dissertation is divided into five parts: Problem definition, theory and literature

survey, economics of reliability engineering, analysis and synthesis of an

electrical motor system, conclusions and recommendations.

Part I describes the environment of an electric motor system and presents some

fundamental concepts of reliability engineering. It emphasizes the importance of

reliability analysis in the design of electric motor systems.

Part II describes some theory and literature about reliability. It emphasizes some

existing reliability analysis methods for development of electric motor systems. The

reliability prediction method is very useful for analysis of electric motor systems.

The author emphasizes that economics of reliability engineering should be taken into

account in the design process in Part III. The analysis of life cycle costs is very

important. Life cycle costs (LCC) usually consist of the initial investment, preventive

maintenance costs, repair costs and the costs for production losses and outages due to

failures and disturbances. Life cycle costing methodology is useful in analyzing the

design, reliability and maintenance during trade off of technical systems and

equipments.

Part IV focuses a specific electric motor system. Some existing reliability analysis

methods are used to analyse reliability of electric motor systems. It is highlighted how

to improve the reliability of electric motor systems. Some economics considerations

are also presented in this section.

The main conclusion reached in this dissertation is that failure data feedback, and

accurate records are very important for reliability engineering. The author makes

some recommendations for reliability of an electric motor system in design.

This dissertation may contain direct information from sources indicated generally by

ii

[ ]. This is however generally contextualized within the main aim of the research. This

is the result of specific communication obstacles.

iii

LIST OF ACRONYMS

Part I

LCC Life cycle cost

Part II

NPV Net present value

LCC Life cycle cost

FMECA Failure mode, effects and criticality analysis

FMEA Failure mode and effects analyses

FTA Fault tree analysis

MTBF Mean time between failure

MTTF Mean time to failure

MTTR Mean time to repair

FRACAS Failure reporting and corrective action system

R&M Reliability and maintainability

CA Corrective action

NFF No-fault-found

NEOF No-evidence-of-failure

RCM Reliability centered maintainability

Part IV

AC Alternating current

DC Direct current

MTBF Mean time between failure

FMEA Failure mode and effects analyses

iv

LIST OF SYMBOLS

Part II

A Failure rate

R (t) Reliability function

F(t) Probability failure function

f(t) Probability density function

Fs(t) Cumulative distribution function

M(t) Maintainability function

i.t. 1/MTTR

t Time period

A(t) Instantaneous availability

0 Mean life

Part IV

A

Failure rate

t

Time period

v

LIST OF FIGURES

Part I

Figure 1.1 Perception of risk

Part II

Figure 2.1 Product life cycle phases

Figure 2.2 A typical system life cycle

Figure 2.3 Reliability and life cycle costs

Figure 2.4 Expenditures and cost during system life cycle

Figure 2.5 System failure components

Figure 2.6 Series system

Figure 2.7 Parallel systems

Figure 2.8 Product phase vs product costs/ flexibility

Figure 2.9 Operational cycles for Intrinsic and system availability

Figure 2.10 Availability and reliability of a single element

Figure 2.11 Failure occurrence

Figure 2.12 Reliability and product life cycle

Figure 2.13 The cost of making changes to a product grows rapidly with the phase

of the product cycle.

Figure 2.14 The efficiency of reliability improvement efforts is greatest in the early

phases of the product cycle.

Part III

Figure 3.1 Cost curve of a system

Figure 3.2 The relationship between life cycle cost and effectiveness

Part N

Figure 4.1 Electric motor system

Figure 4.2 Parts of an electric motor

Figure 4.3 Reliability over time

Figure 4.4 Reliability block diagram of an electric motor system

vi

Figure 4.5

Parallel reliability

Figure 4.6 MIL failure rate model motor

Figure 4.7

Characteristic shape of the familiar bath-tube curve

VII

Contents

Part I Problem definition 1.1 Introduction 1

1.2 Background 2

1.3 Problem statement 4

1.4 Approaches and scope of study 5

1.5 Conclusion 6

Part II Theory and literature survey

2.1 Scope 7

2.2 Background 8

2.3 Foundation of reliability 10

2.4 The life cycle 10

2.4.1 Life cycle cost 13

2.4.2 Methods of estimating life cycle cost 15

2.5 Failure mode, effects and criticality analysis (FMECA) 17

2.5.1 System failure components 19

2.5.2 Failure modes 20

2.5.3 Cause of failure 21

2.5.4 Fault tree analysis 24

2.6 Reliability mathematics 25

2.6.1 Mean time between failure (MTBF) 25

2.6.2 Mean time to failure (MTTF) 26

2.6.3 Reliability functions and failure rate 27

2.6.4 Systems in series 28

2.6.5 Systems in parallel 28

2.7 Reliability prediction and modelling of system 29

2.8 Relex reliability tools and services 31

2.9 Reliability allocation in an electric motor system 32

2.10 Maintainability and availability 34

2.10.1 Maintainability concepts 35

VIII

2.10.2 Design for maintainability 36

2.10.3 Advantages of improved maintainability 38

2.10.4 Availability analysis 39

2.11 Reliability testing in an electric motor system 42

2.12 Failure reporting and corrective action system 44

2.12.1 Field data collection 45

2.12.2 Reliability and maintainability evaluation 45

2.13 Reliability management 47

2.13.1 Development of reliable designs 48

2.13.2 Integrated reliability program 50

2.13.3 Reliability and cost 50

2.14 Conclusion 52

Part DI Economics of reliability engineering

3.1 Background 54

3.2 Introduction 55

3.3 Reliability costs 56

3.4 Effect of reliability on cost 56

3.5 Conclusion 60

Part N Analysis and synthesis of an electrical motor system

4.1 Background 62

4.2 Introduction 63

4.3 Description of the electric motor system 64

4.4 The design of high reliability electric motors 66

4.5 Reliability prediction of the electric motor system 68

4.6 Failure modes, effects and analysis (FMEA) of electric motor systems 74

4.6.1 Failure modes 74

4.6.2 Failure effects 74

4.7 Improving motor system reliability 75

4.8 Economic considerations 76

4.9 Conclusion 78

ix

Part V Conclusions and recommendations

5.1 Introduction 80

5.2 Recommendations 80

5.2.1 System reliability, prediction and evaluation 80

5.2.2 Reliability growth management 81

5.3 Conclusion 82

References 83 APPENDIX A 88 APPENDIX B 91

r

Reliability of an Electric Motor System

PART I PROBLEM DEFINITION

1.1 Introduction

The growth of present day societies in terms of population, transportation, communication

and technology points towards the use of larger and more complex systems. The

importance of reliability has assumed new dimensions in the recent years primarily

because of the complexity of larger systems and the implications of their failure. Besides

unreliability in the modern age of technology causing operational inefficiency and

uneconomical maintenance, It can also endanger human life. Manufacturers often suffer

high costs of failure under warranty. Arguments begin when people endeavor to pursue

reliability values or other benefit values to levels of reliability m. Therefore more and more

manufacturers focus on reliability of their products or systems.

This dissertation will employ a reliability engineering process for the development of

electric motor systems. Specific emphasis is on the development of these systems in an

environment of limited development resources, and where small production quantities are

faced.

An electric motor system may be defined as a system using integrated electronic, electric

and mechanical subsystems, which together provide a functional solution to a customer's

or market's requirement and make profits for the manufacturers r533 . The electronic portion

of the solution can typically implement functions utilizing analogue and digital electronic

components and will probably in most cases include functions implemented in firmware

and/or software. Electric elements include interface wiring between various subsystems

and electric actuators such as electric servo motors.

The history of risk assessment has been considered in some detail by Moss [2] who has also

described the background and relevance of the now well-established risk assessment

methods such as:

• Hazards and operability studies,

1

Failure mode and effect analysis,

Failure mode effect and criticality analysis and

' Fault tree analysis.

These methods are now extensively applied as a basis for the safety assessment of a wide

range of technological systems and are well documented in texts such as that by Andrews

and Moss[31 . It has also been indicated by Moss 123 that there is some difficulty with electric

reliability prediction and it has been pointed out that the techniques developed many years

ago for the evaluation of electronic systems and components must be used with some

caution when dealing with electric systems. As electric elements are usually an integral

part of most technological systems, this prediction difficulty is clearly an important

problem. Fortunately, despite this difficulty with prediction, modern design engineering

approaches concerned with the reduction in uncertainty have resulted in many examples of

highly reliable electric system elements and reliable electric systems.

A system which typifies an electric motor system is a guided missile system. The author

will use the reliability development of such a system as the case study for the

implementation of the reliability management process discussed in this dissertation.

1.2 Background

In recent years people have endeavored to reduce development budgets, to launch their

products to market with faster times, to shorten test time and decrease technical human

resource requirements. Reaching high reliability for complex systems is very difficult but

critical. This is because reliability affects not only technical system performance but also

operating and support costs. Achieving high reliability is receiving increased interest [41 .

However, some industries including the military have been decreasing and eliminating

many of the organizations that specialized in reliability. For instance, Guilin Measuring &

Cutting Tool Works ( The author used to work there) designed a type of digital GEM gauge

which was used to measure diamonds in 1998. Not taking proper care of organization of

2

reliability resulted in this product failing. That presents a problem. Cutting reliability

efforts may be monetary attractive in the short-term but will increase the long-term costs.

Unfortunately, these long-term costs may go beyond directly measurable monetary value.

This may include decreased availability and readiness, more accidents, and even loss of

human lives, for that high reliability new products, new technologies, new applications,

and changing operating conditions make such a solution short-lived at best [51 . Products,

even those used in the home and office, are becoming increasingly more complex, and

society is increasingly vulnerable when those products do not work reliably. Those

organizations that are sustaining and increasing their efforts to design and develop reliable

products will not only be individually successful but may contribute to a strong and robust

society and economy.

Traditionally, the view taken is that cost reduction is a matter for the financial,

procurement and project management functions of a business — that it's simply a matter of

striking a better deal. Designers and engineers were tasked with meeting customer

technical specifications. Their focus was on ensuring that designs provided the required

capability above all else. The design for cost principle suggests that low cost cannot be

managed into a product — it must be engineered into a product. Economically viable

designs can be obtained, if engineers and designers take financial implications of their

decisions into account from the beginning of the design process. The design of

value-for-money products has to be taken into account by engineers, not accountants r61 .

For a product or a system, life cycle cost should be considered. Life cycle cost (LCC)

includes development and acquisition cost (procurement cost) and in-service cost

(operational, support and disposal costs). For instance in-service cost for a typical naval

ship can be twice as high as procurement cost r81 . Warships have a very long life span, so

that service cost will continue to use limited resources available to modern navies for a

long time, running into 20, 30 or even 40 years. It is therefore essential to consider full life

cycle costs, and not only procurement costs, to ensure life-long value for money and enable

customers to maximize the capability within available budgets.

If design is optimized for the procurement costs then it can provide some short-term

savings (reduced unit purchase price) but it may lead to significantly increased in-service

costs, thus increasing total ownership costs. (typically more than 85%). Actually all life

3

Market pressure

Management emphasis

AL

Customer requirements

Legal, statutory

Competition

Safety

Warranty and

service costs

cycle costs are determined by the end of design stage while incurred costs are still very low

at this point [81 .

Another problem that must be taken into account is how to control risk in a product.

Competition, the pressure of schedules and deadlines, the cost of failures, the rapid

evolution of new materials, methods and complex systems, the need to reduce product

costs, and safety considerations all increase the risks of product development. Figure 1.1

shows the pressures that may lead to the overall perception of risk. Reliability engineering

has developed in response to the need to control these tisks E7 ' 111 .

Public liability

Development risks

Source: D. T. O'Connor figure 1.1 P4[7]

Figure 1.1 Perception of risk

1.3 Problem statement

It is generally that the importance of reliability has assumed new dimensions in the recent

years, primarily because of the complexity of larger systems and the implications of their

failure El ' 3] . Unreliability in the modern age of technology besides causing the operational

inefficiency and uneconomical maintenance can also endanger human life. The transition

towards thinking about difficulty does not mean impossibility. The complexity of electric

4

motor systems is increasing rapidly and this dissertation attempts to illustrate the practical

means that can be used to provide reliability and maintainability in such electric motor

systems.

In its wider sense, the word reliability has a very important meaning: re-liability which

simply means that it is liability, not once but again and again, from designers,

manufacturers, inspectors, vendors to users and on all those who are involved with a

system in any way to make it reliable. Much attention is being paid, more than ever before,

to the quality and reliability of engineering systems, such as an electric motor system".

The problems to be addressed by this dissertation include the following:

What is the electric motor system?

Why does the system often fail?

How to control the system?

How to develop reliability engineering approaches to analyse the system?

How to design for sustained performance ( reliability ) ?

How to design for detection and isolation of faults (maintainability)?

These problems of complexity can be resolved through reliability technology approaches.

1.4 Approach and scope of study

The research in this dissertation is focused on assessing a strategy of reliability

management of an electric motor system. Within severe time and resource constraints, this

strategy is structured to provide capability in the context of developing this system. In

order to reach this goal of analysing reliability of an electric motor system, some

approaches should be used:

Apply the engineering reliability philosophy

5

Investigate and calculate the reliability of an existing design for an electric motor

system.

To ensure that life-cycle cost of an electric motor system is optimized through

good reliability design choices.

Through applying the theory of reliability, the development of an electric motor

system can be fulfilled.

Ensure that products meet performance objectives.

Identify potential failure mechanisms during product design using for example

fault tree analysis.

Estimate product warranty costs.

Optimize benefits from design alternatives using reliability optimization.

Find the best reliability allocation to meet system reliability objectives

Predict product reliability prior to making changes.

1.5 Conclusion

This dissertation attempts to introduce reliability engineering to improve an electric motor

system. However it is necessary to first study the concepts and theory of reliability

engineering. Some reliability engineering techniques will be discussed that can be used to

improve the design of an electric motor system.

In the next chapter, the basic background theory on reliability is discussed which is in

preparation for analyzing reliability of an electric motor system.

