The coexistence of mankind and the environment and the sustainable development of the global economy make people eager to find a low-emission and efficient use of resources. The use of electric vehicles is undoubtedly a promising solution.

Modern electric vehicles are comprehensive products that integrate a variety of high-tech products such as electricity, electronics, mechanical control, materials science, and chemical technology. The overall operating performance, economy, etc. firstly depend on the battery system and the motor drive control system.

A typical electric vehicle drive system is powered by a battery to the inverter, and there is usually a gearbox to drive the entire vehicle. This control system is equipped with sensors, and the controllers are now all developed into digitization, and there are not many changes in the motors. Compared with industrial applications, automotive applications are a different concept. Industrial application space is not limited, and it is feasible to use standard packaging mode. However, for automotive applications, space is limited. Each system is customized according to specific models, especially hybrid vehicles. In terms of reliability, industrial applications are highly reliable, but no matter from which aspect, the level of industrial applications is still inferior to automotive applications because their purposes are different. In industrial applications, it is mainly to ensure the reliability of application efficiency, but in automotive applications, the reliability of the motor application system involves the safety of the occupants, so the reliability requirements are very high. The cooling method is air-cooled for industrial applications, and water-cooled for automotive applications. In terms of control performance, industrial applications are mostly variable frequency speed control, which has poor dynamic performance. In automotive applications, precise torque control is required and the dynamic performance is good.

The current development trends of electric drive systems for vehicles mainly include permanent magnetization, digitalization and integration.

The permanent magnet reluctance motor has high efficiency, large specific power and high power factor. Digitization is the core of the electric drive system. There are two ways to integrate the electric motor system, one is the combination of the electric motor and the engine, and the other is the combination of the electric motor and the gearbox. There is also a trend to integrate power electronics. Among the drive controller products, the highest international level is 17.2kW. Using hybrid power electronic integration technology, the core is the use of high-function integrated modules, and the use of new film capacitor integration technology.

The motor drive system of an electric vehicle generally consists of 4 main parts, namely the controller, the power converter, the electric motor and the sensor. The electric drive system is the heart of electric vehicles (EV) and hybrid electric vehicles (HEV). The electric motor converts electrical energy into mechanical energy to propel the vehicle, or vice versa, converts mechanical energy into electrical energy for regenerative braking and charging of on-board energy storage devices. At present, the motors used in electric vehicles generally include DC motors, induction motors, switched reluctance motors, and permanent magnet brushless motors.

1. The basic requirements of electric vehicles for electric motors

The operation of electric vehicles is different from general industrial applications and is very complicated. Electric motors used in electric vehicles generally require frequent starting and stopping, high rate of change acceleration/deceleration, high torque and low speed climbing, low torque and high speed driving, and a very wide operating speed range. Therefore, the requirements for the drive system are very high, mainly due to the following characteristics:

1.1 The electric motor used in electric vehicles should have the characteristics of large instantaneous power, strong overload capacity, overload coefficient (should be 3~4), 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 high speed of the car on flat roads. Require.

1.3 The electric motor for electric vehicles should be able to realize regenerative braking when the vehicle is decelerating, and recycle energy and feed it back to the battery, so that electric vehicles have the best energy utilization rate, which cannot be achieved on internal combustion engine vehicles.

1.4 The electric motor for electric vehicles should have high efficiency in the entire operating range to increase the driving range of one charge.

In addition, the motors for electric vehicles are required to have good reliability, be able to work in harsh environments for a long time, have a simple structure, adapt to mass production, low noise during operation, convenient use and maintenance, and low price.

2. Types and control methods of electric motors for electric vehicles

2.1 DC motor

Brushed DC motors are widely used in occasions that require adjustable speed, good speed regulation performance, and frequent starting, braking and reverse rotation. Its main advantages are simple control, mature technology, and excellent control characteristics that are unmatched by AC motors. It has been widely used in different electric traction application systems. In the early development of electric vehicles, DC motors were mostly used. Even now, some electric vehicles are still driven by DC motors. However, the existence of brushes and mechanical commutator not only limits the further improvement of the motor's overload capacity and speed, but also if it runs for a long time, it is bound to maintain and replace the brushes and commutator frequently. In addition, because 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 shortcomings 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 sliding rings, commutators and other components that contact each other between the stators. Simple structure, reliable operation and durable. The power coverage of AC induction motors is very wide, and the speed can reach 12000~15000r/min. Air cooling or liquid cooling can be used, with high cooling freedom. It has good adaptability to the environment and can realize regenerative feedback braking. Compared with the DC motor of the same power, the efficiency is higher, the quality is reduced by about half, the price is cheap, and the maintenance is convenient.

