I. Basic Knowledge of Electric Motors
Electric motors have become an essential part of our lives. They are found in everything from electric cars and drones to robotic medical devices, home appliances, toys, and a wide variety of other electronic devices.
Electric motors can be divided into two main categories based on the type of power source they use: AC motors and DC motors.
AC motors are powered by AC power (single-phase or three-phase) and are mainly used in industrial applications requiring high torque. DC motors are based on batteries or DC power supplies. AC motors have a simple structure and reliable operation, but their starting characteristics and regulation performance are poor, requiring frequency converters to control their speed. DC motors, on the other hand, have superior starting characteristics and speed regulation performance, mainly manifested in good control performance, a wide speed range, and high efficiency, and are widely used in industrial and civil applications.
DC motors can be divided into three different types: 1) brushed DC motors; 2) brushless DC motors; 3) servo DC motors.
The working principle of electric motors is based on two fundamental laws: Ampere's law and Faraday's law. Simply put, a current-carrying conductor in a magnetic field experiences a force (left-hand rule: let the magnetic field lines pass through the palm of your hand; the direction of your fingers indicates the direction of the current, and the direction of your thumb indicates the direction of the magnetic force). The second law states that if a conductor moves in a magnetic field, the conductor will generate an electromotive force (EMF) by cutting the magnetic field lines due to the force acting on it (1. Right-hand rule: let the magnetic field lines pass through the palm of your hand; the direction of your thumb indicates the direction of motion, and the direction of your fingers indicates the direction of the generated EMF. 2. Right-hand screw rule: grasp a current-carrying solenoid with your right hand, bend your four fingers in the direction of the current, and the end pointed to by your thumb is the N pole of the solenoid).
Our research focuses on motor control, and studying parameters related to motor design such as magnetic circuits, permeability, air gap saturation, and demagnetization curves is not particularly relevant. Understanding the basic structure and principles of a motor is sufficient. An electric motor consists of two main components: a permanent magnet and a coil made of conductors, commonly referred to as the stator and rotor. The essence of motor motion is based on the fact that like poles of magnets repel each other and unlike poles attract each other, achieving rotational motion; in reality, it's a process of one magnetic field chasing another.
The working principle diagram of a brushless DC motor is shown below:
1. First, the working principle of a two-phase two-pole brushless motor will be explained using the magnetic circuit analysis method.
In the diagram above, when current flows through the coils at both ends, according to the right-hand screw rule, an external magnetic induction intensity B pointing to the right will be generated (as shown by the thick arrow in the diagram). The rotor in the middle will try to align the direction of its internal magnetic field lines with the direction of the external magnetic field lines to form the shortest closed magnetic field loop, thus causing the inner rotor to rotate clockwise. The rotor experiences its maximum torque when the direction of its magnetic field is perpendicular to the direction of the external magnetic field.
In the diagram above, the rotor experiences its maximum magnetic force when its magnetic field aligns with the external magnetic field. At this point, the rotor is horizontal, with a lever arm of 0 and a torque of 0. Although no longer subject to rotational torque, it will continue to rotate clockwise due to inertia. If the direction of the current in the two solenoids is changed, the rotor will continue to rotate clockwise forward.
By continuously changing the direction of the current in both solenoids, the inner rotor will rotate continuously. This action of changing the direction of the current is called commutation. When commutation occurs depends only on the rotor's position and not on its speed.
2. Working principle of a three-phase two-pole brushless motor.
Taking the most common "three-phase star connection with two-by-two conduction mode" as an example, the specific wiring diagram of the stator three-phase winding star connection mode is shown in the figure below (the rotor is not shown, but is imagined as a two-pole magnet). The entire motor has three wires A, B, and C. When they are energized in pairs, there are six possible combinations: AB, AC, BC, BA, CA, and CB.
