An improved BEMF detection method for sensorless BLDC motors

There are many ways to test the sensorless of the DC brushless motors, in which the counter electromotive force (EMF) is of the obvious superiority, become a well rounded. Here, the principle of counter EMF is introduced as well as a new program of counter and its modified model are analyzed. This method directly extracts the true back electromotive force 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 30deg. Its advantage is that the precise crossover point can be tested without building up a suppositional neutral point and a filter of low frequency&voltage division circuit. As a result, this approach makes it possible to detect the rotor position over a wide speed range, especially at a lower speed.