TORQUES IN INDUCTION MACHINES DUE TO LOW-FREQUENCY VOLTAGE/CURRENT HARMONICS

E.F. Fuchs and M.A.S. Masoum

Keywords

Time and space harmonics, lowfrequency harmonic voltages/ currents, harmonic torques, voltagesourcefed induction motor, currentsourcefed induction motor

Abstract

Harmonic torques in induction machines can be generated by spatial and time harmonics. Stator and rotor slotting, rotor eccentricity, and multiple armature reactions cause space harmonics. Time harmonics originate in the non-sinusoidal wave shape of the terminal voltage. In addition, the interaction of space and time harmonics may generate side-band frequency torques, which can be of integer, non-integer (inter, fractional), or sub-harmonic order h within the frequency range from 0 to 60 Hz – appearing within the fundamental torque characteristic near zero speed. This paper analyzes and computes harmonic torques of induction machines due to low-frequency (fh < 60 Hz) components of the terminal voltage/current. The analysis suggests that the amplitude of low-frequency voltage/current harmonics must be restricted to a few percentage of the fundamental voltage not to jeopardize the start-up of induction motors.

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