The coexistence of human beings with the environment and the sustainable development of the global economy make people eager to seek a low-emission and resource-efficient means of transportation, and the use of electric vehicles is undoubtedly a promising solution.
Modern electric vehicles are comprehensive products that integrate various high-tech technologies such as electricity, electronics, mechanical control, material science, and chemical technology. The overall operating performance, economy, etc. first depend on the battery system and the motor drive control system. The motor drive system of an electric vehicle generally consists of four main parts, namely the controller. Power converters, motors and sensors. At present, the motors used in electric vehicles generally include DC motors, induction motors, switched reluctance motors, and permanent magnet brushless motors.
1. Basic requirements of electric vehicles for electric motors
The operation of electric vehicles, unlike general industrial applications, is very complex. Therefore, the requirements for the drive system are very high.
1.1 Motors for electric vehicles should have the characteristics of large instantaneous power, strong overload capacity, overload coefficient of 3 to 4), good acceleration performance and long service life.
1.2 Motors for electric vehicles should have a wide range of speed regulation, including constant torque area and constant power area. In the constant torque area, high torque is required when running at low speed to meet the requirements of starting and climbing; in the constant power area, high speed is required when low torque is required to meet the requirements of high-speed driving on flat roads. Require.
1.3 The electric motor for electric vehicles should be able to realize regenerative braking when the vehicle decelerates, recover and feed back energy to the battery, so that the electric vehicle has the best energy utilization rate, which cannot be achieved in the internal combustion engine vehicle.
1.4 The electric motor for electric vehicles should have high efficiency in the entire operating range, so as to improve the cruising range of one charge.
In addition, it is also required that the electric motor for electric vehicles has good reliability, can work for a long time in a harsh environment, has a simple structure and is suitable for mass production, has low noise during operation, is easy to use and maintain, and is cheap.
2 Types and Control Methods of Electric Motors for Electric Vehicles
2.1 DC
Motors The main advantages of brushed DC motors are simple control and mature technology. It has excellent control characteristics unmatched by AC motors. In the early developed electric vehicles, DC motors are mostly used, and even now, some electric vehicles are still driven by DC motors. However, due to the existence of brushes and mechanical commutators, it not only limits the further improvement of the motor’s overload capacity and speed, but also requires frequent maintenance and replacement of brushes and commutators if it runs for a long time. In addition, since the loss exists on the rotor, it is difficult to dissipate heat, which limits the further improvement of the motor torque-to-mass ratio. In view of the above defects of DC motors, DC motors are basically not used in newly developed electric vehicles.
2.2 AC three-phase induction motor
2.2.1 Basic performance of AC three-phase induction motor
AC three-phase induction motors are the most widely used motors. The stator and rotor are laminated with silicon steel sheets, and there are no slip rings, commutators and other components that are in contact with each other between the stators. Simple structure, reliable operation and durable. The power coverage of the AC induction motor is very broad, and the speed reaches 12000 ~ 15000r/min. Air cooling or liquid cooling can be used, with a high degree of cooling freedom. It has good adaptability to the environment and can realize regenerative feedback braking. Compared with the same power DC motor, the efficiency is higher, the quality is reduced by about half, the price is cheap, and the maintenance is convenient.
2.2.2 The control system
of the AC induction motor Because the AC three-phase induction motor cannot directly use the DC power supplied by the battery, and the AC three-phase induction motor has nonlinear output characteristics. Therefore, in an electric vehicle using an AC three-phase induction motor, it is necessary to use the power semiconductor device in the inverter to convert the direct current into an alternating current whose frequency and amplitude can be adjusted to realize the control of the AC three-phase motor. There are mainly v/f control method and slip frequency control method.
Using the vector control method, the frequency of the alternating current of the excitation winding of the AC three-phase induction motor and the terminal adjustment of the input AC three-phase induction motor are controlled, the magnetic flux and torque of the rotating magnetic field of the AC three-phase induction motor are controlled, and the change of the AC three-phase induction motor is realized. The speed and output torque can meet the requirements of load change characteristics, and can obtain the highest efficiency, so that the AC three-phase induction motor can be widely used in electric vehicles.
2.2.3 Shortcomings of
AC three-phase induction motor The power consumption of AC three-phase induction motor is large, and the rotor is easy to heat up. It is necessary to ensure the cooling of the AC three-phase induction motor during high-speed operation, otherwise the motor will be damaged. The power factor of the AC three-phase induction motor is low, so that the input power factor of the frequency conversion and voltage conversion device is also low, so it is necessary to use a large-capacity frequency conversion and voltage conversion device. The cost of the control system of the AC three-phase induction motor is much higher than that of the AC three-phase induction motor itself, which increases the cost of the electric vehicle. In addition, the speed regulation of the AC three-phase induction motor is also poor.
