Preliminary Study on the Applicability of Water-Based Lubricant as a Direct Cooling Medium in Rotating Electrical Machines

Direct oil cooling is particularly popular in modern electric drive systems as it enables an effective cooling performance. Subsequently, the power density of the electric drive can be significantly increased. However, the cooling efficiency is severely limited due to the significantly lower heat capacity and thermal conductivity of oils than the conventional cooling medium, water. To overcome the limitation of oil cooling, a water-based lubricant possessing much higher heat capacity and thermal conductivity is developed as a direct cooling medium. Considering the high electric conductivity, the applicability of the water-based lubricant is discussed in this paper under the aspects of material compatibility and the effects on the electric behavior of the insulation system. The results indicate sufficient material compatibility of the lubricant with the enameled wire. The lubricant affects the insulation system during operation as its presence enhances the likelihood of electric leakage and leads to electric field concentration on the insulation materials.