The traditional brushless DC motor adopts a square wave control method, which is simple to control and easy to implement. At the same time, there are problems such as torque ripple and commutation noise, and there are limitations in some applications that require noise. For these applications, the use of sine wave control can solve this problem well.
Introduction to Sine Wave Control of Brushless DC Motor
The sine wave control of the DC brushless motor is to apply a certain voltage to the motor winding to generate a sinusoidal current in the motor winding, and to control the motor torque by controlling the amplitude and phase of the sinusoidal current. Compared with the traditional square wave control, the motor phase current is sinusoidal, and continuously changes, and there is no sudden change in the commutation current, so the motor running noise is low.
According to the complexity of the control, the sine wave control of the DC brushless motor can be divided into: simple sine wave control and complex sine wave control.
(1) Simple sine wave control:
A certain voltage is applied to the motor windings to make the motor phase voltage a sine wave. Since the motor winding is an inductive load, the motor phase current is also a sine wave. The phase and amplitude of the current are controlled by controlling the amplitude and phase of the motor phase voltage, which is a voltage loop control, which is relatively simple to implement.
(2) Complex sine wave control:
Different from simple sine wave control, the target of complex sine control is the motor phase current, and a current loop is established to achieve the purpose of controlling the motor by directly controlling the phase and amplitude of the phase current. Since the motor phase current is a sinusoidal signal, the current decoupling operation is required, which is more complicated, and the common ones are Field Oriented Control (FOC) and Direct Torque Control (DTC).
This article will mainly introduce the principle and realization of simple sine wave control.
Simple sine wave control principle
Simple sine wave control is to control the motor current by controlling the amplitude and phase of the motor's sinusoidal phase voltage. Usually by applying a certain form of voltage to the motor terminal line to generate a sinusoidal phase voltage across the winding. The common generation methods are: sine PWM and space vector PWM. Since the principle of sine PWM is simple and easy to implement, it is usually used as a PWM generation method in simple sine wave control. Figure 1 is a BLDC control structure diagram, where Ux, Uy, Uz are the bridge arm voltages, and Ua, Ub, Uc are the phase voltages of the motor windings. The following descriptions of different types of PWM modulation methods will be based on this structure diagram.
Fig. 1 Control block diagram of DC brushless motor
(1) Three-phase sinusoidal modulation PWM
Three-phase SPWM is the most common sinusoidal PWM generation method, that is, a sinusoidal voltage signal with a phase difference of 120 degrees is applied to the three terminal lines of the motor. Since the neutral point is 0, the motor phase voltage is also sinusoidal, and the phase is the same as the applied sinusoidal voltage. the same. as shown in picture 2.
Figure 2 Three-phase modulation SPWM terminal line voltage
(2) Minimum switching loss sinusoidal PWM
Different from the common SPWM, when using the sinusoidal PWM with minimum switching loss, the voltages Ua, Ub, Uc applied to the motor terminal line are not sine wave voltages. At this time, the motor center point voltage is not 0, but the motor phase voltage is still sine. Therefore, this type of control method is line voltage control. See Figure 3:
Figure 3 Minimum switching loss sinusoidal PWM terminal line voltage
Among them, Ux, Uy, and Uz are the motor terminal line voltages, and Ua, Ub, and Uc are the motor phase voltages. It can be seen that the phase voltage phase difference is 120 degrees. The relationship between Ux, Uy, Uz and Ua, Ub, Uc is as follows:
After the merger, Ux, Uy, Uz are as follows:
It can be seen that when using sinusoidal PWM with minimum switching loss, Ux, Uy, Uz have a phase difference of 120 degrees, and are in the form of a piecewise function, not a sinusoidal voltage, and the motor phase voltages Ua, Ub, Uc are still sinusoidal voltages. And the terminal line voltage is 0 in the 120 degree area, that is, the corresponding switch tube is normally open or normally closed. Therefore, compared with three-phase sinusoidal PWM, the switching loss is reduced by 1/3.
By controlling the phase and amplitude of Ux, Uy, Uz, Ux, Uy, Uz can be controlled to achieve the purpose of controlling current.
Realization of Simple Sine Wave Control of Brushless DC Motor
The system structure is shown in Figure 4. The working principle is as follows: The Hall input signal is automatically filtered and sampled to obtain a reliable commutation signal. This information can be used to estimate the rotor angle and speed. The speed PI regulator calculates the Modulation of the sinusoidal PWM according to the given speed value and the feedback speed value. The position estimation unit uses the speed and commutation information to estimate the rotor position angle Angle. Through the advance angle adjustment unit, the advance angle Δ is compensated to obtain Angle. The SPWM unit uses the Modulation and Angle information to generate the SPWM with the minimum switching loss and output it to the inverter unit. The following content introduces the principle and realization of each unit.
Figure 4 System block diagram
Generation of sinusoidal PWM with minimum switching loss
Since Ux, Uy, and Uz are 120 degrees out of phase, take Ux as an example for analysis.
The amplitude and phase of the motor phase voltage can be controlled by controlling M and x.
