Semiconductor power losses in AC inverters

The AC induction motor is the workhorse of modern industry. Worldwide about 50 million motors are installed every year which are greater than 1/2 hp. Three-phase induction motors are very simple and inexpensive to build. An AC motor may be used for variable speed applications with the addition of an AC inverter. An accurate model of semiconductor power losses is necessary to design a reliable invertor system using a minimum amount of silicon. Equations for the average and RMS transistor and diode currents are derived based on a quantitative analysis of the current waveforms. Conduction losses in the semiconductor devices are then approximated using a piecewise linear approximation of the device's on-voltage characteristics. Measurements of switching energy versus current are used to obtain a curve of losses versus current. Integration of these losses over a full sine wave period is used to obtain a closed form expression for switching losses as a function of sine wave current. Total losses, conduction losses and switching losses are compared to thermal measurements. The model is further tested by comparing predicted and actual losses at different PWM frequencies.