Magnetic losses of electrical iron sheet in squirrel-cage induction motor driven by PWM inverter

Abstract

In order to understand the magnetic behavior of electrical iron sheets assembled into an actual motor core driven by a PWM inverter, we have observed induced voltages and magnetic inductions in the stator core, the stator teeth, and the rotor core of a 3-phase squirrel-cage induction motor driven by both a sinusoidal power supply and a V/f=const PWM inverter power supply, and estimated the magnetic losses in the motor. Though the waveforms of the magnetic flux at the stator core in both driving conditions were approximately sinusoidal, the induced voltage waveforms in each driving condition were similar to the source voltage waveforms. The induced voltage waveform of the stator teeth contains higher harmonics, due to the rotor slots and the power supply waveform, and differs depending on the positions of the teeth only in the case of the inverter supply. The magnetic flux in the rotor core showed a sinusoidal waveform with the slip frequency multiplied by pole pair number, which contained higher harmonics due to the stator slots, the power supply waveform, the primary voltage imbalance, and the nonuniform air-gap between the stator teeth and the rotor core. In spite of the complex distribution of the magnetic induction in the motor core for PWM inverter drive, the magnetic losses of the motor are estimated to be proportional to the running frequency of the inverter and increase from those at the sine drive by a certain rate corresponding to the eddy current losses.