2.2.2 Control system of AC induction motor

Since the AC three-phase induction motor cannot directly use the DC power supplied by the battery, in addition, the AC three-phase induction motor has non-linear output characteristics. Therefore, in electric vehicles using AC three-phase induction motors, power semiconductor devices in the inverter need to be used to convert DC power into AC power with adjustable frequency and amplitude to achieve control of AC three-phase motors. There are mainly v/f control method and slip frequency control method.

The vector control method is used to control the frequency of the field winding alternating current of the AC three-phase induction motor and the end adjustment control of the input AC three-phase induction motor, control the magnetic flux and torque of the rotating magnetic field of the AC three-phase induction motor, and realize the change of the AC three-phase induction motor The speed and output torque can meet the requirements of load change characteristics and achieve the highest efficiency, so that AC three-phase induction motors can be widely used in electric vehicles.

2.2.3 Shortcomings of AC three-phase induction motors

The AC three-phase induction motor consumes a lot of power, and the rotor is prone to heat. 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 variable frequency transformer device is also low, so a large-capacity variable frequency transformer device is required. 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 AC three-phase induction motor has poor speed regulation.

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 a mechanical commutator and a brush, so the mechanical friction loss is low and the efficiency is high. In addition, it uses a permanent magnet rotor, no excitation loss, that is, the heating 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 restricted by mechanical commutation. 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 Permanent magnet brushless DC motor control system

A 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 suitable for operation in the constant torque region, and generally uses current Hysteresis control or current feedback type SPWM method to complete. In order to further expand the speed, permanent magnet brushless DC motors can also adopt field weakening control. The essence of field weakening control is to advance the phase angle of the phase current and provide a direct-axis demagnetization potential to weaken the flux linkage in the stator winding.

2.3.3 The shortcomings of permanent magnet brushless DC motors

Permanent magnet brushless DC motors are affected and restricted by the technology of permanent magnet materials, making the power range of permanent magnet brushless DC motors smaller, with a maximum power of only tens of kilowatts. When the permanent magnet material is subjected to vibration, high temperature and overload current, its magnetic permeability may be reduced or demagnetization phenomenon will occur, which will reduce the performance of the permanent magnet motor, and even damage the motor in severe cases. It must be strictly controlled during use to make it No overload occurs. 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 characteristics: its structure is simpler than any other motor. There are no slip rings, windings and permanent magnets on the rotor of the motor, but on the stator. There are simple concentrated windings, the ends of the windings are short, and there is no jumper between phases, which is easy to maintain and repair. Therefore, the reliability is good, and the speed can reach 15000r/min. The efficiency can reach 85%~93%, which is higher than that of AC induction motors. The loss is mainly in the stator, the motor is easy to cool; the rotor element is permanent magnet, the speed range is wide, the control is flexible, it is easy to realize the torque-speed characteristics of various special requirements, and it maintains high efficiency in a wide range. It is more suitable for the power performance requirements of electric vehicles.

2.4.2 Control system of switched reluctance motor

The switched reluctance motor has a high degree of non-linearity, so its drive system is more complicated. Its control system includes power converters, controllers and position sensors.

a. Power converter

The field winding of the switched reluctance motor, regardless of the forward current or the reverse current, the torque direction is unchanged, and the phase is reversed. Each phase only needs a power switch tube with a small capacity, and the power converter circuit is relatively simple. There will be a through failure, good reliability, easy to realize the soft start of the system and four-quadrant operation, with strong regenerative braking ability. The cost is lower than the inverter control system of AC three-phase induction motor.

b. Controller

The controller is composed of microprocessors, digital logic circuits and other components. According to the command input by the driver, the microprocessor analyzes and processes the motor rotor position fed back by the position detector and the current detector at the same time, and makes a decision 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 relationship between the performance of the microprocessor's software and hardware.

c. Position detector

Switched reluctance motors need high-precision position detectors to provide the control system with the change signals of the motor rotor position, speed and current, and require a higher switching frequency to reduce the noise of the switched reluctance motors.

2.4.3 Shortcomings of switched reluctance motors

The control system of the switched reluctance motor is more complicated than that 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 has a double salient pole structure, there is inevitably torque fluctuation, and noise is the main disadvantage of the switched reluctance motor. However, recent studies have shown that the noise of switched reluctance motors can be well suppressed by adopting reasonable design, manufacturing and control technology. In addition, because the output torque of the switched reluctance motor fluctuates greatly, and the DC current of the power converter fluctuates, a large filter capacitor is required on the DC bus.

3. Performance comparison of various drive motors used in electric vehicles

Electric vehicles have adopted different electric vehicles in different historical periods. They have adopted DC motors 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 brush DC motors and switched reluctance motors show superior performance than DC motors. In electric vehicles, these Electric motors gradually replaced DC motors. At present, the cost of AC 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 be rapidly reduced, which will be able to meet the requirements of economic benefits and make The price of electric vehicles has been reduced.