Selection of Electrical Motor

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Selection of Electrical Motor

Selection of Motor

  1. The primary technical consideration defining the motor choice for any particular application is the tong required by the load, especially the relationship between the maximum torque generated by the motor and the torque requirements for start-up and during acceleration periods.
  2. The duty/load cycle determines the thermal loading on the motor.
  3. One consideration with Totally Enclosed Fan Cooled (TEFC) motors is that the cooling may be insufficient when the motor is operated at speeds below its rated value.
  4. Ambient operating conditions affect motor choice, special motor designs are available for corrosive or dusty atmospheres, high temperatures, restricted physical space etc.
  5. An estimate of the switching frequency (usually dictated by the process), whether automatic or manually controlled, can help in selecting the appropriate motor for the duty cycle.
  6. The demand of a motor will place on the balance of the plant electrical system is another consideration -if  the load variations are large, for example, as a result of frequent starts and stops of large components like compressors, the resulting large voltage drops could be detrimental to other equipment.
  7. Reliability : In many cases, however, designers and process engineers seeking reliability will grossly oversize equipment leading to sub – optimal energy performance.
  8. Good knowledge of process parameters and a better understanding of the plant power system can aid in reducing over sizing with no loss of reliability.
  9. Inventory : Many large industries use standard equipment which can be easily serviced or replaced thereby reducing the stock of spare parts that must be maintained and minimizing shut – down time.
  10. This practice affects the choice of motors that might provide better energy performance in specific applications. Shorter lead times for securing individual motors from suppliers would help to see the need for this practice.
  11. Price : Many users are first – cost sensitive leading to the purchase of less expensive motors that may more costly on a life cycle basis because of lower efficiency.
  12. For example, energy efficient motors or other specially designed motors typically save within a few years amount of money equal to several times the incremental cost for an energy efficient motor, over a standard efficiency motor.

Some salient features are given below :


1. Reactive power drawn (kVAR) by the motor.
2. The Indian Standard IS 8789 addresses technical performance of Standard Motors while IS 12615 addresses the efficiency criteria of High Efficiency Motors. Both follow IEC 34-2 test methodology wherein, stray losses are assumed as 0.5% of input power. By the IEC test method , the losses are understated and if one goes by IEEE test methodology, the motor efficiency values would be further lowered.
3.nIndian Standard 325 for standard motors allows 15% tolerance on efficiency for motors upto 50 kW rating and 10% for motors over 50 kW rating.
4. In the selection process, the power drawn at 75% of loading can be a meaningful indicator of energy efficiency.
5. The cost benefits can be worked out on the basis of premium required for high efficiency versus worth of annual savings.
6. The energy savings by motor replacement can be worked out by the simple relation : 
kW saving = kW output x [1/ηold – 1/ηnew]

where, ηold and ηnew are the existing and proposed motor efficiency values.
7. It would be prudent for buyers to procure motors based on test certificates rather than labelled values.

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