PWM schemes for Average line to line voltage based sensorless control of BLDC motor

The Average line to line voltage based bipolar pulse width modulation (PWM) technique is used for the sensorless control of permanent magnet brushless dc (PM BLDC) motors. For increasing the performance of the drive in terms of reduced switching losses and EMI, the unipolar PWM technique can advantageously be used. But, this has a problem that it doesn't give the actual line to line voltages in all the reported unipolar PWM technique based schemes. In this paper, investigations are carried out with the generally used four types of unipolar PWM schemes and it is shown that for the scheme namely the ON-PWM scheme wherein for the first half of the conduction period, the switch is ON continuously and for the remaining half period, the switch is under PWM, the average line to line voltages at the commutation instants are almost the same as in the case of bipolar PWM scheme and can thus give reliable and acceptable sensorless operation of the PM BLDC motors. Simulated voltage waveforms, the derived hall signals and the actual hall signals are given for all the investigated schemes.