Whether pure electric vehicle or hybrid electric vehicle, motor drive system is an important part of its power system.
The task of the motor drive system is to efficiently convert the energy of the battery into the kinetic energy of the whole vehicle under the control of the driver, or feed back the kinetic energy of the whole vehicle to the battery. According to the functional division, the motor drive system of new energy vehicles can be divided into electrical and mechanical systems. The electrical system consists of three subsystems: drive motor, power converter and controller; The mechanical system is mainly composed of mechanical transmission (optional) and wheels. The controller is divided into three functional units: sensor, connecting circuit and processor. The sensor converts various measured data, such as current, voltage, temperature, speed, torque and electromagnetic flux, into electrical signals, which can be switching, digital or analog. These electrical signals are adjusted to appropriate values through the connecting circuit and input to the processor. The output signal of the processor is usually amplified to drive the semiconductor element of the power converter. In the process of driving and energy regeneration (the energy refers to the electric energy stored in the battery), the energy flow between the energy source and the motor is regulated by the power converter. The motor and the wheel are connected together through a mechanical transmission device, which is optional because the motor can also be directly installed on the wheel, that is, the electric wheel drive.
- Characteristic requirements for electric drive system of new energy vehicles
The performance requirements of the vehicle’s driving system are determined by the following three aspects: the performance requirements of the vehicle’s on-board driving system and the performance constraints of the vehicle’s new energy system. The requirements of driving performance are determined by the driving mode including vehicle power performance, braking performance and driving range; Vehicle performance constraints mainly refer to vehicle type, vehicle weight and load, etc; The performance of energy system is related to battery, fuel cell, super capacitor, flywheel and various hybrid energy sources. Therefore, the performance determination and overall matching of motor drive system should be optimized at the system level, and the matching between each subsystem must be carefully studied.
The development of electric drive system is closely related to the development of motor, high-power electronic devices, microprocessor technology and control strategy.
1.1 driving motor characteristics and characteristics of new energy vehicles
- Driving motor characteristics of new energy vehicles
Various driving motors used for new energy vehicles are different from ordinary industrial motors. They are usually required to start and stop frequently, accelerate and decelerate. They require high torque at low speed or climbing, low torque at high speed and large speed range; Industrial drive motors are usually optimized at the rated operating point. Therefore, the driving motors of new energy vehicles should be classified into one category. The main characteristics in terms of load requirements, technical performance and working environment are as follows:
(1) The driving motor of new energy vehicles usually needs 4-5 times overload to meet the requirements of driving power during short-term acceleration and maximum climbing gradient; Generally, the industrial drive motor can meet the requirements with twice the overload.
(2) The maximum speed of the driving motor of new energy vehicles is required to reach 4-5 times of the base speed (the base speed of the inherent mechanical characteristics of separately excited DC motor refers to the no-load speed under the rated armature voltage and rated excitation current; while the base speed of permanent magnet motor and asynchronous motor refers to the synchronous speed, 2-pole 3000r / min, 4-pole 1500r / min, 6-pole 1000r / min, 8-pole 750r / min, etc., which is related to the power frequency); Industrial drive motors are only required to reach twice the base speed at constant power.
(3) The driving motor of new energy vehicles requires high specific power and excellent efficiency (high efficiency in a wide range of speed and torque), so as to reduce the dead weight of the vehicle and prolong the driving range; Industrial drive motors are usually considered comprehensively compared with power, efficiency and cost.
(4) When there are multiple motors working together, the driving motors of new energy vehicles are required to have high controllability, high steady-state accuracy and good dynamic performance; The industrial drive motor only meets a specific performance requirement.
(5) The driving motors of new energy vehicles are often installed on motor vehicles. Limited by the volumetric efficiency of vehicles, they work under working conditions such as high temperature, bad weather and background vibration, while industrial driving motors are usually fixed.
- Technical characteristics of driving motor of new energy vehicle
In addition to the above special requirements, from a technical point of view, the following points should be paid attention to.
1) Single motor or multi motor structure
A single motor drives the wheels through the transmission and differential. The multi motor structure is that each driving wheel is driven separately. The advantages of single motor structure are small volume, small mass and low cost. The multi motor structure can reduce the current and power rating of a single motor, make full use of the space inside the wheel, and balance the size and quality of the motor. Because these two structures have their own advantages, they are used in modern new energy vehicles, but now the application of single motor structure is the mainstream.
2) Fixed ratio or variable ratio gear reduction
It is also generally divided into single speed transmission and multi speed transmission. The former adopts fixed speed ratio gear transmission, while the latter adopts multistage gear transmission with clutch and transmission. For fixed speed ratio variable speed transmission, the designed motor is required to provide both high instantaneous torque (3 ~ 5 times of rated value) in the constant torque area and high operating speed (3 ~ 5 times of base speed) in the constant power area. Variable ratio gear transmission has the following advantages: the application of conventional drive motor system can obtain higher starting torque in low gear and higher driving speed in high gear, but its disadvantages are large quality and volume, high cost, low reliability and complex structure. At present, China’s new energy vehicle industry still adopts multi speed transmission or even stepless transmission to make up for the lack of motor performance.
