Design and Testing of a Direct Oil-Cooled Homopolar Synchronous Generator With Dual DC-Link via Diode Rectifiers

The drivetrains of large transportation systems, such as aircraft or ships, are evolving towards hybrid configurations with DC primary power distribution thanks to their higher efficiency. In this context, synchronous homopolar machines equipped with diode rectifiers are promising solutions for on-board generation units as they eliminate the need for active power electronic converters. In this paper, a synchronous homopolar generator with a dual DC-link configuration, where each power bus is supplied via two series-connected rectifiers, is designed and tested. A direct oil-cooled excitation coil provides the full voltage-control capability of the two independent DC links. The manuscript addresses the challenge of the analytical electromagnetic sizing of this machine topology, which has an intrinsic three-dimensional magnetic behaviour. The electromagnetic design is validated through a 3D-FE analysis, while the thermal sizing of the solenoid hollow oil-cooled excitation coil is analytically carried out and validated through CFD analysis. A 1.2 kW, 6000 rpm proof-of-concept demonstrator has been built and tested, and the reported measurements match the predicted electromagnetic and thermal operation. Although the experimental validation is performed on a small-scale prototype, this work may provide a foundation for designing an active power electronic-free electric generation unit for future large transportation systems.