Abstract
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 causes the temperature rise, depends on the current and voltage wave-forms, 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. One approach for calculation of losses is by detailed modelling of the switching waveform, from which the losses can be calculated directly (voltage multiplied with current). Such an approach requires a complicated mathematical model which again leads to laborious calculations during the simulation, especially if the switching frequency/fundamental frequency ratio is high. 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. The loss calculation are performed simultaneous to the simulation. Each turn-on and turn-off of the device implied a given energy loss. The on-state losses are given by the instantaneous current and chip temperature. The dependency between current, temperature and losses are found from data-sheets and/or loss measurements on a single switch.
