A novel method of detecting for rotor position of a sensorless brushless DC motor

In this paper, the author introduces a new rotor position detecting method for the sensorless control of brushless DC motor (BLDCM). It applies a new circuit to get fundamental wave from three phase terminal voltage. A special switched capacitor filter is designed to achieve constant delay 90 electrical degrees. Since the phase lag is not related to the speed of the rotor, it needn´t phase compensation. The filtered signal can be converted to six commutation signals for BLDCM just through zero-crossing comparator and decoder, so the detection circuit is simplified. The hardware-implemented system has low cost and high reliability, which is suitable for oil well detecting.