Factors To Be Considered For Motor Selection

CONTENTSINTRODUCTIONPROBLEMS ENCOUNTEREDDESIGN CONSIDERATIONSCONSTRUCTIONAL FEATURES OF MOTORMECHANICAL DESIGN FEATURESCONCLUSION

IntroductionThe characteristics of motors vary widely with the 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 that specific application.

Like one mentioned above, a number of other 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 subsequently drawing out the technical specifications of the motor.

PROBLEMS ENCOUNTERED

The usual abnormal conditions encountered by the motors are given below.

1. Abnormal System Conditions

VoltageUnder voltageOvervoltageUnbalance in 3-phaseSingle phasingVoltage surgesFrequencyLow frequencyHigh frequency

2. Abnormal Operating Conditions

Locked rotor or stalled rotorRe switching/Frequent start-

stopsMomentary interruption/Bus

transferOverloadingImproper cable sizing

3. Environmental conditions

High/low ambient temperatureHigh altitudeHigh humidityCorrosive atmosphereHazardous

atmosphere/surroundingsExposure to steam/salt-laden,

air/oil vapour

4.Mechanical problems

Seized bearingsExternal shock due to load Incorrect fixing of couplingHigh vibration mountingIncorrect alignment or

foundation levelling

5. Condition at location

Poor ventilationDirt accumulationExposure to direct sunlight

Increase in mechanical stresses leading to:

Shearing of shaftsDamage to winding overhangBearing failuresInsulation failures

Increase in stator and rotor winding temperature leading to:Premature failure of stator or rotor insulation (For wound rotor motor)Increased fire hazardBreakage of rotor bar and/or end ring (For squirrel cage motors)

DESIGN CONSIDERATIONSFollowing are the most important design factors required to be considered when selecting a motor for any of the diversified industrial applications.Output in kW/HPSpeed of the MotorPower Supply Voltage and Frequency VariationsDifferent Rotor ClassesMotor EfficiencyAmbient Temperature AltitudeMethod of Starting and Number of StartsDuty CycleInsulation Class

Output in kW/HPThere are two principle limitations for selecting the motor output:

1. Mechanical limitation

The breakdown torque, which is the maximum torque that the motor can produce when operating without stalling. This is a critical design factor in motor applications, particularly for the motors subjected to occasional extreme load conditions.

Another critical factor is the locked-rotor torque, which is the maximum torque that the motor can produce during start up from steady-state condition, a critical 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 till the winding insulation is able to withstand the prescribed temperature rise over an ambient for that particular class of insulation. Life of the motor greatly depends on the temperature rise of the windings.

Speed of the Motor

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

In order to meet the speed of the driven equipment, the devices like gearbox, chains or belts are introduced between motor and driven equipment. In this case, it may be necessary to provide the rotor shaft suitable for its attachment with the speed decreasing or increasing device and hence the specification should include such specific requirement.

Power Supply Voltage & Frequency VARIATIONSThe permissible voltage variation is ±5 to ±10%, permissible

frequency is 50Hz ± 3%, and permissible combined variation is ±6%.The effect of under voltage is more serious than that of

overvoltage.Continuous operation with under voltage condition increases the

current at the rate of about 20% for every 5% reduction in the supply voltage, increasing the rated copper loss.

This results into heating and prolonged temperature rise, and finally the burning of winding.

The motor offers reduced efficiency at either overvoltage or under voltage.

• It is, therefore, of utmost importance to consider the combined effect of variation in voltage and frequency both when purchasing the motor.

• 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 sequence currents in the rotor causing its overheating and premature failure.

DIFFERENT ROTOR CLASSESThe rotor classes indicate against what quantum of the load torque

the motor would be able to start easily.The motor with KL10 class of rotor, when started direct-on-line,

would accelerate safely to its rated speed against the load torque of 100 % of its rated torque. Similarly, the motor with KL16 class of rotor would be capable of starting against the load torque of

160 % of its rated torque.Though, KL10 class rotor could take maximum starting torque up

to 180 % of the full load torque, and for KL16 class, it could go up to 200 %, but for very minimum time exerting more stress to the rotor.

Such practice reduces the motor life in no uncertain terms

MOTOR EFFICIENCYThe new IEC 60034-30 motor efficiency standard could have major

energy-saving impact for industrial motors worldwide.Though standard motors are now available with a better efficiency,

this factor (motor efficiency) requires due attention when making the selection of the motor for a specific application in view of substantial quantum of power consumed by the motors in the industries.

Ambient Temperature As per normal standards, the motor output is given by the vendors

based on 40°C ambient temperature.In order to maintain the motor output at higher temperature as per the

power requirement of driven equipment, it may be necessary that the motor with a higher frame size for the same rating is selected to avoid adverse effect of derating.

Altitude

The standard motor outputs are specified by the manufacturers for site altitude up to 1000 m.

For altitudes of more than 1000 m, the motor ratings required to be checked for its suitability to maintain the specified output, or otherwise the duration factor is to be applied to know actual anticipated output at higher altitude.

METHOD OF STARTING & NUMBER OF STARTSThe starting performance of the motor depends on the method of

starting deployed, i.e. direction-line, star-delta, high resistance, auto transformer, variable frequency drive, etc.

