Let's talk about the basic principle of the motor first. The basics can be skipped directly.
Everyone has played with magnets when they were young. Different poles attract each other, and the two magnets collided as soon as they approached.
Now suppose your hands are fast enough to lure in front of you with one magnet, and the other magnet follows you all the time.
You hold the magnet in your hand and draw circles, and the other magnet follows you in circles.
The above is the basic principle of motor rotation. It's just that the "magnet" used to seduce is not a real magnet, but a magnetic field generated by the coil energized.
1. Introduction of brushless DC motor
Brushless DC motor, English abbreviation is BLDC (Brushless Direct Current Motor). The stator (the moving part) of the motor is the coil, or winding. The rotor (the part that turns) is a permanent magnet, which is a magnet. According to the position of the rotor, the single-chip microcomputer is used to control the energization of each coil, so that the magnetic field generated by the coil changes, so as to continuously seduce the rotor in the front to make the rotor rotate. This is the rotation principle of the brushless DC motor. Let's dive in.
2. The basic working principle of brushless DC motor
2.1. Structure of brushless DC motor
Let's start with the most basic coils first.
As shown below. A coil can be understood as something that grows like a spring. According to the right-hand spiral rule learned in junior high school, when the current flows from the top to the bottom of the coil, the upper polarity of the coil is N, and the lower polarity is S.
Now make another coil like this. Then fiddle with the position. This way, if the current passes through it, it will act as if there are two electromagnets.
Get another one to form the three-phase winding of the motor.
Coupled with the rotor made of permanent magnets, it is a brushless DC motor.
2.2. Current commutation circuit of brushless DC motor
The reason why the brushless DC motor only uses direct current and no brushes is because there is an external circuit to specifically control the energization of its coils. The main component of this current commutation circuit is the FET (Field-Effect Transitor). A FET can be thought of as a switch. The diagram below labels the FETs as AT (A-phase Top), AB (A-phase Bottom), BT, BB, CT, CB. The "opening and closing" of the FET is controlled by the microcontroller.
2.3. Current commutation process of brushless DC motor
The timing of "opening and closing" of the FET is controlled by the microcontroller. The most commonly used current commutation method is Six-step Commutation, which translates as "six-step commutation". Now create a coordinate system. The six-step commutation process is as follows.
2.4. How does the rotor of the brushless DC motor rotate?
It relies on six-step commutation to generate a rotating magnetic field that continuously seduces in front of the rotor. Just like the hand at the beginning of the article holding the magnet and drawing circles. If you look at the resultant magnetic field direction and where the rotor is located, it's clear at a glance.
You see, the S pole of the resultant magnetic field has been waiting in front of the N pole of the rotor.
As long as the timing of energizing the coil is grasped, the direction of the synthetic magnetic field is always ahead of the position of the rotor, and the rotor will always follow.
3. How to determine the timing of commutation?
As mentioned above, the key to controlling the rotation of the rotor is to commutate the current passing through the coil when the rotor turns to an appropriate angle, so that the direction of the generated magnetic field changes, attracting the rotor and making the rotor rotate.
How should the timing of this current commutation be grasped? That is, how do I know where the rotor is turning now? Only when I know where the rotor is can I know which two-phase electricity to connect to.
In fact, there are many ways to judge the position of the rotor, either with a sensor or without a sensor. Let's talk about the sensor first, and the sensor generally uses a Hall sensor.
3.1. Confirm the rotor position with the sensor
3.1.1. Hall Sensors
Hall sensors can detect changes in magnetic field strength through the Hall Effect. According to the left-hand rule learned in high school physics (used to determine the force direction of a charged conductor in a magnetic field), in the loop where the Hall sensor is located, the magnetic field deflects the motion of the charged particles, and the charged particles "hit" the Hall There is a potential difference between the two sides of the sensor. At this time, a voltmeter can be connected to both sides of the Hall sensor to detect this voltage change, thereby detecting the change of the magnetic field strength. The principle is shown in the figure below.
3.1.2. How do Hall sensors get the rotor position?
With the Hall sensor, the position of the rotor can be roughly known. Hall sensors are generally installed every 120°, or every 60°. The following assumes that the installation is every 120°.
It is assumed that when the N pole of the rotor crosses the sensing area of the Hall sensor, the output voltage of the Hall sensor is high (generally 5V). Otherwise it is low.
According to the levels of HA, HB, and HC, the angle of the position of the rotor can be known. For example, if HA is high, HB is low, and HC is low, we can know that the rotor is in an electrical angle between 180 degrees and 240 degrees (the relationship between the electrical angle and the actual mechanical angle will be discussed later). When using 3 Hall sensors, the resolution is 60 degrees of electrical angle. That is to say, I can only know that the current position of the rotor is within the range of 60° electrical angle, but we don't know exactly how many degrees.
3.1.3. Relationship between electrical and mechanical angles
Although it is a bit strange to insert such a small knowledge here, I still feel it is necessary because I felt that it was not easy to understand when I was learning. It may be easier to understand with the example of the Hall sensor here.
The mechanical angle is the angle that the motor rotor actually turns.
The relationship between the electrical angle and the mechanical angle is related to the number of pole pairs of the rotor.
Because the magnetic field generated by the coil actually attracts the magnetic poles of the rotor. So for the rotation control of the motor, we only care about the electrical angle.
Electrical angle = number of pole pairs x mechanical angle
3.2. Method for estimating rotor position without sensor
This pit is a bit big, and this answer will be skipped first.
4. Rotational speed and direction of rotation of the brushless DC motor
4.4. How to control the direction of rotation of the brushless DC motor?
The order of current commutation can be changed. Let the magnetic field synthesized by the coil rotate in the opposite direction.
4.5. How to control the speed of brushless DC motor?
The greater the voltage across the coil, the greater the current through the coil, the stronger the generated magnetic field, and the faster the rotor rotates.
Because the connected power is DC, we usually use PWM (Pulse Width Modulation) to control the voltage across the coil. The simple principle of PWM is as follows.
Therefore, when the brushless DC motor is energized, the PWM generated by the single-chip microcomputer is used to continuously control the opening and closing of the FET, so that the coil can be repeatedly energized and de-energized. If the energization time is long (Duty is large), the equivalent voltage at both ends of the coil will be large, the strength of the generated magnetic field will be stronger, and the rotor will rotate quickly; if the energization time is short (Duty is small), the equivalent voltage at both ends of the coil will be small, and the generated magnetic field strength will be small. The weaker it is, the slower the rotor turns.
The PWM waveform is connected to the Gate of the FET to control the opening and closing of the FET. Assume that when the voltage on the Gate is high, the FET is closed and turned on; when the voltage on the Gate is low, the FET is turned off and not energized.
In addition, the upper and lower FETs on the same phase must be controlled by opposite-phase PWM waveforms to prevent the upper and lower FETs from being turned on at the same time, causing the current to not pass through the motor but to be the same up and down, resulting in a short circuit. The PWM waveform that controls the FET is as follows.
