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.

The rotor position detector mounted on the motor shaft is an important part of the brushless DC motor. It determines the moment when the windings of each phase of the armature begin to energize. Its function is equivalent to a brush in a general DC motor. Changing the timing (phase) of the signal generated by the position detector is equivalent to changing the position of the brush in the space in the DC motor, which has a great influence on the characteristics of the brushless DC motor. The structure of the position detector is many and it usually includes a set of stationary detection elements and a position signal former that rotates with the rotor of the motor. In a position detector consisting of a Hall element, the Hall element is a detection element and the rotor pole of the motor itself is a position signal former. In other structures, such as electromagnetic induction, photoelectric, and proximity switching, a notched disk is often used as a position signal generator. For example, in the photoelectric type, this notch is used to cause light to shine on the photocell to generate a signal; in the electromagnetic induction type, the notch is used to change the magnetic circuit of the open transformer, so that an electromotive force is generated in the detection coil, and the like.