Basic knowledge of permanent magnet brushless DC motor
The high-density, high-efficiency, wide-speed vehicle traction motor and its control system are not only the heart of electric vehicles but also one of the key technologies in the development of electric vehicles. Before the 1980s, almost all vehicle traction motors were DC motors. This is because DC traction motors have the advantages of large initial acceleration traction and simpler control systems. The disadvantage of the excited DC motor is that there is a mechanical commutator. When running at high speed and heavy load, sparks will be generated on the surface of the commutator, so the running speed of the motor cannot be too high. Since the commutator of the DC motor needs maintenance and is not suitable for high-speed operation, it is generally not used at present except for small cars.
Compared with the original excitation DC traction motor system, the permanent magnet DC motor has obvious advantages. Its outstanding advantages are small size, light weight (the specific mass is 0.5~1.0kg/kW), high efficiency, basically maintenance-free, and speed regulation range Wide, high power density, low rotor inertia, low armature inductance, high operating efficiency, and no slip rings and brushes on the rotating shaft, have been widely used in electric vehicles.
A permanent magnet motor is a motor that uses permanent magnet materials to replace the excitation winding (or rotor winding) of the excitation motor. Permanent magnet motors are divided into permanent magnet AC synchronous motors and permanent magnet DC motors.
If the DC excitation winding of a DC motor is replaced with a permanent magnet, the motor is called a permanent magnet DC motor. In order to overcome the shortcomings of constant magnetic flux, electromagnetic windings that excite the magnetic field are embedded in the permanent magnet stator, which is called a permanent magnet composite motor, which is characterized by both permanent magnets and excitation windings.
For a three-phase asynchronous motor, if a permanent magnet is used to replace its cage-shaped induction rotor, the corresponding motor is called a permanent magnet synchronous motor. In order to overcome the shortcoming of constant impact flux, a cage-shaped electromagnetic winding is embedded in its rotor, which is called a permanent magnet composite motor, which is characterized by both permanent magnets and cage-shaped windings.
Permanent magnet DC motors are divided into permanent magnet brushed DC motors and permanent magnet brushless DC motors. Permanent magnet brushed DC motors are widely used in small electrical appliances. Due to the existence of brushes and commutators, permanent magnet brushed DC motors are more complicated than permanent magnet brushless DC motors in terms of maintenance and manufacturing. The commutation sparks and mechanical noise in the application also make it difficult to operate in harsh environments. Under use. Since permanent magnet brushless DC motors have no brushes, they make up for the shortcomings of permanent magnet DC motors and traditional DC motors. Therefore, permanent magnet brushless DC motors are increasingly used in servo systems, CNC machine tools, inverter air conditioners, and Electric cars.
(1) Basic structure
The permanent magnet brushless DC motor is mainly composed of the motor body (including the rotor position sensor) and the electronic switch drive circuit. Among them, the electronic switch drive circuit includes two parts: an electronic commutation circuit (power conversion circuit) and a control circuit.
①Motor body. The three-phase symmetrical stator winding is fixed on the stator, and the armature winding on the rotor is replaced with rare-earth permanent magnet materials (steel drill, neodymium iron boron). For high-speed permanent magnet brushless DC motors, a non-magnetic guard ring needs to be installed. Its structure is shown in Figure 2.
a. Stator winding. The main electrical parameters and winding forms of the stator windings of permanent magnet brushless DC motors are the same as those of wound three-phase synchronous motors. Each coil is energized in turn and generates a rotating magnetic field, as shown in Figure 3.
b. Rotor. The rotor of the permanent magnet brushless DC motor adopts the long magnet as the magnetic pole, as shown in Figure 4. Under the action of the rotating magnetic field, the rotor will follow the rotating magnetic field to rotate synchronously, and the speed of the rotating magnetic field depends on the frequency of the power supply. Similar to the synchronous motor of the three-phase AC motor, the permanent magnet brushless DC motor can produce an ideal constant torque.
