Analysis And Calculation On Cogging Torque And Temperature Field Of External Rotor Permanent Magnet Synchronous Motor

As one kind of permanent magnet synchronous motors,in addition to the advantages of high power factor,high power density and high efficiency,the external rotor permanent magnet synchronous motor also has higher moment of inertia and higher torque.These advantages make the external rotor permanent magnet synchronous motor widely used in electric vehicles,flywheel energy storage and other fields.However,as an unique problem in permanent magnet motor,cogging torque restricts the development of external rotor permanent magnet synchronous motor.Acoustic noise and speed ripples caused by excessive cogging torque will affect the operating performance of the motor seriously.In addition,if the eddy current loss in the permanent magnet is too large,the temperature of the permanent magnet will rise,and the excessive temperatures will cause the demagnetization phenomenon of the permanent magnet.In this paper,taking a 3kW external rotor permanent magnet synchronous motor as an example,the cogging torque and temperature field are studied.Aiming at the problem of torque ripple in external rotor permanent magnet motor caused by cogging torque,a method of reducing cogging torque in external rotor permanent magnet synchronous motor by using eccentric structure of stator addendum is proposed according to the structure characteristics of external rotor permanent magnet synchronous motor.Based on the energy method and the theory of motor parameters,the analytical expression of cogging torque is deduced,and the effect of eccentric structure on reducing cogging torque is demonstrated.We establish four kinds of two-dimensional finite element simulation models with different eccentricities,and calculate the cogging torque of them.To verify the eccentricity structure of stator addendum can reduce the cogging torque effectively,the simulation results of two-dimensional transient field are compared with the theoretical analysis results.The effects of eccentric structure on the performance of the motor are studied.The current waveform,back electromotive force(EMF),torque ripple,various losses and magnetic field distribution of different eccentricity motor models are calculated and compared.Aiming at the problem of increasing eddy current loss of permanent magnet caused by eccentricity of stator addendum,a method of weakening eddy current loss by circumferential section of permanent magnet is proposed.The effectiveness of the method is demonstrated by simulation.Four three-dimensional temperature field simulation models of external rotor permanent magnet synchronous motors with different eccentricities are established.The heat source of the motor is set up according to the losses calculated by the two-dimensional transient field.The temperature field distribution of the motor model with different eccentricities is calculated by the finite volume method.The influence of eccentric structure on the temperature rise of external rotor permanent magnet synchronous motor is analyzed.The influence of eccentric structure on the temperature of permanent magnet is also studied.