Research Of Dual-Stator Permanent Magnet Brushless Motors Used In Electric Vehicles

With the intensification of the environmental pollution and oil crisis, widespread concern is focused on the research and development of electric vehicles. Unfortunately, the breakthrough of the vehicle-mounted batteries is still not achieved. So the electrical drive system is still the the key of the research. Evidently, the electrical drive system depends on different driving motor types. According to the demand analyses for electric vehicles’ drive motor, the permanent magnet brushless motors are predominant in the appliances. However, the indigenous permanent magnet structure of the motors impedes the flux weakening speed regulation; moreover, the low-speed-high-torque and the high-speed-low-torque operations are usually unattainable under a low capacity inverter. Accordingly, in pace with the continuously upgrades of the electric vehicles’ performances and other demands, the permanent magnet motors with new structures and new concepts are desired. In recent years, a dual-stator permanent magnet brushless motor is proposed, which significantly promotes the motor performances due to the dual-stator structure. At present, the novel dual-stator motors are sparsely carried out in applications or researches. Therefore, the aim of this dissertation is to carry out an in-depth study into the dual-stator permanent magnet brushless motor’s applications in electric vehicles.The main contents of this study are divided as follows:(1) Analyses on the dual-stator motor’s structural principle and on its predominant performances in electric vehicle applications.The dual-stator motor comprises an inner stator, an intermediate rotor, and an outer stator. The total structure can be regard as a combination of an inner-rotor motor and an outer-rotor motor, and is more complicated than a conventional permanent magnet brushless motor. Except inheriting the advantages of the conventional permanent magnet brushless motors, the dual-stator motor attains a much higher power density under the same volume. Furthermore, when the dual-stator motor operates in the generator mode, it could promote the generating capacity; when the motor operates in the motor mode, it could multiply the torque outputs. Analyzing through the characteristics of the typical rotor structures, the surface-mounted series magnetic circuit structure is identified to be of the least material usage and the smallest volume and weight. Because, little magnetic flux passes through the yoke of the rotor core in the circumferential direction, giving the allowance for a very thin rotor yoke that just depends on the mechanical strength.The windings in the inner and outer stators are independent, and the winding turns of the outer stator usually outnumbers that of the inner stator. The windings of both stators can be connected either in series or independently. If both windings are connected in series, the number of turns per phase is increased, bringing about an increased phase EMF and a reduced base speed. And this kind of connection also produces a high electromagnetic torque, which is beneficial for the low-speed-high-torque operation. If the windings are used independently, the number of turns per phase is decreased, bringing about a decreased phase EMF and electromagnetic torque but a high base speed. This kind of connection realizes a high speed operation without the flux weakening speed regulation. Besides, if one of the independent windings breaks down, the other winding can still maintain the motor fault-tolerance running independently. Therefore, according to the multiple winding connections, the dual-stator motor can work under various operating conditions and provide more flexibility for the electric vehicles drive.(2) According to the structural features of the dual-stator permanent magnet brushless motors, a design method of the dual-stator permanent magnet brushless motor is proposed.The dual-stator permanent magnet brushless motor with surface-mounted series magnetic circuit has a complex structure, which not only has the mutual restrained design parameters between the inner and outer stators, but also has the electromagnetic coupling of the inner and outer stators. It means that this kind of motors is difficult to be designed by available design methods, which is restricting their popularity. Accordingly, based on the structural features of the dual-stator permanent magnet brushless motor, a design method is proposed, regarding the dual-stator permanent magnet brushless motor as individual inner and outer single-stator motors, and then, compounding the two individual motor schemes into the final design scheme of the dual-stator permanent magnet brushless motor. As to the parameters matching between the inner and outer motor, the optimization algorithms such as the response surface method (RSM) and genetic algorithm (GA) are applied to achieve a faster design procedure and a more rational result. This method can be used both in the design of dual-stator permanent magnet brushless generator and dual-stator permanent magnet brushless motor. Taking a dual-stator permanent magnet synchronous generator’s design as an example, this dissertation amply introduces the design method and procedure, and then the design scheme is verified by the finite element analysis (FEA) and prototype machine experiments.(3) According to the analyses of the dual-stator permanent magnet brushless motor’s performances, the principles for selecting the connection modes of the inner and outer windings are proposed, when the motor is used in electric vehicle.Basing on the dual-stator permanent magnet brushless DC motor designed for electric vehicles, this dissertation performed the FEA on the motor with different winding connection modes. Then, the torque-speed curve is charted based on the three rated operating points. Furthermore, the operating region is subdivided, and under each operating subdivision, the performances of the motor with different winding connection modes are analyzed by the FEA. According to result, it can obviously determine which winding connection mode has the optimal performances under various operating states. Then, the principles for selecting the connection modes are obtained to adjust the motor working in a wide operation region with high efficiency, which is satisfying the demands of the electric vehicles’ drive system. At last, the experiments are carried on the prototype machine.(4) The winding switching method of the dual-stator permanent magnet brushless motor is proposed, and the winding switching dynamic procedure is analyzed.In order to maintain a high efficiency under different operating states of the electric vehicles, the winding connection modes of the motor should be rationally switched. According to the analyses, the common method is to adjusting the all currents of the motor reducing to zero or nearly zero before the winding switching, which causes significant interrupt to the current and the electromagnetic torque. In order to avoid the interrupt state, this dissertation proposes a winding switching method that every phase winding should be successively switched when every phase current is crossing zero. The whole control system of the dual-stator permanent magnet brushless motor with the winding switching circuit is designed, and in the winding switching circuit, the triode AC switches (TRIAC) are utilized as switches to reduce the complexity of the circuits. Individually, taking a square-wave driven dual-stator permanent magnet brushless DC motor and a sine-wave driven dual-stator permanent magnet synchronous motor as examples, the winding switching dynamic procedures are analyzed. And the results of the field-circuit coupled time-step FEA show that the winding switching method and circuit are feasible.