The prices of mid-and low-end cars in China are declining. In June 2004, Otto and Geely competed to launch extremely low-priced cars, with a price of less than 30,000 yuan/car. In contrast, the cost of electric vehicles is still high. The reason is that electric vehicles are still in the research and development stage, prototype and trial operation stage, and there is no batch at all, which is incomparable with fuel vehicles produced by assembly line. This is realistic and understandable.
At the same time, all kinds of electric vehicles that can be demonstrated at present are modified on the basis of the chassis and carriage of the original fuel vehicle, that is, the engine, fuel tank and other systems are all removed, and then the motor, battery and other related supporting equipment are installed to form an electric vehicle, while the hybrid power is to add a battery and electric drive system on the basis of the crude oil system to form an oil-electric hybrid drive system. Then, the cost of electric vehicles is mainly composed of batteries, chargers, drive motors, controllers and power conversion equipment, accounting for about 50-60% of the total vehicle cost.
At present, taking pure electric vehicles as an example, the batteries include lead-acid batteries, nickel-hydrogen batteries and lithium batteries. Some use DC power to drive DC motors, and some use inverters to convert DC power on board into AC three-phase 380V for three-phase asynchronous motors, and adopt frequency conversion equipment for speed regulation.
Different types of batteries, different storage capacity, the overall cost varies greatly. In addition, the improvement of the storage capacity of electric vehicles also doubles the cost. For example, if a lithium battery is equipped with a car, the cost of the battery is about 40,000 yuan, and the cost of the battery with a mileage of more than 300km is more than 80,000 yuan. The idea of this research and development is to drive during the day and charge at night. In order to make the mileage not lower than that of fuel vehicles, the battery cost is high.
The cost of driving motors for electric vehicles varies greatly. If DC brush motor is used, the vehicle-mounted power supply can directly supply power to the motor, and the thyristor controller is used for chopping speed regulation. At present, DC brush motors for electric vehicles can meet the requirements of electric vehicles. However, due to the limited output, high cost, few varieties and specifications, and small choice, the thyristor controller was originally made by foreign companies such as Italy and the United States, and now it can be localized, with high cost, and the key components are controlled by foreign companies.
If DC brushless motor is used, it must be integrated with the controller, and the cost is high. Adjusting the motor speed by adjusting the pulse width of the power supply has the advantages of small volume and light weight. The motor can be made in China, and the key components of the controller are produced by foreign companies, so it is unlikely to reduce the cost. At present, this kind of motor is in the research and development stage. Like electric vehicles, it is reasonable that it will not be mass-produced and the cost is high.
If AC asynchronous motor is used as the driving motor of electric vehicle, its advantages are: small size, light weight and not bad domestic quality. Because the vehicle power supply is DC, it needs to be converted into AC by inverter. The voltage of automobile motor is about 380V, and the power varies from tens of kw. Inverter power is not small, and the cost will not be low. AC motor adopts frequency conversion regulation, and AC asynchronous motor adopts frequency conversion control (VVVF) and field oriented control (FOC). The variable frequency controller is made in China and imported, but the key components are imported, so it is impossible to reduce the cost.
As for the reluctance motor under development, electronic controller is also needed to control the speed, and its cost is the same as above. Switched reluctance motor adopts fuzzy sliding mode control (FSMC) to control the motor and speed regulation. Without this electronic control device, the motor can't work.
The higher the motor speed, the faster the armature winds and cuts the magnetic field, and the higher the back electromotive force. On the contrary, it limits the current and reduces the torque, but it can output a lot of torque at low speed. Therefore, when the resistance on the road or uphill is high, the torque is large, so the current consumption is large. In other words, when the motor runs at a low speed, the current output is not small, but the voltage is reduced.
To adjust the speed of motor, it is necessary to change the voltage, which is the theoretical basis of motor speed regulation. It is a waste to reduce the voltage of vehicle-mounted power supply to the low voltage of motor speed regulation and consume limited power supply in frequent speed regulation.
The highest efficiency of the motor is at the rated speed, and the lower the speed, the lower the efficiency. In order to improve the utilization rate of vehicle-mounted power supply, it should be hoped that the higher the efficiency of the motor, the better.
The driving motor of electric vehicle requires large torque when starting and climbing, small torque when driving at high speed and large speed range. At present, DC brushless motor, DC permanent magnet brushless motor, AC asynchronous motor and reluctance motor are the mainstream technologies and preferred models of electric vehicle drive motors. They have an unavoidable equipment, electronic control equipment and microcomputer control technology, which is the main part of the cost of electric vehicles and one of the technical obstacles. At present, the core technology is in the hands of foreigners, and we still have to buy it. In the future, all kinds of electric vehicles in China will be pushed aside to form an industry. Or one day China can export electric vehicles, and foreign owners of controller core technology will charge royalties like color TV sets and DVDs, which is another story, but this is not necessarily impossible.
