What is the role of brushless DC motors?
Brushless DC motors use self-controlled inverters. It is different from the general inverter. Its output frequency is not independently adjusted, but is controlled by a rotor position detector mounted on a synchronous motor shaft. Each time the rotor rotates over a certain position, the position detector generates a corresponding signal that acts on the corresponding semiconductor element and energizes the corresponding phase winding to produce a torque. Each pair of magnetic poles of the rotor of the motor rotates, and each semiconductor element alternately conducts one cycle, and the AC output from the inverter changes by one cycle accordingly. Therefore, the output frequency of the self-controlled inverter and the rotation speed of the motor are always synchronized and the out-of-step phenomenon does not occur.
In a small brushless DC motor, the inverter consists of transistors. Since the transistor has self-interruption ability, as long as the control signal on its base disappears, the transistor turns off itself, so the control is relatively simple. In large-capacity brushless DC motors, the inverter consists of thyristors. Thyristors have no self-interruption ability and cannot be turned off by removing the trigger signal. Therefore, when the current of one phase needs to be cut off and the other phase is energized, how to turn off the previously turned-on thyristor and transfer the current to the new one-phase thyristor, that is, the commutation problem between the thyristors is the technical key of the thyristor motor.