Thermal Management

Of

Large Industrial Motors

Prepared by:

Danaraj Chandrasegaran KGH070029

Mahyar Silakhori KGH090001

For

Applied Thermodynamics (KXGM 6103)

Dr. Saidur Rahman

October 2009

Contents

1.0 Background .............................................................................................................. 3

2.0 Working Principle.................................................................................................... 7

3.0 Mathematical Models............................................................................................. 10

4.0 Results and Discussion .......................................................................................... 12

5.0 Conclusion ............................................................................................................. 15

6.0 References.............................................................................................................. 16

1.0 Background

An electrical motor uses for several reason such as, electrical energy to product

mechanical energy, usually through the interaction of magnetic field and current-carrying

conductors. The reverse process, producing electrical energy from mechanical energy, is

accomplished by a generator or dynamo. Traction motors used on vehicles often perform

both tasks. Electric motors can be run as generators and vice versa, although this is not

always practical. Electric motors are ubiquitous, being found in applications as diverse as

industrial fans, blowers and pumps, machine tools, household appliances, power tools,

and disk drives. They may be powered by direct current (for example a battery powered

portable device or motor vehicle), or by alternating current from a central electrical

distribution grid. The smallest motors may be found in electric wristwatches. Medium-

size motors of highly standardized dimensions and characteristics provide convenient

mechanical power for industrial uses. The very largest electric motors are used for

propulsion of large ships, and for such purposes as pipeline compressors, with ratings in

the thousands of kilowatts. Electric motors may be classified by the source of electric

power, by their internal construction, and by their application.

The physical principle of production of mechanical force by the interactions of an electric

current and a magnetic field was known as early as 1821. Electric motors of increasing

efficiency were constructed throughout the 19th century, but commercial exploitation of

electric motors on a large scale required efficient electrical generators and electrical

distribution networks.

Unlike internal combustion engines, electrical motors are generally characterized by very

high efficiency. In fact, ongoing engineering efforts have pushed the degree of modern

motors to within a few percentage points of 100, depending on the size of given a motor.

Poor thermodynamic performance is principally the result of exergy losses

Cooling the electric motors is a challenging task. Thermal dissipation becomes a

key consideration in the operation of large motors, since overheating will result in a

decrease of the motor’s lifetime. Having information for the flow and temperature fields

inside the heat exchangers and the temperature distributions of rotor and stator through

experimental testing is expensive and difficult. Therefore, a Computational Fluid

Dynamics (CFD) software package is useful for addressing this type of problems.

The present paper discusses the thermal management system for large industrial

motors. It is divided into three parts discussing the industry practices, thermodynamics

performances analysis and conclusion.

1.1 Large Industrial Motors

Large industrial motors are used for demanding applications in sectors such as

mining and cement, utilities and water works. Each of this type of motor is custom

engineered to suit the applications and fairly involve large capital investments. The

requirements for these motors include:

a) High starting torque,

b) High inertia – low starting current

c) High torque through entire speed range

d) Suitable for starting at weak network

e) Adaptable for variable speed drives

The usual range for large industrial motors as follows:

a) 300 to 12 000 kW at 50 Hz

b) 200 to 9000 HP at 50 Hz

c) Voltage range 380 to 13 800 V

Due to the fact of large power output from large industrial motors, heat

dissipation from this type of equipment is substantial and permits for attention from the

manufacturer themselves. Heat dissipation problems are addressed via a thermal

management system which is integral of the motor.

Before dwelling further, it is imperative that large industrial motors are described

fairly to have a better overall view and the requirements which surround it. There are

many types of large industrial motors that are available on the market. It varies in terms

of construction, type of starting, insulation class and others. For this paper, the

discussions will be limited to slip ring motor (of air cooled type). A detailed example

from the industry will be discussed in relation to this motor.

