[Abstract] While in-wheel motors offer numerous advantages in electric vehicles, their structural size imposes particularly high requirements on power per unit volume. Single-phase switched reluctance motors possess this characteristic, but lack self-starting capability. Through analysis and comparison of the starting mechanisms of various motors, this paper proposes an innovative combined motor that utilizes DC motor principles for starting, operates according to variable reluctance principles, and possesses enhanced electromagnetic braking capabilities. This fully leverages the motor's advantages of simple structure, robust reliability, low cost, and high efficiency.
[Keywords] Hub motor, electromagnetic braking, power per unit volume, DC starting winding
I. Fully leverage the technological advantages of in-wheel motors in electric vehicles
With the deepening of energy conservation and emission reduction efforts, replacing traditional vehicles with electric vehicles will inevitably become a trend. Electric vehicles, including fuel cell vehicles (FCVs), hybrid electric vehicles (HEVs), and pure electric vehicles (EVs), all use electric motors as actuators to drive the wheels. Fully leveraging the advantages of electric motors, such as their rapid response, wide speed range, and considerable short-term overload capacity, to improve the cost-effectiveness of electric vehicles and facilitate their rapid commercialization is particularly important. Using in-wheel motors to directly drive the wheels best utilizes their technological advantages. Based on this summary, the application of in-wheel motors in electric vehicles has the following eight characteristics:
1) Simplifying the mechanical transmission mechanism and reducing the vehicle's weight: Using hub motors to directly drive the wheels greatly shortens the mechanical transmission chain, achieving "zero transmission." This will fundamentally change the structure of the car. For pure electric vehicles, it not only eliminates the corresponding auxiliary devices such as the engine, cooling water system, exhaust muffler system, and fuel tank, but also eliminates mechanical transmission devices such as the gearbox, universal joint transmission components, and drive axle. This not only saves a lot of mechanical component costs but also reduces the vehicle's weight, which is conducive to improving the overall driving efficiency of the vehicle and is beneficial to energy saving and noise reduction.
2) It is beneficial to the structural layout of automobiles. Since a large number of mechanical devices are eliminated, a lot of effective space can be freed up to facilitate the overall layout of automobiles. The added batteries can be appropriately distributed as counterweights and the structural layout can be carried out in accordance with the requirements of vehicle dynamics, such as minimizing the height of the vehicle's center of gravity [1].
3) Improve the off-road performance of automobiles. The 4WD front and rear four-wheel drive mode of high-end cars can be easily realized by using hub motor drive. According to vehicle dynamics[1] analysis, only four-wheel drive can make full use of the adhesion of the wheels to the ground, thereby greatly improving the off-road performance, anti-skid braking, and fast steering performance of the vehicle.
4) Improved rapid response of wheel control: According to control theory, the dynamic response of each component of the entire closed-loop system is a crucial factor limiting its performance. Typically, the electrical response speed is 1-2 orders of magnitude faster than that of mechanical mechanisms with friction damping. For example, in a drive speed control system, the response time of a traditional car, from throttle control through the engine's combustion process to the various mechanical transmission mechanisms, is hundreds of times longer than that of a car using hub motors to directly drive the wheels. This allows for the easy implementation of various high-performance control functions that are difficult to implement in traditional high-end cars, significantly improving vehicle safety, handling, and stability.
5) Improve the kinetic energy recovery rate of the wheel. As we all know, only the drive wheel can realize the recovery of braking energy. The use of hub motors to drive directly eliminates mechanical transmission losses and makes the recovery of wheel kinetic energy more direct. Therefore, four multi-functional hub motors [2] that combine electric power, power generation feedback and electromagnetic braking are used for driving. During the car's coasting, deceleration braking and downhill process, the power energy recovered will be at least more than double that of the general method of existing electric vehicles.
