Thermal Analysis and Management of Switched Reluctance Machines

With limited resources of rare earth materials, electric motor manufacturers have been driven towards non rare earth solutions like switched reluctance machines (SRMs). Due to their simplicity, robust structure, low cost, high torque density and wide speed/constant power range, SRMs are the center of attention for electric machine researchers and manufacturers.

Considering the fact that there is no winding on the rotor of an SRM, major heat sources are the stator windings, and the stator itself. This makes the SRMs capable of having a relatively simpler cooling system. On the other hand, development of smaller and more efficient motors calls for the need of precise thermal analysis along with the traditional electromagnetic design.

In this dissertation, fundamentals of thermal analysis in electrical machines are briefly introduced first. Next, heat generation in switched reluctance machines (SRMs) is investigated and the effects of temperature on their steady state performance is studied. Accordingly, an input-output model based on the thermal impulse response of the SRM is developed. This model is then verified through comparison of simulation and experiments using two different methods. The first method is utilized in design stage where Finite Element Analysis (FEA) tools are used to estimate the temperature and the results are verified with experiment. The second method is faster in computing and it is utilized where the prototype is available and the results are verified using the measurement for a conventional SRM.

Next, thermal modeling and analysis of a double stator switched reluctance motor (DSSRM) is studied. This analysis shows the conventional design used in the state-of-the art electric machines used for traction applications. However, the use of cooling jackets will increase the size of the machine significantly. Therefore, to conclude, a novel direct in-winding conductor cooling system is designed and analyzed for switched reluctance machines. This cooling system allows for boosting the current density in conductors while keeping the dimensions of the machine the same, therefore allowing for higher torque density compared to standard SRMs. Numerical methods have been used to analyze the design and the cooling performance. Finally, the proposed cooling system have been compared with the conventional cooling methods and the advantages/disadvantages of each system is studied at various operating conditions.