Wednesday, 10 February 2016

Motor Selection

The primary technical consideration defining the motor choice for any particular 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 / load 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 below its rated value.

Ambient operating conditions affect motor choice; 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 components like compressors, the resulting large voltage drops could be detrimental to other equipment.

Reliability is of prime importance - in many cases, however, designers and process engineers seeking reliability will grossly oversize equipment, leading to sub-optimal energy performance. Good knowledge of process parameters and a better understanding of the plant power system can aid in reducing oversizing with no loss of reliability.

Inventory is another consideration - 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 choice of motors 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 issue - 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 salient selection issues are given below:
  • In the 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 IS12615 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 × [ 1/ηold – 1/ ηnew ] where ηold and ηnew are the existing and proposed motor efficiency values. 
  • The cost benefits can be worked out on the basis of premium required for high efficiency vs. worth of annual savings.