Thermal and Electromagnetic Analyses of Interior Permanent Magnet Machines Accounting for AC Copper Loss

This thesis presents novel methods for AC copper loss calculation and analysis, and investigates the influence of AC copper loss on electromagnetic and thermal performance, with particular emphasis on torque/power-speed characteristics of interior permanent magnet (IPM) machines for low voltage and high-speed operation.
The influences of position and size of both solid and hollow rectangular conductors on the AC copper loss and thermal management capability are firstly investigated by the finite element method. It is shown that the AC copper loss accounts for a major part of the total loss in the IPM machine with solid rectangular conductors. There is a tradeoff between high maximum torque capability and low AC copper loss at high speed. Thus, it is essential to consider AC copper loss in the calculation of torque/power-speed characteristics. With the liquid coolant flows directly inside the hollow conductors, the hollow region should be carefully designed since it affects not only the AC copper loss but also the thermal management capability. The hollow shape with the best overall performance can be obtained by considering the variable coolant flow rate.
An efficient and accurate hybrid analytical and finite element model has been proposed for calculating the AC copper loss, accounting for the influences of temperature, frequency, current angle, and saturation. Comparing with a pure finite element method, it is much faster to predict the AC copper loss under different frequencies, current levels, and temperatures. By utilizing the proposed method, together with the general lumped thermal parameter model, the torque/power-speed characteristics accounting for AC copper loss can be obtained within a desirable computing time.
Furthermore, a novel lumped thermal parameter network is proposed based on heat transfer physics. It eliminates the difference in temperature rise between the distributed loss model and the concentrated loss model. Comparing with the existing models, it utilizes full loss and full resistance and maintains the same parallel flux divide ratio comparing with the distribution loss model. The thermal network element with internal heat generation is relatively isolated from other thermal network elements to prevent over-prediction. It has been shown that the proposed method is relatively more accurate than the existing models for the prediction of hot-spot temperature.
All developed analytical and hybrid models, as well as the lumped thermal parameter network, have been validated by finite element analyses.