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Losses, torques and magnetic noise in induction motors with static converter supply, taking multiple armature reaction and slot openings into account

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1 Author(s)
Oberretl, K. ; Fac. Elektrotechnik, Univ. of Dortmund, Dortmund

A system of n complex equations, for which n=14, ..., 30 suffices, is used to determine the n unknown stator - and rotor current harmonics for each of the given terminal voltage harmonics. Correct superposition of harmonics yields the air-gap field, losses, torque and radial force. As an illustrative example, the systems of simultaneous equations are solved for an eight- pole squirrel-cage motor with different numbers of rotor slots. Parasitic effects such as supplementary losses, synchronous torques, magnetic noise or line-current harmonics depend highly on the slot number difference Z1-Z2 and on winding connections. Owing to the voltage distortion of the static converter, copper losses increased by 61%. In the case of parallel winding branches, equalising currents with 511 Hz and 37% magnitude occurred, which increased the copper losses by 152%. With Delta-connected windings, circulating currents with a magnitude of 33% have been detected, thus increasing the losses by 21%. Rules to avoid these additional losses are given. Parasitic synchronous torques are increased and emerge at additional speeds. Asynchronous dips in the torque-speed curve because of voltage harmonics are of minor importance, even for large supply voltage harmonics. The accurate determination of radial force waves enables the prediction of core vibrations and the magnetic noise, even at resonance frequency. It has been found by experiments that sound radiation of an oscillating sphere or cylinder seem not to be equivalent to that of an induction motor.

Published in:

Electric Power Applications, IET  (Volume:1 ,  Issue: 4 )