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Selection o f Induction Moto rs f or Industrial Applications (photo by http://www.iecee.org)

http://electr ical-engineering-portal.com/selection-of- induction-motors-for- industr ial-applications-part-1 May 13, 2013

Selection of Induction Motors for Industrial Applications(part 1)

 Ashok Parikh

Introduction

 All types of industries are invariably required to install dif ferent t ypes of electric mot ors as primemover for driving process equipment participating in their respective product ion line up. Thecont inuous process of technical development has resulted into availability o f highly diversifiedtypes of electric mot ors.

Hence, an utmost care should be exercised in selection of most appropriate type of motor considering number of technical factors for each application, so t hat the motor wouldprovide desired and optimum performance.

The characteristics of motors vary widely with t he nature of their application and the type of duty they are expected to perform. For example, the applications like constant speed, constant

torque, variable speed, continuous/intermittent duty, steep/sudden starts, frequent start/stops,etc. should be taken into consideration carefully when deciding for the type of a motor for thatspecific application.

Moreover, the motors are required to perform quite often under abnormal conditions duringtheir to tal service life.

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Stator and Roto r Damages

In view of above, an incorrect selection of motor always lands the industrial buyer into all sortsof problems, including premature failure of the motor, causing severe product ion curtailments.

Like one mentioned above, a number of ot her factors and design features like weather conditions, stringent system conditions, abnormal surroundings, hazardous area, duty cycle, motor efficiency , etc. should be considered while deciding the rating and subsequentlydrawing out the technical specif ications of the mot or.

Abnormalconditions andeffects

The usual abnormal conditionsencountered by themotors are givenbelow.

1. AbnormalSystemConditions

1. Voltage

1. Undervoltage

2. Overvoltage

3. Unbalance in 3-phase

4. Single phasing

5. Voltage surges

2. Frequency 

1. Low frequency

2. High frequency

2. Abnormal Operating conditions

1. Locked rotor or stalled rotor 

2. Reswitching/Frequent start-stops

3. Momentary interruption/Bus transfer 

4. Overloading

5. Improper cable sizing

3. Environmental conditions

1. High/low ambientt emperature

2. High altitude

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3. High humidity

4. Corrosive atmosphere

5. Hazardous atmosphere/surroundings

6. Exposure to steam/salt-laden air/oil vapour 

4. Mechanical problems

1. Seized bearings

2. Incorrect alignment/foundat ion levelling

3. Incorrect fixing of coupling

4. High vibration mount ing

5. External shock due to load

5. Condition at location

1. Poor ventilation

2. Dirt accumulation

3. Exposure to direct sunlight

Though, above ment ioned abnormalities may prevail for short or long duration or maybe transient in nature, major impact of the listed abnormal conditions is overheating of themotor along with one or several of the other eff ects as fo llows.

Change in the motor performance characteristics like drawl of more power andconsequent deterioration in mot or ef f iciency, etc.

Increase in mechanical stresses leading to:

1. Shearing of shafts

2. Damage to winding overhang

3. Bearing failures

4. Insulation failures

Increase in stator and rotor winding temperature leading to:

1. Premature failure of stator or rotor insulat ion (For wound rotor motor )

2. Increased f ire hazard

3. Breakage of rotor bar and/or end ring (For squirrel cage motors)

 All the motors encounter few or several of these abnormalities during the course of their 

service lives. Consideration of listed abnormal conditions at design stage greatly helps tominimise the effects of abnormal conditions to maintain a consistent performance.

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Design Considerations

Following are the most important design factors required to be considered when selecting amotor for any of the diversified industrial applications.

Output in kW/HP

There are two principle limitations for selecting the motor output:

1. Mechanical limitation

The breakdown torque, which is the maximum torque that the mot or can produce whenoperat ing without stalling. This is a crit ical design factor in motor applications, particularly for the motors subjected to occasional extreme load conditions.

 Another critical factor is the locked-rotor to rque, which is the maximum torque that the motor can produce during startup from steady-state condition, a crit ical design feature for conveyor drives.

2. Electrical limitation due to insulation provided on the motor windings

The electrical load on the motor can be imposed t ill the winding insulat ion is able to withstandthe prescribed temperature rise over an ambient for that particular class of insulat ion. Life of the motor greatly depends on the temperature rise of the windings.

 Ant icipated life-span of the motor can be achieved provided it is operated at its rated outputwithout overloading and the prescribed preventive maintenance practices are religiouslyfollowed.

Speed of the Motor 

Most of the motors are directly coupled with the driven equipment where in the speed of themotor and the driven equipment will be same.

In order to meet t he speed of the driven equipment, the devices like gearbox , chains or beltsare introduced between mot or and driven equipment. In this case, it may be necessary toprovide the rotor shaft suitable for its attachment with t he speed decreasing or increasingdevice and hence the specification should include such specific requirement.

In case a variable speed drive is to be used for the speed variation, the motor shouldbe compatible for this specif ic application. The standard mot or may not provide desiredperformance when operated via variable speed drive.

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Power Supply Voltage and Frequency Variations

Variations in the power supply parameters, i.e. voltage and f requency significantly af fect overallperformance of the motor. As provided in IS:325-1996, the permissible voltage variation is ±5 to ±10%, permissible frequency is 50Hz ± 3%, and permissible combined variation is ±6 %.

The effect of undervoltage is more serious than that of overvoltage.

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Large burned out induction motor 

The higher torque, result ing from overvoltage, can handle a lit t le overload without undueheating of the winding, but only fo r a short duration. Cont inuous operation with undervoltagecondition increases the current at the rate of about 20% for every 5% reduction in the supplyvoltage, increasing the rated copper loss.

This results into heating and prolonged temperature rise, and finally the burning of winding .During a motor start-up, the torque reduces by 10% for each 5% reduction in the supplyvoltage, causing more start ing current and consequently more rapid heat ing of the

winding.

Themotor offers

reduced efficiency at either overvoltage or undervoltage. Power factor drops sharply withhigher voltage and improves with lower voltage. Even when mot or is lightly loaded, over-voltagecause rise in current and t emperature thus reducing the life of motor. The variation in frequencyby +5 % decreases the torque by about 10% and vice-versa at – 5% f requency, the torqueincreases by about 10%.

It is, therefore, of utmost importance to consider the combined effect o f variat ion in voltage

and frequency bot h when purchasing the mot or.

Unbalance in the supply voltage results into a current unbalance of 6 to 10 times the percentage voltage unbalance. This in turn results into generation of negative sequencecurrents in the rotor causing its overheat ing and premature failure.

It is therefore vital to specify the permissible limits of variations in the power supply parametersfor the motor in accordance with the requirement of the driven equipment. However, thepermissible limits should never be more than provided in the applicable Indian Standard IS:325-1996 (Reaff irmed in 2002).

Will be continued very soon…