A study of inverter operation in small induction motor drives

Small (< 2 kW) variable speed induction motor drives increasingly find application in many domestic and light industrial applications such as small machine tools, pumps and domestic appliances where they can replace single-phase induction motors or universal motors. Their benefits are widely recognised, particularly improved performance and lower energy consumption. However, the cost and complexity of the inverter remain barriers to the wider adoption of brushless drives. Voltage source inverters using IGBTs are the industry standard for induction motor drives in this range. It is therefore important to study inverters to optimize their design to minimize losses. This reduces the thermal management required, which has a significant effect on the cost of the drive. The losses in an inverter depend on the choice of IGBT and diode, the switching frequency and modulation scheme. Circuits to limit the dV/dt and di/dt also affect the losses. It is therefore important to explore the IGBT´s operation under real circuit conditions. The authors present a modelling exercise in PSpice to understand the effect of these factors on inverter losses as this understanding allows inverter designs can be optimized for low power losses without compromising reliability, cost and EMC standards. A model which accurately represents real circuit conditions and would enable the design to be optimized without building and testing numerous inverter prototypes.