Operational Design Analysis of a Shaft Oil Spray Cooling in Electrical Machines

The design of electrical machines used in traction applications trends towards smaller and more efficient motors. Therefore, the thermal design aspect becomes more relevant. Conventional cooling methods may not have sufficient cooling capability and novel cooling concepts, like oil spray cooling of the end windings, are needed. However, no reliable correlations for the prediction of the cooling capability of oil spray cooling methods are available. This paper contributes with a design approach for the pressure difference of a shaft oil spray cooling system, which is essential for the regulation of the pumping system as well as the determination of the outlet pressure. Dimensionless numbers and analytical relationships are used to reduce the parameter space. The impact of the number of radial holes and the rotational speed can be approximated analytically. With the use of the non-dimensional Euler and Hagen number, the results are applicable to every Newtonian fluid at any temperature as long as the fluid properties are known. A sensitivity analysis is performed for the shaft geometries, showing that the diameter of the radial holes and the volumetric inlet flow have a major influence on the pressure difference, whereas the inner and outer diameter of the shaft exhibit a minor influence. The results of the above-mentioned approach are used to establish a series of tests to determine the heat transfer coefficients of liquid impingement jets on a plain surface using orifice-like nozzles at varying input pressures and volumetric inlet flow rates. The measurements showed that a smaller hole diameter of the nozzles and a higher volumetric inlet flow led to a higher heat transfer coefficient. The higher the temperature of the fluid, the higher the heat transfer coefficient. The nozzle distance, on the other hand, has no influence on the heat transfer coefficient and can therefore be neglected in the design process.