In order to reduce the total cost of electric vehicles, we can only make a fuss about batteries, chargers, motors and controllers. We should change this situation with the idea of technological innovation, invent new motor drive, speed change mechanism system and battery charging method, and take our own unique road.
If the battery of the electric vehicle is low, under the premise of ensuring the normal operation of the motor and not damaging the battery life under various road conditions, the battery-driven vehicle can continuously charge for about 200km and drive for 2-3 hours, and then supplement the power supply on the fast charger, which requires that the battery can be charged at a current of 1C or above 2C-3C. In addition, electric vehicles should travel in a city and an area, and there should be public charging stations within the driving range. In a very short time, such as 10 minute and 15 minute, the battery pack can be charged to 80%-90% and can run 100 km- 150 km. The electric car itself is equipped with a car charger, so you can charge it slowly in the garage when you go home. The vehicle battery is less, the vehicle weight is small, the load can be effectively increased, and the cost is low.
The motor should be a DC brush motor, which is directly driven after a slight improvement, without inverter power supply, and this piece of cost is cut off. Motor speed regulation does not adopt the usual methods of temporary wave, pulse adjustment and frequency adjustment, but adopts the principle of adjusting the throttle of internal combustion engine. The power of vehicle driving motor is decomposed into several low-power motors to form a combined motor. In this combination, the power of each motor is equal or different, and it is different in starting, accelerating, light load, heavy load and climbing. That is, the driver adjusts the workload and total power of the motor according to the actual operation of the electric vehicle, and the working motor always outputs the speed and torque at the rated speed without speed regulation, so that the electronic controller and governor are no longer used.
Multi-motor drive can reduce the current and rated power of the main motor of the vehicle, and reduce the impact of large current on the vehicle battery when driven by a single motor, which is particularly important and key for batteries that have been used for a long time and batteries that have little power in the vehicle battery pack, and can prolong the service life of the battery.
At present, some driving mechanisms of the electric vehicle under study still retain the mechanical transmission and clutch in the original vehicle, mainly because the motor speed control is not ideal, so it is retained. It is necessary to cancel the original mechanical gearbox and clutch and adopt magnetic drive to realize stepless speed change. By adjusting the distance between the driving device and the driven device, the function of the gearbox clutch can be realized, and an organic whole motor drive system can be formed by combining with two motors. The speed regulation driven by magnetic force can be matched with several small power motors of a single large motor or a combined motor. In this matching, the motor always works at the rated speed. Due to the adjustment of magnetic drive, the speed of the electric vehicle changes quickly and slowly, and then the load and torque of the motor also change, that is, if the whole vehicle needs large torque, the motor or motor group outputs large torque, otherwise it outputs small torque, and the torque change of the motor changes with the needs of the whole vehicle. The power consumption of the motor also changes accordingly, so that the motor can output how much torque and electricity the whole vehicle needs, which not only saves energy, but also does not need to go through a complicated motor control system. When the motor is running, the higher the speed, the smaller the torque, and the lower the speed, the greater the torque. This means that the load is heavy, or when climbing a hill, the speed should be reduced to increase the torque, which is the same as that of a motor. China's rare earth permanent magnet materials are dominant in the world and should be developed and applied. With DC rare earth permanent magnet brush motor and magnetic driver, rare earth permanent magnet materials are fully utilized, and China has independent intellectual property rights, and the overall cost is far lower than that of "motor, controller, mechanical gearbox and clutch". And will not be subject to foreign companies in the future.
Electric vehicles include pure electric vehicles, hybrid vehicles and fuel cell vehicles, all of which are driven by electric motors. If the combined motor and magnetic drive can be applied to these vehicles, the total cost of such vehicles will be greatly reduced, which will be easily accepted by the market and more competitive than diesel locomotives. Therefore, the research and development of the combination of motor and magnetic drive will play a positive role in promoting the research and development and industrialization of electric vehicles.
If the motor, clutch, gearbox, fuel tank and fuel supply system are removed with extremely low-priced cars such as Otto and Geely, the cost of deducting this piece is about 5,000-6,000 yuan, then the cost of the whole vehicle is about 25,000 yuan/vehicle, plus the battery pack, car charger, motor and magnetic drive, the total cost is 30,000-35,000 yuan. This kind of car is competitive. The electricity charge is estimated to be about 10 yuan/100km, and the power consumption per 100km is about 18 kW/h, while the average car consumes 8 liters per 100km/100km, according to the oil price of 3.63 yuan in June 2004. The former is 1/3 of the latter. If the fuel tax is implemented in 2005, the fuel consumption will further increase. At present, electric vehicles should be supported, and the electricity bill will not change much in 2005, and its power consumption will not increase. Compared with the two, electric vehicles are more competitive in operating costs.
