High speed drive using a slotless PM motor

In high speed applications, slotless permanent magnet (PM) motors appear an attractive solution, being almost insensitive to magneto-motive force harmonics and to pulsewidth modulation (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 found during the realization of a 30-krpm 1-kW PM motor prototype are also highlighted. A low-cost commercial sensorless brushless do 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 integrated circuit, an additional control circuit has been designed to detect that anomalous condition and to restart the motor when it occurs.