Brushless Motor Current Control Method

Brushless motors have different current control methods depending on the drive system. In addition, in order to control the speed and torque generated, the magnitude of the motor current needs to be controlled. Here, we will explain the PWM method of controlling the motor current.

Current control method

An electrical control system called PWM (Pulse Width Modulation) is used as a control method for the voltage applied to the motor windings. PWM control is a method of controlling the output voltage by repeatedly turning on and off the switching element in the circuit and generating a pulse-shaped voltage.

Figure 3.12 shows the PWM control circuit model for a DC motor. Figure 3.13 shows the voltage and current waveforms when the pulse width is varied (modulated) during the ON phase. Modulating the pulse width and changing the duty cycle of the ON/OFF switching element controls the average voltage.

At this time, the inductance causes the current to lag behind the increasing voltage, and when the applied voltage is removed, the current gradually decreases.

3.31 Modulation

Let's explain the modulation method of PWM control explained in Figure 3.13. The PWM control duty cycle is determined by comparing the standard triangle wave signal and the reference voltage. If the triangle wave signal voltage is lower than the reference voltage, the switching element is turned on, and if it is higher than the reference voltage, the switching element is turned off. In order to increase the motor current, the reference voltage needs to be increased. This means that the switching element duty cycle becomes larger, and the motor current increases due to the increase in the average voltage. If the reference voltage is reduced, the switching element duty cycle becomes smaller, and the motor current decreases due to the decrease in the average voltage.

Brushless motors with square wave drive systems regulate the motor current by PWM controlling the switching elements that excite the motor windings, thereby controlling the speed and torque generated.

3.3.2 Sine wave drive system modulation method

Let's explain the modulation method of the sine wave drive system explained in Figure 3.10. If the reference voltage is made into a sine wave, the duty cycle of the switching element is continuously modulated, and a sine wave current can flow. In other words, in PWM control, the reference voltage waveform and the current waveform present the same form. (The reference voltage of the waveform you want to output is called the modulation signal.) The brushless motor of the sine wave drive system performs PWM control like the square wave drive system, but the method of generating the reference voltage is different.

The sinusoidal motor current obtained by formula (3.6) is a function of the current waveform value im and the rotor angle θ. By creating a modulation signal based on the rotor angle information and the current command value, the current flowing to the motor winding can be adjusted, and the speed and torque generated can be controlled.

Therefore, to perform sinusoidal drive in a brushless motor, the driver must accurately detect the rotor magnet angle and thus "close the loop". This can be done with Hall Effect sensors or encoders. Dongfang Motor uses both to meet different speed accuracy requirements.