Calculation of temperature conditions in semiconductors for variable speed induction motor drive

In a variable speed PWM induction motor drive the maximum allowed current will be limited by the maximum allowed temperature in the semiconductors. The losses, which cause the temperature rise, depends on the current and voltage waveforms which varies with applied load, speed and motor parameters. Thus it is a complicated matter to estimate accurate temperature conditions in the semiconductor devices of a PWM induction motor drive. This paper describes an alternative method to be used in a simulation program applying ideal switch models. Instead of a detailed modelling of the switches, simple switch models are used in the simulation (low or high resistance, depending on the state of the device). This makes it possible to simulate several thousand switching intervals, which may very well be required because of the relative large time constants in the thermal calculation