Performance comparison of electric vehicle drive motor
Abstract: The drive motor system is one of the key technologies of electric vehicles. In this paper, several typical electric vehicle drive systems are qualitatively analyzed, their performances are compared, and their advantages and disadvantages are pointed out.
Keywords: electric vehicle; Drive motor; Analysis; performance comparison
The survival of human beings and the environment and the sustainable development of the global economy make people eager to find a low-emission and effective way to use resources, and the use of electric vehicles is undoubtedly a promising solution.
Modern electric vehicle is a comprehensive product that integrates many high and new technologies such as electric power, electronics, mechanical control, material science and chemical technology. The overall performance and economy depend on the battery system and motor drive control system. The motor drive system of electric vehicles generally consists of four main parts, namely the controller. Power converter, motor and sensor. 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 for electric vehicle motors
The operation of electric vehicles is very complicated, which is different from general industrial applications. Therefore, the requirements for the drive system are very high.
1. 1 The motor for electric vehicles should have the characteristics of high instantaneous power, strong overload capacity (overload coefficient 3 ~ 4), good acceleration performance and long service life.
1.2 The motor for electric vehicles should have a wide speed range, 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, it is required to have a high speed when the torque is low, so as to meet the requirements of high-speed driving on flat roads.
1.3 the motor used for electric vehicles should be able to achieve regenerative braking when the vehicle slows down and recycle energy back to the battery, so that the electric vehicle has the best energy utilization rate, which is impossible in internal combustion engines.
1.4 The motor for electric vehicles should have high efficiency in the whole operating range, so as to increase the driving range of 1 charging.
In addition, it is required that the motor for electric vehicles has good reliability, can work for a long time in harsh environment, has simple structure, is suitable for mass production, has low noise during operation, is convenient to use and maintain, and is cheap [1-2].
2 Types and control methods of motors for electric vehicles
2. 1 DC car
The main advantages of brushed DC motor are simple control and mature technology. It has incomparable excellent control characteristics of AC motor. In the early development of electric vehicles, DC vehicles were widely used. Even now, some electric cars are still driven by DC Motor Company. However, the existence of brush and mechanical commutator not only limits the further improvement of motor overload capacity and speed, but also requires frequent maintenance and replacement of brush and commutator if it runs for a long time. In addition, because the loss exists in the rotor, it is difficult to dissipate heat, which limits the further improvement of the torque-mass ratio of the motor. In view of the above defects of DC motor, newly developed electric vehicles basically do not use DC motor [3].
2.2 AC three-phase induction motor
2.2. 1 Basic performance of AC three-phase induction motor
AC three-phase induction motor is the most widely used motor. Its stator and rotor are laminated by silicon steel sheets, and there are no slip rings, commutators and other contact parts between the stators. Simple structure, reliable operation and durability. AC induction motor covers a wide range of power, and the speed reaches 12000 ~ 15000 r/min. Air cooling or liquid cooling can be used, and the cooling freedom is high. It has good adaptability to the environment and can realize regenerative feedback braking well. Compared with the DC motor with the same power, it has higher efficiency, reduced weight by about half, low price and convenient maintenance.
2.2.2 AC induction motor control system
Because the AC three-phase induction motor can't directly use the direct current provided by the battery, in addition, the AC three-phase induction motor has nonlinear output characteristics. Therefore, in the electric vehicle with AC three-phase induction motor, it is necessary to use the power semiconductor device in the inverter to change DC into AC with adjustable frequency and amplitude to control the AC three-phase motor. There are mainly v/f control method and slip frequency control method.
The vector control method is used to control the frequency of alternating current in the excitation winding of the AC three-phase induction motor and the terminal adjustment of the input AC three-phase induction motor, and to control the magnetic flux and torque of the rotating magnetic field of the AC three-phase induction motor, so as to change the speed and output torque of the AC three-phase induction motor, meet the requirements of load change characteristics, obtain the highest efficiency, and make the AC three-phase induction motor widely used in electric vehicles.