The following describes the direction of the magnetic field strength generated by each energized coil (indicated by red and blue arrows) and the direction of the combined magnetic field strength of the two coils (indicated by green arrow) in these six cases. See the diagram below:
When phases AB are energized and phase C is not energized, the direction of the magnetic induction intensity generated by the stator coil of phase A is shown by the red arrow; the direction of the magnetic induction intensity generated by the stator coil of phase B is shown by the blue arrow; the direction of the combined magnetic induction intensity of the stator coils of phases A and B is shown by the green arrow. When phases AB are energized, the middle rotor will try to align itself in the direction of the green arrow (the middle rotor will try to keep its internal magnetic induction direction consistent with the external magnetic induction direction). When the rotor reaches the position of the green arrow in Figure (a) above, the outer coil changes phase, and phases AC are energized. At this time, the rotor will continue to move and align itself in the direction of the green arrow in Figure (b), and so on. After the outer coil completes six phase changes, the inner rotor has rotated exactly one revolution.
In the diagram above, commutation is performed following the sequence AB->AC->BC->BA->CA->CB. The rotor rotates counterclockwise. To make the motor rotate clockwise, the electronic method involves reversing the commutation sequence, while the physical method involves directly swapping any two wires. For example, if A and B are swapped, the sequence would be BA->BC->AC->AB->CB->CA. Brushed DC motors use mechanical commutation; the magnetic poles remain stationary while the coils rotate. The permanent magnet is the fixed stator, and the wound coils form the rotor. Brushes and a commutator achieve commutation. The advantage of brushed motors is their simple drive; simply adding a DC power supply controls the motor's rotation. However, friction between the brushes and the commutator reduces efficiency, increases electromagnetic interference, shortens lifespan, and complicates maintenance. Brushless motors, as the name suggests, have no brushes. They use electronic commutation; the coils remain stationary while the magnetic poles rotate. With the coil fixed, how can a changing magnetic field be generated? First, a Hall element must be used to sense the position of the permanent magnet poles. Then, the order in which the three sets of bridge power MOSFETs are turned on or off is determined according to the stator winding. The direction of the coil current is switched in a timely manner to ensure that the correct magnetic force is generated to drive the motor, thus eliminating the shortcomings of brushed motors.
The main advantages and disadvantages of brushed and brushless motors are summarized in the table below:
II. System Composition of DC Brushless Motor
A brushless DC motor is a new type of mechatronics product that emerged with the development of semiconductor electronics technology. It is a product of the combination of modern electronic technology, control theory, and motor technology. Its system mainly consists of four parts: a DC power supply, a controller, a motor, and a Hall sensor that determines the rotor position. See the diagram below:
1) The power supply can be directly input as DC; if it is input as AC, it must first be converted to DC by an AC-DC converter.
2) Controller: Whether it is AC power or DC power, the DC voltage must be converted into three-phase voltage by an inverter before it is input to the motor coil to drive the motor. This is achieved by a controller, which consists of three parts: main switching circuit, drive circuit, and control circuit.
a) The main switching circuit consists of 6 discrete power devices connected to the motor via upper and lower arms to control the current flowing through the motor coil. Commonly selected power devices include power transistors (GTRs), power MOSFETs, insulated transistors (IGBTs), turn-off thyristors, etc.
b) The drive circuit is the circuit that reliably turns the power devices on and off.
c) Control circuit MCU: The MCU determines the switching frequency of the power transistors and the timing of electronic commutation through PWM (Pulse Width Modulation) signals.
3) The electric motor, in this case, is a three-phase brushless DC motor.
4) Rotor position sensor: For a brushless motor to rotate, it must commutate to generate a rotating magnetic field. To determine the rotor's position during commutation, the rotor's position must be known. Brushless motors typically use three switch-type Hall effect sensors, spatially separated by 120 electrical degrees, to measure the rotor's position. These sensors output a 3-bit binary coded signal, which is then fed back to the MCU. During electronic commutation, maximum torque is achieved if the stator and rotor magnetomotive forces are perpendicular.