6) The requirement is to implement an electromechanical integrated control mode, using two or four hub motors to achieve dual front-wheel drive, dual rear-wheel drive, or 4WD (front and rear four-wheel drive) modes. Distributed drive allows for a smaller motor pulling a larger load. The drive control modules for each motor can be integrated within the wheels. To reduce hardware wiring between control components and improve reliability, modern automotive control systems increasingly employ microcomputer multi-CPU bus control, especially for 4WD modes using hub motors. This bus control technology simplifies the internal wiring layout of the electric vehicle, improves reliability, and facilitates fault diagnosis and maintenance. While implementing this modular structure for electromechanical integrated control will increase R&D difficulty and initial investment, the cost will decrease significantly with increased production volume once the technology matures.
7) The use of hub motors to achieve electronic differential steering eliminates the mechanical differential and the left and right half-shafts that run through the axle, thereby lowering the overall vehicle height. The design allows for a low floor, making it easier for passengers to get in and out of the vehicle. More importantly, it lowers the vehicle's center of gravity, thus improving driving stability.
8) Significantly Improves Vehicle Steering Performance: Four-wheel hub motors, driven by electronic differential control, easily enable four-wheel steering, comprehensively improving vehicle steering performance. This reduces the low-speed turning radius and increases high-speed steering stability. Furthermore, since the wheels are driven by in-hub motors, the need for external power transmission mechanisms such as drive axles and differentials is eliminated. This allows for significant improvement and simplification of the vehicle's steering system, enabling wheels to rotate ±180° to achieve lateral or arbitrary rotational driving.
Given the numerous advantages of hub motors in electric vehicles, hub motors are limited by their structural volume, and therefore require a relatively large driving power for general electric vehicles, which places special high demands on the power per unit volume of the motor. Single-phase switched reluctance motors [3][4] have the characteristics of high power per unit volume, i.e., high magnetic circuit utilization, and also have the advantages of simple structure, robust reliability, low manufacturing cost, the lowest cost of drive controller, and high efficiency. However, the biggest drawback of single-phase switched reluctance motors is that they do not have a self-starting function. Although there are structures that use permanent magnet materials or embed aluminum and copper blocks between rotor poles to use eddy current reaction torque for auxiliary starting, these methods reduce the power per unit volume of the motor, and the effect is not ideal. In addition, the motor can only run in one direction. As a result, this cheap and efficient motor is difficult to put into practical use.
Based on the analysis and comparison of the starting mechanisms of various motors, it is proposed to add a DC winding specifically for starting on the motor rotor, that is, to start using the principle of DC motor and operate according to the principle of variable reluctance. Based on the analysis of the improvement ideas of the multi-functional hub motor that combines electric motor, power generation feedback and electromagnetic braking[2], in order to better perform its three functions, the number of motor phases should be as small as possible. Therefore, the single-phase switched reluctance motor will have a better multi-functional effect. The switched reluctance (SR) motor is also called the SR motor. Its structure and principle have been described in detail in many monographs[3][4]. They can be consulted when needed.
II. Structure of a Single-Phase SR Multifunctional Motor with Starting Winding
Figure 1 shows the structural principle diagram of a single-phase reluctance multi-functional hub motor with a starting winding. In a single-phase structure, both the stator and rotor have the same number of poles, as shown in the figure (6 poles). Increasing the number of poles helps reduce torque fluctuations at low speeds, but the motor speed also decreases accordingly at the same frequency. The salient pole tooth pitch and slot pitch of the stator and rotor can also be equal. Increasing the salient pole tooth pitch, even with a smaller slot pitch, helps increase the electromagnetic braking torque, but it affects the driving rotation efficiency. The inner stator winding is a ring coil wound in the outer circular slots of the stator core. When the winding is energized, it forms a mixed axial and radial magnetic flux, which improves the efficiency of a motor of the same volume. The outer rotor is divided into upper and lower parts to form a closed loop magnetic circuit. Both the upper and lower outer rotors have dedicated DC armature windings for starting. According to the working principle of a DC motor, the current direction of the starting winding should be tangential to the magnetic field lines formed by the inner stator excitation winding. As shown in the diagram, the starting windings are radially distributed on the salient poles and are led out through inner and outer loops. The polarities of the leads from the upper and lower outer rotors are exactly opposite; that is, the inner and outer loops of the upper outer rotor are connected to the outer and inner loops of the lower outer rotor, respectively. Since the windings are on the rotor, the power input needs to pass through brushes. However, since they are only used during startup, the inherent drawbacks of brushes have a minimal impact.