6

Reliability of an Electric Motor System

PART II THEORY AND LITERATURE SURVEY

2.1 Scope

Before the financial impact on the reliability of products is assessed, a reliability

engineering program must be used to test and report the reliability of a product. A

reliability engineering program can consequently improve the financial strength of an

organization, and the overall product reliability i23 .

Reliability assessment is based on the results of testing from in-house laboratories and data

pertaining to the performance results of the product in the field. The data produced by these

sources is to be utilized to measure and improve the reliability of the products being

produced. It is often a temptation to cut corners and save initial costs by using cheaper parts

or curbing testing programs. Unfortunately, cheaper parts may be less reliable, and

inadequate testing programs can allow products with undiscovered flaws to be distributed

into the field. A quick saving in the short term through the use of cheaper components or

small test sample sizes will usually result in higher long-term costs in the form of warranty

costs, or loss of customer confidence. The proper balance should be struck between

reliability, customer satisfaction, time to market, sales and features.

Through proper testing and analysis the in-house testing laboratories, as well as collection

of adequate and meaningful data on a product's performance in the field, the reliability of

product and system, such as an electric motor system, can be measured, tracked and

improved, leading to a balanced organization with a financially healthy outlook for the

future i81 .

The need for improving the reliability of products and systems has become very important.

The degree of interest one has in the reliability of a system and the standard of reliability to

be achieved are closely coupled to the consequences of unreliable behaviour. Improving

reliability will, in general, cost more at beginning. However, if it has been achieved, the

reliability usually saves money and sometimes saves lives [93 . Accordingly, the need to

7

maintain an "economic balance" determines the level of reliability one should aspire for in

the design of components and systems. K.K.Aggarwal Ei 11 pointed out that the socio-ethical

aspects of products with a reliability that is too low cannot be underestimated. These

low-reliability disposable products lead to a waste of labour, energy, and raw materials that

are becoming more and more scarce. In order to get good application of engineering

reliability, a fundamental grasp of the theory of probability and statistics is requiree' l 11 .

2.2 Background

Continuous operation of electric motors is essential to the function of modern power,

manufacturing, and process plants. Electric motors should be quite reliable (151 . Standard

practice has often been simply to run the motors until they fail. Periodic visual inspection,

servicing, and lubrication have been the limit of the preventive maintenance practice. In

this dissertation, how to improve the reliability and to decrease maintenance of an electric

motor system will be discussed.

As plants have become larger and more integrated, requirements for reliable motor

operation have increased. Many large plants have electric motor populations in the

hundreds or even in the thousands which are vital for the plants. As well the necessity for

continuous operation, some motors are essential to safety. An example of motor systems

where safety is crucial, is the continuous operation of large pumps driven by motors in

nuclear power plants, which are critical to keep the reactor in an undisturbed state [151 .

Reliability engineering has not developed as a specific discipline, but has grown in number

of activities which were previously the field of the engineer.

By balancing the value of the enhanced product against the cost of failure reduction, during

the initial design phase of reliability engineering, the necessary design compromise can be

obtained i561 .

According to Serope Kalpakjian [1°I , reliability can be defined as: "the probability that a

product will perform its intended function in a given time environment and for a specified

period of time, without failure".

8

The Electronics Industries Association, (EIA) USA states: "the reliability of an item (a

component, a complex system, a computer program or a human being) is defined as the

probability of performing its purpose adequately for the period of time intended under the

operating and environmental conditions encountered."" II

These definitions stress four elements:

Probability

Adequate performance,

Time

Operating and environmental conditions.

In fact reliability can be viewed as the assessment of a product against a set of attributes or

a specification, and the successful delivery of the approved product to the customer" °1 .

After the product has been accepted by the customer, the customer should accept that the

product might cease to operate at some time in the future. Whether failures occur or not and

their times to occurrence, can seldom be forecast accurately. Reliability is therefore an

aspect of engineering uncertainty. Whether an item will work for a particular period is a

question which can be answered as a probability.

The satisfactory performance of a product, has a mutually exclusive relationship with the

concepts of stoppages or failures of a product, which actually means that the specific

operating equipment works satisfactorily or has failed. Since reliability is a yardstick of

capability to perform within required limits when in operation, it normally involves a

parameter which measures time.

In recent years it has been found that reliability prediction can not only be based on the

concept of validly repeatable component failure rates. It has been acknowledged that there

are an extremely wide variability of failure rates of identical components under identical

operating and environmental conditions. It has also been shown that the complexity of

modem engineering systems, ensures that component part failure does not necessarily

result in system failureM .

Therefore to ensure better products for customers, mathematical and statistical methods

should be used for quantifying reliability (prediction, measurement) and for analysing

9

reliability data. The reliability manager must ensure that the number of tests allocated to

the system would be useful for the reliability calculations. This is ensured by collecting

relevant data as accurately as possible. From these test results the calculation of the

reliability can be determined more accurately.

2.3 Foundation of reliability

In order to be able to successfully implement a reliability program of an electric motor

system, there are a certain number of activities that need to be undertaken. Most of these

activities are relatively simple, but they are vital to the design and implementation of a

reliability program. It is like the foundation of a house: relatively inexpensive and largely

forgotten once the major construction has begun, but vital to the structure. Smith and

Crawford {81 addressed the reliability "foundation building" concepts as follows:

Fostering a culture of reliability [81

The most important part of developing a reliability program is having a culture of

reliability in the organization. It is vital that everyone involved in the product's

production, from upper management to assembly personnel, understands that a

sound reliability program is necessary for the organization's success.

Production mission i81

The underlying concept in characterizing the reliability of a product involves the

concept of product mission (e.g., operate for 36 months, or complete 1000 cycles).

Universal failure definition t81

Another important foundation for a reliability program is the development of

universally agreed-upon definitions of product failure. It should be fairly obvious

whether a product has failed or not, but such a definition is quite necessary for

various reasons.

2.4 The life cycle

All systems and products regardless of technology pass through four basic phases from

their creation to their removal from service [163 . These for basic phases are illustrated in

figure 2.1, and are:

10

A need for the product is established and its basic characteristics defined, usually

in the form of a systems or an equipment specification in the concept and

definition phase

The product or systems hardware and software are developed to perform the

functions described in the product specification in the design and development

phase.

The design is placed into production for service by one or more customers in the

manufacturing and installation phase

The product or system is operated throughout its useful service life in the

operation and maintenance phase. During this phase the essential repair and

maintenance actions are taken and performance of the system is monitored.

1

Time

Concept and Definition

Design and Develop

Manufacture and Install

Operate and Maintain

I --Quality Control-- I

Reliability Life Phase

1

Source: Arsenault and Roberts[16] Figure 2.1 Product Life Cycle Phases

Patrick O'Connorm states that the system's performance depends mainly on the failures,

which may occur during the life cycle of the product. The life cycle of a system is

illustrated with the bathtub curve, as shown in Figure 2.2.

11

Raleigh Distribution

Weibull Distribution

•

Exponential Distribution

Development Phase Support phase End of Life phase

If one considers the bathtub curve for a system, one can clearly differentiate between three

phases in the lifecycle. In the development phase, the system would mainly fail due to

inherent design weaknesses and defects in the system. This can be attributed to the lack of

proper quality control during the manufacturing phase of the system. The failure rate will

decrease during the first phase, and most of failures would occur during the early life of the

system. ( O'Connor M, Blanchardr01 )

Failure Rate — A (t) Life Cycle of a System

System life (t) Source: O'Connor[7]

Figure 2.2 A typical system life cycle

The second phase which one identifies is the useful life of the system. During this phase,

the failure rate normally remains constant, but there is still the possibility of random

failures occurring. The number of failures occurring is due to inherent design weaknesses,

which survived the development phase. However most of the causes for failures are

changes in the working conditions of the system. These changes are outside the initial

design parameters of the system, according Blanchard [20I , Wright[211 ,and Barlow 1221

During end of life phase, the failure rate of the system is increasing due to for instance

fatigue, old age and wear out of the product. The failure occurs mainly due to the natural

wear out and the lack of proper maintenance programs for the system or product.

12

RamakumarI91 describes this stage in the system life cycle by means of a Raleigh

distribution function.

2.4.1 Life cycle cost

What is life cycle cost analysis?

Relax Software 1311 states that life cycle cost analysis is a method of calculating the cost of a

system over its entire life span.

To ensure that the specific design configuration reflects the appropriate economic

considerations, one has to evaluate alternative designs by implementing life cycle cost

(LCC) analysis to varying levels of depth throughout the system development process 1411

The life cycle cost concept is addressed in the British Standards [361 as Terotechnology. The

British Standard BS:3811 defines terotechnology as: "A combination of management,

financial, engineering, building and other practices applied to physical assets in pursuit of

economic life-cycle costs."

The objective of life cycle cost analysis is to choose the most cost-effective approach from

a series of alternatives so that the least long term cost of ownership is achieved. Life cycle

cost analysis helps engineers justify equipment and process selection based on total costs

rather than the initial purchase price of equipment or projects. Life cycle cost provides best

results when both art and science are merged together with good judgment, as is evident

with most of the engineering tools available today. Remember the first alternative for

accountants is the "Do Nothing" case. However this is the last alternative for engineers.

Engineers must make their first alternative a computation of the "Do Nothing" case to form

the datum for their improvement alternatives. Also remember that the single life cycle cost

number for the figure of merit is net present value (NPV). It can be positive or negative. In

general, since most engineers are only working with small parts of a projects, their NPV's

will be negative. Therefore the lesser negative value is the preferred course of action r11] .

The analysis of a typical system could include such costs as system planning and concept

design, preliminary system design cost, design and development costs, product costs,

maintenance costs, and disposal costs. This type of life cycle assessment often uses values

13

calculated from other reliability analyses like failure rate, cost of spares, repair times, and

component costs. Relex automates a life cycle cost analysis by making all of these

calculated values available through its fully integrated interface 1311 .

Life cycle costs have two major elements [371 :

Acquisition costs and

Sustaining costs.

Acquisition and sustaining costs are not mutually exclusive. Sometimes acquisition cost

contains little sustaining cost. Frequently the cost of sustaining equipment is 2 to 20 times

the acquisition cost. Often 65% of the total LCC is set when equipment is specified (even

though only 10% of the expenditures have been made) 1371 .

B.S. Blanchard 1201 states that LCC includes all system costs and may be broken down into

various categories to include design and development cost, construction and / or

production cost, system operation and maintenance cost, and system retirement and

material recycling or disposal cost.

A company could use a life cycle cost analysis to determine warranty costs, for instance.

This type of life cycle assessment could be based on anticipated failures, repair times, and

costs of repairs. Many companies are finding that a life cycle cost analysis is a valuable

tool during the design phase of a project in order to determine the most cost effective

solutions before substantial costs are incurred. As an example, given the list of motor

failures in APPENDIX A, a life cost analysis can be completed in respect of the failures.

After a system has been designed, produced and commissioned into service, it must be

supported throughout the duration of its life cycle. The result is that the capital required to

support the system in the future, is often more than the initial acquisition cost 1I61 .

Usually the disposal value is small. By considering the total cost of the system incurred

over its life cycle, rather than only the initial cost factor, system procurement is satisfied,

and the cash flow over the system life cycle can be better foreseen by management. Figure

2.3 is a commonly described representation of the theoretical cost-benefit relation ship of

effort expended on reliability ( or production quality) activities 171 . This figure shows the

failure cost will decrease with reliability going up, and quality /reliability program cost will

14

7*

Total Costs Quality/Reliability

Program Costs

Failure Costs

increase. Similarly, Ramakumar 191 analyses the relationship between reliability and the life

cycle cost in another figure. This figure shows the relationship between reliability, failure

rate, operation and maintenance cost, production and/ or acquisition cost, and total cost.

From figure 2.3, a specific degree of reliability results in a minimum value of total cost of a

system, and prudence dictates the aim for that value in design choice, unless there are

compelling countervailing reasons r91 . It is a different tool from Figure 2.3 by O'Connor m .

Cost

Quality / Reliability

100%

Source: O'Connor[7] Figure 2.3 Reliability and life cycle costs

2.4.2 Methods of estimating life cycle cost

For a system or a product, how to estimate the life cycle cost is very important. Before

embarking on any life cycle cost study one should ask the following questions 116) :

What is the purpose of the estimate?

What is the impact of accuracy and precision in the estimates?

Who is involved in the cost project?

What are the constraints?

What data are available?

What details are required of the cost structure?

How are the uncertainties to be treated?

15

12

35%

OperationeSuppart

Designitleveiop rt 0%

Life Cycle Phases

50%

% L

ife C

ycle

Cos

ts

109% 90%

80%

70%

60% 50%

40%

30%

20%

What are the general cost fund limitations?

What responsibility has the cost analyst?

In order to estimate life cycle cost one must make a comparison between existing costs and

new costs. There is little sense in developing life cycle cost for a system and then have

nothing to compare or relate it to. Historical data is therefore often used as the base for

comparison. There are two typically methods which one can use to develop cost estimates.

A simple method is the use of existing catalogue data. Although the price for a single item

in a catalogue may not reflect the actual market price, it is a base from which one can

operate. In a sense it is historical information which once must have been correct and used.

The comparison of one catalogued item and the same from another catalogue may yield a

good estimated price. Another method is the use of information which was generated for

the purpose of the initial plan. This cost data may be very preliminary but sometimes may

be all that is available, especially for a newly developed product. Adjustments to such cost

data will have to be made progressively [17 .

Reliability should be designed and built into products at the earliest possible stages of

product development [16 '30'561 . As Figure 2.4 shows, it's the most sound approach to take.

J.E. Arsenault and J.A. Roberts [161 suggest that although only 15% of the expenditures were

made prior to production, about 90%-95% of the life cycle costs were determined. The

design specifications that were approved prior to production determined how it would

proceed and, therefore, determined the costs to be incurred in that phase.

Source: [30] Figure 2.4 Expenditures and Cost during System Life Cycle

16

2.5 Failure mode, effects and criticality analysis (FMECA)

Failure mode and effects analyses (FMEA) or failure mode, effects, and criticality analysis

(FMECA) are of the most common reliability methods used today. A FMEA (or FMECA)

is a systematic bottom-up approach of identifying potential failure modes of a system, and

analyzing the possible effects of those failures as well as the severity of the effects.

