Motor drive control is about controlling the rotation or stopping of the motor, as well as the rotation speed. The motor drive control part is also called an electronic speed controller, abbreviated as ESC. Depending on the motor used, ESCs can be either brushless or brushed.
The permanent magnets of a brushed DC motor are fixed, and the coils are wound on the rotor. By intermittently contacting a commutator via a brush, the direction of the magnetic field is changed to keep the rotor rotating continuously. A brushless DC motor, as the name implies, does not have brushes or a commutator. Its rotor is a permanent magnet, and the coils are fixed, directly connected to an external power source. The problem arises: how does the direction of the coil’s magnetic field change? In fact, a brushless DC motor requires an external electronic speed controller, which is essentially a motor driver. It continuously changes the direction of the current inside the fixed coils, ensuring that the force between them and the permanent magnet is repulsive, allowing continuous rotation.
A brushed motor can work without a speed controller by directly supplying power to it, but this way, the speed cannot be controlled. A brushless DC motor must have a speed controller; otherwise, it cannot rotate. The brushless speed controller converts direct current (DC) into three-phase alternating current (AC) to power the brushless DC motor.
The earliest speed controllers were not like the ones today; they were all brushed speed controllers. At this point, you might ask, what is a brushed speed controller, and how is it different from today’s brushless controllers? The differences are vast. Brushed and brushless controllers are named based on the motors they control. Today’s motor rotors, which are the rotating parts, are all magnet blocks, and the coils are fixed stators. Because there are no carbon brushes in between, this is a brushless DC motor.
Conversely, a brushed motor, as the name implies, has carbon brushes, making it a brushed DC motor. For instance, the motors used in common low-cost toy remote control cars are brushed DC motors. The speed controllers are named brushed or brushless based on these two types of motors. From a professional perspective, a brushed speed controller outputs DC, while a brushless speed controller outputs three-phase AC.
AC (alternating current) comes with a certain frequency, which, in simple terms, means positive and negative swap back and forth on a single wire. DC (direct current) has a constant positive and negative pole. Once you understand the difference between AC and DC, the next question is, what is "three-phase power"? Theoretically, three-phase AC is a type of electrical transmission, abbreviated as three-phase power. It consists of three AC voltages of the same frequency and equal amplitude, each with a phase difference of 120 degrees.
A brushless speed controller inputs DC through a filter capacitor to stabilize the voltage. Then, it splits into two paths: one for the controller's BEC (Battery Eliminator Circuit), which powers the receiver and the controller’s own microcontroller (MCU). The power lines output to the receiver are the red and black wires on the signal line. The other path goes to the MOSFETs (metal–oxide–semiconductor field-effect transistors). When powered on, the MCU starts and drives the MOSFETs, causing the motor to make beeping sounds.
After startup, the controller enters a standby mode. Some speed controllers feature throttle calibration. Before entering standby, they check the throttle position—if high, it enters throttle range calibration; if center, it emits an alarm; if low, it enters normal operation. Once ready, the MCU within the speed controller adjusts the output voltage's magnitude, frequency, and drive direction based on the PWM (pulse-width modulation) signal to control the motor's speed and direction. This is the principle of a brushless speed controller. When driving the motor, the controller uses three sets of MOSFETs, each with a positive and a negative output. When one outputs positive, the other does not output negative, and vice versa, thus forming AC. All three sets work this way, with a frequency of 8000Hz. In essence, a brushless speed controller is similar to a frequency converter or speed controller used in industrial motors.
The ESC inputs DC power, usually supplied by lithium batteries. The output is three-phase AC, which can drive the motor directly. Additionally, model aircraft ESCs have three signal input lines for inputting PWM signals to control motor speed. Due to the unique requirements of model aircraft, especially quadcopters, they need specially designed model aircraft ESCs.
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