2.3 Permanent magnet brushless DC motor
2.3.1 Basic performance of permanent magnet brushless DC motor
Permanent magnet brushless DC motor is a high-performance motor. Its biggest feature is that it has the external characteristics of a DC motor without a mechanical contact structure composed of brushes. In addition, it adopts permanent magnet rotor, and there is no excitation loss: the heated armature winding is installed on the outer stator, which is easy to dissipate heat. Therefore, the permanent magnet brushless DC motor has no commutation sparks, no radio interference, long life and reliable operation. , easy maintenance. In addition, its speed is not limited by mechanical commutation, and if air bearings or magnetic suspension bearings are used, it can run at up to several hundred thousand revolutions per minute. Compared with the permanent magnet brushless DC motor system, it has higher energy density and higher efficiency, and has a good application prospect in electric vehicles.
2.3.2 The control system of the permanent magnet brushless DC motor The
typical permanent magnet brushless DC motor is a quasi-decoupling vector control system. Since the permanent magnet can only generate a fixed-amplitude magnetic field, the permanent magnet brushless DC motor system is very important. It is suitable for running in the constant torque region, generally using current hysteresis control or current feedback type SPWM method to complete. In order to further expand the speed, the permanent magnet brushless DC motor can also use field weakening control. The essence of field weakening control is to advance the phase angle of the phase current to provide a direct-axis demagnetization potential to weaken the flux linkage in the stator winding.
2.3.3 Insufficiency of
Permanent Magnet Brushless DC Motor The permanent magnet brushless DC motor is affected and restricted by the permanent magnet material process, which makes the power range of the permanent magnet brushless DC motor small, and the maximum power is only tens of kilowatts. When the permanent magnet material is subjected to vibration, high temperature and overload current, its magnetic permeability may decrease or demagnetize, which will reduce the performance of the permanent magnet motor, and even damage the motor in severe cases. Overload does not occur. In the constant power mode, the permanent magnet brushless DC motor is complicated to operate and requires a complex control system, which makes the drive system of the permanent magnet brushless DC motor very expensive.
2.4 Switched Reluctance Motor
2.4.1 Basic Performance of Switched Reluctance Motor
The switched reluctance motor is a new type of motor. The system has many obvious features: its structure is simpler than any other motor, and there are no slip rings, windings and permanent magnets on the rotor of the motor, but only on the stator. There is a simple concentrated winding, the ends of the winding are short, and there is no interphase jumper, which is easy to maintain and repair. Therefore, the reliability is good, and the speed can reach 15000 r/min. The efficiency can reach 85% to 93%, which is higher than that of AC induction motors. The loss is mainly in the stator, and the motor is easy to cool; the rotor is a permanent magnet, which has a wide speed regulation range and flexible control, which is easy to achieve various special requirements of torque-speed characteristics, and maintains high efficiency in a wide range. It is more suitable for the power performance requirements of electric vehicles.
2.4.2 Switched reluctance motor control system
Switched reluctance motor has a high degree of nonlinear characteristics, therefore, its drive system is more complex. Its control system includes a power converter.
a. The excitation winding of the switched reluctance motor of the power converter, no matter the forward current or the reverse current, the torque direction remains unchanged, and the period is commutated. Each phase only needs a power switch tube with a smaller capacity, and the power converter circuit is relatively Simple, no straight-through failure, good reliability, easy to implement soft start and four-quadrant operation of the system, and strong regenerative braking capability. The cost is lower than the inverter control system of the AC three-phase induction motor.
b. Controller
The controller consists of microprocessors, digital logic circuits and other components. According to the command input by the driver, the microprocessor analyzes and processes the rotor position of the motor fed back by the position detector and the current detector at the same time, and makes decisions in an instant, and issues a series of execution commands to control the switched reluctance motor. Adapt to the operation of electric vehicles under different conditions. The performance of the controller and the flexibility of adjustment depend on the performance cooperation between the software and hardware of the microprocessor.
c. Position detector
Switched reluctance motors require high-precision position detectors to provide the control system with signals of changes in the position, speed and current of the motor rotor, and require a higher switching frequency to reduce the noise of the switched reluctance motor.
2.4.3 Shortcomings of Switched Reluctance Motors
The control system of the switched reluctance motor is a little more complicated than the control systems of other motors. The position detector is the key component of the switched reluctance motor, and its performance has an important influence on the control operation of the switched reluctance motor. Since the switched reluctance motor is a doubly salient structure, there is inevitably torque fluctuation, and noise is the main disadvantage of the switched reluctance motor. However, research in recent years has shown that the noise of the switched reluctance motor can be completely suppressed by adopting reasonable design, manufacturing and control technology.
In addition, due to the large fluctuation of the output torque of the switched reluctance motor and the large fluctuation of the DC current of the power converter, a large filter capacitor needs to be installed on the DC bus. Cars have adopted different electric motors in different historical periods, using the DC motor with the best control performance and lower cost. With the continuous development of motor technology, machinery manufacturing technology, power electronics technology and automatic control technology, AC motors. Permanent magnet brushless DC motors and switched reluctance motors show superior performance over DC motors, and these motors are gradually replacing DC motors in electric vehicles. Table 1 compares the basic performance of various electric motors used in modern electric vehicles. At present, the cost of alternating current motors, permanent magnet motors, switched reluctance motors and their control devices is still relatively high. After mass production, the prices of these motors and unit control devices will decrease rapidly, which will meet the requirements of economic benefits and make the The price of electric vehicles is reduced.
Post time: Mar-24-2022