An approach to position sensorless drive for brushless DC motors

The authors describe a brushless DC motor without a position sensor. Variable speed is achieved by adjusting the average motor voltage, just like chopper control of DC motors. The position sensorless drive proposed is based on detection of the conducting interval of free-wheeling diodes connected in antiparallel with power transistors. This approach makes it possible to detect the rotor position over a wide speed range, especially at a lower speed. Experimental results obtained from a prototype brushless DC motor of 300 W rating are shown to confirm the validity of the sensorless drive from 45 to 2300 r/min. A starting procedure of the motor is also discussed because it is impossible to detect the rotor position at standstill