Localised losses in stator laminations of an induction motor under PWM excitation

Electronically generated voltage waveforms have increasingly been used to energise small induction motors for speed and torque control due to low cost. The most popular such waveform is a pulse width modulation (PWM), which is obtained by logically comparing a reference voltage waveform with a carrier wave of different frequency. When a small induction motor is driven by a typical PWM voltage supply, additional power losses occur in its stator core due to a magnetisation process, which is very different to standard sine wave excitation. In this investigation flux density and power loss distribution under PWM and sinusoidal voltage excitations were measured in a small induction motor stator core using a system which set up flux distribution as would occur in a rotating 3-phase stator core. The iron loss increase was between 15 and 20% under PWM excitation. Surprisingly, the increase in iron loss in a typical induction motor stator core lamination was 3% lower than that in Epstein strips of the same electrical indicating an effect of the magnetic circuit geometry as well as the voltage waveform.