The main goal of the analysis is usually to obtain the critical failure modes to be reduced in

probability of occurrence or severity, given limited engineering resources. When

identifying potential design weaknesses through analysis of the different ways a

component/equipment could fail, the results will be provided in the necessary format. This

includes the identification of the cause of failure, and its effect on the functions of a

product or system i341 . Each mission phase of the equipment or system would normally be

taken into consideration. The FMECA should be initially performed, in the early project

phase, at the level of system functions and/or functional paths 1381 . As the design advances,

the FMECA should be refined and completed down to unit level, with the circuit

interfacing external elements for which FMECA should be performed down to component

level.

The objectives of the FMECA include 1381 :

Identification of failure effects, which include determination of the need for

redundancies or fail-safe features, and the identification of hazards in support

of safety analysis.

Demonstration of compliance. It is with applicable safety and reliability failure

tolerance requirements.

Identification of the necessary monitoring devices for the observation of the

symptoms of a failure via telemetry of on-board monitoring.

Identification of ground maintenance activity inputs as well as on-orbit inputs.

The following failure modes should also be considered in the FMECA E34'381 :

Premature operation

Failure to operate at prescribed time

17

Failure to cease operation at prescribed time

Failure during operation

Degradation or out of tolerance operation

For failure of electric, electronic and electro-mechanical parts:

short circuit

open circuit

incorrect function

Incorrect commands or sequence of commands

Incorrect software functions

Maximum impact and influence will be obtained in the final design, if FMEA/ FMECA are

implemented during the design phase. The FMEA/ FMECA serves to input and support

other engineering design activities, such as [341 :

Safety Engineering: The FMECA would support the safety engineering efforts in analysis

such as the Fault Tree Analysis.

Testability Engineering: In the development of the FMECA, a column is reserved to

explain the method of failure mode detection. This information can be used to support a

fault diagnostics procedure or effect an equipment / a system built in test capability.

Additionally critical failure modes associated with safety may be identified that would

present themselves as potential hazards r381 .

Maintainability Engineering: As part of the maintainability analysis is the importance that

detection and isolation is accurately reflected in the overall mean time to repair

calculations. It is critical to its undertaking [381 .

Logistics Engineering: For each failure mode occurrence, a resulting corrective

maintenance task would be implemented. Of equal importance the FMEA/ FMECA plays a

significant supporting role in the development of preventative maintenance tasks through a

reliability centered maintenance (RCM) approach. Therefore the occurrence of failure

18

Design Defects

Ar"----

Quality Defects

Wearout Reliability 41-- Defects

Defects

modes caused by wearout characteristics would be identified and used to supplement the

RCM effort 1381 .

Availability Engineering: The FMECA ensures that there are no failure modes in the

architecture, after a complex system is developed employing the use of redundant elements.

This FMECA could be advantageous in redundant cross over points (potential single point

failures) etc L381 .

Design Engineering: The FMECA would support the design engineering effort to ensure

that program design requirements are addressed. These could be in the support of

requirements [383 .

2.5.1 System failure components

Ronald T. Anderson and Lewis Neril L71 point out that the possible failure of components

must be known before analysis of a system reliability. System failure components are

defined in the following manner:

Quality defects—represent early failures and have a decreasing hazard rate.

Reliability (or stress related ) defects—represent failures during the early and

useful life period; have a constant (or slightly decreasing) hazard rate.

System Hazard Rate z(t)

Time t -0.

Source: Ronald T. Anderson and Lewis Neril[17] Figure 2.5 System Failure Components

19

Wearout defects—represent failures during the normal and end-of-life period; have

an in creasing hazard rate.

Engineering (or design) defects—normally represent early failures and have a

decreasing hazard rate; however, an immature design can allow these defects to

dominate all other defects.

2.5.2 Failure modes

The identification and analysis of potential failure modes, serves as an important aspect of

the process of using FMECA, to establish the initial basis for formulating corrective

maintenance requirements" 71 . The main objective is to stepwise identify the failure modes,

the criticality of each failure on function, safety and maintenance, and the identification of

the possible effects of each failure.

Failure rate modes included in this section are based upon identified failure modes of the

individual parts. Failure rate models or estimates for the following component parts are

discussed or referenced in Part N.

Bearings

Windings

Brushes

Armature (shaft)

Stator housing

The models developed in this section will be based on an AC fractional horsepower type

motor being analysed may not need to include all the failure rates due to its lack of certain

parts (e.g., brushes).

The results of a failure mode analysis are tabulated in such a manner that enables the

design engineer to iI71 :

Design improvement, design review and structure control after identification of

reliability, safety critical areas and single failure points [ ' ?1

20

Determine the necessity for fail-safe design, redundancy, design simplification,

and more reliable materials" 71

The test program should be responsive to safety hazards and identified failure

modes.

Identify specific areas required for concentrating quality control, manufacturing

process controls, and environmental stress screening.

Data recording requirements identification.

Failure mode analysis can also be used to facilitate the investigation of actual field

failures and the determination of their impact on mission success and overall reliability,

such as the electric motor system.

2.5.3 Cause of failure

Failure mode analysis involves determining what parts in a system or component can fail,

the modes of failure that are possible, and the effect of each mode of failure on the

complete system f171 . The need for a systematic and formal process to identify and classify

effects are increased, if the system is more complex, resulting in an greater interaction

between its constituent components, such as a cooled system in nuclear reactor pile.

It is often necessary to identify the original causes of subcomponent failure, and an

example of this may be to apply FMECA to obtain the failure modes of a typical electric

motor: the circumstances during design of large electric motor, manufacturing, assembly,

installation, operation, maintenance or testing that have led to failure. Failure causes are

surely the most difficult to fill in correctly as it requires design knowledge and analysis

capability and tools usually beyond the competence of the failure report authors. Failure

causes are typically classified as defective component, poor installation, inadequate

maintenance/ testing, inadequate physical protection, inadequate electric protection,

personnel error, normal wear/ aging, and other i341 .

21

Component Failure Mode Failure Cause

Motor:: Bearings., Fail Worn ,Bearing, Lubrication Problem

Brushes Fail Open Dirty Worn Brushes

Coil Fails Open Open Circuit Coil

Coil Fails Short Insulation Breakdown

Source: [34] Table 2.1 Failure modes and cause of an electric motor

The specific causes of failures of components in large electric motors can be many. Some

main causes of failures are shown as in table 2.1. Some are known and others are unknown

due to the complexity of the system and its environment. A few of causes of failure are

listed below:

> Poor design, production and use [18]

Poor design and incorrect manufacturing techniques are obvious reasons of the low

reliability. Some manufacturers hesitate to invest more money on an improved

design and modern techniques of manufacturing and testing. It is very important to

determine the correct materials to implement is a design, otherwise it could be

catastrophic at the outcome. It is therefore necessary to have a complete knowledge

of material characteristics, limitations, and applications, to minimize the occurrence

of failures. All failures have a cause and the lack of understanding of these causes is

the primary cause of the unreliability of operation of a large electric motor.

Reliability is a critical factor that needs to be considered in every industrial product

design. By ignoring it could result in huge financial /corporate image implications on

the company's part.

> System complexity" Elms]

Developing a complex system results in overall production costs to be increased, and

reliability to be decreased. Users of complex systems often find it difficult to

22

maintain and understand such products. By producing simple cost effective products,

product failures and costs are decreased, resulting in more reliability of the product.

> Poor maintenance [11,18]

The important period in the life cycle of large electric motors is its operating period.

Since no product is perfect, it is likely to fail. However its life time can be increased

if it can be repaired and put into operation again. By implementing a

preventitive-maintanence policy on the manufactured products, could result in the

minimization of failures of products. Early prevention is better than cure applies late

to products and equipments as well.

The key components of an electric motor maintenance and management program

include:

Electric motor system inventory control in software.

Replace decisions versus pre-made repair.

Preventitive/predictive maintenance program.

Include top management commitment.

Have obtained employee buy-in.

Have pre-set energy conservation goals.

Partnerships between vendors and owners implemented with pre-planned

decisions

> Human reliability in an electric motor tis] The contribution of human-errors during the operation and maintenance of electric

motors, leads to the increase in product unreliability, after some effects have been

made to decrease the human-error effect by increasing automation and application

techniques. Failures due to the human-error can be due to:

Lack of understanding of large electric motors.

Lack of understanding of the process of producing a large electric motor

Carelessness

Forgetfulness

Poor judgmental skills, such as installation of motor

Absence of correct operating procedures and instructions

23

Although, it is not possible to eliminate all human-errors, it is possible to minimize some of

them by the proper selection and training of personnel, standardization of procedures,

simplification of control schemes and other incentive measures. Simply user friendly

products should be designed to ensure minimization of the probability of an error, and the

smooth operation of a product. The following checklist should prove useful to the design [. engineers II] .

Is the operator position comfortable for operating the controls?

Do any of the operations require excessive physical effort?

Is lighting of the workplace and surrounding area satisfactory?

Does the layout ensure the required minimum movement of operator?

Can the operator's judgment be further minimized?

In this case operation refers to all those events in which the component function output is

false or unstable, etc. It refers mainly to failures of motor drives, instrumentation,

automation and components of electric power supply. Also those failures, where the motor

rotated in wrong way due to poor installation, such as these failures as shown in Table 2.1.

2.5.4 Fault tree analysis

There is another basic approach to failure mode analysis : fault tree analysis (FTA). The

fault tree analysis process is a tool that lends itself well to analyzing potential failure modes.

Its objectives are described by Ronald T. Anderson and Lewis Neril [171 as follows:

The assessment of the magnitude of potential safety failures at the beginning of

system development;

The formulation and induction of effective corrective measures prior to system

deployment, after the identification and prioritization of all possible failure

modes/hazardous conditions.

FTA starts by using a specific failure condition, and proceeds to define possible system

faults and conditions, by implementing a top-down technique. Logic diagrams are used to

describe these basic faults, conditions and events.

24

The steps of fault tree stated by J.E. Arsenault and J.A. Roberts 1163 generally include:

Understand the system to be evaluated

Define the bad event, critical hazard, loss of an account, etc.

System analysis to determine logical interrelationships of lower and higher

functional events, resulting in a system fault condition.

Apply logical relationships to input fault events which are defined in terms of

basic, independent and identifiable faults.

Using fault tree symbols connect this information.

Reduce the fault tree if possible.

Eliminate any feedback paths.

Check to ensure all faulty rules have been followed.

2.6 Reliability mathematics

Engineers often meet some problems which involve the accumulation and analysis of data

on random phenomena. It is very difficult for engineers to analyse single data in resolving

complicated problems. Therefore, probability theory and statistics should be used to

establish mathematical models. For instance, how can the reliability in an operating

electric motor be measured? In this case, measuring the reliability of an electric motor is

very important so as to avoid some faults before its manufacturing.

The concept of reliability as a probability means that any attempt to quantify it must

involve the use of statistical methods. When one is calculating the reliability of a system, it

is necessary to measure the failures occurring in the system. The principal parameter of

failure rate ( ), is defined by Patrick D. T. O'Connor ri as "the mean number of failures in

a given time".

2.6.1 Mean time between failure (MTBF)

Mean time between failure is defined by R. Ramakumar i83 as "the expected or mean value

of the random variable". Mean time between failures is almost the same as mean time to

25

failure, if the repair time is short compared to the time between failures. These two terms

are used interchangeably; otherwise, the mean time between failures is equal to the sum of

the mean time to failure and the mean time to repair.

2.6.2 Mean time to failure (MTTF)

Mean time to failure is the total number of life units of an item divided by the total number

of failures within the population during a particular measurement interval under stated

conditions. (Source: Mil-Std 7210 E241 )

Total Operating Hours

MTTF= ...[2.1] Total Failures

2.6.3 Reliability functions and failure rate

In reliability engineering concern is with the probability that an item will survive for a

stated interval (e.g. time, cycles, distance, etc.), that there is no failure in the interval (0 to t).

This is the reliability, and it is given by the reliability function R (t). From this definition, it

follows that

R(t) =1— F (t) = f f (t)dt ... [2.2]

Assuming that the time to failure is described by an exponential density function, then

f (t) = 1 -e_tie ... [2.3]

where 0 is the mean life, t is the time period of interest, and e is the natural logarithm base. The reliability at time t is

26

1 e R(t)= f e - t le

dt = -d e ...[2.4]

Mean life (8) is the arithmetic average of the lifetimes of all items considered. The mean life (8) for the exponential function is equivalent to mean time between failure (MTBF). Thus,

At-t/M -

R(t) = e =e ...[2.5]

2.6.4 Systems in series

A system is in series if the total system will fail when any single element fails. The

reliability of the system depends upon the conditional reliability of the elements [7 '9 ' 11,1420] .

If one then considers a system consisting of n components in series, as shown in Figure 2.6.

It then follows that the reliability of a system in series is,

n

Rs(t) = H Ri(t) =R1(t) • R2(t) • R3(t) • • • • • Rn(t) =i

...[2.6]

Figure 2.6 Series system

In terms of the cumulative distribution function Fs(t), it follows that

n Fs(t) =1— Rs(t) = 1 — Hp. _ fi(t),

i=1 ...[2.7]

27

The failure rate can be represented as

Number of failures in a given time

— ... [2.8] Total operating time in same period

The failure rate for a series system is calculated by using (exponential distribution),

As= Al+ A.2± A3-1-...-1-An ... [2.9]

2.6.5 Systems in parallel

A system, which operates until the last of its component fails, is a parallel or redundant

system. Parallel systems offer advantages in reliability, especially early in life. This is

especially the case where it has higher priority than component costs. In addition, it is

important to design in redundancy for the system, i.e. computer system of a space shuttle,

etc. if one then considers a system consisting of n components in parallel, as shown in

Figure 2.7, the reliability of the system is calculated by,

Rs(t)= 1- Fs(t) ...[2.10 ]

But the cumulative distribution function Fs(t) for a parallel system is

Fs(t)= Flo F20 F3...• Fn = H Fi(t) =1

Rs(t) =1 —11Fi(t) ...[2.11]

but Fi(t) =1- Ri(t)

Rs(t) = 1 — H[1 _Ri (t )] =1— Fi(t) . . . [2.12] i =1 1=1

28

system

n

Figure 2.7 Parallel systems

The concept of stand-by redundancy or back up is to switch over from one element to

another only when the first has failed. The success of the switch over depends on the

reliability of the mechanism to detect the failure and switch over from one element to the

other, e.g. main compressor and stand-by compressor for a compressed air plant E7 ' 111 .

