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High efficiency motor ratings

Q: On premium efficiency motors, the FLA is higher on IEEE841 motors than on standard high efficiency motors. Yet the efficiency is higher on 841. More amps is more watts. What makes a premium efficiency motor more efficient? I'm looking at a 50-hp, 1,800, 3/60/460 motor; (a) 841 motor, FLA: 59.9, Efficiency: 94.5% and a (b) High efficiency motor, FLA: 55.9, Efficiency: 93.6%.

A: The input power to an ac motor is determined by volts, amps, efficiency and power factor. The formula for three-phase power is: watts = 1.732 x volts x amperes x power factor. Applying the formula, the reason for the higher current of the more efficient motor is that its power factor is lower than the standard motor. The premium motor power factor is 0.827 and the standard motor's is 0.895.