According to the different installation positions of the permanent magnets on the rotor, permanent magnet brushless DC motors can be divided into several structural forms such as surface type, built-in type and embedded type. The advantage of the surface type brushless DC motor is its simple structure. Since the permeability of the permanent magnet is close to that of air, the permanent magnet brushless DC motor has a larger effective air gap and lower armature response. The built-in brushless DC motor has high magnetic dominance, can generate additional reluctance torque component, and maintain mechanical stability during high-speed operation. The permanent magnets of the embedded motor can have a variety of inlay methods (see Figure 5), and its performance is between the surface motor and the built-in motor.
c. The rotor position sensor is shown in Figure 6. The rotor position sensor is used to determine the position of the rotor magnetic pole in the brushless DC motor, and provides the correct commutation information for the logic switch circuit, which is to convert the position signal of the magnetic steel pole of the rotor into The electrical signal is then used to control the commutation of the stator winding.
There are many types of rotor position sensors, and each has its own characteristics. Common position sensors in brushless DC motors include electromagnetic position sensors, proximity switch position sensors, photoelectric position sensors and magnetic sensitive position sensors.
②Electronic switch drive circuit. As shown in Figure 6, the stator winding of the permanent magnet brushless DC motor is connected to the DC power supply by the “external commutator” (inverter) in the electronic switch drive circuit, which can be classified as a part of the DC motor. kind. From the perspective of the inverter, the current change in the armature winding of the permanent magnet brushless DC motor is completed by the electronic switch drive circuit, and its frequency is consistent with the speed change, so it belongs to a permanent magnet synchronous motor. The main difference between it and the sinusoidal permanent magnet synchronous motor: the current flowing in the armature winding of the brushless DC motor changes in the form of a square wave, so it is also called “square wave AC permanent magnet motor”. Therefore, its working principle is the same as that of a permanent magnet synchronous AC motor.
The electronic switch drive circuit is mainly composed of high-performance semiconductor power devices (such as GTR, MOSFET, IGBT, IPM, etc.) to form a full-bridge or half-bridge switch drive circuit.
(2) Working principle
As shown in Figure 3(a), when the A, B, and C phases of the stator are supplied with currents in time sequence, the magnetic field of the stator A, B, and C phases will rotate in a certain direction, so the permanent magnet can rotate. The stator of a common permanent magnet brushless DC motor is a three-phase symmetrical winding, which has the same structure as a three-phase asynchronous motor. There are rare earth permanent magnets on the rotor. The driver is an AC-DC-AC voltage inverter, through sine wave pulse width modulation (PWM), the output frequency is f, three-phase sine wave voltage with variable voltage. The three-phase sine wave voltage generates a symmetrical three-phase square wave current in the three-phase winding of the stator, and generates a rotating magnetic field in the air gap. The rotation speed of the rotating magnetic field is n1=2πf/P. This rotating magnetic field and the permanent magnet rotor drive the rotor to rotate synchronously with the rotating magnetic field, and try to align the stator and rotor magnetic field axes. When the load torque is applied, the rotor magnetic field axis lags behind the stator magnetic field axis by a power angle θ. The power angle θ is proportional to the load. The greater the load, the greater the power angle until the power angle is large enough to stop the rotor from rotating. It can be seen from this that when the permanent magnet brushless DC motor is running, its speed must rotate in strict proportion to the frequency, otherwise it will lose step and stop. Therefore, the speed of the permanent magnet brushless DC motor is synchronized with the rotating magnetic field, and its static error is zero. Under load disturbance, only the power angle is changing, but the speed is unchanged, and the response time is real-time. This is the operating characteristic of the permanent magnet brushless DC motor. But when the power angle is at a certain value, the motor will stop due to out-of-step. Therefore, the motor is not suitable for use under heavy load, and it is not easy to start quickly.
(3) Drive process
As shown in Figure 6, the three-phase stator A, B, and C windings of the three-phase two-pole permanent magnet brushless DC motor are respectively connected to the corresponding power transistors V1, VT2, VT3 in the electronic switch drive circuit. The tracking rotor of the position sensor is connected with the motor shaft.
When a certain phase of the stator winding is energized, the current interacts with the magnetic field generated by the permanent magnet of the rotor to generate a torque, which drives the rotor to rotate, and then the position of the rotor magnet is converted into an electrical signal by a position sensor to control Electronic switch circuit, so that the stator phase windings are turned on in a certain order, and the stator phase current changes in a certain order with the change of the rotor position. Since the conduction sequence of the electronic switch circuit is synchronized with the rotor angle, it plays the role of commutation of the mechanical commutator.