Figure 1 : Slip ring motor

1.2 Rotor

There are two types of rotors for induction motors, which are squirrel cage type

and wound type. The most common rotor is a squirrel-cage rotor. It is made up of bars of

either solid copper (most common) or aluminum that span the length of the rotor, and are

connected through a ring at each end. The rotor bars in squirrel-cage induction motors are

not straight, but have some skew to reduce noise and harmonics. A slip ring rotor

replaces the bars of the squirrel-cage rotor with windings that are connected to slip rings.

When these slip rings are shorted, the rotor behaves similarly to a squirrel-cage rotor;

they can also be connected to resistors to produce a high-resistance rotor circuit, which

can be beneficial in starting.

1.3 Stator

The stator consists of wound 'poles' that carry the supply current to induce a

magnetic field that penetrates the rotor. In a very simple motor, there would be a single

projecting piece of the stator (a salient pole) for each pole, with windings around it; in

fact, to optimize the distribution of the magnetic field, the windings are distributed in

many slots located around the stator, but the magnetic field still has the same number of

north-south alternations. The number of 'poles' can vary between motor types but the

poles are always in pairs (i.e. 2, 4, 6, etc.)

1.4 Thermal Management System (Cooling System)

This is discussed in depth in the next section.

2.0 Working Principle

For large motors, a distinct cooling system is required due to the high heat

dissipation from the motors. High heat dissipation is a result from the high voltage and

current consumption of the motors compared to other smaller range motors.

The cooling system utilized for electric motors are defined by EN 60034-6

(European Code). In principle there are three types of cooling systems for large motors:

a) Air-to-air heat exchanger (IC611)

IC611 cooling type represents the established standard in the industry. It mainly

used in cement industry, smelting plants, mills, crushers and fans, where use of water is

avoided. Figure 2 shows the inner workings of it.

Figure 2 : Workings of IC611 Air-to-air heat exchanger

b) Air-to-water heat exchanger (IC81W)

IC81W cooling is not a common system found in the industry and generally more

expensive to construct. It is ideal for power plant, paper and steel industry. Figure X

shows the inner workings of it. The internal air circulation is provided by a shaft mounted

fan or a separate blower. In air-to-air cooled motors, the external cooling air is circulated

by a shaft mounted fan or a separate blower. Figure 3 shows the inner workings of it.

Figure 3 : Workings of IC81W Air-to-water heat exchanger

c) Open circuit ventilation (IC01)

Very few motors use this type of cooling method. However, this type offers the lowest

capital cost for deployment. Figure X shows the inner workings of it.

Figure 4 : Workings of IC01 Open Circuit Ventilation

The selection of the cooling system is usually driven by cost factor, available

facilities, environmental aspects and maintenance requirements. Disadvantages and

advantages of the different cooling system is shown in Table 1.

Table 1 presents the advantages and disadvantages of different cooling system.

Table 1: Advantages and disadvantages of different cooling system

Cooling type IC611 IC81W IC01

Advantages System is

independent from

any cooling water

circuit

Winding is

protected against

environmental

impact

Winding is

protected against

environmental

impact

Can withstand

polluted

Ambient

temperature has

System involves the

lowest procurement

environment negligible impact cost

Disadvantages Require regular

cleaning effort

System involves the

highest procurement

cost

Ambient airborne

pollutant drawn into

motor

A cooling water

system is required

Require increased

cleaning effort

3.0 Mathematical Models

A typical schematic is shown in Figure 5. This shows the flow paths and component for

the motors with air- to-air heat exchanger(IC611).

Figure 5 : Schematic View of flow paths and components for the motor

Heat dissipation from an electric motor can be calculated as follows, when the driven

machine is outside of the airstreams:

Hloss = P* [ (1.0- EM)/EM)]* FUM* FLM [1]

where

Hloss = heat loss from motor ( dissipated to the surroundings), W

P = motor power rating, W

EM = motor efficiency, decimal fraction < 1.0

FUM = motor use factor, 1.0 or decimal fraction < 1.0

FLM = motor load factor, 1.0 or decimal fraction < 1.0

The mass flow of air needed for transporting heat from the electric motor can be

expressed as

mair = Hloss / cp (tout - tin) [2]

where

mair = mass flow of air (kg/s)