2.7 Reliability prediction and modelling of system

Recently, most attention has been attracted towards the prediction of reliability, by

deriving mathematical models for reliability m. Anderson EI71 describes reliability prediction

as the process of estimating system reliability starting at the lower level assemblies and

proceeding up to the higher levels of assembly.

The evaluation of a electric motor system design from the conceptual stage to manufacture

can be simplified if reliability can be predicted EI21 . Prediction provides a rational basis for

design decisions, involving choice between alternative concepts, variations in part quality

levels, appropriate application of derating factors and use of proven vs state-of-art methods

and other related factors E133 .

29

Reliability prediction has many purposes L171 :

Basis for selection among competing designs.

Reliable and critical limiting item disclosure in the design, such as the size of

copper wire in an electric motor.

Design sensitivity to thermal stress and parts quality.

Reliability trade-offs among system components.

Numerical description of the reliability of design.

Provide inputs to design review, failure mode effects and criticality analysis,

maintainability analysis, safety analysis, logistic support and thermal design.

Reliability prediction is classified as follows:

Feasibility prediction

It could be used in the conceptual phase of item development. During this phase

the level of detailed design information is generally restricted to overall aspects

of the item.

Preliminary design prediction

Configuration information is documented by preliminary drawings and

engineering sketches in the early design phase.

Detailed design prediction

It is intended for use in and subsequent to the detailed design phase. This phase

is characterized by drawings which identify all parts, materials, and processes

needed to produce the item.

A reliability prediction for a system containing many parts is likely to be more accurate

than for small system m, because the engineers often analyse the complicated systems more

carefully than simple systems. In this case, these types of reliability prediction are very

useful for a large electric motor system.

US MIL-HDB K-2 1 7 E231 , 'Reliability Prediction of Electronic Equipment', establishes

uniform methods for predicting system reliability. It presents failure rate models for most

30

electronic systems in specific applications. It provides a common basis for reliability

predictions and a basis for comparing and evaluation reliability predictions of related or

competing designs.

The prediction of reliability is basically that system failure is a reflection of part failure.

The overall system failure rate is then simply the sum of the individual part failure rates,

which is based on a series reliability model.

These models vary with part types, however , their general form is :

A P = A t) nE nAnQ•••nn ...[2.13]

where:

A p is the total part failure rate. A b is the base failure rate.

n E is the environmental adjustment factor, which accounts for the influence of

environment other than temperature.

n A is the application adjustment factor.

n Q is the quality adjustment factor

it n represents adjustment factors which are used to account for cycling effects,

construction type, and other design and application characteristics.

2.8 Relex reliability tools and services

In general, design for high reliability defined by Ronald T. Anderson and Lewis Neri1 1171

means:

Parts and components selection with proven reliability and life characteristics

Part deration to minimize deterioration

Using redundancy of a form appropriate to considered hardware

To plan, perform and document carefully reliability qualification tests,

environmental stress screens and acceptance tests

To develop extensive and effective controls, disciplines, and provisions

employed in a well-designed reliability program; and

31

To require that all failures be analysed when they occur, with rapid feedback of

test and failure analysis results to the designers for correction of inadequate

design

In the electric motor system, Relex reliability tools and services can be used. Because

quality must be built into motors from the very beginning, a set of tools and services should

be used to help firms improve quality, reduce cost of poor quality, and improve reliability

at each point in the product lifecycle. In the electric motor system, quality and reliability

must be managed to ensure the entire life cycle effectively.

The Relex Reliability Suite - which includes reliability prediction software and reliability

analysis software t271 can be used to improve the design, build, and test phases of product

development.

By using the Relex tools and services in an electric motor system, firms may:

Reduce their Cost of Poor Quality.

Achieve improved electric motor design with predictive analyses and life data.

Succeed using quality-controlled processes to build and deliver products.

Test and gain insight into product quality.

Produce more reliable electric motors

Improve the reliability of electric motors.

2.9 Reliability allocation

Anderson t17' states that reliability allocation is an important technique to apply during the

design of a complex system. Reliability allocation is the process of apportioning the overall

system reliability requirement down to the subsystems and lower levels of assembly.

The objectives of reliability allocation are to :

Assign the system reliability requirement among the subsystems and

components before a commitment is made to a particular design approach.

32

Focusing on the various subdivision systems relationships.

Reliability design target setting for lower subdivisions.

Determine the need for incorporation of specific reliability design features, e.g.,

the need redundancy can be established during the conceptual phase and then

reflected in the specification for system development.

In a large electric motor system, it is necessary to translate overall system characteristics,

including reliability, into detailed specifications, for the numerous units that make up the

system. The process of assigning reliability requirements to individual units to attain the

desired system reliability is known as reliability allocation. The allocation of system

reliability involves solving the basic inequality:

f (R1*, R2*, ..., Rn*) __---R*

where,

f : a function

R*: system reliability requirement

Ri*: i th subsystem reliability requirement

For a series system, the above equation is simplified as

R1*. R2*.....Rn* R*

...[2.14]

...[2.15]

Some of the advantages of the reliability allocation program are:

The reliability allocation program may force system designer and development

personnel to understand and develop the relationships between component,

subsystem, and system reliabilities. For instance, a designer of electric motor have

to know what factors result in the bearing seizure of electric motor. It may be old

lubricant, or corrosion due to entrance of moisture, etc. This leads to an

understanding of the basic reliability problems inherent in the design.

Weight, cost, and performance characteristics should also be considered with

reliability.

The reliability allocation program ensures adequate design, manufacturing

methods, and testing procedures.

33

In electric motors systems, reliability allocation factors to consider include [461 :

Complexity

Cost

Redundancy introduction

Maintenance

Time of operation

2.10 Maintainability and availability

Reliable, maintainable and available systems are achieved through a disciplined systems

engineering approach employing the best design, manufacturing and support practices. In

order to achieve the user reliability, maintainability and availability requirements, Bill

Mostia Jr., PE E281 emphasizes in the following:

Understanding the user's system readiness and mission performance

requirements, physical environments (during use, maintenance, storage,

transportation, etc.) the resources (people, dollars, etc.) available to support

the mission, the risks associated with these requirements, and translating

them into system requirements that can be implemented in design and

verified;

Managing the contributions to system relibility, maintainability and

availability that are made by hardware, software, and human elements of

the system;

Preventing design deficiencies (including single point failures), precluding

the selection of unsuitable parts and materials, and minimizing the effects

of variability in the manufacturing and support processes; and

Developing robust systems, insensitive to the environments experienced

throughout the system's life cycle and capable of being repaired under

adverse or challenging conditions.

34

2.10.1 Maintainability Concepts

Ronald T. Anderson and Lewis Neri1 1171 define maintainability as the probability that a

hardware item will be retained in or restored to a specified operating condition, within

allowable time limits, using available test equipment, facilities, personnel, spare parts and

prescribed procedures. Maintainability prediction is an analytical process of estimating the

parameters that describe this probability. Maintenance features and design characteristics

of a system are accounted for, as well as the provision of the ease at which failures are

diagnosed, and maintenance operations performed.

Patrick D. T. O'Connorm states that most engineered systems are maintained, i.e. they are

repaired when they fail, and work is performed on them to keep them operating. The ease

with which repairs and other maintenance work can be carried out determines a system's

maintainability. Maintained systems may be subject to corrective and preventive

maintenance.

Maintainability is only one part although a very important part of the measurement of

overall system worth. The USA Department of Defence 1241 definition of maintainability is

quoted as follows:

"Maintainability is a quality of the combined features and characteristics of equipment

design which permits or enhances the accomplishment of maintenance by personnel of

average skills, under the natural and environmental conditions, in which it will operate."

From these points above, maintainability can be expressed in terms of mean time to repair

(MTTR) E7' 111 .

The principal objectives of maintenance are defined by K.K.Aggarwal Eill as follows:

Life extension of assets, where this is important in view of lack of resources.

Optimum availability assurance of installed equipment for production.

The assurance of operational equipment required for emergencies, for example:

firefighting and standby units.

To ensure the safety of personnel using facilities.

35

In line with this reasoning, several possible indices were suggested which may be useful in

the quantitative description of maintenance activity in an electric motor system [131 . Among

these are:

Ratio of satisfactory operation to total required time in motor system.

Average down time per unit of calendar time( or any other stated time)

Mean time to repair motor system

Man-hour requirements per unit of operating time.

Total man-hour requirements per unit of calendar time.

Waiting time per unit of time.

Material requirements per unit of time.

Cost of support per unit of calendar time.

Ronald T. Anderson and Lewis Neril U7] point out that in general, to design for a high

degree of maintainability means:

Accessible and interchangeable module, assembly and unit incorporation.

Scanning of selected measures of performance.

Providing alert systems to warn workers when tolerance levels have been

exceeded.

Automatic logging of selected performance parameters to permit trend detection.

Incorporation of features for detection location and diagnosis of failures.

Use extensive and effective controls, disciplines and provisions employed in a

well-designed maintainability program

2.10.2 Design for maintainability

The Ministry of Defence [29} of USA illustrate that the reliability and maintainability

achievement of a system depend on all aspects of that system including hardware,

software, people and the man/machine interfaces. Reliability and maintainability is a

systems engineering discipline and so the R&M program must adequately address all these

aspects above. Therefore design for maintainability should be addressed.

36

Many durability products require maintenance throughout their useful life. Thoughtful

consideration of product maintenance features early in the design process can reduce or

eliminate maintenance costs, reduce downtime and improve safety.

What is design for maintainability?

Andy FitzGerald states i251 :

"First , design is the transformation of an idea into a product, process, or service that meets

both the designer's requirements and end user's need. Second, maintainability is the degree

to which the design can be maintained or required easily economically and efficiently."

The objectives of design for maintainability should include:

Identify and prioritize maintenance requirements (25 '28)

Increase product availability and decrease maintenance time r25 '281

Increase customer satisfaction [281

Decrease logistics burden and life cycles costs [20)

How to decrease life cycle costs is very important. It is already mentioned in section 2.4.1.

In design for maintainability, a product's maintainability should be given strong

consideration in the initial product development stage. The notion is driven by the fact that

maintenance and the associated costs are accrued over the entire life of the product.

Another reason is that maintenance costs can be a significant factor in a product's overall

cost. If design flexibility is high and design change costs are low, it is essential that

maintenance is considered early in the design process. As shown in Figure 2.8, design

flexibility is greatest in the conceptual stage of the product and design change costs are low.

As the product nears production, design flexibility decreases and design change costs rise[25,26] .

37

•

Design Change Costs

Design Flexibility

Concept I Design Prototype Production

Design Cost And

Flexibility

Source: Andy FitzGerald[25]

Figure 2.8 Product Phase vs Product Costs/ Flexibility

2.10.3 Advantages of improved maintainability

The purpose of maintainability engineering is to increase the efficiency and safety as well

as to reduce the cost of equipment maintenance. To accomplish this, it should be evident

that the achievement of significant cost reductions in maintenance begins with improved

equipment design. Although maintainability engineering will not eliminate the need for

service and repair on for example mining equipment, it provides some of the following

advantages [321 :

Scheduled /unscheduled maintenance time reduction.

Accessibility improvement for inspection and servicing, to minimize the

frequency of unscheduled maintenance.

Maintenance error reduction and incorrect installation minimization.

38

A Total System Uptime + Total System Downtime Total System Uptime

Post maintenance inspection improvement.

Maintenance-related injury reduction.

Maintenance training requirements minimization.

Troubleshooting performance improvement.

2.10.4 Availability analysis

Availability is the probability of a hardware system or component item being in service

when required. Ronald T. Anderson and Lewis Neril [171 state that availability provides a

single combined measure of the reliable operation of the system and its ability to be

efficiently maintained. It has a similar meaning for repairable equipments to that of

reliability for non-repairable equipments.

The difference is that reliability only accounts for the single event failure and availability

accounts for both failure and repair events. Availability is defined mathematically by

Ronald T. Anderson and Lewis Neril [171 as :

...[2.16]

Various other definitions of availability have been established based on the time elements

included in total system downtime [171 . Intrinsic availability is defined as consisting only of

the actual active repair time and neglects any other logistic or personnel factors. On the

other hand, system operational or system availability includes the following: waiting,

repair time, administrative time, and logistic time. Their operational cycles and the various

time elements are as shown in Figure 2.9.

39

Logistics Time

• •

System Availability Operational Cycle

Intrinsic Availability Operational Cycle

Active Waiting epair Time Time

► ∎

Total Uptime

Total Downtime

Uptime

Downtime

Source: Anderson[17]

Figure 2.9 Operational Cycles for Intrinsic and System Availability

From the figure it can be seen that system availability takes into consideration all delay

factors and hence provides a realistic picture of the actual time the system will be available

to perform its intended function. Because of the difficulty in evaluating total downtime,

care must be exercised in assessing system availability, due to the large number of factors

that will affect its actual value.

Availability is associated with the concept of maintainability as discussed in the previous

paragraph. The maintainability function M(t) is defined as the probability that the

equipment will be restored to operational effectiveness within a specified time when the

repair is performed in accordance with the prescribed conditions. It is a function of repair

time. Availability thus depends upon both failure and repair rates.

In a previous paragraph availability was defined only for maintainability in a system.

However, in general, the availability of a system is a complex function of reliability and

maintainability. This can be expressed as

A = f(R, M) ... [2.17]

40

where A = system availability

R = system reliability

M = system maintainability

This equation can be viewed as an input and output relation, where R and M are the inputs

and A is the output. In the electric motor system, the maintainability and availability should

also be considered as mentioned above.

