Influence of the Shaft Cooling System on the Performances of a High Speed Wound Rotor Motor: a CFD-thermal Network Coupled Approach

Being the high performances electric motors demand increasing for automotive traction purposes, the in-depth study of ad-hoc rotor cooling solutions (in particular for wound rotors configurations) plays a crucial role in keeping the hotspot temperature under the required limits while developing powerful high-speed machines. This paper presents a Computational Fluid Dynamic – Lumped Parameter Thermal Network methodology to study the advantages deriving from the use of a recirculating hollow shaft cooling system in terms of power and torque levels that can be achieved in comparison to the standard configuration (with no shaft cooling system inserted). A numerical analysis of the shaft cooling system at different rotational speeds and for different coolant flow rates have been performed to determine the cooling system heat transfer coefficient trend as a function of the regime and the coolant flow rate. Then, the obtained function has been applied at the shaft node of an in – house thermal network model. The subsequent analysis allowed the Authors to determine the maximum loss that can be generated at the rotor without overcoming the insulation class limit of the motor and to compare it with the corresponding maximum rotor loss achievable by the configuration with no shaft cooling system. A general increase of about 50% in rotor losses has been observed for the shaft cooled configuration, which will turn into a large increase in torque and output power.