Direct rotor position detection method for sensorless brushless DC motors

Rotor position sensors are eliminated in a rotor position detection method developed for sensorless,brushless DC motor drives.This method directly extracts the true back electromotive force(EMF) zero-crossing point by detecting the voltage difference between the floating phase terminal and the midpoint in the DC link.The commutation signal of the floating phase is obtained by lagging the back EMF zero-crossing point 30°.The method provides good motor performance over a wide speed range,especially at lower speeds.Experimental results from a prototype brushless DC motor verify the analytical results and confirm the validity of the method.