Figure 1. Structure of a single-phase SR multi-functional hub motor with starting winding.
III. Various operating processes of a single-phase SR multi-functional motor with a starting winding
The working process of the motor in each operating state—start-up, drive rotation, power generation feedback, and electromagnetic braking—is described in detail below:
1. Motor starting process
Before starting, the motor is usually in a balanced position, meaning the stator and rotor salient pole teeth are aligned, which can be determined by the rotor position angle detection signal. At this time, a DC excitation current is applied to the stator winding, and the motor generates a closed magnetic field. According to the direction of the magnetic field and the required direction of rotation, a current in the corresponding direction is applied to the rotor winding. The motor starts according to the left-hand rule, based on the principle of a DC motor where a current-carrying conductor generates electromagnetic force in a magnetic field. When the motor has rotated through a small balance angle, that is, when the rotor salient pole approaches the stator salient pole, the rotor current is immediately cut off, causing the stator winding to operate according to the "principle of minimum reluctance" of a variable reluctance motor. If starting under load is difficult due to high load torque, a method of multiple energizations of the rotor windings can be used. This involves using the rotor position angle detection signal to apply current to the rotor windings when the stator and rotor salient poles are about to align, causing the motor to operate like a DC motor. When the stator and rotor salient poles are close to align again, the rotor is de-energized, and the stator windings rotate according to the principle of a variable reluctance motor. This cycle is repeated until the motor is fully started. Because the current is always unidirectional throughout the rotor's operation, the brushes do not suffer from the commutation sparking issues of DC motors, resulting in a considerably long brush life.
2. Driving rotation working process
After starting, the motor operates according to the principle of variable reluctance. Whenever the rotor salient pole approaches the stator salient pole, the stator winding is energized. The resulting magnetic field aims to reduce magnetic reluctance, meaning the magnetic lines of force strive to follow the path of least reluctance. The rotor experiences reluctance torque, causing it to rotate through the corresponding angle. Immediately afterward, the winding is de-energized to prevent the rotor and stator salient poles from aligning and generating braking torque. The rotor rotates by inertia until its salient pole approaches the next stator salient pole, at which point energization is restored. This is achieved by controlling the winding's energization in pulses via angle detection, allowing the motor to rotate continuously. For larger loads, a combination of DC and variable reluctance motors can be used. Based on the rotor angle detection signal, the rotor is de-energized when the stator and rotor salient poles approach each other, allowing the stator winding to rotate according to the variable reluctance motor principle. When the stator and rotor salient poles are about to align, current is supplied to the rotor winding, causing the motor to operate like a DC motor. Similarly, to meet the speed regulation requirements of electric vehicles, a current chopper control method is required at low speeds to achieve constant torque speed regulation characteristics; while an angle position control method is used at high speeds to achieve constant power speed regulation characteristics.
3. Power generation feedback process
When an electric vehicle needs to slow down for braking or is going downhill, it can utilize its kinetic inertia to generate electricity regenerate. Based on the rotor angle detection signal, when the outer rotor groove is about to approach the stator salient pole, the stator winding is immediately switched on, generating a braking torque on the rotor and converting its kinetic energy into magnetic energy stored in the magnetic field. When the rotor salient pole is about to approach the stator salient pole, the stator winding circuit is switched off. At this point, the magnetic energy stored in the magnetic field is converted into electrical energy and fed back to the battery through a freewheeling diode. This process is repeated to charge the battery in pulses.
