Design And Research On The Magnetic Structure Of Direct-driving Ferrite Permanent Magnet Motor

As the direct-drive permanent magnet synchronous motors (DDPMSM) replacedtraditional asynchronous motors with gear-box drive system, the overall efficiency ofsystem improved, but manufacturing cost of permanent magnet motor increased for theprice rise of rare earth permanent magnet material, which hindered the promotion ofDDPMSM. In consequence, how to use permanent magnet material of low price todecrease the manufacturing cost of PMSM become the study highlight in recent years, onthe condition of ensuring the power density and performances of permanent magnet motor.In this paper, taking a DDPMSM of8.8kW,20-pole and24-slot as an example, forusing interior tangential structure and ferrite permanent magnet material of low price, themanufacturing cost decreased with the same power density.Firstly, as the big difference of magnetic property between ferrite and NdFeB and thelarge flux leakage of interior structure, how to decrease the flux leakage become thedifficulty of the subject, on the condition of ensuring the flux density in the air gap and theperformances of motor are nearly the same. The structure of cutout in rotor slot adopted todecrease the flux leakage and improve the utilization of permanent magnet. Using finiteelement software-Ansoft, the curves of leakage factor, fundamental amplitude of no-loadflux density in the air gap and cogging torque under different magnetizing thicknesses ofpermanent magnet and different cutout lengths of rotor slot are analyzed.Secondly, because of the large total harmonic distortion(THD) of no-load flux densityin the air gap of interior structure, which resulted in the increase of torque ripple, vibration,electromagnetic noise and no-load loss, and directly affected the performances ofpermanent magnet motor, relative measurements should be adopt to improve the waveformof flux density in the air gap. In this paper, non-uniform air gap adopt to improve thewaveform of flux density in the air gap. Through the simulation and contrast of differentnon-uniform air gaps, the relationship curves between THD of no-load flux densitywaveform in the air gap and offset is gotten. Choosing appropriate value of non-uniform air gap makes the less THD of no-load flux density waveform in the air gap and less fluxleakage to guarantee the motor’s flux density.Finally, comparing the interior ferrite permanent magnet motor with the surfaceNdFeB permanent magnet motor(efficiency, noise, overload factor, volume and effectivematerials cost, etc), the manufacturing cost of PM motor decreased while maintaining thevolume and the performances of motor.