• Composition and working principle of brushless DC motors
(1) Composition of a brushless motor
A brushless DC motor consists of two main parts: a rotor and a stator, as shown in Figure 3.
(2) Working principle of brushless DC motor
The brushless DC motor adopts a square wave self-controlled permanent magnet synchronous motor, replacing the brush commutator with a Hall sensor. The signal line of the Hall sensor transmits the position of the magnet inside the motor relative to the winding coil. Based on the signals of the three Hall sensors, it can know how to supply power to the motor coil at this time (different Hall signals should provide current in the corresponding direction to the motor winding). That is to say, the direction of the coil current is different when the Hall sensor state is different.
The Hall signal is transmitted to the controller, which supplies power to the motor windings through a thick wire (not a Hall wire). The motor rotates, and the magnet and the windings (more accurately, the coils wound on the stator; Hall sensors are usually mounted on the stator) rotate. The Hall sensor detects the new position signal, and the controller supplies power to the motor windings, which have changed the direction of the current, through the thick wire. The motor continues to rotate (when the position of the windings and magnets changes, the windings must change the direction of the current accordingly so that the motor can continue to move in one direction; otherwise, the motor will oscillate left and right at a certain position instead of rotating continuously). This process is called electronic commutation.
Brushless DC motors are powered by DC power supplies and use position sensors to detect the rotor position. The detected signals trigger the corresponding electronic commutation circuits to achieve contactless commutation.
Brushless DC motors use electronic switches and position sensors instead of brushes and commutators to convert DC power into analog three-phase AC power. By modulating the pulse width, the current magnitude is changed to change the speed.
Control structure of DC brushless motor
A brushless DC motor is a type of synchronous motor, meaning that the rotor speed is affected by the speed of the rotating magnetic field of the stator and the number of rotor poles (P).
N= 120 F/P. With a fixed number of rotor poles, changing the frequency of the stator rotating magnetic field alters the rotor speed. A brushless DC motor is essentially a synchronous motor with added electronic control (driver). This controls the frequency of the stator rotating magnetic field and feeds the rotor speed back to the control center for repeated adjustments, aiming to achieve characteristics close to a DC motor. In other words, a brushless DC motor can maintain a certain rotor speed within its rated load range even when the load changes.
The DC brushless driver includes a power supply section and a control section as shown in Figure (1): the power supply section provides three-phase power to the motor, and the control section converts the input power frequency as needed.
The power supply can accept direct current (DC) input (typically 24V) or alternating current (AC) input (110V/220V). If the input is AC, it must first be converted to DC by a converter. Regardless of whether the input is DC or AC, the DC voltage must be converted to a three-phase voltage by an inverter before it can drive the motor coils. The inverter typically consists of six power transistors (Q1-Q6), divided into upper arms (Q1, Q3, Q5) and lower arms (Q2, Q4, Q6), which connect to the motor and act as switches to control the flow of electricity through the motor coils.
The control unit provides PWM (Pulse Width Modulation) to determine the switching frequency of the power transistors and the timing of commutation in the inverter. Brushless DC motors are generally intended for speed control that maintains a stable speed at a set value without significant fluctuations when the load changes. Therefore, the motor is equipped with a Hall sensor that senses the magnetic field, serving as a closed-loop speed control and also as a basis for phase sequence control. However, this is only used for speed control and cannot be used for positioning control.
Control principle of DC brushless motor
To make the motor rotate, the control unit must first determine the current position of the motor rotor as sensed by the hall-sensor, and then decide the sequence of turning on (or off) the power transistors in the inverter (inverter) according to the stator windings, as shown in Figure 2. This involves AH, BH, and CH (these are called upper arm power transistors) and AL, BL, and CL (these are called lower arm power transistors) in the inverter, causing current to flow sequentially through the motor coils, generating a clockwise (or counter-clockwise) rotating magnetic field that interacts with the rotor's magnets. This allows the motor to rotate clockwise/counter-clockwise. When the motor rotor rotates to a position where the hall-sensor senses another set of signals, the control unit turns on the next set of power transistors. This cycle continues until the control unit decides to stop the motor rotor, at which point the power transistors are turned off (or only the lower arm power transistors are turned on); to reverse the motor rotor's direction, the power transistors are turned on in the reverse order.
The basic methods for turning on a power transistor can be illustrated as follows:
AH, BL in one group → AH, CL in one group → BH, CL in one group → BH, AL in one group → CH, AL in one group → CH, BL in one group, but never AH, AL, BH, BL, or CH, CL. Furthermore, because electronic components always have a switching response time, the switching time of the power transistor must take into account the component's response time. Otherwise, if the upper arm (or lower arm) is not fully closed before the lower arm (or upper arm) is already open, it will cause a short circuit between the upper and lower arms, resulting in the power transistor burning out.
Once the motor starts rotating, the control unit compares (or calculates via software) the command (comprising the speed set by the driver and the acceleration/deceleration rate) with the speed of change of the HALL-SENSOR signal to determine which group of switches (AH, BL, AH, CL, BH, CL, etc.) should be turned on, and for how long. If the speed is insufficient, the switch will be turned on for a longer duration; if the speed is excessive, the switch will be turned on for a shorter duration. This part of the operation is handled by PWM.
PWM (Pulse Width Modulation) determines the speed of a motor, and generating such PWM is key to achieving precise speed control. High-speed control requires consideration of whether the system's clock resolution is sufficient to handle the processing time of software instructions. Furthermore, the method of accessing data related to HALL-SENSOR signal changes also affects processor performance, accuracy, and real-time performance. For low-speed control, especially during low-speed starts, the feedback HALL-SENSOR signal changes more slowly. Therefore, the signal acquisition method, processing timing, and appropriate configuration of control parameters based on motor characteristics become crucial. Alternatively, speed feedback can be modified to use ENCODER changes as a reference, increasing signal resolution for better control. The smooth operation and good response of the motor also depend on the appropriateness of P.I.D. control.
As mentioned earlier, brushless DC motors use closed-loop control. Therefore, the feedback signal essentially tells the control unit how far the motor speed is from the target speed; this is the error (ERROR). Knowing the error naturally requires compensation, which can be achieved through traditional engineering control methods such as P.I.D. control . However, the control state and environment are complex and ever-changing . To achieve robust and durable control, the factors that need to be considered are probably beyond the complete control of traditional engineering methods. Therefore, fuzzy control, expert systems, and neural networks are being incorporated into important theories of intelligent P.I.D. control .
Wiring principle of 48V brushless motor electric vehicle
There are eight wires: three thick wires (yellow, blue, green – colors may vary slightly depending on the model) and five Hall effect wires (red, black, blue, green, yellow). The red and black Hall effect wires must be connected correctly; the other three thin wires must be connected according to their colors. Even with the three thick wires connected correctly, the motor may still vibrate, not turn, or reverse. Therefore, you can connect the three thick wires in any combination. For the thin wires, except for the red and black wires which must be connected correctly, the other three can also be connected in any combination.
Electric vehicle brushless motor controller drive circuit diagram
