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

Modern electric vehicles are a combination of high-tech products such as power, electronics, mechanical control, materials science and chemical technology. The overall operational performance, economy, etc. depend first on the battery system and the motor drive control system.

A typical electric vehicle drive system, powered by a battery to an inverter, usually has a gearbox to drive the entire vehicle. This control system has sensors, and the controllers are now digital, and there are not many changes in the motor. Automotive applications are a different concept than industrial applications. There is no limit to the industrial application space. It is still feasible to use the standard package mode. However, for automotive applications, the space is limited. Each system is customized according to the specific model, especially the hybrid vehicle. In terms of reliability, industrial applications are highly reliable, but in any case, industrial applications are not as good as automotive applications because their purpose is different. In industrial applications, the reliability of application efficiency is mainly ensured, but in automotive applications, the reliability of the motor application system involves the safety of the occupant, so the reliability requirements are very high. Industrial applications for cooling are air-cooled and automotive applications are water-cooled. Industrial applications in control performance are mostly variable frequency speed control, which has poor dynamic performance. In automotive applications, precise torque control is required and dynamic performance is good.

At present, the development trend of automotive electric drive systems mainly includes permanent magnetization, digitization 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 motor system. One is the combination of the motor and the engine, and the other is the combination of the motor and the gearbox. There is also a trend to integrate power electronics. Today, the highest level of drive controller products is 17.2 kW. Using hybrid power electronics integration technology, the core is to use high-performance integrated modules, using a new film and capacitor integration technology.

The motor drive system of an electric vehicle generally consists of four main components, namely a controller, a power converter, a motor and a sensor. The heart of electric drive system electric vehicles (EV) and hybrid electric vehicles (HEV). The electric motor converts electrical energy into mechanical energy to propel the vehicle, or conversely converts mechanical energy into electrical energy for regenerative braking and charging of the onboard energy storage device. At present, electric motors used in electric vehicles generally include a direct current motor, an induction motor, a switched reluctance motor, and a permanent magnet brushless motor.

First, 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 for electric vehicles typically require frequent starting and stopping, high rate of change acceleration/deceleration, high torque and low speed hill climbing, low torque and high speed driving, and a very wide range of operating speeds. Therefore, the requirements for the drive system are very high, mainly due to the following characteristics:

1.1 Motors for electric vehicles should have the characteristics of large instantaneous power, strong overload capability, overload factor (should be 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 zone and constant power zone. In the constant torque zone, it is required to have large torque at low speed to meet the requirements of starting and climbing; in the constant power zone, high speed is required when low torque is required to meet the high speed of the car on a flat road. Claim.

1.3 Motors for electric vehicles should be able to achieve regenerative braking when the car decelerates, recovering energy and feeding back the battery, so that the electric car has the best energy utilization, which is not possible in internal combustion engines.

1.4 Motors for electric vehicles should have high efficiency throughout the entire operating range to increase the driving range of one charge.

In addition, the electric motor for electric vehicles is required to have good reliability and can work for a long time in a harsh environment. The structure is simple and suitable for 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 applications requiring adjustable speed, good speed regulation, and frequent starting, braking, and reversing. Its main advantages are simple control, mature technology, and excellent control characteristics unmatched by AC motors. It has been widely used in different electric traction applications. In the early development of electric vehicles, DC motors were used. Even now, some electric vehicles still use DC motors to drive them. However, due to the presence of brushes and mechanical commutators, the motor overload capability and speed are further limited, and if it is operated for a long time, it is necessary to frequently maintain and replace the brushes and commutators. In addition, since the loss exists on the rotor, heat dissipation is difficult, which further limits the motor torque quality ratio. In view of the above drawbacks of DC motors, DC motors have not been basically 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 the rotor are made of silicon steel sheets and the sliding rings, commutators and the like are not in contact with each other. Simple structure, reliable operation and durability. The AC induction motor has a wide power coverage and a speed of 12,000 to 15000 r/min. Air cooling or liquid cooling can be used, and the degree of freedom of cooling is high. 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 AC induction motor control system

Since the AC three-phase induction motor cannot directly use the DC power supplied from the battery, the AC three-phase induction motor has a nonlinear output characteristic. Therefore, in an electric vehicle using an AC three-phase induction motor, it is necessary to apply a power semiconductor device in the inverter to convert the direct current into an alternating current whose frequency and amplitude can be adjusted to realize control of the alternating current 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 control of the input AC three-phase induction motor, controlling the magnetic flux and torque of the rotating magnetic field of the AC three-phase induction motor, and realizing the change of the AC three-phase induction motor Speed and output torque to meet load variation characteristics and to achieve maximum efficiency, making AC three-phase induction motors widely used in electric vehicles.

