MAGNETIC LOSSES OF ELECTRICAL IRON SHEET IN SQUIRREL-CAGE INDUCTION-MOTOR DRIVEN BY PWM INVERTER

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 fore, the stator teeth, and the rotor core of a 3-phase squirrel-cage induc tion motor driven by both a sinusoidal power supply and a V/f = const PWM inverter power supply, and estimated the magnetic losses in the mo tor. Though the waveforms of the magnetic flux at the slater sore in b oth driving conditions were approximately sinusoidal, the induced volt age waveforms in each driving condition were similar to the source vol tage waveforms. The induced voltage waveform of the stator teeth conta ins higher harmonics, due to the rotor slots and the power supply wave form, and differs depending on the positions of the teeth only in the case of the inverter supply. The magnetic flux ire the rotor core show ed a sinusoidal waveform with the slip frequency multiplied by pole pa ir number, which contained higher harmonics due to the stator slots, t he power supply waveform, the primary voltage imbalance, and the non-u niform air-gap between the stator teeth and the rotor core. In spite o f the complex distribution of the magnetic induction in the motor core for PWM inverter drive, the magnetic losses of the motes are estimate d to be proportional to the running frequency of the inverter and incr ease from those at the sine drive by a certain rate corresponding to t he eddy current losses.