List of Fundamental motor selection criteria

The fundamental technical consideration defining the motor selection for any specific application is the

torque required by the load, especially the relationship between the maximum torque generated by the motor (break-down torque) and the torque requirements for start-up (locked rotor torque) and during acceleration periods. 

The duty cycle determines the thermal loading on the motor. One consideration with totally enclosed fan cooled (TEFC) motors is that the cooling may be insufficient when the motor is operated at speeds lower than the designed value.

 

Ambient operating conditions also affects the motor selection; special motor designs are available for corrosive or dusty atmospheres, high temperatures, restricted physical space, etc. 

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. 

The demand 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 equipment like compressors, the resulting large voltage drops could be detrimental to other equipment.

 

Reliability is the key in many cases, however, designers and process engineers seeking reliability will grossly oversize equipment, leading to sub-optimal energy performance. Fair knowledge of process parameters and a better understanding of the plant power system anatomy can help in reducing over sizing with no loss of reliability.

 

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. This practice affects the motor selection that might provide better energy performance in specific applications. Shorter lead times for securing individual motors from suppliers would help reduce the need for this practice. 

Price is another major and important issue as a nod has to come from both technical and management - Many users are first-cost sensitive, leading to the purchase of less expensive motors that may be more costly on a lifecycle basis because of lower efficiency. For example, energy efficient motors or other specially designed motors typically save within a few years an amount of money equal to several times the incremental cost for an energy efficient motor, over a standard-efficiency motor. Few of notable motor selection issues are given below: 

• In the motor selection process, the power drawn at 75 % of loading can be a meaningful indicator of energy efficiency. 
• Reactive power drawn (kVAr) by the motor. 
• Indian Standard 325 for standard motors allows 15 % tolerance on efficiency for motors upto 50 kW rating and 10 % for motors over 50 kW rating. 
• 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. 
• It would be prudent for buyers to procure motors based on test certificates rather than labeled values. 
• The energy savings by motor replacement can be worked out by the simple relation :                      kW savings = kW output x [ 1/(Old Efficiency)  - 1/ (New Efficiency) ] 
• The cost benefits can be worked out on the basis of premium required for high efficiency vs worth of annual savings.