Effect of PWM voltage excitation on iron loss of inverter fed motors

A growing number of induction motors are used in energy efficient, variable speed drive systems fed from PWM inverter sources. Losses in electrical steel used in stator cores increase by as much as 100% compared to sinusoidal excitation when magnetised under PWM conditions. Laboratory data taken under PWM conditions are used to show how the efficiency of typical motors can fall drastically at certain points of a duty cycle due to variation of the PWM modulation index. The core loss is 20% to 80% higher than under sinusoidal voltage excitation over a typical speed range, which can have a major impact on the claimed efficiency of drive systems.