Brushless DC motor is a kind of synchronous motor, that is to say, the speed of motor rotor is affected by the speed of motor stator rotating magnetic field and the number of rotor poles (P)

n=120．f / p。 When the number of rotor poles is fixed, the rotor speed can be changed by changing the frequency of stator rotating magnetic field. Brushless DC motor is a kind of synchronous motor with electronic control (driver), which controls the frequency of stator rotating magnetic field and sends the speed of motor rotor back to the control center for repeated correction, in order to achieve close to the characteristics of DC motor. In other words, brushless DC motor can maintain a certain speed when the load changes within the rated load range.

The brushless DC driver consists of a power supply unit and a control unit. The power supply unit supplies three-phase power to the motor, and the control unit converts the input power frequency according to the demand.

The power supply unit can input direct current (usually 24 V) or alternating current (110 V / 220 V). If the input is alternating current, it must be converted to direct current through converter. Whether it is DC input or AC input, it is necessary to convert DC voltage from inverter to three-phase voltage to drive the motor before it is transferred to the motor coil. Inverter is generally divided into six power transistors (Q1 ~ Q6), the upper arm (Q1, Q3, Q5) / the lower arm (Q2, Q4, Q6) is connected to the motor as a switch to control the flow through the motor coil. The control unit provides PWM (pulse width modulation) to determine the switching frequency of the power transistor and the commutation time of the inverter. Brushless DC motor generally wants to use the speed control that the speed can be stable at the set value when the load changes without too much change, so the motor is equipped with Hall sensor which can sense the magnetic field, as the closed-loop control of speed, and also as the basis of phase sequence control. But this is only used as speed control, not as positioning control.