1. Hub motor technologyHub motor technology, also known as wheel built-in motor technology, is a hub device technology that integrates the motor, transmission system and braking system. The layout of the drive system using this technology is very flexible, allowing electric vehicles to be driven in various combinations according to 2 front-wheel drive, 2 rear-wheel drive or 4-wheel drive. Compared with vehicles driven by traditional internal combustion engines and central motors, due to the cancellation of clutches, transmissions, drive shafts and differentials, the chassis structure is greatly simplified, the weight of the vehicle is reduced, and the vehicle's lightweight goal is well achieved. It also provides a guarantee for realizing the electronicization and intelligence of the chassis.In addition, the electric wheel drive system can complete the control of the driving force of each wheel only through the electric motor and the control system. Compared with the internal combustion engine, whether it is accelerating or decelerating, the electric motor torque response is fast and easy to measure. At the same time, due to the reduction of intermediate links in power transmission, the noise of the electric wheel drive motor is extremely low. Judging from the current development trend and the characteristics of various driving technologies, in-wheel motor will be the final driving form of electric vehicles, and it is also one of the hot and difficult points of electric vehicle research at this stage.【Advantage】(1) The power control is changed from a hard connection to a soft connection type. Through the electronic wire control technology, the stepless speed change from zero to the maximum speed of each electric wheel and the differential speed requirements between each electric wheel are realized, which omits the requirements of traditional vehicles. Mechanical manipulation of shifting devices, clutches, transmissions, transmission shafts and mechanical differentials, etc., makes the drive system and the whole vehicle simple in structure, large in effective use of space, and high in transmission efficiency.(2) The driving force of each electric wheel is directly and independently controllable, making the dynamic control more flexible and convenient, and can reasonably control the driving force of each electric wheel, thereby improving the driving performance under severe road conditions.(3) It is easy to realize electric braking, electromechanical compound braking and braking energy feedback of each electric wheel, saving energy.(4) The chassis structure is greatly simplified.(5) In the 4-wheel electric vehicle using the in-wheel motor drive system, if the wire-control four-wheel steering technology (4WS) is further introduced to achieve high-performance vehicle steering and effectively reduce the steering radius, or even achieve zero steering radius, Greatly increased steering flexibility. Judging from the current development trend and the characteristics of various driving technologies, the in-wheel motor will be the final driving form of electric vehicles, and it is also one of the hot spots and difficulties in electric vehicle research at this stage.The drive mode of the hub motor can be divided into two categories: reduction drive and direct drive. Reduced driveIn this driving mode, the motor generally runs at high speed, and there are no special requirements for other performances of the motor, so an ordinary inner rotor motor can be used. The deceleration mechanism is placed between the motor and the wheel to reduce the speed and increase the torque.The advantage of deceleration drive is that the motor runs at high speed, has high specific power and efficiency, is small in size and light in weight, and after boosted by gears, it has large torque and good climbing performance. In addition, it can ensure that the car obtains a large stable torque when running at low speed. The disadvantages are that it is difficult to achieve liquid lubrication, the gear wears faster, the service life becomes shorter, it is not easy to dissipate heat, and the noise is too high.direct driveIn this driving mode, the motor mostly uses an external rotor, that is, the rotor is directly installed on the rim. In order to make the car start smoothly, the motor is required to provide high torque at low speed. In addition, in order to make the car have better power, the motor needs to have a wider speed range.The advantage of direct drive is that there is no deceleration mechanism, which not only makes the structure of the entire driving wheel simpler and more compact, but also reduces the axial size, and further improves the efficiency and speed of response. The disadvantage is that a large current is required when starting, facing the wind or climbing a slope, etc., when carrying a large torque, and it is easy to damage the battery and permanent magnet; the peak area of the motor efficiency is very small, and the efficiency drops sharply when the load current exceeds a certain value. It is suitable