However, instantaneous availability, A(t), is defined by Anderson and Lewis Neril Ern as

the probability that a system will perform a specified function under given conditions at a

prescribed time.

The instantaneous availability is bounded such that

R(t) A(t) 1 ...[2.18]

A(t) = R(t) for an item, that does not undergo repair. An important difference between A(t)

and R(t) is their behavior for large times. As t becomes large, R(t) approaches zero,

whereas availability functions reach some steady-state value 1171 . The relationship of

availability and reliability with time is shown in Figure 2.10. Then availability can be

expressed as:

A( 00 ) = X-Ftt

where:

1/MTBF µ=1/MTTR

If mean time between failure (MTBF) or mean time to failure (MTTF) is very large

compared to the mean time to repair (MT1R) or mean time to replace (MTTR), then

availability will be high. If mean time to repair or replace is very short, then availability

will be high. As reliability decreases (i.e., MTTF becomes smaller), better maintainability

(i.e., shorter MTTR) is needed to achieve the same availability. Maintainability is not so

important to obtain the necessary reliability, when reliability is increased. Thus tradeoffs

can be made between reliability and maintainability to achieve the same availability.

Therefore the two disciplines must work hand-in-hand to achieve the objectives.

41

A(t), K(t)

A(t) = P-±x±iLem where m= -( X + u )t

X =1/MTBF g=1/MTTR

A(00)

When /-( =0 (no repair)

A(t)=R(t) =641

AV

0 Time t

Source: Anderson[17] Figure 2.10 Availability and Reliability of a Single Element

2.11 Reliability testing in an electric motor system

Reliability testing has become a prominent part of a typical encompassing reliability

program. Reliability is a tool which attempts to prove the viability of the theoretical

reliability calculations or the expectations associated with a design. Although it may not

always be possible to duplicate the operational environment exactly for test purposes, most

of the time it will be possible to simulate the operational environment to a satisfactory

level. Ronald T. Anderson and Lewis Neril Eri emphasize that the attainment of a specified

reliability level, within allocated resources, is largely dependent on the extent to which

reliability testing is applied during development. Since the reliability of a new system

would be much lower than the required level, reliability tests and other improvement

techniques should be applied to grow reliability to the specified level.

In general, reliability testing has four underlying principles which are stated by A.H.K.

Ling and J.E.Arsenault in Reliability and Maintainability of Electronic Systems " 63 :

42

"Statistically efficient tests are chosen to minimize cost and time to an

accept/reject decision. After each failure r moment of time the test data are

examined and a decision is made either to accept, reject or continue testing

Test data is carefully recorded, is available for review, and there is redundancy

in the forms used so that data may be cross-checked

All failures occurring during testing are to be thoroughly analysed to determine

the failure mechanism. I.e., the fundamental cause of failure traced to its

chemical, physical, design or workmanship origin

Corrective action is taken in the event that the test reveals evidence of

systematic failures and the required changes are reflected in the final design"

In brief, reliability testing is a technique. It demonstrates that the reliability characteristic

of interest will not be less than a certain specified minimum acceptable value to some level

of confidence 131 .

The steady, safe, and efficient operation of electric motors is essential to the productivity of

most plants and facilities. Some facilities, including electric utilities, pulp and paper mills,

and many others, have critical and/or expensive motors 13 ' 131 . A motor failure could be

catastrophic, causing lost production and costly emergency repairs [131 . For example, a

motor failure at a nuclear plant can cost up to one million dollars a day for critical motors

and may have a disastrous, long-lasting impact. Motors fail due to numerous operational

circumstances including power condition, mechanical influences, and environmental

hazards, etc" 31 .

Correct testing of all components of a motor requires a series of tests designed to emulate

the conditions the motor will see in the field. It has been proven in numerous studies that

low-voltage testing, including capacitance, inductance, impedance, etc., is not an effective

tool in detecting weakness in the insulation 1161 .

How may achieve a cost-effective test program be achieved? It requires careful planning

followed by well governed executed tests results in a cost-effective test program. The

necessary planning of test activities must be integral part of the overall reliability program,

and the test activities should also be evaluated and optimized relative to the specific needs

of the system under development r173 . This requires establishing adequate test requirements,

43

assigning responsibilities and providing necessary resources(test units, facilities, personnel,

etc.).

Testing is an essential part of any engineering development program. If the development

risks are high, the test program becomes a major component of the overall development

effort, in terms of time and other resources. The objective of a reliability test program is to

gain information concerning failures, for instance, the tendency of systems to fail and the

resulting effects of failure. Therefore in a sense, reliability tests are distinguished from

most other types of test, which are generally concerned with normal operation of

equipment. Reliability testing should be considered as part of an integrated test motor

program, which should include [123 :

Functional testing

Environmental testing

Reliability testing

Safety testing

Although all testing contributes data for reliability calculations, they could be considered

in an electric motor system to be reliability testing. After reliability testing for an electric

motor system, failure reporting and corrective action should be continuing.

2.12 Failure reporting and corrective action system

From a reliability perspective the failure reporting and corrective action system (FRACAS)

understand how a product or a system actually performs. The benefits of implementing

FRACAS can be realized in a number of ways. A FRACAS is very importance for a

manufacturer how to operate and to perform a system. The effectiveness of the FRACAS

will be seen by the manufacturer in an operational environment, and the data obtained from

the FRACAS will enable correlation to be made between predicted and actual performance

R&M data. This information will permit better informed decisions for new designs and

provide confidence over issues associated with warranties etc as shown in Figure 2.11.

For an organization operating a system or a multiple of systems, an effective FRACAS,

should reveal when there are potential reliability (and maintainability and logistical)

44

nrnhiPmc Fnr example a small airline company nnoratinc. a fleet ..F several a

commercial jets, this information could be used to determine the optimum operations and

maintenance costs 1351 .

L .Cc

ArM 4-71111Mtrrl,4%,

dith- biin"

Source: [35] Figure 2.11 Failure occurrence

2.12.1 Field data collection

The responsible person will collect the required field data. The phase and requirements of

the program determine the required field data collected, and the same amount of data will

be collected as has been done for maintenance performed in the field. The action of the

responsible person may include i281 :

Initiate the failure reporting sequence;

Collect the field data as listed in FRACAS form;

Forward the failure report to the reliability engineering manager.

2.12.2 Reliability and maintainability evaluation

After the collection of "field data" elements the reliability engineering team should review

each failure report for the following [351 :

45

Whether or not the field data completed is sufficient in detail and quality;

Determine if there are potential problems and/ or unwarranted trends;

Dispose and close out the failure report if no action is required;

Initiate a failure analysis, if required due to potential problems or trends;

Report potential problems and trends to the corrective action (CA) team

members;

Annotate failure reports with CA disposition and the CA log number;

Monitor the timely response of and submission of vendor failure reports;

Monitor the status of all open failure reports; and

Preparing the FRACAS data for inclusion into the R&M reports (if required

and/ or detailed in a R&M Plan).

A reliability engineer should review the collected information, to ensure that the potential

problems are identified, and the necessary actions are taken. This problem review activity

will be typically involved in Table 2.2.

Reliability Monitor the MTBF data in the case of unwarranted failure patterns.

Establish possible common failure cause.

Maintainability Monitor maintainability characteristics such as, BIT effectiveness,

accessibility, fault diagnostics problems etc.

Vendor

Monitor vendor failure report for unwarranted failure trends e.g. same

CA and/ or component, no-fault-found or no-evidence-of-failure,

indicates inadequacies in diagnostics capability (BIT, test procedure etc.)

Customer Focus on grey areas such as physical and functional interfaces for

modified between the equipment and system

Source: [35]

Table 2.2 Problem review activities

46

2.13 Reliability management

Reliability is no more a subject of interest confined to only academics and scientists. It has

become a serious concern for practicing engineers and manufacturers, sales managers and

customers, economists and government leaders [151 . A.D.S. Carter 1401 also states that any

management must take in connection with reliability. It is to convince the management

itself that a high level of reliability in its equipment/ product/system is a worthwhile

economic objective. The management of a manufacturing concern could well argue that

money spent on achieving high reliability represents an equivalent loss in profits and the

loss of the sometimes high profits associated with spares provisioning. The reliability of a

large electric motor system is for example directly influenced by every aspect, such as

design and manufacturing, quality engineering and control, commissioning and subsequent

maintenance, and feedback of field-performance data. The relationships between these

activities are shown in Figure 2.12.

design preview

design development

prototype approval test manufacture

product quality ► control

-4-

customer

rel'ability prediction and improvement

reliability data and statistics

field service

service —■

information

external sources

Source: K. Aggarwal[11]

Figure 2.12 Reliability and product life-cycle

The reliability requirements should be clearly stated at the design and development stage

itself also in an electric motor system. While setting reliability objectives it is worth

considering the following objectives of the organization ° 11 :

47

Maximize output

Optimize reliability

Minimize waste

Minimize discontent

Maximize customer satisfaction and reputation

Optimize job satisfaction

2.13.1 Development of reliability program

BS:5760 Part 1 (Guide to Reliability Programme Management) {39] divides projects into

four phases:

Definition

Design and development

Production

Function and maintenance

Each of these is further divided into activities, some of which are iterated as shown in

Table 2.3. This is the basis of any program, but circumstances indicate emphasis. The

design engineers complete designs for a product or a system through these activities with

high reliability.

Phase Activities

Definition

Design and

Development

Reliability feasibility study

Statement of reliability objectives and requirements

Reliability specification and contract formulation

Analysis of parts, materials and processes

Analysis of established and novel features

48

Phase Activities

(including initial Failure modes, effects and criticality analysis

manufacture) Incident sequence analysis 9fault tree analysis)

Stress and worst-case analysis

Redundancy analysis

Reliability assessment

Human factors

Design review

Design audit

Safety program

Maintainability program

Parts and sub-assembly testing

Performance testing

Environmental testing

Accelerated testing

Endurance testing

Reliability demonstration

Data collection, analysis and feedback

Production

Function and

Maintenance

Preservation of reliability achievement

Quality conformance verification

Screening of components and assemblies

Data collection, analysis and feedback

Redesign/modification

Maintenance

Source: BS5760[39]

Table 2.3 Program activities during the principal phases of a project

49

2.13.2 Integrated reliability program

The reliability effort should always be treated as an integral part of the product

development and not as a parallel activity unresponsive to the rest of the development

program [591 . It has four project phases mentioned in previous section.

Since production quality will be one of the final determinants of reliability, quality control

is an integral part of the reliability program. O'Connor m states quality control cannot make

up for design shortages, but poor quality can deny much of the reliability effort. Quality

control can be made to contribute most effectively to the reliability effort if:

Quality procedures, such as test and inspection criteria, are related to factors

which can affect reliability, and not only to form and function t7 ' 143 .

Quality control test and inspection data are integrated with the other reliability

datarl .

Quality control personnel are trained to recognize the relevance of their work

to reliability, and trained and motivated to contribute r ' 141 .

2.13.3 Reliability and cost

It is known that achieving high reliability is expensive, especially when the product is

complex or involves relatively untried technology E421 . For example, how to produce a high

reliability spacecraft to land on Mars? It is often necessary for testing purposes that all the

resources of engineers, test equipment, and management time are required for these

techniques. It is most often the case that it is difficult to justify the amount of tasks

performed in the quest for reliability. Several standard references on quality management

suggest considering costs under three headings, so that they can be identified, measured

and controlled E7 ' 17 '421 . These quality costs are the costs of all activities specifically directed

at reliability and quality control. Quality costs are usually considered in three categories:

Prevention costs

Appraisal costs

Failure costs

Why however does Phil Crosby (1979) say " Quality is free"?

50

Reliability and its related factors are all elements of quality to which this adage applies

with even greater force. This is because their values are largely determined at the design

and development phase when it costs little to make changes that would be very expensive

later on in the product cycle.

Good quality can be realized through the implementation of a total quality management

approach. It is an integrated management approach directed at achieving total customer

satisfaction through the continuous improvement of system characteristics by improving

design, organizational activities, and so on 1203 . Quality control has the same relationship to

production as reliability engineering has to design and development m. A good quality

system (such as effective procedures, training, etc.) is necessary and can provide a baseline

for achieving high quality and reliability. If a product is good quality , it must be high

reliability.

The author has already discussed this problem of life cycle cost in Section 2.4.1. There are

two notional graphs ( Figure 2.13 and Figure 2.14) which illustrate this point. It becomes

Cost of Change

Phase of Product Cycle Source : Sherwin[19]

Figure 2.13 The cost of making changes to a product grows rapidly with the phase of the

product cycle.

51

more expensive to make changes as the project advances through the phases of the product

cycle. It implies that the final system reliability can be adjusted less and less and is mainly

and effectively determined in the early stages. The biggest headache is that nobody really

knows what the system reliability is going to be in the end. Figure 2.14 shows that the

efficiency of reliability improvement efforts is much greater in the early phases than in the

later phases of the product cycle.

Proportion of MTBF Effectively Determined

Function and

maintenance Production

Design Definition

Phase of Product Cycle Source : Sherwin[19]

Figure 2.14 The efficiency of reliability improvement efforts is greatest in the early

phases of the product cycle.

2.14 Conclusion

Reliability engineering of a system such as an electric motor system can be viewed as an

essential concept that each manufacturer should have as a core competency. The

manufacturer should develop some tools, models and analysis such as FMECA, LCC

analysis, reliability prediction, reliability allocation, maintainability, availability and

reliability test to improve the reliability of a system or a product. It is important to note that

no manufacturer can afford to sit back and relax and wait for the future. The manufacturer

must start with a reliability management program to ensure that their systems will be

competitive in the marketplace.

52

Quality control is essential for the manufacturer to develop the systems or products.

Quality represents the essence of freedom which makes decisions at work and as a

customer. The products or systems needed today seem to be too complex to be competitive.

Good quality products with high reliability are required by customers at good prices. The

manufacturer makes efforts to develop products or systems at low cost, however they must

face quality problems. The product is more complex, and good quality is more difficult to

achieve. Another factor to account for is that complexity is the enemy of reliability.

