Experimental investigation and thermal modelling of slot cooling improvement for electrical machines

Intensive transport electrification is key to meet the future increasingly stringent emission targets, with legislations spanning all forms of transport including automotive, aerospace, marine and rail. The electrical machine is at the heart of all the electrified transport architectures, and hence improving its performance metrics, being it power density (kW/kg, kW/L), efficiency or cost performance ($/kW) is critical to increase the proliferation of cleaner, greener technologies. Thermal improvements are quite important in improving the performance metrics of electrical machines used for transport, and this research focuses on the aforesaid aspects while keeping a multi-domain perspective. Taking as a case study an existing Interior Permanent Magnet Synchronous Machine used for an EV traction application, firstly thermal models are built and experimentally validated. The thermal models are then used to conduct sensitivity analysis on the constituent elements, from which it is determined that the slot number and the effective slot thermal conductivity are important aspects which merit looking into more detail within this research. By conducting multi-domain studies, including electromagnetic and thermal aspects, the optimal slot number is investigated and experimentally validated, with guidelines provided on the selection of this parameter for temperature reduction for different stator sizes. Subsequently a novel, low-cost, effective way to improve the thermal performance of concentrated-wound electrical machines is proposed by extending a part of the back-iron extension into the slot, with the invention named ‘Back Iron Extension’ (BIE). Comprehensive modeling and experimental validation of BIE is conducted, with a 26.7% peak temperature reduction demonstrated, and general guidelines on its sizing are also provided. The simplicity of the BIE, which requires no additional costly materials and which can be implemented within the lamination punching process make it a strong candidate to be used with the next generation of high power density, high cost-performance electrical machines