3) System voltage
The selected voltage level of the new energy vehicle system will greatly affect the design of the drive motor system. Using reasonable high voltage motor can reduce the cost and volume of inverter. If the required voltage is too high, many batteries need to be connected in series, which will reduce the space in the car and luggage compartment, increase the quality and cost of the vehicle, and reduce the performance of the vehicle.
Because different models adopt different system voltage levels, the design of electric vehicle drive motor needs to be suitable for different electric vehicles. In general, the system voltage is limited by the quality of the battery, which accounts for about 30% of the whole vehicle. In fact, the higher the power of the motor, the higher the voltage level.
The 102kw motor used by General Motors EV1 adopts 312v voltage, while Reva ev13kw motor adopts 48V voltage and Prius hybrid adopts 500V voltage. China’s electric vehicles are divided into high-speed electric vehicles and low-speed electric vehicles. Low voltage is usually selected for low-speed electric vehicles, and the voltage of high-speed electric vehicles is higher.
4) System matching
The matching between motor and converter (also known as frequency converter), controller, transmission, energy, etc. is very important. The designer of new energy vehicle drive motor should fully understand the characteristics of these components, and then design the motor under given conditions. The design of industrial drive motor should be different.
1.2 key points of driving motor design for new energy vehicles
The development of motor has experienced more than a century, with a long development time and slow development speed. In recent years, due to the development of new material technology, topology, computer-aided design (CAD), high-power electronics and microelectronics technology, the motor for new energy vehicles has developed rapidly.
In order to meet the rapid design of different motor structures, motors are generally designed with the help of CAD software. There are two main methods: circuit method and electromagnetic field method. The circuit method is based on equivalent circuit analysis, and the electromagnetic field method depends on electromagnetic field analysis. The advantage of electromagnetic field method is that the result is more accurate, it can better deal with complex mechanical shapes and nonlinear materials, and better determine the critical region. Now finite element method (FEM) is considered as one of the powerful tools for electromagnetic field analysis of electric vehicles. The finite element method is superior to other methods because of its flexibility and applicability in stress and thermal field. Moreover, with the help of computer graphics, the visibility and interactivity of the analysis results are very good.
The basic factors to be considered in the design of the motor include: magnetic load – the peak value of the basic component of the magnetic flux density passing through the air gap of the motor, electrical load – the root mean square of the total current per unit perimeter or the number of ampere turns per unit perimeter of the motor, power and torque per unit volume and per unit mass, magnetic flux density per unit magnetic circuit, speed, torque, power loss and efficiency, as well as thermal circuit design and cooling. The corresponding key points are: better utilization of steel, magnet and copper, better electromagnetic coupling, motor geometry and layout, better thermal design and cooling, understanding the limitations of motor performance, and understanding the relationship between motor geometry, size, parameters and performance. Only in this way can the motor with higher unit mass power, unit mass torque and better performance be designed. For example, compared with the previous generation of Prius 2004 products, the power increases from 33kw to 50KW with little change in volume and mass.
1.3 new energy vehicle drive motor
Because the magnetic field of the excitation winding and the magnetic field of the armature winding are vertically distributed, the DC motor with commutator has very simple control principle, large torque and good speed regulation performance. It has always played a prominent role in the field of electric drive. By replacing the excitation winding of DC motor with permanent magnet material, the radial space is effectively used, so that the stator diameter of the motor can be greatly reduced. Due to the low permeability of permanent magnet materials, the armature reaction decreases and the mutual inductance increases. Series excitation, parallel excitation, separate excitation and permanent magnet DC motors are currently used in electric vehicles. However, the main problem of DC motor is that due to the commutator and brush, its reliability is reduced and needs regular maintenance.