Hloss = heat loss to the surroundings (W)

cp = specific heat capacity of air (kJ/kg o

C) (1.005 kJ/kg o

C standard air)

tout = temperature of air out (oC)

tin = temperature of air in (oC)

The volume flow can be calculated by multiplying [2] with the specific volume or

inverted density:

qair = (1 / ρair) mair [3]

where

ρair = density of air at the actual temperature (kg/m3)

3.1 Efficiency of Thermal Management System

Efficiency of the thermal management can be defined same as heat exchanger

efficiency. Heat exchanger efficiency can be expressed as

ηHX= ( T2 – T1)/( T3 – T1)

where

T1 = Inlet temperature of cold stream

T2 = Outlet temperature of cold stream

T3 = Inlet temperature of hot stream

T4 = Outlet temperature of hot stream

3.2 Second Law Efficiency Analysis

For an adiabatic heat exchanger with two unmixed fluid streams, the exergy

supplied is the decrease in the exergy of hot stream, and the exergy recovered is the

increase in the exergy of the cold stream, provided that the cold stream is not at a lower

temperature than the surroundings. Therefore, The second law efficiency of the heat

exchanger becomes:

ηII= mcold (ψ4 – ψ3)/ mhot(ψ 2– ψ1)

where

mcold = Mass flow rate of cold stream (kg/s)

mhot = Mass flow rate of cold stream (kg/s)

ψ = Stream exergy (kJ/ kg)

4.0 Results and Discussion

4.1 Results

In this section, the performance of the thermal management system will

discussed. Motor data were based from experiments conducted Chih Chung et al. (2009)

as shown in Table 2. Then efficiency of the system is calculated. The results are shown in

Table 3.

Ambient

Temperature (C)

Description

28

ηHX 0.29

ηII 0.92

Table 3: System Efficiencies

There is a noted difference between the two efficiency values. This is due to

exergy loss as the second law efficiency describes the measure of approximation to the

reversible operation .

Motor Ratings kW 2350

Motor Efficiency % 97

Use Factor 1.0

Load Factor 1.0

Cold Stream

Temperature In

C 28

Cold Stream

Temperature Out

C 46

Hot Stream

Temperature In

C 91

Hot Stream

Temperature Out

C 60

Axial Fan Power kW 2.7

Centrifugal Fan

Power

kW 2.7

Table 2: Motor Data

4.2 Exergy Loss

Exergy loss can be attributed to the followings:

a) Air leakage in motor casing

This can affect on the motor cooling performance. As pointed out by Chih Chung et al

(2009) leakage effect inside the motor decreases the flow rate of the internal air. To

overcome this problem, dimensional changes between the rotor and the axial fan have to

be considered for further improvement of cooling performance.

b) Pressure drop in the streams

Flow through the streams also experiences pressure. Higher pressure drop means more

energy translated to the surroundings. This also contributes to the exergy loss.

c) Fouling of Heat Exchanger

Fouling refers to undesired accumulation of solid material (by-products of the heat

transfer processes) on heat exchanger surfaces which results in additional thermal

resistance to heat transfer, thus reducing performance. The fouling layer also blocks the

flow passage/area and increases surface roughness, thus either reducing the flow rate in

the exchanger or increasing the pressure drop or both

5.0 Conclusion

This paper has presented the various type of thermal management system for large

industrial motors. The thermal performance of a motor with air-to-air heat exchanger has

been assessed here and exergy losses have been pointed out. A further development of

mathematical analysing the dimensions within the thermal management system to

improve the cooling performance of the thermal management is suggested here.

6.0 References

American Society of Heating Refrigerating and Air-Conditioning Engineers Inc., 1999,

HVAC Applications Handbook,USA

Cengel, Y. A. & Boles, M. A., 2007, Thermodynamics: An Engineering Approach,

McGraw Hill, Singapore

Chih Chung et al, 2009, ‘Experimental and Numerical Investigations of Air Cooling for

Large Scale Motor’. International Journal of Rotating Machinery, vol. 2009, Article ID

612723