7. What design factors should I consider when choosing a new motor?

Motor Size

Size motors for efficiency. Motors should be sized to operate with a load factor between 65% and 100%. The common practice of oversizing results in less efficient motor operation.

For example, a motor operating at a 35% load is less efficient than a smaller motor that is matched to the same load (see Figure 2).
Of course, some situations may require oversizing for peak loads, but in such cases alternative strategies should be considered, such as a correctly sized motor backed up with a pony motor.

Operating Speed

Select replacement energy-efficient motors with a comparable full-load speed for centrifugal load applications (pumps and fans). Induction motors have an operating speed that is slightly lower than their rated synchronous speed. For example, a motor with a synchronous speed of 1800 rpm will typically operate under full load at about 1750 rpm. Operating speed (full-load rpm) is stamped on motor nameplates.
The difference between the synchronous speed and the operating speed is called slip. Slip varies with load and the particular motor model.
Every pump and fan has a design speed. Centrifugal pump and fan loads are extremely sensitive to speed variations; an increase of just 5 rpm can significantly affect the pump or fan operation, leading to increased flow, reduced efficiency, and increased energy consumption. Whenever a pump or fan motor is replaced, be sure to select a model with a full-load rpm rating equal to or less than that of the motor being replaced.

Inrush Current

Avoid overloading circuits. Energy-efficient motors feature low electrical resistance and thus exhibit higher inrush currents than standard models. The inrush current duration is too short to trip thermal protection devices, but energy-efficient motors equipped with magnetic circuit protectors can sometimes experience nuisance starting trips.


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