2.2.3 Insufficient AC three-phase induction motor

The AC three-phase induction motor consumes a large amount 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 may 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

A permanent magnet brushless DC motor is a high performance motor. Its biggest feature is that it has the external characteristics of the DC motor without the mechanical contact structure composed of the mechanical commutator and the 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 mounted on the outer stator, and the heat dissipation is easy. Therefore, the permanent magnet brushless DC motor has no commutation spark, no radio interference, long life and reliable operation. Easy maintenance. In addition, its speed is not limited by mechanical commutation. If air bearings or magnetic bearings are used, it can run at hundreds of thousands of revolutions per minute. The permanent magnet brushless DC motor system has a higher energy density and higher efficiency, and has a good application prospect in electric vehicles.

2.3.2 Control system of permanent magnet brushless DC motor

A typical permanent magnet brushless DC motor is a quasi-decoupled vector control system. Because permanent magnets can only generate fixed amplitude magnetic fields, permanent magnet brushless DC motor systems are very suitable for operation in constant torque regions. Hysteresis control or current feedback type SPWM method is used to complete. In order to further expand the speed, the permanent magnet brushless DC motor can also adopt weak magnetic control. The essence of the field weakening control is to advance the phase angle of the phase current, providing a direct-axis demagnetizing potential to weaken the flux linkage in the stator winding.

2.3.3 Insufficient permanent magnet brushless DC motor

The permanent magnet brushless DC motor is affected and limited by the permanent magnet material process, so that the permanent magnet brushless DC motor has a small power range and the maximum power is only several tens of kilowatts. When the permanent magnet material is subjected to vibration, high temperature and overload current, its magnetic permeability may be degraded or demagnetization may occur, which will reduce the performance of the permanent magnet motor. In severe cases, it will damage the motor and must be strictly controlled during use. No overload occurs. In the constant power mode, the permanent magnet brushless DC motor is complicated to operate and requires a complicated control system, so that the driving system of the permanent magnet brushless DC motor is expensive.

2.4 switched reluctance motor

2.4.1 Basic performance of switched reluctance motor

Switched reluctance motor is a new type of motor. The system has many obvious features: its structure is simpler than any other motor. There are no slip rings, windings and permanent magnets on the rotor of the motor, just on the stator. There is a simple concentrated winding, the ends of the windings are short, there is no phase jumper, and maintenance and repair are easy. Therefore, the reliability is good and the rotational speed can reach 15000r/min. The efficiency can reach 85% to 93%, which is higher than AC induction motor. The loss is mainly in the stator, and the motor is easy to cool; the rotor element permanent magnet has a wide speed range, flexible control, easy to realize various torque-speed characteristics of special requirements, and maintains high efficiency over a wide range. More suitable for electric vehicle dynamic performance requirements.

2.4.2 Switched Reluctance Motor Control System

Switched reluctance motors have a high degree of nonlinearity and, therefore, their drive systems are complex. Its control system includes power converters, controllers and position sensors.

Power converter

The excitation winding of the switched reluctance motor, regardless of the forward current or reverse current, has the same torque direction, and the commutation period requires only one power switch tube with a smaller capacity for each phase. The power converter circuit is simpler, not There will be a straight-through fault with 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 is composed of components such as a microprocessor and a digital logic circuit. The microprocessor analyzes and processes the position of the motor rotor fed back by the position detector and the current detector according to the command input by the driver, 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 the adjustment depend on the performance of the microprocessor's software and hardware.

c. position detector

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

2.4.3 Insufficient Switched Reluctance Motor

The control system of the switched reluctance motor is more complicated than the control system 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 double salient pole structure, torque ripple is inevitable, and noise is the main disadvantage of the switched reluctance motor. However, recent studies have shown that the noise of a switched reluctance motor can be well suppressed by reasonable design, manufacturing and control techniques. In addition, since the output torque of the switched reluctance motor fluctuates greatly, the DC current fluctuation of the power converter is also large, so a large filter capacitor is required to be mounted on the DC bus.

Third, the performance of various drive motors used in electric vehicles

Electric vehicles have adopted different electric motors in different historical periods, using DC motors with the best control performance and lower cost. With the continuous development of motor technology, mechanical manufacturing technology, power electronics technology and automatic control technology, AC motors, permanent magnet brush DC motors and switched reluctance motors have shown superior performance over DC motors. In electric vehicles, these The motor gradually replaced the DC motor. At present, the cost of the motor, the permanent magnet motor and the switched reluctance motor and their control devices are relatively high. After mass production, the price of these motors and unit control devices will be rapidly reduced, which will meet the economic efficiency requirements and The overall price of electric vehicles is reduced.