The principle of the brushless DC motor half-controlled bridge circuit is shown in Figure 7. This picture uses a photoelectric device as a position sensor, and three power transistors VT1, VT2, and VT3 form a power logic unit. The installation positions of the three optoelectronic devices VP1, VP2 and VP3 differ by 120° and are evenly distributed at one end of the motor. With the help of the rotating light shield installed on the motor shaft, the light from the light source is irradiated on each optoelectronic device in turn, and the position of the rotor magnetic pole is judged according to whether a certain optoelectronic device is irradiated with the light.
When the rotor is in the position shown in Figure 8(a), the photoelectric device VP1 is illuminated by light, so that the power transistor VT1 is turned on, and the current flows into the winding A-A’. The winding current is generated by the action of the rotor pole The torque causes the magnetic poles of the rotor to rotate in the direction of the arrow in Figure 8(a). When the rotor magnetic poles turn to the position shown in Figure 8(b), the rotating light shield directly mounted on the rotor shaft also rotates synchronously and shields VP1 so that VP2 is illuminated by light, so that the transistor VT1 is turned off and the transistor VT2 is turned on. On, the winding A-A’ is disconnected, and the current flows into the winding B-B’, so that the rotor poles continue to rotate in the direction of the arrow. When the rotor magnetic poles turn to the position shown in Figure 8(c), the rotating light shield has covered VP2, causing VP3 to be illuminated by light, causing the transistor VT2 to turn off and the transistor VT3 to turn on, so current flows into the winding C-C’, so The magnetic poles of the drive rotor continue to rotate in the clockwise direction and return to the position shown in Figure 8(a).
In this way, with the rotation of the position sensor rotor sector, the stator windings are sequentially fed phase by phase under the control of the position sensors VP1, VP2, VP3, realizing the commutation of the winding currents of each phase. In the process of commutation, the rotating magnetic field formed by the stator windings in the working air gap is jumping. This rotating magnetic field has three magnetic states within the range of 360° electrical angle, and each magnetic state lasts for 120° electrical angle. The relationship between the winding current of each phase and the magnetic field of the motor rotor is shown in Figure 8. Figure 8(a) is the first state, Fa is the magnetomotive force generated after winding A-A’ is energized. Obviously, the interaction between the winding current and the rotor magnetic field causes the rotor to rotate in a clockwise direction; after 120° electrical angle, it enters the second state, at this time the winding A-A’ is de-energized, and B-B’ is then energized , That is, the magnetic field generated by the stator winding has rotated 120°, as shown in Figure 8(b), the motor rotor continues to rotate in the clockwise direction; another 120° electrical angle will enter the third state, at this time winding B -B’ is powered off and C-C’ is powered on, the magnetic field generated by the stator winding has been rotated through an electrical angle of 120°, as shown in Figure 8(c); after the rotor has been rotated in a clockwise direction through an electrical angle of 120°, it will be Restore to the initial state. The schematic diagram of the turn-on sequence of each phase winding of the three-phase two-pole brushless DC motor is shown in Figure 9.
a. Since high-performance permanent magnets such as rare earth magnets can be used, the motor can achieve larger factors and higher efficiency, especially at low speeds.
b. A higher magnetic flux density can be obtained, and a small high-speed motor can be made.
c. It is easy to be multi-polarized and can be made into a hub motor.
a. In a certain output area, the efficiency will decrease when the excitation is weak. With the adoption of embedded magnets in recent years, this reduction is slowly being resolved.
b. Magnet materials are expensive and difficult to install. Permanent magnets are prone to demagnetization when they heat up.
c. Short-circuit damage to the inverter will cause a short-circuit of the motor terminals, which will generate a lot of braking force.
The advantages of high efficiency at low speeds of permanent magnet motors are very effective for cars that frequently start and stop in the city. In recent years, designs based on this point of view have increased. In particular, the motors used in Japanese electric vehicles have mostly transitioned to permanent magnet motors, such as Toyota RAV4LV EV and Honda EV Plus.