Transient harmonic torques in induction machines: measurement and impact on motor performance

This paper focuses on the measurement of transient torques produced by the field harmonics in three phase squirrel cage motors and on the assessment of the operating conditions under which these torques might become significant. The paper presents first a model of the induction machine with any stator and rotor phase number and configuration, fed by arbitrary voltage waveforms and taking into account the space harmonics. Then, very much attention is paid to a reliable model validation against actual transient torques measurements. In this sense, the actual and great difficulties of measuring pulsating electromagnetic torques of several hundreds of Hertz are brought into light, the possible measurement techniques are critically reviewed and the solution chosen is discussed. Once the model accuracy has been experimentally confirmed, it is used to carry out a systematic plan of more than 250 simulations under clearly distinct operating conditions on five different motors. The analysis of these simulations yields a main practical conclusion: field harmonics may have (depending on the motor structure) a significant impact on electromagnetic torque, but only during transients characterized by high instantaneous frequencies of the rotor currents. For low slip transients, their impact is negligible. This conclusion is also theoretically justified in the paper.