Performance of a 7.5 kW induction motor under harmonically distorted supply conditions

The behaviour of a 7.5 kW high efficiency cage induction motor fed by distorted supply has been investigated in this paper. Experimental data were collected by developing a testing facility consisting of a three phase harmonic generator, a double chamber calorimeter and a PC based data acquisition system. Motor additional losses due to the presence of harmonics were estimated using the calorimetric method. It has been shown that a distorted voltage containing low order harmonics causes more losses in the motor as compared with the high order harmonics. Accordingly, weighted THD (WTHD) has been defined to specify the limits for additional losses in a motor supplied by distorted voltages. The variation of test motor parameters with harmonic order as well as the variation of additional losses with WTHD has led to establishment of derating factor (DF) for the test induction motor. Applicability of the DF on several induction motors with different power ratings has been examined. It has been demonstrated that with a service factor of 1.15, most induction motors are capable to handle distorted voltages as defined by standards