The efficiency of a motor is simply the ratio of the power "out" to the power "in" expressed in percentage.
Power input and output for a three-phase motor is:
Efficiency is expressed by:
Shown here are representative performance curves for typical NEMA Design-B motors. Values shown do not apply to a particular motor.
This illustrates how efficiency typically varies with load. Notice that motor efficiency is relatively flat from rated load to 50% of rated load. Some motors exhibit peak efficiency near 75% of rated load.
