Electromagnetic Performance of Consequent Pole Permanent Magnet Machines, with Particular Reference to Analysis and Minimization of Torque Ripples

Consequent pole permanent magnet (CPPM) machines can save the amount of permanent magnet (PM) while produce comparable torque as conventional surface-mounted PM (SPM) machines. However, due to the unbalanced features between north and south poles, i.e. PM and iron poles, the presence of a large torque ripple can hinder their adoption in some applications. Unfortunately, the basic characteristics and generalized principles have not been fully established. This thesis investigates the electromagnetic performances of CPPM machines, with particular reference to the analysis and minimization of torque ripples.
A general analytical model is developed for the first time to analyze the cogging torque accounting for unequal north and south pole widths as well as pole shifting. It can be applied to CPPM machines and other rotor PM machines. Based on the analytical expression, optimal design parameters for minimum cogging torque with/without pole shifting, including slot opening and pole-arc to pole-pitch ratio, are analytically derived and verified by finite element method (FEM).
By selecting appropriate design parameters, the cogging torque and torque ripple under light load conditions can be suppressed effectively but little torque ripple reduction can be achieved at high currents. To solve this problem, symmetrical and asymmetric pole shaping methods with different PM and iron pole shapes are proposed. It is confirmed that the symmetrical pole shaping methods can only reduce the PM torque ripple while the asymmetric pole shaping method can minimize the overall torque ripple since it can adjust the amplitudes and phasors of torque ripple components, i.e. PM, reluctance, and cogging torques, to make them counteract each other. It is also the first time that the contribution of reluctance torque ripple is revealed and utilized to minimize the overall torque ripple in CPPM machines.
However, the effect of proposed methods will be affected by different slot/pole number combinations and different load conditions. For CPPM and SPM machines with odd number of coils per phase per submachine, the influences of unbalanced pole characteristics can cause additional torque ripple harmonics due to additive effects in windings, but can be canceled in other machines. Compared with symmetrical pole shaping method, asymmetric pole shaping method can achieve lower torque ripple for CPPM machines with odd number of coils per phase per submachine, while these two pole shapes have similar effects on torque ripple reduction for other CPPM machines and all the SPM machines. It also confirms that for all the CPPM machines, the armature reaction will lead to variations of fundamentals and harmonics in performances, including flux density, flux linkage, back electromotive force, inductance, and torque ripple, as well as significant magnetic saturation and limited overload capability.
All the investigations have been carried out by analytical analyses or FEM and experimental verification.