Title:

Investigation of torque characteristics of permanent magnet and electrically excited machines

This thesis is focused on the comparative investigation on the torque characteristics, both average torque and torque ripple, of permanent magnet (PM) and electrically excited (EE) machines for direct drive applications, which require both high torque density and low torque ripple. The effectiveness of two of the most widely used methods, i.e., rotor shaping and skewing, for torque ripple reduction in EE and PM machines is investigated. It is found that the effectiveness of skewing largely depends on the axial variation of torque ripple phase but less on its magnitude under skewing. It is further found that, in both linear and nonlinear cases, the onload torque ripple cannot be fully eliminated by skewing one onload torque ripple period or any other angles, except 360° electrical, which is impractical. The torque ripple may be even increased by skewing, especially when the cogging torque is low and electric loading is high. An improved skewing is developed by optimising both the skewing and current phase advance angles when the conventional skewing method fails. Compared with PM machines, rotor shaping is less effective on torque ripple reduction in EE machines due to the asymmetric magnetic saturation of the stator and rotor cores. However, if rotor skewing is used then EE machines can still achieve the same low level of torque ripple as PM machines. In order to better understand characteristics of onload torque and hence improve it, the onload torque separation of PM machines is investigated as well. It is found that the average torque components can be appropriately separated using the virtual work principle but not by the Maxwell stress tensor. For the onload cogging torque calculation, which is the most difficult challenge for onload torque ripple separation, a new method is proposed by combining the virtual work principle with an improved frozen permeability method. For torque density optimisation, analytical methods are often desirable. This thesis develops an analytical model for PM machines having external rotor. It is shown that the torque density is maximum when the average airgap flux density is slightly lower than half of the maximum flux density in the stator. In order to compare the torque densities between EE and PM machines, a simplified analytical model of EE machines is developed as well. It is shown that, for EE machines, there is an optimal pole number to maximize the torque densities and it is more advantageous for large volume applications. PM machines can exhibit higher than √2 times torque densities of EE machines. The designs of maximum torque per weight are more costeffective and result in a significantly larger airgap diameter than the designs of maximum torque per volume.
