Electromagnetic Characteristic Analysis Of Permanent Magnet And Electromagnetic Hybrid Excitation Starter Generator For Electric Vehicle Range Extender

The hybrid excitation starter generator,which is jointly provided with magnetic field by permanent magnet and electric excitation winding,has high power density and wide magnetic adjustment range,and has become an important development direction of starter generator for the extended-range electric vehicles in the future.The paper proposed a combined-pole permanent magnet and electromagnetic hybrid excitation starter generator topology for the range extender of electric vehicle.The key theories and technologies such as magnetic field characteristics,magnetic regulation characteristics and parameter optimization,main air-gap magnetic density harmonics,armature reaction effect and performance test have been deeply studied.Those have important theoretical guiding significance and practical application value for the application of hybrid excitation starter generator in the field of range extender starter generator.The main works are as follows:The different topologies of permanent magnet rotor and electric excitation rotor were analyzed by using simplified equivalent magnetic circuit method,the magnetic flux equations and analytic formulas were given.Moreover,the results showed that the combined-pole permanent magnet rotor with smaller reluctance could effectively improve the rotor magnetic field strength and power density.Considering the influence of leakage flux and magnetic saturation,the equivalent magnetic circuit models of permanent magnetic field and electric magnetic field with high accuracy were established.The calculation methods of key magnetic circuit permeances and leakage permeances of permanent magnet rotor,electric excitation rotor,main air gap and stator were given.The main magnetic flux,the leakage flux and the leakage coefficients were calculated.The accuracy of the calculation model were verified by the finite element method.According to the calculation results of leakage flux and leakage flux analysis,it was proposed that increasing the magnetic resistance of leakage flux circuit was the effective way to reduce the leakage flux and leakage coefficient.Based on this,the magnetic isolation air gap at the ends of the permanent magnet and the claw-pole with variable cross-section trapezoid were optimized to reduce the leakage flux and improve the rotor magnetic flux density.The output voltage mathematical models of the hybrid excitation starter generator under different excitation currents were established,and the calculation method of the magnetic regulation coefficient was given.The electromagnetic characteristics of the starter generator with different magnetic regulation coefficients were analyzed,and the results showed that the magnetic adjustment coefficient at the same design power mainly affected the minimum armature flux linkage under weak magnetic condition.Moreover,the larger the magnetic adjustment coefficient was,the larger the flux linkage adjustment range was,and the better the adjustment performance of the whole machine was.Based on the analysis of output power,the influence laws of permanent magnet parameters and claw-pole structure parameters on the electromagnetic performance under no-load and load conditions were revealed.It was concluded that larger permanent magnet parameters,reasonable design of claw-pole yoke,flange plate and excitation bracket parameters could obtain larger output power and better magnetic field distribution.Taking the maximum rated-load output voltage and the minimum distortion rate of no-load induced electromotive force waveform as the optimization objectives,the particle swarm optimization algorithm was used to optimize the parameters of permanent magnet and claw-pole structure respectively to obtain the optimal rotor parameters.Based on the magnetic density calculation method of the main air gap,the Carter coefficients of the influence of stator slotting,permanent magnet rotor with unclosed outer circle and the air gap between the claws of the electric excitation rotor on the main air gap were derived.The analytical models of the main air-gap magnetic density of the permanent magnet part and the electric excitation part were established,the influence laws of the parameters of the stator slot,permanent magnet rotor equivalent slot and claw pole on the main air-gap magnetic density waveform,number and amplitude of magnetic density harmonics were revealed.It was concluded that reducing the stator slot width,slot inside width and slot depth,reducing the equivalent slot width of the permanent magnet rotor,and increasing slot spacing could effectively reduce the magnetic density harmonics of the permanent magnet part and improve the sine of the magnetic density waveform.Meanwhile,reasonably designing the pole arc coefficient,increasing the thickness of the claw tip and increasing the thickness of the claw root could effectively reduce the distortion rate of the main air-gap magnetic density waveform and improve the electromagnetic performance of the whole machine.The expression of voltage regulation rate under the armature reaction was deduced by graphical method,and the methods to reduce the voltage regulation rate were to increase the armaturer eaction reactance of cross-axis,reduce the armature reaction reactance of straightaxis,the leakage reactance of stator and the resistance of armature winding.Based on this,the influence laws of various parameters on armature reaction reactance,voltage regulation rate and output power were analyzed,and the results showed that reducing the number of armature winding turns,increasing the length of main air gap and the length of permanent magnet magnetization direction could ensure the output performance and reduce the voltage regulation rate at the same time.Moreover,the variations of the average magnetic density working point and the minimum magnetic density value of the permanent magnet under different load conditions were analyzed to determine the position of each demagnetization point of the permanent magnets and the influencing factors.Based on this,the parameters of the connected magnetic isolation air gap at the end of the permanent magnets were optimized in the consideration of the leakage flux and demagnetization to ensure that the permanent magnets were not demagnetized.The control method of output performance under starting and generating conditions of starter generator by controlling the excitation current was analyzed,and it was verified by the simulation model.The prototype was trial produced and the performance tests under starting and generating conditions were carried out.The results showed that the excitation current could be adjusted under starting conditions to achieve a larger torque adjustment range and a faster response performance,it could meet the starting requirements.Under generating condition,the starter generator had high output power,wide voltage adjustment range and good voltage stabilizing performance.