Temperature effects on torque production and efficiency of PM motors using NdFeB magnets

Recent developments in high energy magnets have created widespread interest in the area of permanent magnet (PM) motors. The use of PM synchronous motors or brushless motors to replace conventional DC or induction type motors has not been as speedy as anticipated earlier. This paper deals with the temperature effects of PM motors using neodymium magnets on the torque production capability and on the efficiency of the motor. When PM motors are designed to operate in a wide temperature range, the reversible demagnetization of the neodymium magnets with temperature and the increase in winding resistance with temperature influence the maximum torque capability at rated speed and efficiency of the PM motor. The maximum torque at rated speed is limited due to the fixed DC link voltage of the inverter feeding the motor. In this paper, it is shown that over an operating range of -40°C to 150°C the maximum torque capability and efficiency of the motor can vary over a wide range. It is also shown that for certain designs, a near flat maximum torque versus temperature characteristic may be obtained. The major factors influencing these variations are identified. The discussion in this paper is concentrated on PM motors with a trapezoidal back EMF waveform. The idea could be extended to sinusoidal back EMF motors and to PM DC motors