It is also important for the manufacturer to gear oneself for reliability engineering, by

deciding on what level of the business this should be done, and to what extent. To get an

effective reliability management program for the manufacturer, there must be a good

performance measurement tool in place and clear performance levels must be set. The

management of the manufacturer must also ensure that it is clear that they are committed to

the success of reliability engineering also in the marketplace. Reliability of a product or

system looks like a chain. To satisfy customers, the manufacturer must continue to keep the

high reliability of products. In the market place, the manufacturer should provide good

services and support.

For a product or a system, high reliability is very important. However, the manufacturer

will make much efforts to achieve it at high cost. Therefore, economics of reliability should

be taken into account. In next chapter the author will discuss the relationship between

economics and reliability engineering.

53

Reliability of an Electric Motor System

PART III ECONOMICS OF RELIABILITY ENGINEERING

3.1 Background

Most business decisions should be made on a sound economic basis. Capital investments

are assessed on aspects including return on investment or cash flow, cash availability, and

perceived importance. Projects are then prioritized and released for construction. For

example, Nuclear power plants are one of the most economical forms of energy

production iI7'211 . Nuclear fuel costs (as a function of power generation potential) represent

only a fraction of the cost of fossil fuels. Including capital and non-fuel operating costs,

the cost of operating a nuclear power plant is roughly equivalent to fossil fuels. Recently,

the average electricity production cost for nuclear energy was recognized as the cheapest

source of electricity". In 2002, the U.S. average cost of power generation by nuclear

plants was 1.71 U.S. cents per kilowatt-hour, for coal-fired plants 1.85 cents, for gas 4.06

cents, and for oil 4.41 cents i421 . Costs for solar and wind are still well beyond that

considered to be competitive to the public. The procedures followed to oversee and

regulate nuclear power generation have become an issue affecting the finances of major

industries. The costs associated by managing these power stations, can be reduced if there

are improvements in reliability and maintenance efficiencies i431 .

Many reliability improvement projects often face these tests. An example might be

deciding to invest in a back-up for a programmable logic controller (computer) that

controls an automated factory line. Being able to identify the cost and savings is important

in justifying the expense. The return on the investment must be compared to the cost of

doing nothing. Documentation of the problems and costs is the most fundamental step in

the analysis of options.

Some of the costs of a reliability issue include [43]

Manufacturing cost

Productivity cost

54

Field service cost

Lost opportunity

Wasted energy

Decreased equipment life

Loss of revenue

This chapter will describe reliability cost. The author will discuss economics in reliability engineering, such as what the effect of reliability on cost can be.

3.2 Introduction

Any manufacturing industry is basically also a profit making organization and no

organization can survive for long without minimum financial returns for its investments.

There is no doubt that the expense connected with reliability procedures increases the

initial cost of every device, equipment or system. However, when a manufacturer's

products are not reliable, the manufacturer can lose potential customers, and he or she has

to become liable for the expense that follows. How much reliability cost is worth in a

particular case depends on the cost of the system and on the importance of the system's

failure free operation. If a component or equipment failure can cause the loss of an

expensive system or of human lives, it is worth improving reliability with the

corresponding incurred cost to eliminate these factors. For the producer, it is a matter of

remaining in the business. However, his business volume and profit will be substantially

increased once his reliability reputation is established. Therefore, from a manufacturer's

point of view, two important economic issues are addressed by K. Aggarwal 1113 :

Financial profit

Customers' satisfaction

If a manufacturer intends to stay in business, he has not only to optimise his own costs and

profits but to maximize customers' satisfaction as well.

55

3.3 Reliability costs

Reliability costs can be divided into five categories suggested by K. Aggarwal [111 :

Costs of internal failures

Prevention costs

Costs of external failures

Administrative costs

Appraisal and detection costs

These costs above effect reliability directly. The author will discuss them in next section.

3.4 Effect of reliability on cost

Analysis of economics of reliability engineering should center on life cycle costs. Life

cycle costs usually consist of the initial investment, preventive maintenance costs, repair

costs and the costs for production losses and outages due to failures and disturbances. The

life cycle costing methodology is useful in analyzing the design, reliability and

maintenance of different technical systems and equipments. Helena Malkki in Reliability

and safety of processes [11 states that life cycle costing methodology makes it possible to

take into account the effect of failures and outages on system life time economy.

LCC-analysis can thus be used in design, procurement and maintenance planning of

technical systems and equipments.

Any effort on the part of manufacturer to increase the reliability of his products should

increase reliability design costs and internal failure costs. However, after some time

internal failure costs will usually start decreasing [111 . For example, if the reliability of an

electric motor is improved, the bearings won't be changed for long time. Then the internal

cost will be decreased. Installation and maintenance costs will show a decline with an

increase in reliability.

In general, it is not profitable to aim for complete perfection by eliminating all failures

(even if it is possible). This is clear from the reliability cost curves given in Figure 3.1 for

various categories of costs for an equipment or system". Up to a certain point, it is

56

Total Cost

ailure Cost

Maintenance Cost

Operating Cost

Manufacturing Cost

worthwhile to make appropriate investments for reliability and further investments will be

advisable only where the reliability has an over-riding importance. Of course, some

reliability cost models can be used to analyse how the system life-cost is affected by

reliability achievement, utility, depreciation and availability.

Cost

Reliability

Source: K. Aggarwal[11]

Figure 3.1 Cost curve of a system

Most often, when dealing with the aspect of cost in the initial investment of a

product/system, on addresses only the short-term costs [6' 111 . There is some historical basis

for predicting the costs of development, design, and manufacturing of products/systems.

However, the long-term costs associated with system operation and support are often

57

HIGH LIFE-CYCLE

COST

LOW EI-.1-,ECTIVENESS (PRODUCTIVITY)

-- DESIGN -CONSTRUCTION COST -OPERATING COST

--MAINTENANCE AND SUPPOTY COST -RETIREMENT/ PHASE-OUT COST

-PERFORMANCE -AVAILABILITY -QUALITY -RELIABILITY

--MAINTAINABILITY -SERVICEABILITY

--OTHER TECHNICAL FACTORS

hidden, and they are very difficult to be found. Experience has indicated that these costs

often constitute a large percentage of the total life-cycle cost for a given system, therefore

much more attention should be taken r61 . For instance, the insulation sub-system in an

electric motor system is easy to be ignored.

Systems often are designed, without taking into account future changes to be made, or

problems to be fixed. The approach can turn out to be very costly for a company, sepecially

when changes are made to the product through its life cycle" 11 . Figure 3.2 illustrates that an

imbalance between the relationship of some measurement of system effectiveness and cost

can exist. Many systems become more complex because of new technologies used in

designr3 '203 . In these modern times, international competitiveness between company's is

strong, and the need to produce cost-effective, high-quality systems have become very

important.

777777 7 7 7 /7 7 Source: Blanchard[20]

Figure 3.2 The relationship between life cycle cost and effectiveness

58

From Figure3.1, maintenance costs will show decline with an increase in reliability. This is

because maintenance is an important part of the life-cycle of systems. And it must be

considered from the design stage through the end-of-life stage of the system. Maintenance

covers two aspects of systems: operation and performance". Maintenance is generally

performed in anticipation of a failure. System performance is restored to specified levels,

when maintenance is performed, and most often incorrect maintenance increases problems

because of faulty parts and maintainer error. A systematic and structured approach to

system maintenance, starting during the design process, is necessary to ensure proper and

cost-effective maintenance

Profits and business models are strongly related to maintenance, and affect design

decisions made. These economic considerations cover a broad range of other topics which

will be discussed below. How is one's business model affected if there is a low availability

of working systems which need to be repaired often? What are the economic benefits and

design considerations of disposal versus repair at the system or component level? Who will

perform maintenance when it is necessary, and how do the choices affect recommended

types of maintenance? What aspects of system maintenance are safety-critical, and how

does that affect the system design? Also, how do maintenance contracts affect design

decisions?

To answer these questions above, some points will be stated according to Adrian Drury t

as follows:

• Repair or replace

Economic benefits of disposal and repair are often approached most easily

from an accounting point of view. If the cost of designing for maintenance is

much higher than the cost of not doing so, and replacement is feasible, then

disposal may be a good option. Considerations about the expected lifetime of

the system must be taken into account as well. If designing for maintenance is

instead of maintaining inventories of replacement, it may in fact be cheaper.

On a system level, mechanical systems are virtually always repaired rather

than they are disposed and replaced, because of the cost associated. Electronic

systems are sometimes repaired, but that repair is often done through the

disposal and replacement of a component. Electronic components are virtually

59

impossible to repair in a cost-effective manner, while larger numbers of

mechanical components are.

Personnel

The issue of who performs maintenance when it is necessary is an important

one from the point of view of profits. There are endless variations on who can

perform, what maintenance, how and when. however three common situations

will be covered. The first approach is for the owner or user of the system to

perform the maintenance themselves. If the owner had special maintenance

training or certification, he or she will be allowed in a safety-critical system to

perform the maintenance. An example of this type of situation is commercial

airplane maintenance. Another approach might be that the system producer

should have an in-house maintenance staff to perform the necessary

maintenance duties. If the maintainers and the designers work closely

together, this approach generally results in the highest quality maintenance.

People knowledgeable about the design and functionality of the system are

arguably best qualified to maintain it. A third approach is for a third-party to

provide maintenance. If the maintenance personnel are well trained, this

approach can result in maintenance as good as would be provided by the

system provider. It may result in quicker service, if the third-party happens to

be located closer to the user's location.

Maintenance contract

System providers profits usually from maintenance contracts, but it affects

other maintenance and design decisions. An example of such a situation is that

if scheduled maintenance visits are required, it is essential to design the system

for preventative tasks.

3.5 Conclusion

The current economic trends of rising inflation and cost growth, combined with the

reduction of available resources, have increased the importance of cost-effectiveness

requirements for all evolving and existing systems. There is an enhanced awareness of the

60

total system life-cycle cost in general, and system operation and maintenance costs in

particular.

Decision made during the early design and development phases usually have a significant

impact on the costs incurred during the subsequent life-cycle phases illustrated in Figure

2.14 and Figure 3.1. The greatest opportunity to influence the cost involved, is during the

beginning of the development and design phases, considering that the total system

life-cycle cost can be attributed to the system phase and operation. For maximum benefit,

when dealing with economic factors in design, a life-cycle approach is recommended. In

keeping with this concept, the first step in performing an economic analysis is to define the

life cycle as it applies to a particular system configuration along with the activities that

constitute the evolving phases. The next step is to develop a cost breakdown structure

tailored to the specific system being evaluated.

Through the analysis of economics related to reliability, the design engineers could make

good decisions how to design the product or system with high reliability. For an electric

motor system, it is essential to take into account economics and reliability. The next

chapter will address electric motor system in more detail on reliability and economics.

61

Reliability of an Electric iviotor System

PART IV ANALYSIS AND SYNTHESIS OF AN ELECTRIC MOTOR

SYSTEM

4.1 Background

In 1887, the induction motor was invented with great success, and thereafter many

commercial designs induction motors in Great Britain accounts for more than sixty percent

of industrial electric power consumption i451, and even more than fifty percent of China's

electricity use [461 . Electric motor systems use 19% of all energy within the United States,

which accounts for 57% of all generated electric power. More than 70% of the electric

energy used by manufacturing, and 90% in process industries, is consumed by motor

systems1471 . Electric motor retrofits, variable frequency drive applications, and other

energy efficiency strategies have been receiving encouraging attention. However,

maintenance and reliability have been critical areas, which have not been given to much

attention over the years. In recent decades, the induction motor has been dramatically

reduced in size-to-output ratios, and this could be due to the necessity of competitiveness in

the global world. This trend has been facilitated by advanced design techniques and

improved materials, especially insulations occupying smaller space and withstanding

higher operating temperatures.

Motor design philosophy has been concerned more to the initial cost of the motor for a

market than the energy. It can be said that characteristics such as low noise, and starting

performance with competitive selling prices, usually had higher priority as design criteria,

since high motor efficiency is an important factor.

Improved equipment design, better system integration, and improved operations and

maintenance practices can reduce motor system energy use by 20% [48] or more. The

bottom line is substantial energy and emissions savings, reducing factory operating costs

and increased economic viability of the factory.

62

Purchasing high efficiency motors, improving motor management practices and system

optimization leads to significant energy cost and greenhouse gas savings. This can improve

product quality, increase motor life and system reliability, and provide quieter and cooler

equipment operation.

This chapter will present some methods to analyse the reliability of an electric motor

system.

4.2 Introduction

It is undoubted that modern processing plants and facilities demand a high degree of motor

reliability. This section seeks to present the most current, effective, and accepted methods

of analyzing electric motor system reliability. The steady, safe, and efficient operation of

electric motors is essential to the productivity of all plants and facilities. Electric utilities

and paper mills are but a few facilities consisting of expensive motors, which is critical to

operate continuously [45,46]. If a motor in these specific system breaks down, it could be

catastrophic, and results in costly emergency and last production repairs. An example of

such a disastrous situation is when a motor failure occurred at a nuclear plant, costing up to

one million dollars a day. Even failures at a wastewater treatment facility can have a huge,

negative environmental effect and can be very costly r521 .

Electric motors convert electric energy, supplied from an A.C. or D.C. source, to

mechanical energy at a rotating shaft. All electric motors have certain basic features in

common. Each has a stationary member, the stator, and a rotating member, the rotor,

separated by an air gap. The stator and rotor each have a magnetic core, usually laminated,

although on some high-speed A.C. machines the rotor may be of solid steel. The core

carries copper or aluminum windings in slots or on salient poles. The windings are

insulated except in the case of cage rotors t45A6'5°I .

Electric motors are inherently quite reliable. Standard practice has often been simply to run

the motors until they fail. Periodic visual inspection, servicing, and lubrication have been

the limit of the preventive maintenance practice 145 '461 .

63

In recent years, the requirements for reliable motor operation have increased, since there

presently exists plants, which consists of hundreds or even thousands of electric motors,

that are very important for the continuous functioning of a plant. In addition to the

necessity for continuous operation, some motors are essential to safety. For instance,

motors running large pumps in nuclear power plants are critical to keep the reactor in

undisturbed operation or even sufficiently cooled' s] .

