Hall-less BLDC motor control

1. Introduction

The basic principle of the BLDC control scheme without Hall effect is similar to that of the BLDC with Hall effect. Both use the so-called "six-step commutation method". According to the current position of the rotor, the stator winding is energized in a certain order to make the BLDC motor rotate. The difference is that the BLDC without Hall effect does not require a Hall effect sensor, and the current position of the rotor is determined by detecting the zero crossing of the back electromotive force of the stator winding. Compared with the scheme with Hall effect, the most obvious advantage is the reduction in cost and size. In addition, the number of motor leads is reduced from 8 to 3, which greatly simplifies wiring and debugging. In addition, the Hall sensor is easily affected by external environments such as temperature and magnetic field, and has a high failure rate. Therefore, the BLDC without Hall effect is increasingly used and is gradually replacing the BLDC with Hall effect in many occasions.

This article introduces the Hall-less control theory of three-phase BLDC motors. The specific implementation method will vary depending on the specific application.

2. Introduction to BLDC motor structure and drive mode

The construction of a simple BLDC is shown in Figure 1. The outer layer of the motor is the stator, which contains the motor windings. Most BLDCs have three Y-connected windings, each of which is composed of many coils interconnected. Inside the motor is the rotor, which consists of magnetically opposite poles around the circumference of the motor. Figure 1 shows a rotor with only two poles (north and south), in actual applications, most motors have rotors with multiple pairs of poles.

Figure 1 Basic structure of BLDC[1]

The basic model of the BLDC motor drive circuit is shown in Figure 2. The power-on state of the three-phase winding of the motor is controlled by the switch tubes Q0~Q5, which can be IGBTs or power MOS tubes. The switch tube located at the top, that is, connected to the positive end of the power supply, is called the "upper bridge", and the switch tube at the bottom, that is, connected to the negative end of the power supply, is called the "lower bridge".

Figure 2 Basic model of BLDC motor drive circuit [2]

For example, if Q1 and Q4 are turned on and the other switches are turned off, the current will flow from the positive end of the power supply through Q1, the A-phase winding, the C-phase winding, and Q4 back to the negative end of the power supply. The current flowing through the A-phase and C-phase stator windings will generate a magnetic field, and the right-hand rule shows that its direction is parallel to the B-phase winding. Since the rotor is a permanent magnet, it will rotate in a direction parallel to the stator magnetic field under the action of the magnetic field force, that is, it will rotate to a position parallel to the B-phase winding, so that the north magnetic pole of the rotor is aligned with the south magnetic pole of the stator magnetic field.

Similarly, by turning on different combinations of upper and lower bridge arm MOS tubes, the direction of current flow can be controlled, magnetic fields in different directions can be generated, and the permanent magnet rotor can be rotated to the specified position. To make the BLDC motor rotate continuously in the specified direction, the stator winding must be energized in a certain order. The switch from one power-on state to another is called "phase change", for example, from AB power to AC power. Phase change causes the rotor to rotate to the next position. There are three switch tubes in each upper and lower bridge arm, for a total of six combinations, so it changes every 60°. After six steps of phase change, the motor can rotate one electrical cycle. This is the so-called "six-step phase change method".

To make the rotor have the maximum torque, the ideal situation is to make the stator magnetic field perpendicular to the rotor magnetic field. But in reality, since the direction of the stator magnetic field changes only once every 60°, and the rotor keeps rotating, it is impossible to keep them at a 90° phase difference all the time. The most optimized method is to make the stator magnetic field lead the rotor magnetic field by 120° electrical angle at each phase change, so that in the next 60° rotation of the rotor, the angle between the stator magnetic field and the rotor magnetic field changes from 120° to 60°, and the torque utilization rate is the highest.

In order to determine which winding will be energized in the power-on sequence, the current position of the rotor must be known. In a BLDC with Hall, the position of the rotor is detected by a Hall effect sensor embedded in the stator. A BLDC motor without Hall does not use a position sensor, but instead uses the characteristic signal of the motor itself to achieve an effect similar to that of a position sensor. The most widely used method is the back-EMF method to be introduced in the next section of this article.