5) Installing Hall sensors increases the size and complexity of the motor, negatively impacting its reliability and manufacturing. Control methods for Hall sensorless brushless motors are becoming increasingly widespread. Hall sensorless brushless DC motors do not require Hall sensors; instead, they determine the rotor's current position by detecting the zero-crossing point of the stator winding's back EMF. Compared to Hall sensor-based solutions, the most significant advantages are reduced cost and size, and the reduction of motor leads from eight to three, greatly simplifying wiring and debugging. However, without Hall sensors to detect rotor position, the back EMF is zero or very small when the motor is stationary or at low speeds, making accurate detection of the winding back EMF difficult and resulting in an inability to obtain a valid rotor position signal. This necessitates an open-loop starting method, which can easily cause vibration during motor startup, making it unsuitable for applications with large load variations or heavy loads.
III. Control Principle of DC Brushless Motors
The stator coils are commutated and energized according to the rotor's magnetic pole position. The most crucial technology involves using three half-bridges composed of six power devices to control the coils' energizing process in six steps, thus creating a rotating magnetic field. The working process is shown in the diagram below:
In the diagram above, A+, B+, and C+ are the upper arm power transistors, and A-, B-, and C- are the lower arm power transistors. By sequentially turning on these six power transistors according to specific requirements, the three-phase windings A, B, and C of the motor are energized in turn, generating a rotating magnetic field. The energizing sequence shown in the diagram is: B+A-, B+C-, A+C-, A+B-, C+B-, C+A-. This cycle produces a clockwise rotating magnetic field. To reverse the motor's rotation, simply reverse the order of the power transistors. It is crucial to note that the upper and lower arm power transistors of the same phase must never be turned on simultaneously; otherwise, a short circuit will occur, burning out the power transistors.
Changing the voltage across the coil windings can adjust the motor speed. In a motor control system, a microcontroller generates a PWM (Pulse Width Modulation) waveform. By controlling different duty cycles of the PWM, the average voltage across the coil windings can be controlled, thus controlling the motor speed. PWM modulation methods are divided into two types: full-bridge modulation and half-bridge modulation. During the 120°C conduction period, both the upper and lower bridges of the power inverter bridge are driven by PWM, i.e., "full-bridge modulation"; during the 120°C conduction period, only the upper (or lower) bridge of the power inverter bridge is driven by PWM, while the lower (or upper) bridge remains constantly on, called "half-bridge modulation".
The switching frequency of MOSFETs in full-bridge modulation is about twice that of half-bridge modulation, resulting in higher losses, and it is rarely used. Half-bridge modulation is more commonly used, and it is further divided into several types, such as H-PWM-L-ON (PWM modulation of the upper bridge arm MOSFET and constant conduction of the lower bridge arm MOSFET within the 120°C conduction range), H-ON-L-PWM (constant conduction of the upper bridge arm MOSFET and PWM modulation of the lower bridge arm MOSFET within the 120°C conduction range), PWM-ON (PWM for the first 60°C, constant conduction for the last 60°C), and ON-PWM (constant conduction for the first 60°C, PWM for the last 60°C), each with its own characteristics. The appropriate control method must be selected according to the application and specific circuit.
IV. Control Scheme for Brushless DC Motors (BLDC)
Currently, there are four main categories of overall solutions for BLDC brushless DC motors.
1) MCU + Pre-driver + Driver MOS
2) MCU + [Pre-driver + Driver MOS]
3) [MCU + Pre-driver] + Driver MOS
4) SOC Solution
The advantages and disadvantages of various control schemes are summarized below:
The traditional solution is MCU + pre-driver + driver, due to high voltage and heat dissipation issues in high-power systems. Therefore, traditional control schemes are typically used in high-voltage and high-power systems. With technological advancements, brushless DC motor control solutions are evolving towards sensorless, highly integrated designs. Various chip manufacturers have developed differentiated products with varying voltage ratings (40V/100V/300V/600V) and drive capabilities for different market segments. These, combined with various motor control solutions, provide customers with highly flexible choices.
