Equivalent loading of induction motors using a new composite power supply with a linear (U/Hz) characteristic

Abstract

This paper describes a mixed variable voltage magnitude variable frequency (VVMVF) power supply with linear U/Hz characteristic that can be used for equivalent loading of ac machines. The proposed voltage source ensures the rated air gap flux as well as forces the rated current (Irms) inside the test machine without the need to connect a mechanical load to the shaft. This means that the motor total losses essentially will be the same as under normal full load operation. A comparison is first made between those of equivalent loading schemes of induction machine that use the frequency and magnitude modulating voltage sources. Then the practical implementation of a 1.5 kW voltage source of the above mentioned type is presented which is achieved by series connection of a regular constant voltage and frequency source (the motor-rated source) and a special PWM inverter (auxiliary source), using an isolating transformer. The mixed single-phase source is used for synthetic loading of a capacitor start single-phase induction motor. Simulation and experimental results obtained are in close agreement. Copyright © 2006 John Wiley & Sons, Ltd.