Design and Verification of an Electrically Excited Synchronous Machine Rotor With Direct Oil Cooling for Truck Applications

The rotor cooling in electrically excited synchronous machines (EESMs) poses significant challenges due to the high power density resulting from active conductors necessary for generating the dc field, which in turn leads to substantial heat generation. Additionally, the rotor rotation further complicates the cooling system and contributes to mechanical losses. This article presents the design of the rotor cooling system for a 200 kW EESM for truck applications. The system employs simultaneous direct cooling of the hollow shaft, rotor lamination, and rotor winding using mineral oil. The design is verified in simulations using conjugate heat transfer (CHT) method by analyzing the oil flow and temperature distribution. Experimental tests are conducted on an eight-pole prototype rotor in stationary condition with a maximum coil heat loss of 4.7 kW, enabling $8\,\, {}\times {}190$ Aturns magneto-motive force (mmf) per pole with 9.2 L/min oil flow rate. The results demonstrate the effectiveness of the cooling method in doubling the possible current density for continuous operation, from 5.05 to 10.1 A/mm2, keeping the Start-up condition for more than 5 min and the maximum power for 20 min when the ventilation effect for the rotor rotation is included in simulation.