Advanced Direct Drive Electric Machine Enabling High Torque at Low Speeds

Ocean and wind energy require reliable, efficient electric conversion systems to be viable resources. Direct drive electric generators offer both benefits; however, it is difficult to generate the required torque at the low speeds typical of these resources. In this DOE sponsored project, ABB US Corporate Research partnered with Resolute Marine Energy (RME) and Texas A&M University to investigate the suitability of direct drive electric power generation for a paddle-type wave energy converter (WEC). This WEC provides high torque in a relatively chaotic, oscillating manner and requires a machine capable of handling high peak torques of approximately 40,000 N·m at speeds typically not exceeding 3 rpm. The baseline concept uses a hydraulic power take-off coupled with a generator and convertor; a direct drive electric solution may be beneficial. As a first step in designing a suitable direct drive generator for this application, we have begun with a smaller scale prototype targeting one tenth of the speed and torque, and we investigated a machine topology that may be promising in the operating regime of the full-scale machine. To this end, we designed and tested an integral generator and magnetic gear which is rated at 3,800 N·m at 30 rpm and completed a paper design for a machine of the required scale. This paper describes the mechanical design and testing of the prototype machine and provides some reflection on necessary design changes for the full-scale machine.