In the permanent magnet synchronous motor control system, the commutation of the motor is determined by the position of the rotor, so there must be a rotor position sensor to detect the position of the rotor in real time. There are many types of rotor position sensors actually available. Permanent magnet synchronous motors driven by sine waves generally use resolvers, absolute electric pulse encoders or incremental photoelectric pulse encoders as position detectors, which require continuous detection of rotor position and resolution. The requirements are high and the cost is also high. In a permanent magnet DC brushless motor driven by a rectangular wave, only discrete rotor position information is needed, that is, a limited number of commutation point moments. For example, the commonly used three-phase six-state working mode, only need to determine 6 under a pair of poles. Just one reversing point moment. Therefore, a simple position detector can be used, and the resolution requirement for detecting the rotor position is much lower, so the cost is also lower.

　　This article is taken from ------"Permanent Magnet Brushless DC Motor Technology"