A super high speed PM motor drive system by a quasi-current source inverter

The authors describe a surface-mounted PM (permanent magnet) motor with the ratings of 5 kW and 180000 RPM for super-high-speed drive applications such as machine tools. A much larger air gap, compared with that of the existing PM motors, is used to reduce the slot ripples and hence the resultant eddy current loss in the metallic sleeve that retains the permanent magnets; otherwise, this loss would be objectionable at such high speeds. The motor is supplied from a quasi-current source inverter with the DC current controlled by a chopper and operated at a near-unity power factor by the use of a simple position sensorless control, to improve the motor efficiency. Analytic results are presented showing the slot ripple reducing effect of increasing the air gap length. To investigate the losses of the sleeve, several motors with different sleeves were manufactured and tested. Typical experimental results on these prototypes are given, which show that a motor efficiency of higher than 90% can be realized at the rated torque over a wide speed range