September 2012 Electrical Apparatus

September 2012 Electrical Apparatus

This is a summary of the Electrical Apparatus September 2012 featured technical article,  by Richard L. Nailen, P.E.

As emphasis on polyphase induction motor efficiency intensified worldwide, engineers developed more accurate methods for testing motor efficiency to verify compliance with energy conservation standards. Because such motors are essentially constant-speed machines operating at constant voltage and frequency, torque (and therefore output power) are the only variables.

For a motor applied to an adjustable-speed drive, powered by a variable-frequency inverter, testing is not that simple. “Full load” is normally defined as a specific output power level. Since that power varies as the product of torque and speed, and speed varies with frequency, the same output can occur at many different combinations of speed and torque. Efficiency at “full speed” and “rated” or “base” torque may be irrelevant, because the sole purpose of the inverter is to permit operation at other speeds.

Consequently, test protocol must first define what operating points will be the loads of interest. Accurate evaluation of individual motor losses at those load points is difficult. Motor core loss and stray load loss both vary with frequency and voltage. Friction and windage loss varies with speed. Even at the motor nameplate voltage, power output, frequency, and RPM, some increase in total loss will occur if either current or voltage waveforms are not completely sinusoidal.

Test programs in both Europe and the United States have therefore attempted to create families of curves from which efficiency can be estimated at any desired load point. Such values are not precise enough to constitute proof that any standard limit is being met.

Efficiency of the inverter itself generally exceeds 95%. That must be multiplied by the motor efficiency to yield an overall system figure.

Fortunately, these things are seldom important, because driving a centrifugal pump or fan at reduced speed (and input power), instead of operating at full power and controlling flow mechanically, saves so much energy input to the motor that losses within the motor are rarely significant.

To order a back issue with the full article, “Can tests prove ASD (Adjustable Speed Drive) motor efficiency?” call 312-321-9440 or visit our online webstore.

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