The rapid development of science and technology has brought a leap in power semiconductor technology. The successful development of switching transistor has brought vitality to the creation of a new type of DC motor – brushless DC motor. In 1955, American Harrison first put forward the idea of replacing motor brush contact with transistor commutation line, which is the dimension of Brushless DC motor. It is composed of power amplification part, signal detection part, magnetic pole body and transistor switching circuit. Its working principle is that when the rotor rotates, periodic signal electromotive force is induced in the signal winding, which makes the transistors turn on in turn to realize commutation. Firstly, when the rotor does not rotate, the induced electromotive force cannot be generated in the signal winding, the transistor is not biased, and the power winding cannot be fed, so this brushless DC motor has no starting torque; Secondly, because the leading edge steepness of the signal electromotive force is small, the power consumption of the transistor is large. In order to overcome these disadvantages, people use the commutator of centrifugal device or place auxiliary magnetic steel on the stator to ensure the reliable start of the motor. However, the former has a complex structure, while the latter requires additional starting pulses. Then, after repeated tests and continuous practice, people finally found a mechanical commutation device that uses position sensor and electronic commutation circuit to replace brush DC motor, which opened up a new way for the development of DC motor. In the early 1960s, proximity switch position sensors, electromagnetic resonance position sensors and high-frequency coupling position sensors came out one after another, followed by magnetoelectric coupling and photoelectric position sensors. With the rapid development of semiconductor technology, people are interested in the Hall effect discovered by American hall in 1879. After years of efforts, a brushless DC machine with the help of Hall element (Hall effect rotor position sensor) to realize commutation was successfully trial produced in 1962. In the early 1970s, a brushless DC motor was successfully developed to realize phase commutation with the help of a magnetic sensitive diode with a sensitivity of about 1000 times higher than that of Hall element. While developing various types of position sensors, people try to find a brushless DC motor without additional position sensor structure. Using permanent magnet material to replace the excitation winding of traditional synchronous motor, permanent magnet synchronous motor can remove the copper loss of traditional brush, slip ring and excitation winding. Permanent magnet synchronous motor is also called permanent magnet brushless AC motor or sinusoidal permanent magnet brushless motor because it adopts sinusoidal AC and brushless structure. Since these motors are essentially synchronous motors, they can be operated by sinusoidal alternating current or pulse width modulation without electronic conversion. When the permanent magnet is embedded on the surface of the rotor, because the permeability of the permanent magnet material is similar to that of air, the operating characteristics of this motor are the same as those of non salient pole synchronous motor. If the permanent magnet is embedded in the magnetic circuit of the rotor, the salient pole will produce additional magnetic pole torque, so that the constant power area of the motor has a wider speed range. If the salient pole of the rotor is intentionally used and the excitation winding or permanent magnet is removed, the synchronous reluctance motor can be obtained. Its structure is simple, the cost is low, but the output power is relatively low. Like induction motor, permanent magnet synchronous motor usually adopts appropriate control methods to meet the high-performance requirements of electric vehicle motor drive.
It should be noted that the term “DC” in the name may be misleading because it does not refer to DC motors. In fact, this kind of motor uses AC square wave power supply, so it is also called permanent magnet brushless square wave motor. The most obvious advantage of this kind of motor is to remove the brush, which also eliminates many problems caused by the brush; Another advantage is that it can produce large torque because its square wave current is perpendicular to the magnetic field. Moreover, this brushless structure makes the armature winding a more representative area. Due to the improvement of heat conduction through the whole structure, the increase of electrical load can produce higher power density. Different from permanent magnet synchronous motor, this kind of permanent magnet brushless DC motor is usually equipped with shaft position sensor. Recently, a decoupling permanent magnet brushless DC motor for electric vehicles has been developed. It has high power density, uninterrupted torque and good dynamic performance. It also adopts advanced induction angle control method to effectively increase its constant power speed range.
Switched reluctance motor has great potential in electric vehicles. Switched reluctance motor is a new type of speed regulation drive system developed with the rapid development of power electronics, microprocessor and control technology in the early 1980s. It has the outstanding characteristics of simple structure, reliable operation and high efficiency. It has become a strong competitor of AC motor speed regulation system, DC motor speed regulation system and brushless DC motor speed regulation system, which has attracted extensive attention of scholars and business circles all over the world. Emerson Electric Company, a multinational motor company, also regards switched reluctance motor as a new technology and economic growth point of its speed regulation drive system in the 21st century. At present, switched reluctance motor has been widely used in industry, aviation, household appliances and other fields.
In 1970, the stepping motor research group of Leeds University in the UK initiated the prototype of switched re reluctance motor (SRM), which is the earliest research on switched reluctance motor. In 1972, the low-power motor with semiconductor switch (10W ~ 1kW) was further studied. Substantial progress was made in 1975, and it has been developed to provide devices for 50KW battery cars. In 1980, switched reluctance motor drive Co., Ltd. (srdltd.) was established in the UK, Specialized in the research, development and design of SRD system. 1983 UK (srdld.) Firstly, SRD series products are launched, which is named outlon.
TASC drive systems also launched their products in 1984. In addition, srdltd A driving system suitable for tram has been developed, which has been running for 500 km by 1986. The emergence of this product has caused great repercussions in the field of electrical transmission. It has reached an unexpectedly high level in many performance indexes, and the comprehensive performance and price indexes of the whole system have reached or exceeded some variable speed transmission systems widely used in industry for a long time.
Recently, a new research direction is to develop permanent magnet hybrid motors for electric vehicles. In principle, there are many permanent magnet hybrid motors, and three of them have been studied, which are the mixing of permanent magnet and reluctance, the mixing of permanent magnet and hysteresis, and the mixing of permanent magnet and excitation winding. First, the permanent magnet is embedded in the magnetic circuit of the rotor, and the permanent magnet synchronous motor generates hydro magnetic torque and synchronous reluctance torque at the same time. In addition, if the water magnet is combined with the switched reluctance structure, another hybrid motor of permanent magnet and reluctance will be produced, which is the so-called doubly salient permanent magnet motor (DSPM). The development of doubly salient permanent magnet motor shows that it has the advantages of high efficiency, high power density and wide speed range. It is a new type of hybrid permanent magnet motor, which has the advantages of stable starting torque, high hysteresis and so on. Third, the permanent magnet is placed in the rotor and the DC excitation winding is placed on the inner stator. By controlling the size and direction of the excitation current, it is easy to adjust the air gap flux of the motor. In this way, it is easy to obtain the torque / speed characteristics that meet the driving requirements of electric vehicles.