High speed drive using a slotless PM motor

In high speed applications, slotless PM motors appear an attractive solution, being almost insensitive to MMF harmonics and to PWM current ripple and exhibiting lower stator iron losses and rotor losses (significant with squarewave current control). This paper illustrates advantages and drawbacks of a high speed drive using a slotless PM motor. The difficulties have been found during the realization of a 30-krpm 1-kW PM motor prototype are also highlighted. A low-cost commercial sensorless BLDC motor controller has been used to force squarewave currents without a significant PWM ripple. In order to recognize a possible unexpected rotor arrest, that is not detected by the IC, an additional control circuit has been designed to detect that anomalous condition and to restart the motor when it occurs.