4. Electromagnetic braking working process
When the motor needs to stop after the aforementioned regenerative braking, the stator windings are continuously energized. The resulting magnetic field causes the stator and rotor salient poles to be attracted to each other by electromagnetic force, and the braking force is uniformly symmetrical around the circumference according to the distribution of the salient poles. Because a single-phase motor is used, when the salient pole pitch and groove pitch are equal, the total arc distance between the stator and rotor salient poles due to electromagnetic attraction reaches 180 degrees. If the ratio of the salient pole pitch to the groove distance is appropriately increased, such as to 6:5, the total arc distance between the stator and rotor salient poles due to electromagnetic attraction during electromagnetic braking reaches 196.36 (360 × 6/11) degrees. If the motor's kinetic inertia is large, it is necessary to repeatedly apply the method combined with regenerative braking based on the angular position detection signal until the angular position detection shows no change, at which point the braking process stops. The electromagnetic braking-regenerative braking process, which involves repeated braking, is similar to the braking process of modern cars' anti-lock braking system (ABS) or anti-slip control (ASR), and can improve vehicle driving stability and steering maneuverability.
Compared with the two-phase 8/12 and three-phase 12/8 variable reluctance motors introduced in the electric vehicle hub motor [2] which combines electric motor, power generation feedback and electromagnetic braking, the single-phase reluctance multi-functional motor with starting winding has the characteristics of high power per unit volume, robust and reliable, simpler structure, and lower cost of motor manufacturing and its drive controller. When realizing electromagnetic braking, its braking effect is also much higher than that of the two-phase 8/12 and three-phase 12/8 motors. In the drive operation, the combination of DC electric motor and variable reluctance electric motor can be used to improve the efficiency per unit volume of the motor. This innovative combined motor that starts with the principle of DC motor and runs according to the principle of variable reluctance will greatly improve the cost performance of electric vehicles. Good electromagnetic braking can greatly reduce the frequency of use of the original mechanical brake. For this reason, it is expected that the original mechanical brake can be modified or moved out, and used only for emergency braking and parking braking of automobiles. This allows more space in the wheel hub for the motor layout.
References:
[1] Yu Zhisheng (ed.). Automobile Theory [M]. Beijing: China Machine Press, 4th edition, May 2006.
[2] Wang Guiming, Wang Jinyi. Adjustable speed rotary motor with electric motor, regenerative braking, and electromagnetic braking functions [P]. Chinese Invention Patent. ZL200810062784.5
[3] Sun Jianzhong and Bai Fengxian, eds. Special Motors and Their Control [M]. Beijing: China Water Resources and Hydropower Press, 2005.
[4] Wang Guiming and Wang Jinyi, eds. Electric Vehicles and Their Performance Optimization [M]. Beijing: China Machine Press, 2010.
First author Wang Guiming: Born in October 1950; Gender: Male; Title: Senior Engineer;
ID number: 330103195010081611; Education: Postgraduate;
Resume: Wang Guiming is a senior engineer at the Vehicle Engineering Research Institute of the School of Mechanical and Electrical Engineering, Zhejiang University of Technology. Since 1984, he has received numerous national, Ministry of Machinery Industry, provincial, and municipal science and technology achievement awards. He has authored two technical monographs and published over 30 papers. In the early 1980s, he worked at the Hangzhou Machine Tool Factory, engaged in CNC retrofitting and new product development. In 1995, he went to Japan to study CNC. In 1996, he returned to the university to continue his research and teaching in mechatronics, with significant expertise in CNC servo systems. For the past eight years, he has conducted in-depth research on electric vehicles, intelligent transportation, and related technologies, applying CNC servo technology to electric vehicle motor drives. Integrating multiple theories, he has proposed several invention patents to improve the performance and cost-effectiveness of electric vehicles, dedicating his remaining years to energy conservation, environmental protection, and comprehensive improvement of transportation.
Contact information:
Wang Guiming's phone numbers: 0571-88320456-803 (office, Monday to Friday), 0571-86099489 (home, holidays), 13655815440 (mobile).
Email: [email protected] or [email protected]
Mailing Address: Room 801, Building 6, Binwenyuan, Binjiang Higher Education Park, Hangzhou, Zhejiang Province (Residential Address); Postcode: 310053
School of Mechanical and Electrical Engineering, Zhejiang University of Technology, Chaohui Sixth District, Hangzhou (Retirement at the end of the year); Postcode: 310032;
Wang Jinyi's phone numbers: 0571-28958888-6750 (office), 13867478826 (mobile).