The objectives of idea electric motor systems are as follows:

Low mass and inertia

High specific output

Low cost

Excellent commutation over a wide speed range

Low noise

Fast response

Low maintenance

High reliability

Flexibility of application

Conformity with international standards

High efficiency

Mechanical strength

4.3 Description of the electric motor system

Rotating electric machines convert electric to mechanical energy, or vice versa, and they

achieve this by magnetically coupling electric circuits across an air gap that permits

rotational freedom of one of these circuits t3 '451 . Electric motor drives are divided into six

categories, and each category is further divided into subcomponents. These items are

defined so that each reported failure can be assigned to one hardware item. The categories

of an electric motor are stator, rotor, bearings, windings, brush and enclosure related

subcomponents (like power supply).

64

winding Brushes Power Supply -11110 Rotor -OP Bearin Stator

The electric motor reliability block diagram as shown in Figure 4.1 appears

unsophisticated, but is actually rather complex, because it possesses components that fail

and repair nonexponentially. In fact, all of the components' failure distributions are

wear-out Weibulls with lognormal repair distributions E4 '7 '571 . In addition, several different

types of dependencies are present within this model. "Dependency" is a reliability concept.

Good components stop functioning because a component they are dependent upon has

failed or transitioned to a standby mode. For the electric motor shown Figure 4.2, the

brushes and armature components are dependent on the power supply component. If power

is lost (i.e., the power supply component fails), then the brushes and armature components

stop functioning and most importantly stop accumulating life. The failure of the power

supply has no effect on the failure modes of the magnets or casing components, because

they fail independently of the motor's rotational effects 1571 .

Figure 4.1 Electric motor system

021101 Horatuf }Weak,

Source: [58]

Figure 4.2 Parts of an electric motor

65

4.4 The design of high reliability electric motors

It is not an easy task to improve on a product where numerous advances have already been

made. Therefore, in the context of the whole motor design, each component has to be

examined thoroughly.

The principal means of increasing output are to increase rotor diameter and upgrade

insulation i45'46'491 . Increasing the core length requires additional material and may create

thermal or mechanical problems. However, it provides an appropriate cost-effective means

of increasing output, at low or no capital investment cost. It enables high output/low center

height motors to be produced, which has high torque-to-inertia ratios and wide field range

power characteristics.

Reducing internal losses should be taken into account in design process. The motor's

losses are separated to five categories by John Malinowski [49] :

Iron Core Losses — Magnetic losses in laminations, inductance and eddy

current losses.

Stator Resistance — Current losses in the windings

Rotor Resistance — Current losses in the rotor bars and end rings

Windage and Friction — Mechanical drag in bearings and cooling fans

Stray Load losses — Magnetic transfer loss in the air gap between the stator and

rotor

B.J. Chalmers i531 states that insulation system must be considered in motor design. In

designing electric machines, many different criteria must be met. No other major item

common to a electric rotating plant is affected, apart from the machine bearings and brush

geart531 . If a reliable, trouble-free service life is to be achieved, then the insulation system

must be designed to meet all foreseeable operating requirements. Consequences of failure

of a vital unit of capital plant can be serious both in economic terms as well as in its effect

on associated equipment. The most important feature of any insulation system is its

reliability.

66

Some approaches can be used in designing high reliability electric motors'''"':

Dependable insulation systems for all applications [531

When operating in the most extreme environmental conditions, these type of

systems is protected, and corona is also prevented when motors operate on

inverter power. Reliable operation is obtained when corona is eliminated,

rather than trying to defend against it.

Low maintenance and life extending bearing systems 145 '531

Contamination can be prevented by using precision controlled shaft-to-bracket

bores. Contamination is the leading cause of failure of a motor, and it has been

shown that motor operating life can be extended by using advanced lubrication

technology.

Motor and drive load vibration protection [4553]

Overall system reliability can be improved by decreasing vibration. A longer

system operation life and a more concentric rotating assembly can be achieved,

if a combination of low tolerance designs and high precision plant processes is

used.

Lower total cost of ownership efficiencies

High quality and thinner steel laminations in the stator

More copper in the windings

Small air gaps

Closer machine tolerances

Manufacturing methods and quality control are very important to design high reliability

electric motors. They provide perhaps the greatest area for consideration in ways to

improve product quality and reliability because, figuratively speaking, it is inherently the

weakest link in the chain. It is the parameter most subject to day-to-day variations. People

have to look very closely at their manufacturing methods to eliminate these variations as

67

100.0 90.0 130.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0

0.0

CI IM CD c CO

CM CO r— CD

,-- MI

03 cn

Hours of Operation

far as possible. People also need quality control to pick up sub-standard equipment or

workmanship at once, to enable corrective action to be taken before it is too late 145-511 .

4.5 Reliability prediction of the electric motor system

It has been shown that designed system operation will decrease over time in all dynamic

systems 145'511 . Electric motors systems consists of several dynamic systems each possessing

a reliability function that decreases over time as a motor system ages. The costs involved

in operating equipment and electric motors are optimized when using a reliability-based

motor program. The reliability portion of a motor management program is placed in the

arena of business impact, by measuring reliability of electric motor system with the costs

associated with unreliability 1511 .

The reliability of the system, is the measure of the chance that the equipment will operate

over a period of time. One of the keys to understanding reliability is known the mean time

between failures. For instance, if an electric motor has a failure rate of one in 40,000 hours,

the MTBF would be 40,000 hours. The failure rate for that motor would be 1/MTBF, or 2.5

x 10-5 (identified as X ) [5°1 .

Source: W. Penrose, BJM Corp[50] Figure 4.3 Reliability Over Time

68

Knowing the failure rate, the information can be applied to the reliability function

(R =e A *t)). Therefore, the chance that the motor system will operate for 50,000 hours

would be: R= eq(-50,000)*(0.000025)] = 0.287, or 28.7%.

In an electric motor maintenance and management program, there are several points in

which the system reliability can be influenced. These points include:

Acceptance of new electric motors

Acceptance of motor vendors

Acceptance of repaired electric motors

Acceptance of motor repair centers

Tracking and correction of minor defects during the life cycle of the system

Time instead of singular visits and measurements should be used to measure the reliability

of a vendor. It is recommended that the vendor should be measured against the series of

specifications over time [50] . The reliability costs of a motor system can be calculated. In the

following example, a motor fails twice per 50,000 hours, it takes 6 hours to repair the

system upon each failure, the system operates 8,760 hours per year, production costs are

$10,000 per hour and maintenance costs are $100/ hour (energy, motor repair / replacement

and waste costs not considered) E509 :

Over 50000 hours (life cycle)

MTBF = 50000/2 =25000 hours

Failure rate ( ) = 1/MTBF

= 2/50000

= 40 x 10-6

Repair time = 6 x 2 = 12 hours

Over 8760 hours (annual)

Failures = (2/50000) (8760) = 0.351 times

Repair time = (12/50000) (8760) = 2.1 hours

Production cost = [(10000 x 12)/50000] x 8760 =$21024

Maintenance cost = (8760/50000) (121060) = $ 21210

69

Stator housin Armature -1110 Bearin

windings Brushes

Table 4.1: Reliability Cost Example [5°1

Lifecycle Annual

Interval (hours) 50,000 8,760 / year Failures 2 0.351/yr MTBF (hours) 25,000 25,000 Failure Rate (10-6), A 40 40/hr Repair Time (hours) 12 2.1 Production Cost $10,000 / hour $21,024 / year Maintenance Cost $100/hour $210 / year Availability 13.5% over 50,000 hours 70.4%/year Costs $121,060 over 50,000 hours $21,210 / year

If a maintenance and reliability program (for this one system of many) reduce the failures

by half, the impact would be a cost of $58,300 over 50,000 hours, a reduction is:

$121060 - $58300 = $62,760

The reduction rate is :

$62,760 /$121060 =52%

For a large electric motor in series, the predicted reliability is the probability that it will

continue to work to its quality specification, under stated conditions, for a stated period of

time. The reliability block diagram is shown in Figure 4.3.

Figure 4.4 Reliability block diagram of an electric motor system

By using equation above, the reliability of a large electric motor, Rmain, is calculated using,

Rmain = R1*R2*R3*R4 *R5 ...[4.1]

70

Where

R1 = reliability of the stator housing work failing

R2 = reliability of the armature work failing

R3 = reliability of the bearing work failing

R4 = reliability of the windings work failing

R5 = reliability of the brushes work failing

And assume these failures to belong to the exponential distribution, it also follows that the

reliability is equal to R91 :

R =e A (- *t) ...[4.2]

If one then considers equation [4.2], the following failure rates can be substituted into

equation [4.1] for a period of t=8760 hours,

A 1= failure rate of the stator housing failing = 0.001 x 10 -6 failureper hour

X 2= failure rate of the armature failing =8 x 10 -6 failure per hour

A 3= failure rate of the bearing work failing =10 x10 -6 failure per hour

X 4= failure rate the windings failing = 2.6 x 10 -6 failure per hour

A 5= failure rate the brushes failing =3.2 x 10 -6 failure per hour

By substituting the above-mentioned failure rate into equation [4.2] for each component, it

follows that

R1 = e A (- = e A(-0.001 x 10-6 x 8760) =0.99999

R2 = e A (- e A(-8 x 10-6 x 8760) =0.932

R3 = e A (- A *t) = e A(-10 x10-6 x 8760) =0.916

R1 = e A (''" = e A(-2.6 x 10-6 x 8760) =0.977

R1 = e A (- e A(-3.2 x 10-6 x 8760) =0.972

Rmain = Rl*R2*R3*R4*R5

= (0.99999)(0.932)(0.916)(0.977)(0.972)

= 0.811

71

100.0

1:10.10

--t5. 801

70.0

s 60.0

50.0 •..=

series reliabili 30.0 —

20.0

10.0

0.0

8 0

parallel reliably

I Hours of Operation

Cf3 40.0

The reliability of the operation of a large electric motor is then calculated as 0.811 or

81.1%, for a period of 8760 hours.

For a large electric motor system, in a redundant (parallel) system, the overall system

reliability increases. The result of a single parallel system is as follows i7 '9 ' 111

R = Ra + Rb —(Ra)(Rb)

= 0.811 +0.811 — (0.811)( 0.811)

= 0.964

Source: W. Penrose, BJM Corp[50]

Figure 4.5 Parallel Reliability

Using the example above, the parallel system has a 96.4% chance of operating through

8760 hours.

A.C. Brombacher1541 maintains that mechanical properties of the motor will be taken into

account. MIL-HDBK-217E uses the following reliability model for electro-mechanical

components such as motors shown in Figure 4.6.

72

,2 L

motor — 3 + a [4.3]

t: motor operating time. (total elapsed time before replacement)

ab: Weibull characteristic of bearing life constant (mechanical)

aw: Characteristic life constant of winding (electric)

This reliability model is different from the example above which used the exponential

distribution. It combines some characteristic of the Weibull distribution to be developed.

In this case the time-dependent failure rate is approximated by its (worst-case) failure rate,

that is the failure rate of the mechanical parts of the motor at the end of its lifetime, at the

moment that motor replacement is expected.

Mech. degradation

4.0E+03 8.0E+03 1.2E+04 1.6E+04

Time (hours)

Time of motor replacement

Source: Brombacher [54]

Figure 4.6 MIL failure rate model motor

Failure rate (failures/hr)

1.8E-06

1.6E-06

1.4E-06

1.2E-06

1.0E-06

0.0E+00

73

4.6 Failure modes, effects and analysis (FMEA) of electric motor systems

FMEA is discussed in Section 2.5. It is probably the most widely used and most effective

design reliability analysis method. FMEA is used to assess the effects on system operation

of each failure mode after considering each mode of failure of every component. An

example where failure effects can be considered at more than one level is overall system

and subsystem level. Failure modes are examined in relation to the severity of their

effects [17 '381 .

4.6.1 Failure modes

To analyse failure of an electric motor system, some theory of failure mode should be

reviewed first.

The failure modes are subdivided into two general classes: demanded change of state is not

achieved and change in conditions (state). In the first class denominator is usually the

number of demands or cycles, in the second, the operating time. The first class is then

further subdivided into three self-explanatory codes such as: fails to start, fails to stop and

fails to reach design specifications. In the second class " change of conditions", first two

categories describes whether the failure is sudden or incipient, that is, whether the

unavailability of the motor is coincident with the detection of the failure or of the

abnormality, or whether the repair unavailability of the motor can be deferred. For example,

typical sudden failures were short-circuit and earth fault.

The detail motor failures are shown in Appendix A and B.

4.6.2 Failure effects

The failure of a subcomponent may affect the operation of the motor drive locally or the

function of a whole system. Failures, affecting a higher system level, are classified as

system down. An example of such a situation might be the automatic forced shut-down of a

paper machine. Machine stop is classified, when the operation of a motor is stopped by a

failure, but did not affect other systems. In many cases, the failure of a subcomponent has

no effect on the motor operability but the effect of the repair actions must still be

considered. Failure effects which were not possible to identify are classified as other [34'381 .

74

Some causes and effects of motor failure include:

Broken rotor bars,

Abnormal air gap eccentricity,

Shorted turns in stator windings,

Shaft/coupling

Power supply

Circuit short

4.7 Improving motor system reliability

Electric motor system failure is due to numerous operational circumstances including

power condition, mechanical influences, and environmental hazards. According to recent

IEEE studies, at least 35 to 45 percent of motor failure are electricity related 15°'521 .

Determining a trend for the historical operating condition of a motor makes early detection

of any decline in the motor's health possible. Therefore Noah Bethel [513 and Timothy M.

Thomas1521 illustrate that motor system reliability may be improved through static (off-line)

and dynamic (on-line) testing.

The advantage of utilizing both static and dynamic technologies for improved motor

reliability is tremendous. First and foremost, the combination of technologies enables a

facility to test during any plant condition. Dynamic is used for the obvious: the collection

of data under operation without the interruption of production. This allows a manager to

make more informed decisions as to which motors to focus on during the turn around.

