Raising the Thermal Limits of an Electrical Motor Based on an Air Gap Winding by Applying Improved Laminated Layers and Optimized Cooling Duct Geometries

Abstract

Due to their electro-mechanical behaviour, electrical motors are able to generate high power densities. For a 20 kg motor based on a modified air gap winding, a mechanical load of 80kW is theoretically possible. However, the achievable power density is generally limited by the thermal loads induced by Joule heating due to the Ohmic resistances in the motor's wiring. This paper discusses the potentials of cooling channel modifications, such as the overall geometry as well as surface variations, to enhance the convective heat transfer from the solid material in the coolant with focus on motors using an air gap winding. The convective heat transfer acts as a relevant thermal resistance, which becomes critical at elevated loads, because it is lies between the wiring, acting as a heat source, and the liquid coolant, acting as heat sink. The thermal resistance of the interface material between the wiring and the stator is in the same order of magnitude as the convective heat transfer. These non-metallic materials such as foils, coatings and glues for electrical insulation and mechanical bonding are highly limited in their temperature resistance. To evaluate the behaviour of modified cooling channels and investigate novel interface materials under motor-like conditions, experimental and numerical methods are employed to determine thermo-physical properties, heat and mass transfer and finally the temperature distributions. The experimental results are discussed in comparison with numerical and analytical models and validated thereby. Because of the limited possibilities for experimental investigations within a real electrical motor, a flat test bed, based on the boundary conditions of an existing 16" wheel hub motor was developed. The Ohmic heating was realised, applying a thin sheet foil of Inconel, so that specific heat loads up to 100 kW/mK could be realised. Here, the surface temperature could be measured contact-free at a high optical resolution, using infrared thermography. The most advantageous test bed feature is the optical accessibility of cooling channels which allows the visualisation of fluid flow regimes. As results, the influence of large-scale coolant duct geometry modifications on the motor temperatures and hydraulic characteristics as well as of small-scale modifications will be presented in comparison to non-modified geometries. Also, the outcomes and capabilities of novel thermal interface materials consisting of laminated foils will be discussed and an outlook on a modified motor design to increase the thermal limits will be given.