Operating Limits of Inverter-Driven Permanent Magnet Motor Drives

Recent developments of high-energy permanent magnet (PM) materials such as neodymium-iron-boron have focused attention on the use of PM synchronous motors supplied from inverters for a wide variety of speed control applications. PM motor drives are readily applicable where full torque is required up to full or base speed. They are, however, limited in their ability to operate in the power-limited regime where the available torque is reduced as the speed is increased above its base value. In contrast with the field weakening approach used in dc drives, the magnet is permanent. The torque-speed operating limits of PM motor drives, of conventional design are explored, and then design measures which can substantially extend operation into the power-limited regime are proposed. A trade-off between the low speed-torque limit and the extent of the speed range above base speed is achieved by variations in magnet dimensions and the depth of inset of the magnets in the rotor iron. Experimental results are presented for two motors of different design, each using neodymium magnets and each supplied from a current-controlled inverter with hysteresis control of current waveshape.