for flat roads, light loads, Transportation and other occasions.2. Types and advantages and disadvantages of hub motor drive motorsTo make electric vehicles have better performance, the drive motor should have a wide speed range, high speed, large enough starting torque, small size, light weight, high efficiency, and strong dynamic braking and Features such as energy feedback. At present, there are four main types of motors used in electric vehicles: induction motors (IM), permanent magnet brushless motors (PMBLM), switched reluctance motors (SRM), and transverse field motors (TFPM). Various motors have their own advantages and disadvantages, the following are for comparison:1. Asynchronous motorSimple structure, sturdy and durable, low cost, reliable operation, small torque ripple, low noise, no need for position sensor, high speed limit; the disadvantage is that the drive circuit is complex, high cost, compared with permanent magnet motor, the efficiency and power of asynchronous motor The density is low.2. Permanent magnet brushless motorThe permanent magnet synchronous motor has the characteristics of high efficiency and torque density, stable torque and low vibration and noise, and has become an ideal choice for electric vehicle electronic control components. The permanent magnet hub motor drive can be flexibly arranged on the front and rear wheels of the electric vehicle, and even directly drive the hub to rotate. It has been applied in various electric vehicles at home and abroad.3. Switched reluctance motorSwitched reluctance motors have the advantages of simplicity and reliability, efficient operation over a wide speed and torque range, four-quadrant operation, fast response, and low cost. But there are many disadvantages: large fluctuations in torque, large vibration, and large noise; nonlinear system, difficult modeling, high control cost; low power density, etc.4. Transverse magnetic field motorCompared with other motors, the advantages of transverse magnetic field motors are very prominent: the decoupling of the circuit and the magnetic circuit is realized, and the design freedom is greatly improved; high torque density, and is especially suitable for applications requiring low speed and high torque; winding form Simple, there is no end of the traditional motor, the winding utilization rate is high; the phases are independent of each other; the efficiency is high; the control circuit is the same as the permanent magnet brushless motor, and the controllability is good. But it also has many disadvantages: the number of permanent magnets is large, the amount is large; the structure is relatively complex, the process requirements are high, the cost of the motor is high; the magnetic flux leakage is serious; the power factor is low; the self-positioning torque is relatively large.3. Development status of in-wheel motors at home and abroad1. JapanJapan's research on in-wheel motor technology started earlier and is in a leading position in the world. In the past 10 years, the electric vehicle research group led by Professor Shimizu Hao of Keio University in Japan has developed electric vehicles such as IZA, ECO, and KAZ, all of which use in-wheel motor drive technology. KAZ's electric wheel drive system uses a high-performance high-speed inner-rotor electric motor, which is matched with a linear gear reduction mechanism. Hub motor parameters: 6-phase permanent magnet synchronous motor, maximum torque 100N m, maximum torque 12000r/min, maximum power 55kW. The rear-wheel drive electric vehicle ECO adopts a permanent magnet brushless DC motor with a rated power of 6.8KW and a peak power of up to 20KW. Japan's Bridgestone Corporation and Toyota Corporation had launched the dynamic damping type wheel built-in motor system and the fuel cell concept car FINE-N respectively at the 2003 Tokyo Motor Show, which applied the wheel hub drive technology. The 4WD Bus developed in Japan in recent years also adopts the outer rotor structure, the stator winding is a concentrated winding, and the maximum power and torque are 75kW and 2046N·m respectively.2. Europe and AmericaFrench TM4 Company The structure of the integrated in-wheel motor designed and manufactured by French TM4 Company is shown in the figure. It adopts an outer rotor type permanent magnet motor, and the motor rotor shell is directly consolidated with the rim, and the motor shell is used as a component of the wheel rim, and the motor rotor is integrated with the brake drum of the drum brake to realize the motor rotor, rim And the integration of the three rotary moving objects of the brake greatly reduces the quality of the integrated hub motor system, and the degree of integration is quite high. The permanent magnet brushless DC motor of the integrated in-wheel motor systemThe rated power of the engine is 18.5kw, the peak power can reach 80kw, the peak torque is 670N·m, the rated speed is 950rpm, the maximum speed is 1385rpm, and the average efficiency under rated working conditions can reach 96.3%.French company TM4 has