1, structural differences, brushless DC motors usually rotor poles using tile magnets, after the magnetic circuit design, you can get trapezoidal wave of air gap magnetic density, stator winding more concentrated whole pitch winding, so the induction counter-electromotive force is also trapezoidal wave. Brushless DC motor control requires position information feedback, and must have position sensors or use position sensorless estimation technology to form a self-controlled speed control system. The current of each phase is also controlled as square wave as much as possible, and the inverter output voltage is controlled according to the method of PWM for brushless DC motors. In essence, brushless DC motor is also a kind of permanent magnet synchronous motor, speed regulation actually belongs to the category of variable voltage variable frequency speed regulation.

Usually AC permanent magnet synchronous servo motor has stator three-phase distribution winding and permanent magnet rotor, the magnetic circuit structure and winding distribution ensure that the waveform of induction potential is sinusoidal, and the applied stator voltage and current should also be sinusoidal, which is generally provided by AC variable voltage inverter. Permanent magnet synchronous motor control systems are often self-contained and also require position feedback information, and can use advanced control methods such as vector control (magnetic field directional control) or direct torque control.

The difference between the two can be thought of as a difference in design philosophy due to square wave and sine wave control. Finally, to clarify a concept, the so-called “DC frequency conversion” of brushless DC motors is essentially AC frequency conversion through the inverter, and in terms of motor theory, brushless DC motors are similar to AC permanent magnet synchronous servo motors and should be classified as AC permanent magnet synchronous servo motors; however, they are customarily classified as DC motors because of their control and However, it is customarily classified as a DC motor, because from the perspective of its control and driving power supply and control object, it is also appropriate to call it a “brushless DC motor”.

Only for the motor structure, the difference between the two is really not much, I think the difference between the two mainly lies in.

1 conceptual difference. Brushless DC motor refers to a system, to be precise, it should be called “brushless DC motor system”, which emphasizes the integration of motor and controller design, is a whole, the interdependence is very high, the motor and controller can not exist independently and work independently, the assessment is also their overall technical performance. The AC permanent magnet synchronous motor refers to a motor, emphasizing that the motor itself is an independent device, which can leave the controller or inverter and work independently.

2 From the design and performance point of view, “brushless DC motor system” design mainly considers how to maintain the advantages of mechanical commutation motor after changing the ordinary mechanical commutation into electronic commutation, and the focus of the assessment is also the DC motor characteristics of the system, such as speed control characteristics; while the AC permanent magnet synchronous motor design mainly focuses on the performance of the motor itself, especially the AC motor performance, such as speed control characteristics. Especially the performance of the AC motor, such as voltage waveform, motor power factor, efficiency power angle characteristics, etc.

3 From the counter potential waveform, brushless DC motors are mostly square wave, while AC permanent magnet synchronous motor counter potential waveform is mostly sine wave.

4 From the control point of view, brushless DC motor system basically does not need any algorithm, just based on the rotor position to consider the winding current, while the AC permanent magnet synchronous motor needs to be inverter speed control requires a certain algorithm, need to consider the reactive and active armature current, etc.

5 Regarding the question “Can a three-phase brushless DC motor use three-phase sinusoidal AC power? If so, can the Hall device be eliminated?”

In principle, a three-phase brushless DC motor can run on three-phase sinusoidal AC power, but the operating performance may be very poor. If the counter potential waveform of a three-phase brushless DC motor is a square wave, a large harmonic loss will be generated when using three-phase sinusoidal AC power, and the temperature rise will be very high. Whether the need for the Hall device and the use of what power supply (three-phase sinusoidal AC or square wave pulse power) has nothing to do with the motor control algorithm, control strategy and control mode and other factors, if the control mode without position sensor, the Hall device can be used, if the control mode with rotor position sensor, the position sensor is still necessary, of course, can not use the Hall device and use other Of course, you can use other sensors, such as encoders, resolvers, etc. Have measured the brushless DC motor counter-electromotive force, is not a perfect square wave, is like a clipped sine wave, with DC frequency control, the motor speed is relatively smooth, but depends on the accuracy, in your acceptance range.

The motor body of brushless DC motor: the stator winding is concentrated winding and the permanent magnet rotor forms a square wave magnetic field; the motor body of permanent magnet synchronous motor: the stator winding is distributed winding and the permanent magnet rotor forms a sinusoidal magnetic field.

2, the position sensor of brushless DC motor: low resolution, 60 degree resolution, Hall element, electromagnetic, photoelectric; position sensor of permanent magnet synchronous motor: high resolution, 1/256,1/1024, resolver, optical code disk.

3. Different control.

Brushless DC motor: 120 degree square wave current, PWM control.

Permanent magnet synchronous motor: positive metric wave current, using SPWM SVPWM control.

PS: According to the current open source control algorithms, SVPWM is also applicable to the control of three-phase brushless DC motors. There is no need to dwell on the difference between the two, it is not significant. In general, if BLDC is to be made into a high-performance, you can use PMSM control, slightly lower cost, small torque and miniaturization.


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