Brushless dc motors winding generate a trapezoidal back EMF synchronized to the position of the rotor magnet.
Hall effect sensors detect the rotor magnet position and provide signals indicating the “flat top” portion for each winding’s back emf.
Six Switching segments can be identified.
Star Connection Control
For any one segment,two winding will be in the “flat top” portion of the back emf and a third winding will be switching between a positive and negative iutput.
Electronic control leaves one winding open circuit,connects one winding to the lower dc rail,and controls the voltage applied to the third winding using PWM.
The fill factor of the applied PWM controls the speed of the motor
Delta Connection Control
For any one segment,two winding are connected to the positive voltage supply and a third winding is connected to the negative voltage supply.
The fill factor of the applied PWM controls the speed of the motor.
Sensor-less control can be achieved by detecting the zero crossing of the BEMF for each phase.
Sensorless Control Benefits - Lower system cost Increased reliability
Sensorless Control drawbacks - BEMF zero crossing cant be reliably detected at low motor speed.