Finite Element Analysis And Control Of Interior Composite-Rotor Controllable-Flux Permanent Magnet Synchronous Machine

It is difficult to adjust the permanent magnetic flux which leads to the hardness of rising speed above rated value by PM-flux weakening in traditional PMSM. Many kinds of PMSM structure have been proposed to increase the PM-flux weakening range, and one of them is Ostovic's variable-flux PMSM (memory motor). But the memory motor still has shortcomings. Its sandwich-shaped rotor structure leads to the low percentage of pole embrace and poor mechanical robustness. And AlNiCo is costly compared to NdFeB which has high Br and Hc. The controllable-flux PMSM proposed in this paper makes full use of the high remanent flux density, high coercive force of NdFeB, and the high remanent flux density, low coercive force of AlNiCo. The size and placement of PMs were elaborately designed to guarantee the cooperation of the two PMs on magnetic performance. The speed is regulated by using the PM-flux weakening in a wide speed range while maintaining the basic electrical properties of motors.The vector control strategy is applied to the system, which means the d-axis current id=0 and then the armature current equals to q-axis current iq. The direction of q-axis armature reaction MMF(magnetic motive force) is orthogonal to the magnetization direction of AlNiCo and have little impact on its magnetization status. By imposing a d-axis current pulse whose direction and amplitude can be changed in the stator windings, the closed-loop speed control system can change the magnetization direction and remnant flux density of AlNiCo, and control the air-gap PM-flux ultimately. When AlNiCo has the same magnetization direction with NdFeB, it contributes little to the air-gap PM-flux, but force the flux contributed by NdFeB to stator windings to make the strongest air-gap flux. When they have opposite magnetization direction, AlNiCo can make a part shunt circuit of the flux contributed by NdFeB in the rotor and weaken the air-gap flux.A series of motor models with different PM width were set up on the basis of finite element analysis to get the interior distribution of magnetic field and gap-flux density in different working conditions. The impact of load current on d-axis PM magnetic field and gap-flux harmonic component were both analyzed to get the rule how the magnet size influences the multiple of flux weakening. And for the 6-pole controllable-flux PMSM, air-gap magnetic field waveforms in different motor conditions were demonstrated to show the necessity of increasing q-axis magnetic reluctance.For the characteristics of that the interior composite-rotor controllable-flux PMSM needs id to change its gap-flux, a control strategy was designed. And on the basis of the control strategy for the controllable-flux PMSM, a sound control system was designed.