Brushless DC motor vector control (two)-the working principle and internal structure of brushless DC motor

I believe that many students were confused about the specific working principle when they first started to learn about the brushless DC motor, so let's sort it out together. (The picture in this article comes from Freescale Technical Document PZ104)

1 BLDC working principle

First, let’s look at the picture above. When the two coils are energized, follow the right-hand spiral rule ( hold the energized solenoid with your right hand, and the four fingers bend in the same direction as the current, so the direction caused by the thumb is N pole ). Under the action of the magnetic field, the rotor (permanent magnet ) in the middle will try to keep the direction of its internal magnetic field lines and external magnetic field lines in the same direction, so the rotor will move. The direction of movement of the rotor is related to the current direction of the energized solenoid. The energized direction shown in the figure above, the rotor rotates clockwise.

When the rotor position rotates to the horizontal position, the torque on the rotor is 0, but due to the effect of inertia, the torque of the motor will continue to rotate clockwise at this time. If at this time, the direction of the current is reversed, then the rotor of the motor It will continue to rotate clockwise , as shown in the figure below. By the way, the principle of this place reminds me of the zero-crossing detection loop in the BLDC detection circuit, which I believe has a greater relationship with this. If we don't care about the current commutation at the critical point where the rotor is too horizontal, the motor's rotor will continue to rotate. This operation of changing the direction of the current is called commutation. Many times, if you don't explore the principle of motor rotor movement, it is actually difficult to understand why the motor needs commutation operation. I also sorted out today to know why this place needs to be done.

One thing to note here: the commutation operation is only related to the rotor position of the motor, not to the speed of the motor.

2 Internal structure and actual operation process

The interior of the motor will certainly not be such a simple DC structure. The interior of the motor is generally combined through three-phase coil windings and combined by star connection or corner connection. Then the power supply method involved will be more complicated than the above principles. The picture below shows the internal structure of a common three-phase two-pole star connection motor.

It can be seen from the figure that the three windings are connected together through the center point Y. The whole motor leads to three wires ABC. When they are energized in pairs, they will form the small unit in the first section. There are 6 cases in total, AB\AC\BC\BA\CA\CB (One point needs to be paid attention to here. Although AB and BA are both A and B two-phase energization, but the current direction is different, so the effect obtained is also different). The above six situations will produce six different working magnetic circuits, as shown in the figure below. Take one of them as an example for analysis. When AB is energized, and A is positive and B is negative, according to the right-hand spiral rule, the direction of the magnetic field generated by phase A is shown by the red arrow, and the direction of the magnetic field generated by the phase B coil is shown by the blue arrow. If the direction is shown, the motor will generate a synthetic magnetomotive force in the direction of the green arrow. The permanent magnet rotor of the motor will rotate towards the position of the green arrow. In these six states, switching in a specific order, the motor will continue to rotate. (The position of the rotor is in the center)

summary:

1. The energized conductor is forced in the magnetic field, and the energized coil will generate a magnetic field, which is the basic physical principle of the operation of the motor.

2. The permanent magnet rotor will follow the direction of the magnetic field of the energized coil under the action of the magnetic field. When the rotor position reaches the level, the direction of the current is changed, that is, the direction of the magnetic field is changed, and the motor can rotate.

3. In a three-phase motor, there are 6 phase states AB\AC\BC\BA\CA\CB in the whole 360° , and different states correspond to different magnetomotive force positions, which are divided by 60° in space, so that The motor can rotate.

The content of this section, I think it is more critical, because we have learned PWM, the principle of modulation should be combined with the magnetic field control of the motor itself, so for BLDC, understanding its specific working principle will help us Control him later, hehe, of course, before the complicated control theory, modulation is the most critical asynchronous. You need to learn to send waves and send out PWM that can make the motor rotate, so you don’t have to worry about the latter.