Static testing facilitates quality assurance assessments, diagnostic testing on de-energized

motors and comprehensive analysis during plant shutdowns. The following defects should

be monitored:

High Resistance Connections

Stator Faults

Rotor Bar Defects

75

Static and Dynamic Eccentricity

The recent debate about the quality and reliability of electric motors has brought into sharp

focus critical elements of the motor insulation system. Insulation system quality is key to

electric motor system reliability. Therefore the insulation system should be considered.

4.8 Economic consideration

For an electric motor system, economic consideration should be taken into account. The

general aim of an economic policy relating to the maintenance and replacement of electric

motors is to minimize their life-cycle costs. If the mortality characteristics of a particular

electric motor are known then it is possible to formulate optimal maintenance strategies. If

annual motor running costs are known than an economic model can be developed to

determine the optimal electric approaches to be valid it is essential that suitable and reliable

data are available [42 '43] .

Reliability has already been defined in section 2.1. The reliability of most devices can be

characterized by the typically shaped curve of Figure 4.7.

The sectors of this model for an electric motor system are described as follows:

The sector A—B states early life mortality due to erroneous design, fabrication

and malfunctioning parts. With many electronic components, failure due to

early-life mortality is minimized by using burn-in techniques, i.e. running the

devices for a time which takes them beyond point B on the curve during the

manufacturing process. In this case of electric motor (particularly the larger

ones), rigorous tests are generally undertaken at various stages of manufacture,

hence ensuring the quality of the final product and minimizing the difference in

hazard rate between A and B.

The sector B—C states chance (random) failures due to the environmental

stress exceeding the strain capabilities of the device. Some causes of these

types of failures are listed in Appendix A.

76

• The sector C—D states fatigue and wear-out phenomena. The point at which

this sector is entered will depend upon the complexity of the motor. Typically,

a complex D.C. motor with commutator and brushgear assembly will enter this

sector much earlier than the rugged and robust cage induction motor.

Mortality rate

Elapsed time

A—B Early life mortality

B—C Random failure

C—D Fatigue and wearout phenomena

Source: Chalmers[53]

Figure 4.7 Characteristic shape of the familiar bath-tube curve

An energy and reliability program for electric motor systems should be developed to

decrease the cost of energy, production, and maintenance overheads associated with the

production of a product—in effect, reducing the cost per production unit as effectively as

possible.

The key components of a motor maintenance and management program include:

• Control of the electric motor system inventory in software

■

77

Pre-made repair versus replace and retrofit decisions

Predictive and preventive maintenance program implementation with a

continuous improvement component

Top management commitment

An in-house energy coordinator

Employee buy-in

Pre-set energy conservation goals

Partnerships between vendors and owners implemented with pre-planned

decisions and shared information.

Such a program can result in improvements of 10-15 percent or more. For instance, if the

electric motor is maintained regularly, it may avoid replacement of a component such as a

bearing. If bearings are replaced, the cost becomes more. These opportunities result from

simple improvements. These simple improvements include: replacing failed electric

motors with energy efficient or premium efficient electric motors; scheduling proper

electric motor bearings greasing, impedance unbalance correction in motor windings of

electric motor systems, testing questionable equipment before/after repair, correcting

alignment and belt tension, electric motor system friction loss reduction, and properly

sizing electric motors to the load. These examples and other related benefits can have

energy, reliability, waste stream, and production financial impacts that more than justify

the combined energy and reliability effort [45 '501 .

4.9 Conclusion

As the industry continues to develop more advanced ways to detect the faults that plague

electric motors, facilities will have fewer unplanned failures and increased production.

Therefore it is very important that high reliability of electric motor systems should be

delivered to customers. Through the analysis of electric motor system reliability, in order

to obtain an efficient motor system, the components must be taken into account, such as

stator, rotor, bearing, winding, power supply system, insulation system. The life-cycle cost

78

should be considered for economics. For a manufacturing, the high reliability of electric

motor system can attract more customers.

For every design engineering should develop a reliability program for products or systems.

Economics should also be considered related to his products or systems. The author has

discussed some problems about reliability of an electric motor. Some recommendations

about reliability management is described in the next chapter.

79

Reliability of an Electric Motor System

PART V

CONCLUSIONS AND RECOMMENDATIONS

5.1 Introduction

In section 1.4 it was pointed out that the purpose of this study was to,

Apply the engineering reliability philosophy

Investigate and calculate the reliability of an existing design for an electric

motor system.

To ensure that life-cycle cost of an electric motor system is optimized through

good reliability design choices.

Through applying the theory of reliability, the development of an electric

motor system can be fulfilled.

Ensure that products meet performance objectives.

Identify potential failure mechanisms during product design using for example

fault tree analysis.

Estimate product warranty costs.

Optimize benefits from design alternatives using reliability optimization.

Find the best reliability allocation to meet system reliability objectives

Predict product reliability prior to making changes.

The purpose of this section is to briefly highlight the results of this study and also to

identify what future steps the supplier considered must implement to ensure that the

reliability of the electric motor system is increased.

5.2 Recommendation

5.2.1 System reliability prediction and evaluation

The author believes it is very important to do proper calculations of component or system

reliability.

80

A relatively standard procedure for evaluating the reliability of a system should be to

decompose it into its components. The reliability of a system can be evaluated by using a

standard procedure of estimating the reliability of each component after decomposing the

system into its components. The overall reliability of the system can then be estimated by

combining the component reliabilities using one or more numerical techniques. The

reasonable and acceptable precision of the reliability of resulting components, determines

the level of the decomposition undertaken ill ' 171 .

5.2.2 Reliability growth management

The author concurs that the supplier must formulate a reliability growth management

program to ensure that all their reliability is achieved and that overall system quality

improves. The analysis of actual test and field data can provide a wealth of information.

Gathering appropriate failure data from testing or from the field and performing a Weibull

analysis should be developed. If desired, additional data can also be acquired from the

history of prior products as well as from reliability prediction results. Results derived from

this data can be used to evaluate trends, pinpoint problem areas, compare various failure

modes, determine optimal replacement of components, and ultimately demonstrate

reliability growth in order to predict future reliability. The may entail:

Gathering field and test and determine optimal replacement times

Prediction of future reliability with specifics such as most common failure

modes

Identification of problem areas using industry-accepted Weibull techniques.

The approach to a reliability and maintainability program is dependent upon many factors

that include the customer's requirements, therefore careful task selection must be done for

each particular program, to ensure that the reliability and maintainability requirements and

objectives are achieved. These are just a few of many questions that need to be asked and

answered prior to implementing a reliability program as follows:

81

How to determine the reliability of a system, taking into consideration the

mission operation profiles?

How to optimize the reliability (and availability) of a system with respect to

the life cycle cost?

Where to focus engineering efforts to minimize program cost?

5.3 Conclusion

The study of reliability engineering in this dissertation has been aimed at general electric

motor systems. The intention has been to present a theoretical background to the subject

and build on that to more general aspects. Some reliability methods and models were used

for analysis of electric motor systems. The necessity of having an effective reliability

management program was highlighted in this study. The author analysed reliability of

motor systems through existing reliability prediction methods and life cycle of motor

systems.

One could not study reliability without reference to economics , although this is another

vast ancillary subject. Economics has therefore been introduced, but only as essentially

required and at the simplest level. For an electric motor system, the economic

consideration was taken into account by the author through life cycle cost and cost

efficiency. The general aim of an economic policy relating to the maintenance and

replacement of electric motors is to minimize their life-cycle costs. Especially, an energy

and reliability program for electric motor systems should be developed to decrease the cost

of energy, production, and maintenance overheads associated with the production of a

product—in effect, reducing the cost per production unit as effectively as possible.

One can not leave the subject of electric reliability without some comment on the

respective roles of engineers and statisticians. The author believes that electrical engineers

must recognize that virtually every quantity that they deal with is distributed. Statistical

methods are available to deal with distributed quantities and they should be used.

82

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00-40/Issue 4, 22 October,1999

http://reliability.sandia.gov/Reliability/design_for_reliability.htm

Relax Software, http://www.relexsoftware.com/resources/lcc.asp 2005

Maintainability, http://www.quality-one.com/services/reliabilitymaint.cfm,

2005

Design for Reliability Service Offerings,

http://www.relexsoftware.com/services/reliabilityconsult.asp Relex

Consulting Services, 2005

Failure Mode Effects and Criticality Analysis (Reliability engineering)

http://www.mtain.com/relia/relfmeca.htm , November 2001

Failure Reporting and Corrective Action System,

http://www.mtain.com/relia/relfracas.htmnfo@mtain.com November,2001

The British Standard BS:3811, Maintenance Management Terms in

Terotechnology, British Standards Institution, London, 1984

Life Cycle Cost Issues, http://www.barringerl.com/lcc.htm, 2005

Reliability and Maintainability,

http://productiveenergy.corn/clients pubs/pubs/aceee study.pdf, 2004

85

The British Standard BS:5760, Reliability of Systems Equipments and

Components: Part 1, Guide to Reliability Programme Management. British

Standards Institution, London, 1984

A.D.S. Carter, Mechanical Reliability, Second Edition, Macmillan Education

Ltd, 1986

B.S. Blanchard, Dinesh Verma, and Elmer L. Peterson, Maintainability : A Key

to Effective Serviceability and Maintenance Management, John Wiley & Sons,

INC., 1995, ISBN 0-471-59132-7

Economics and Reliability,

http://www.converdyn.com/industry/excerpts/economics.html, 2005

Large Business - Programs & Services - Electric Reliability - Reliability

Economics, http://www.sdge.com/business/reliability economics.shtml,

2002

Adrian Drury, Maintenance and Dependability, Carnegie Mellon University,

18-849b Dependable Embedded Systems, Spring 1999,

http://www.ece.cmu.edu/-koopman/des s99/maintenance/

Drives, Motors, Controls'82, University of Leeds, UK, 29 June - 1 July 1982,

Organized and Edited by Professor Peter Lawrenson

Qin Hongbo, Shanghai Energy Service Center, Motor Systems Optimizations

Case Studies in the Peoples Republic of China

The True Benefits of Motor Circuit Analysis, By Howard W. Penrose,

Ph.D,BJM Corp. http://www.ecmweb.com/mag/electric true benefits motor/,

March 1, 2003

China Motor System Project: More Productivity and Less Pollution,

http://www.unido.org/file-storage/download/?file id=18338, 2003

Finding Efficiency and Reliability in Motor Selection, by John Malinowski,

07/01/2004,

86

http://www.pollutionengineering.com/CDA/ArticleInformation/coverstory/BNP

CoverStoryItem/0,6646,127945,00.html

Howard W. Penrose, Ph.D,BJM Corp , Electric Motor Energy and Reliability

Analysis, http://www.mt-online.com/current/10-00mr.html, 2005

Noah Bethel, Improving Motor Reliability Through Static and Dynamic Testing,

http://www.pdma.com/Correlating2.html, 2004

Timothy M. Thomas, Baker Instrument Company, On-line and Off-line Testing

of Electric Motor

B.J. Chalmers, Electric Motor Handbook, First Buttterworth & Co. Ltd., 1988,

ISBN 0-408-00707-9

A.C. Brombacher, Reliability by Design, John Wiley & Sons Ltd., 1992, ISBN

0-471-93193-4

Insulation Quality is Key to Motor Reliability,

http://www.engineeringtalk.com/news/weg/weg104.html, 2005

Wessels A., The Management of Reliability in a Mult-level Support

Environment, RAU, October, 1997

Raptor Reliability Software Example: Electric Motor

http://www.arinc.com/products/raptor/examples/electric motor.html

Inside an Electric Motor, http://electronics.howstuffworks.com/motorl.htm

Roos S.D., A Model for Complex Product Development Using Integrated

Product and Support Development Criteria, PHD Thesis, Rand Afrikaans

University, 2001

87

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APPENDIX B Intim:moe ofuser reci %.i11

rcr1

a1rt

0 and cowe L ■

aLl motor s ecnicauon Aspect 1 Relevant Rritich Comments

supply

Supply voltage

BS4999: Part 30

Choice of supply voltage is related to motor output power. Above 1100V, the motor size and cost increases with voltage

Voltage and frequency fluctuations

BS4999: Part 31 Motors can generally cope continuously with voltage fluctuations of ±6%. Other variations by special agreement

Voltage transients

•

nsulation

Votage transients due to switching or lightning may impose additional stress on main and interturn insulation

Neutral earthing

BS4999: Part 31 The period when operating with one line at earth potential should be limited to prevent the danger of corona attack

System fault level

It is good practice to supply motor terminal boxes which can deal with the system fault level.

Waveform BS4999: Part 31 the instantaneous values of phase voltage should not differ by more than 5% from the fundamental sine wave

Phase balance

BS4999: Part 31 The negative-sequence and zero-sequence components should not exceed 2% of the positive sequence component of voltage.

User requirement

Noise level BS4999: Part 51 Describes test methods and gives tabulation of sound power level limits

vibration BS4999: Part 50 Gives tables of vibration velocity limits for shaft heights up to 400 mm and vibration amplitude limits for larger motors

Dimensions and mounting

BS4999: Part 10 BS4999: Part 22

Leading dimensions are given for foot-mounted machines. Standard codings for mounting arrangements are presented

Cooling methods

BS4999: Part 21 A classification of cooling methods is presented, using coded nomenclature

Duty type BS4999: Part 30 Different duty types are defined to represent actual operating cycles including stopping, starting and speed changing

Torque and Torq current limits

BS4999: Part 41

Design letters define combinations of torque and current at starting. Low starting current and high starting torque need larger motors

Load inertia Drives with large load inertia, such as fans, require special design consideration in order to incorporate sufficient thermal capacity.

Starting frequency

BS4999: Part 41 Cage motor drives involving frequent starting require special design consideration. Normal drives can withstand two consecutive starts.

overloads BS4999: Part 41

Momentary overloads for all types of motor are tabulated. Unless otherwise specified, motors have no sustained overload capacity

overspeed BS4999: Part 41 Motors are designed to withstand 1.2 times the maximum rated speed.

111

91