recently developed a new type of in-wheel motor, MΦTIVE, which has been selected by India's TATA Motor Company as its drive system for electric vehicles. The car will go on sale in 2011. The structure of the motor is shown in the figure below. It adopts a permanent magnet synchronous motor with an outer rotor structure, a concentrated winding, and a water-cooled structure. The peak torque is 170N m, the continuous torque is 600N m, the maximum torque is 10000r/min, the maximum power is 120kW, and the continuous power is 37kW. The weight of the motor is 26Kg, and the efficiency of the motor can reach 96% at the rated speed.Volvo cars use Protean hub motor four-wheel drive, and can accelerate to 0~60mph within 5.1s. The breakthrough technology of Protean's PD18 series in-wheel motor is that the motor and inverter are integrated together, and no additional power electronic devices or mechanical gears are needed. Motor parameters: peak torque 825N m, maximum speed 1400rpm, maximum power 83kW, continuous power 54kW . The weight of the motor is 31Kg, and the outer diameter of the motor is 420mm. Wheel size is 18 inches.Michelin's Active Wheels have two electric motors in the hub, one of which outputs torque to the wheels, and the other is used to control the active suspension system to improve comfort, handling and stability. Michelin Active Wheels can be used for electric vehicles and fuel cell electric vehicles. Compared with ordinary internal combustion engine vehicles, gearboxes, clutches, drive shafts, and universal joints are no longer required. There is also a braking device between the two electric motors, and the power, braking and suspension are all integrated together, and the structure is quite compact. Because the torque of the electric motor is easy to control, if it is equipped with four Michelin active wheels, it becomes a four-wheel drive system, and the torque of any wheel can be adjusted independently through the computer.3. DomesticCompared with foreign countries, my country's research in this field is relatively backward, but in recent years, with the deepening of the research on the major topics of the national "863" program of electric vehicles, and the deepening of the understanding of the advantages of the electric wheel drive system, the research and development of the new drive system by various universities Research has also intensified. Tongji University School of Automobiles first proposed the research and development of the vehicle power platform "Chunhui No. 1" driven by in-wheel motors, and on this basis, developed a prototype car "Chunhui No. 2" in 2003. The parameters of the hub machine: permanent magnet brushless with outer rotor DC motor, rated power 800W. The 2.2kW in-wheel motor and its dynamic system are also under development.4. Key technical issues in hub motor researchThe in-wheel motor system integrates multiple functions such as driving, braking, and bearing, and the internal space of the wheel is limited, so the technical requirements for the driving motor are mainly: small size, light weight, high power density and torque density; Within the range of speed regulation, both the motor and the drive controller have high efficiency; they have good control performance and overload capacity to improve the starting and acceleration performance of the vehicle.1. Improve the torque characteristics of the motor Increase output torque and reduce torque ripple. There are two reasons for torque ripple, one is the torque ripple generated by electromagnetic factors. This type of torque ripple results from the interaction of the stator current and the rotor magnetic field. The suppression methods include: motor optimization design method, optimal opening angle method, harmonic elimination method and torque closed-loop control method, etc. The second is torque ripple caused by cogging. This type of torque is the torque generated by the permanent magnet magnetic field and the cogging of the stator core in the circumferential direction, and it can also be called positioning torque or reluctance torque. The methods for suppressing cogging torque include: magnetic slot wedge method and closed slot method, auxiliary slot method, auxiliary tooth method and fractional slot method, inclined slot method and oblique pole method, etc.2. Increase specific powerThe built-in radial permanent magnet motor is applied, and the mechanical loss and inverter loss are reduced by increasing the reluctance torque and related optimization, so as to increase the output power and maintain high efficiency. Concentrated winding and stator segment technology can also be used to increase the power density of permanent magnet motors.3. Improve the speed expansion ability of magnetic field weakeningSince the rotor of permanent magnet synchronous motor is excited by permanent magnets, it is difficult to weaken the field at high speeds, and the speed regulation characteristics are not as good as those of DC motors and induction motors.
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