Thermal modelling of an induction motor to study local temperature rise differences between sinusoidal and PWM supply

Abstract

Induction motors in modern drive systems are commonly supplied from voltage-source inverters, where pulse-width modulation (PWM) introduces harmonic distortion that increases losses and modifies their spatial distribution. Measurements on a 5 kW SEMTEC induction motor showed that the temperature rise under PWM supply exceeded the level predicted by the increase in total losses alone, suggesting that additional local heating mechanisms were present. This observation motivated the development of a steady-state lumped-parameter thermal network to study local temperature-rise differences between sinusoidal and PWM excitation.

A 22-node motor-specific thermal network was created, including separate nodes for end-windings, end-space air regions and the stator–frame interface. Internal air-gap convection was modelled as speed dependent, enabling the representation of inverter-fed operation at reduced speed. Because experimental measurements did not provide a full PWM loss breakdown, a residual-loss allocation method (“Path B”) was introduced, where the measured PWM loss increment was distributed to nodes most sensitive to waveform distortion.

The model was implemented in MATLAB and evaluated against eighteen temperature-sensor measurements across seven operating points. The results showed that the network reproduced the measured temperature ordering for both supply types, and when PWM residual losses were assigned to winding and near-winding regions, the higher temperature rise under PWM was captured. The main sources of uncertainty were the frame-to-ambient convection coefficient, the stator–frame contact resistance and the proportion of PWM-related losses.

The work provides a transparent and reproducible method for comparing sinusoidal and PWM operation in induction motors and demonstrates that a measurement-supported thermal network can describe local temperature-rise differences under inverter excitation with good consistency.