• Where the starting torque requirement is not so critical, the star-delta starting or any other reduced voltage method of starting is used.

• Where the starting with very heavy load, such as with hoist or crane drives, and speed control over a wide range is required, it is advantageous to consider the slip-ring type (or wound rotor type) motor with a drum controller starter or resistance starter.

Duty Cycle

• The Duty Cycle is a fixed repetitive load pattern over a given period of time which is expressed as the ratio of on-time to cycle period. When operating cycle is such that electric motors operate at idle or a reduced load for more than 25% of the time, Duty Cycle becomes a factor in sizing electric motors.

Also, energy required to start electric motors (that is, accelerating the inertia of the electric motor as well as the driven load) is much higher than for steady-state operation, so frequent starting could overheat the electric motor. so frequent starting, in most probability, is likely to overheat the motor.

When the motor is supposed to operated at idle or reduced load for more than 25% of the time in accordance with its operating cycle, the Duty Cycle becomes a vital factor in sizing the motor.

Also, the energy required to start the motors, i.e. during accelerating along with driven load, is much higher than that required for steady-state operation, so frequent starting, in most probability, is likely to overheat the motor.

Insulation class

For every 10°C rise in operating temperature, the insulation life reduces by 50% of its usual life.

Hence it is essential to specify proper class of insulation for the motor based on design ambient temperature, if it is more than standard design temperature of 40°C.

The endurance of the insulation is adversely affected by many other ageing factors, such as surroundings, electrical and mechanical stresses, vibration, deleterious atmospheres and chemicals, moisture, dirt and radiation.

CONSTRUCTIONAL FEATURES OF MOTORBased on the application requirements, the constructional features of the motor are to be selected as follows:Mounting ArrangementsEnclosuresFrame Size

Mounting ARRANGEMENTSMotors must be securely mounted to drive a piece of equipment,

safety, adequately and efficiently. Mounts for electric motors are available in a variety of styles, each serving a different purpose and application. By determining the current mounting for your motor, you are ensuring proper performance of your electric motor.

Different types of mounting arrangement for the motors are Horizontal foot mounted (B3),Horizontal flange mounted (B5), Flange-cum-foot mounted (B3/B5), Vertical flange mounted with shaft downwards (V3), etc. and so on.

International standards specify mounting arrangement with some different notation.

EnclosuresThe environment in which the motor is operating dictates the type of

enclosure needed for the motor.It is selected based on the specific application and location of the

motor.In terms of motor enclosures, there are two types:

a. Open Enclosure: May not be used in any situation where moisture or explosive dust or gases are present.

b. Totally Enclosed Enclosure: Must use a higher degree of insulation on windings to prevent overheating, and should be used on all motors in animal confinement situations

Standard motor enclosures include Open Drip-proof (ODP), Totally-enclosed fan-cooled (TEFC)—used in outdoor or dirty applications, totally-enclosed air-over (TEAO) (used in grain handling applications), totally-enclosed non-ventilated (TENV) (used in milk pumps and similar applications), and explosion-proof.

Open Type Totally Enclosed Type

 1. Screen protected drip proof (SPDP)  1. Totally enclosed fan cooled (TEFC)

 2. Splash waterproof  2. Totally enclosed surface cooled

 3. Pipe ventilated  3. Totally enclosed pipe ventilated

 4. Weather protected  4. Hose and splash proof

 5. Increased safety motor with enclosure ‘e’ for hazardous areas

 6. Totally enclosed for marine applications

FRAME SIZEThe frame size of the motor is to be selected considering ambient

conditions and environment in surroundings, where it is to be installed.

Mechanical design features

Coupling Arrangement With LoadService Factor Or Overload CapacityLocation Of Motor

Coupling Arrangement with Load

It is necessary to mention whether the motor is to be directly coupled with the driven equipment, or coupled through a belt/chain drive, or gearbox.

SERVICE FACTOR OR OVERLOAD CAPACITY

The service factor of a motor indicates how much it could be overloaded without immediately failing. Generally, the motors are designed with 1.15 service factor with the development of high quality insulating materials that can withstand higher temperatures.

LOCATION OF MOTOR

It is essential to include all the possible details of location and surroundings in the technical specification.

The motor with a suitable enclosure should be selected based on its location, whether it is indoor, outdoor, semi-outdoor, prone to dust, corrosive vapours, chemically hazardous explosive area, salt laden air, etc.

Corrosion to metal body is prevented by applying epoxy based resin paint.

• Most of the motors do not operate at maximum efficiency when overloaded. The optimum efficiency is obtained from the standard motors when operated at around 80 to 95% of their rating and the energy efficient motors offer optimum efficiency when operated between 65% to 95% of their rating.

Operating conditions such as duty cycle, number of starts, ambient conditions, and data for location, environment, driven equipment, etc. are important considerations for the motor efficiency and reliability. It is also imperative to seek advice of the motor manufacturer many a times if motor will be operated under any unusual service conditions to support the selection procedure.

****************************************************************************

CONCLUSION

PRESENTED BY GROUP #8MEMBERS:14T110 – JAYANTHI14T116 – KIRAN KUMAR14T127 – SIDHARTH14T226 – SHANMUGESHWARI14T230 - YOGESHWARAN