Investigation of High-Speed Permanent Magnet Motors with Toroidal Windings

This thesis investigates the electromagnetic performance of low-power and small-size toroidally wound 2-pole high-speed permanent magnet (HSPM) motors. The influence of various design parameters, such as stator slot number, split ratio, slot ratio, and stator iron flux density, has been investigated. It is found that 6-slot/2-pole HSPM motor with toroidal windings is eminently suitable for high-speed applications due to no UMF and low rotor eddy current loss. The analytical predictions show that when the slot ratio, i.e. the ratio of the inner slot area to total slot area, is 0.5, the maximum torque can be achieved, and the optimal split ratio increases with the decrease of slot ratio, as confirmed by the finite element (FE) analyses. Considering modular design for auto-manufacturing, the influence of stator gap and misalignment on the electromagnetic performance is investigated. It shows that the stator gap results in unbalanced three phase back-EMFs, not only amplitudes, but also phase angles. The stator gap increases the equivalent air-gap length and PM flux leakage, which lead to lower air-gap flux density, back-EMF, and average torque. Moreover, the stator gap results in asymmetric air-gap length, which results in large cogging torque. Further, it shows that the misalignment of two stator parts mainly affects the uneven equivalent air-gap length and symmetry of winding configuration, which lead to unbalanced three phase back-EMFs, especially phase angles, and self-/mutual-inductances, as well as rotor PM loss. Experimental results of a prototype motor with/without stator gap and/or misalignment are given to validate the finite element predicted analyses. The electromagnetic performance between 2-pole slotted and slotless HSPM motors with toroidal and tooth-coil windings, together with the influence of the tooth-tip, has been compared. It is found that the slotless motor with 6-coil toroidal windings has advantages of low stator iron loss and rotor eddy current loss due to no slotting effect, but has significantly higher AC copper loss and lower output torque. In addition, considering the thermal limitation, compared to the motor with tooth-coil windings, toroidal windings have the advantage of relatively large torque due to smaller stator core volume and stator iron loss.