The permanent-magnet(PM) spherical motor has special configuration and complicated distribution of electromagnetic field.The spatial distribution of the effective flux density produced by permanent magnets is obtained,and the end-effect of the permanent magnets is analyzed.In the 3-dimensional(3-D) finite element(FE) analysis,the calculation of Back-EMF charac-teristics of the PM spherical motor needs to be conducted by steps,which leads to large computational burden.A novel ana-lyzing method to quantize the end-effect of the PM spherical motor is proposed,that is,the end-effect coefficient of the PM spherical motor is calculated to revise the back-EMF waveform obtained from the 2-D FE model.Under different outer radii of the stator coil and different pole arc coefficients of the PM spherical motor,the revised 2-D results are validated by the results obtained from the 3-D FE model and the experimental results.With this method,the special structural feature of the PM spherical motor is fully considered;the pure 3-D calculation of electromagnetic field is conducted only once;higher precision and less computational burden can be achieved.The method provides a new approach to investigating the back-EMF charac-teristic of the PM spherical motor.
This paper presents a mathematical model of three-level voltage PWM rectifier,and derives a power control model from the theory of instantaneous power.In the vector-space,the influences on instantaneous power exercised by all the switching vectors are studied and illustrated separately,then a direct power control(DPC) scheme for three-level PWM rectifier which uses multistage band hysteresis comparator is proposed,and a novel switching table is designed.Meanwhile,the neutralpoint voltage unbalance is inhibited by selecting the redundancy switching states of small voltage vectors.Simulation and experimental results show that the proposed strategy can not only stabilize the DC bus voltage but also realize the unity power factor operation and the balance of neutral-point voltage.Moreover,the proposed method can improve the performance of the three-level rectifier.