3. The principle of controlling BLDC motor by back electromotive force method

When the BLDC motor rotates, the rotation of the permanent magnet rotor generates a changing magnetic field inside the motor. According to the law of electromagnetic induction, each phase winding will induce a back electromotive force (BEMF). The BEMF waveform of the BLDC motor changes with the position and speed of the rotor, and is generally trapezoidal.

Figure 3 shows the waveforms of the current and back electromotive force during one electrical cycle of the motor. The solid line represents the current, the dotted line represents the back electromotive force, and the horizontal axis is the electrical angle of the motor rotation. According to the "six-step commutation" control theory of BLDC, we know that at any time, only two phases of the three-phase BLDC are energized, and the other phase is open. The three phases are energized in pairs, and there are six combinations in total. They change every 60° in a certain order, thus generating a rotating magnetic field, pulling the permanent magnet rotor to rotate. The 60° here refers to the electrical angle, and one electrical cycle may not correspond to a complete rotor mechanical rotation cycle. The number of electrical cycles to be repeated to complete one mechanical rotation depends on the number of rotor pole pairs. Each pair of rotor poles needs to complete one electrical cycle, so the number of electrical cycles/number of revolutions is equal to the number of rotor pole pairs.

Figure 3 BLDC motor current and back EMF waveforms

The key to controlling BLDC is to determine the moment of commutation. As can be seen from Figure 3, there is a point where the polarity of the back EMF changes between each two commutation points, that is, the point where the back EMF changes from positive to negative or from negative to positive, which is called the zero-crossing point. Using this characteristic of the back EMF, as long as we can accurately detect the zero-crossing point of the back EMF and delay it by 30°, it is the moment when commutation is required.

4. Detection method of back electromotive force

As can be seen from Figure 3, each time the back electromotive force passes through the zero point, it occurs in the phase that is not energized. For example, in the first 60° in Figure 3, the current of phase A is positive, the current of phase B is negative, and the current of phase C is zero, which means that the motor phases AB are energized, the current flows from phase A to phase B, and phase C is open. The zero point of the back electromotive force happens to appear in phase C. And because phase C is not energized and has no current, its phase voltage has a direct corresponding relationship with the back electromotive force. Therefore, as long as the voltage of the phase that is not energized is detected in each 60°, the back electromotive force can be detected.

4.1 Reconstructing the virtual neutral point

Due to the Y-shaped connection of the BLDC motor, all three phases are connected to a common neutral point, and the phase voltage cannot be measured directly. Only the terminal voltage of each phase, that is, the voltage of each phase to the ground, can be measured, and then compared with the neutral point voltage. When the terminal voltage changes from greater than the neutral point voltage to less than the neutral point voltage, or from less than the neutral point voltage to greater than the neutral point voltage, it is the zero crossing point. The schematic diagram is shown in Figure 4(a).

Figure 4 Detecting the zero crossing of back EMF

However, general BLDC motors do not have external leads for the neutral point, so the neutral point voltage cannot be directly measured. The most direct way to solve this problem is to reconstruct a "virtual neutral point" by connecting the three-phase windings to a common point through voltages with equal resistance. This common point is the virtual neutral point, as shown in Figure 4(b).

The method of reconstructing the virtual neutral point is practical to a certain extent, but it also has great shortcomings. Since the BLDC motor is driven by PWM, PWM outputs "ON" and then "OFF" in one cycle; when PWM is "ON", the motor winding is energized, and when it is "OFF", it is turned off.

The voltage at the end of the switch is constantly switching between high and low levels, and the neutral point voltage contains a lot of switching noise. If the neutral point voltage is filtered, on the one hand, the circuit complexity is increased, and on the other hand, the filtering circuit will cause a phase shift of the signal, causing the detected zero crossing to shift later than the actual time, making it impossible to accurately guide the commutation.