The relationship between the minimum switching loss sinusoidal PWM control and the Hall position sensor
Generally, the DC brushless motor uses Hall sensors to locate the rotor position. Since the traditional control method is square wave control, 3 Hall sensors can meet the requirements. The relationship between the position of the Hall sensor and the back EMF of the rotor is shown in Figure 5. That is, the Hall sensor is installed at the position where the back EMF is 30°, 90°, 150°, 210°, 270°, 330°. The specific Hall output value is related to the specific installation method of the Hall.
Figure 5 The relationship between BLDC Hall sensor output and back EMF
When using the minimum switching loss sinusoidal PWM to control the BLDC, the relationship between the motor terminal line voltage and the Hall sensor output is shown in Figure 6.
Figure 6 The relationship between terminal line voltage and Hall state when using sinusoidal PWM with minimum switching loss
It can be seen from Figure 2 that the motor terminal line voltage is 30° ahead of the phase voltage when using the sine PWM with the minimum switching loss, so the motor phase voltage and the back EMF can be synchronized with the sine wave control.
Since the phase voltage leads the phase current, the phase current lags behind the back EMF.
The rotation speed calculation relies on the Hall sensor. In an ideal state, the interval between two adjacent Hall states is 60°. In actual applications, due to installation errors, the actual interval is not 60°, which will introduce calculation errors. In this document, the output of a Hall sensor is used as a reference for speed calculation, as shown in Figure 7. Among them, the high and low levels are respectively 180 degrees, which will not introduce installation errors. Using this information, the motor speed can be calculated.
Different from the square wave control, the angle in the sine wave control changes continuously, and the common 3 Hall sensors in BLDC can only provide 6 angle information, namely 0°, 60°, 120°, 180°, 240°, 300 °, other angle information cannot be obtained directly. The average speed method is usually used. Assuming that the motor speed is stable within a certain period of time, the angle and speed information during the previous Hall commutation are used to interpolate other angle information, as shown in Figure 8.
Figure 8 Angle estimation
Therefore, it can be seen that the fluctuation of the motor speed will directly affect the error of angle calculation. In the scheme, the average value of three adjacent 180° commutation times is used to calculate the speed information, as shown in Figure 9.
Figure 9 Calculating speed by multiple averaging method
That is, to reduce the angle error caused by the fluctuation of the rotation speed.
Speed control adopts PI regulator, the input is speed setting and speed feedback, and the output is the amplitude Modulation of sinusoidal PWM with minimum switching loss. The formula is as follows:
Among them: is the proportional gain, is the integral gain, and y is the output of the PI regulator. In specific implementation, the integration link adds the anti-integration saturation function to limit the maximum and minimum output of the integrator, and at the same time increases the saturation limit for the output value of the entire PI regulator. The implementation block diagram is as follows.
Figure 10 PI mediator block diagram
Before the brushless DC motor is started, the rotor is at a standstill, and only the Hall sensor can be used to obtain the absolute position information of the motor. Since there is no commutation, the motor speed information cannot be obtained, so the average speed method cannot be used to calculate the angle required for sinusoidal control. information. Therefore, during the motor start-up phase, the sine control mode cannot be cut directly, and the square wave control mode is used to start. When the motor is started and reliable commutation information is obtained, it can switch to sine wave control. In order to prevent large speed fluctuations, it is necessary to pay attention to the smooth transition of the phase and amplitude of the current before and after switching.
The current waveform diagram 11 before and after ideal switching is as follows.
Lead angle adjustment
It can be seen from the foregoing that the output of the Hall sensor reflects the back EMF information of the rotor, and the sine wave phase voltage generated according to the Hall state is in phase with the back EMF of the rotor. Since the motor is an inductive load, the motor phase current lags behind the phase voltage. That is, the motor phase current lags behind the back EMF. When the Hall maximum torque is output, the motor phase current is synchronized with the back EMF, so it is necessary to adjust the voltage phase so that the generated phase voltage is ahead of the back EMF, that is, the lead angle Δ. Proper adjustment of Δ can make the phase current and the back EMF have the same phase, increase the output torque, and improve the system efficiency. The adjustment of the lead angle can be adjusted manually through experiments, or automatically adjusted by a certain algorithm.
Figure 11 Ideal switching from square wave control to sine wave control
The control method proposed in this paper is implemented using Infineon's high-performance 8-bit microcontroller XC866. XC866 integrates a dedicated motor control unit CCU6E (providing a dedicated BLDC control mode) and a high-performance ADC module, which is an ideal choice for controlling DC brushless motors. The motor is a brushless DC fan with a rated power of 35W, and the number of pole pairs: 4. It adopts square wave control when starting, and cuts into sine wave control when the speed is stable. Figure 12 shows the motor phase currents running under sinusoidal PWM control with minimum switching loss.
Figure 12 BLDC phase current controlled by sine wave with minimum switching loss
This paper introduces a sine wave control scheme of brushless DC motor based on sine PWM with minimum switching loss, and implements and verifies the system based on Infineon's high-performance 8-bit microcontroller XC866. Compared with the traditional square wave control, due to the use of sine wave drive technology, the motor has low operating noise, and the switching loss is reduced by 1/3 compared with SPWM, which can well meet the noise and efficiency requirements in the application of brushless DC fan. This kind of control scheme will have great application prospects.