2.2.3 shortcomings of AC three-phase induction motor
The AC three-phase induction motor consumes a lot of power and the rotor is prone to heat. The AC three-phase induction motor must be cooled at high speed, otherwise it will damage the motor. The low power factor of AC three-phase induction motor makes the input power factor of frequency conversion transformer low, so it is necessary to adopt large-capacity frequency conversion transformer. The control system cost of AC three-phase induction motor is much higher than that of AC three-phase induction motor itself, which increases the cost of electric vehicle [2-4]. In addition, the speed regulation performance of 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 kind of high performance motor. Its biggest feature is that it has the external characteristics of DC motor and has no mechanical contact structure composed of brushes. In addition, it uses a permanent magnet rotor, and there is no excitation loss: the heated armature winding is installed on the outer stator, so the heat dissipation is easy. Therefore, the permanent magnet brushless DC motor has no commutation spark, no radio interference, long service life, reliable operation and simple maintenance. In addition, its speed is not limited by mechanical commutation, and it can run at hundreds of thousands of revolutions per minute if air bearings or magnetic bearings are used. Compared with brushless DC motor system, permanent magnet motor system 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
The typical permanent magnet brushless DC motor is a quasi-decoupled vector control system. Because the permanent magnet can only produce a fixed amplitude magnetic field, the permanent magnet brushless DC motor system is very suitable for running in the constant torque region, which is generally completed by current hysteresis control or current feedback SPWM. In order to further expand the speed, the permanent magnet brushless DC motor can also adopt weak magnetic control. The essence of flux weakening control is to lead the phase angle of phase current, provide direct axis demagnetization potential and weaken the flux linkage in stator winding.
2.3.3 Deficiencies of Permanent Magnet Brushless DC Motor
The permanent magnet brushless DC motor is influenced and limited by the process of permanent magnet materials, which makes the power range of permanent magnet brushless DC motor smaller 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 be reduced or demagnetized, which will reduce the performance of the permanent magnet motor and even damage the motor in serious cases. Therefore, it must be strictly controlled in use to prevent overload. In the constant power mode, the operation of permanent magnet brushless DC motor is complex, which requires a complex control system, making the driving system of permanent magnet brushless DC motor expensive [5- 10].
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 characteristics: its structure is simpler than any other motor. There are no slip rings, windings and permanent magnets on the motor rotor, and there are simple concentrated windings on the stator. The winding end is short, and there is no interphase jumper, which is convenient for maintenance and repair. Therefore, the reliability is good, and the rotating speed can reach 15000 r/min. The efficiency can reach 85% ~ 93%, which is higher than that of AC induction motor. The loss is mainly in the stator, and the motor is easy to cool; The rotor element has a wide range of permanent magnet speed regulation, flexible control, easy realization of torque-speed characteristics with various special requirements, and high efficiency in a wide range. And is more suitable for that pow performance requirements of electric vehicle.
2.2.4 Switched reluctance motor control system
Switched reluctance motor is highly nonlinear, so its driving system is more complicated. Its control system includes a power converter.
A. rectifier
Whether the excitation winding of switched reluctance motor is forward current or reverse current, the torque direction is constant and the period is commutated, and each phase only needs a small capacity power switch tube. The power converter has simple circuit, no through fault, good reliability, easy soft start and four-quadrant operation of the system, and strong regenerative braking ability. The cost of inverter control system is lower than that of AC three-phase induction motor.
B. Controller
The controller consists of microprocessor, digital logic circuit and other components. According to the instructions 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 sends out a series of execution instructions to control the switched reluctance motor to run under different working conditions of the electric vehicle. The performance and adjustment flexibility of the controller depend on the performance matching relationship between the hardware and software of the microprocessor.
C. position detector
Switched reluctance motor needs high-precision position detector to provide motor rotor position, speed and current signals for control system, and needs high switching frequency to reduce the noise of switched reluctance motor.
2.4.3 disadvantages of switched reluctance motor
The control system of switched reluctance motor is more complicated than that of other motors. Position detector is the key device of switched reluctance motor, and its performance has an important influence on the control and operation of switched reluctance motor. Because switched reluctance motor has double salient pole structure, torque fluctuation is inevitable, and noise is the main disadvantage of switched reluctance motor. However, recent research shows that the noise of switched reluctance motor can be well suppressed through reasonable design, manufacture and control technology. In addition, because the output torque of switched reluctance motor fluctuates greatly and the DC current of power converter fluctuates greatly, it is necessary to install a large filter capacitor on the DC bus [2, 1 1- 13].
3 Performance comparison of backup drive motors for electric vehicles
Electric vehicles have adopted different motors in different historical periods, especially 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 motor. Permanent magnet brushless DC motor and switched reluctance motor show better performance than DC motor, and gradually replace DC motor in electric vehicles. Table 1 is the basic performance comparison of various motors used in modern electric vehicles. At present, the costs of alternator, permanent magnet motor, switched reluctance motor and their control devices are still relatively high. After mass production, the prices of these motors and unit control devices will be rapidly reduced to meet the requirements of economic benefits and reduce the price of electric vehicles [2].