Response of a Trapped Flux Superconducting Motor to Two Drive Methods

Abstract

This work presents the starting of a trapped flux superconducting motor considering two drive methods: direct starting (using an ac power source) and soft starting (using a frequency inverter). These two methods present different characteristics that are expected to affect the behavior of the superconducting motor. For the direct starting, there is inrush current but no harmonics. On the contrary, for soft starting, there are harmonics but no inrush current. To understand the impacts of the starting current or the harmonics on the performance of superconducting motors, different tests were carried out on a superconducting motor prototype under the aforementioned drive methods. This prototype, using commercial rare-earth barium copper oxide (REBCO) tapes, was first connected to a frequency inverter (harmonics tests), and then to an ac power source (starting current tests) and tested at 77 K in liquid nitrogen. The torque under load variations, mechanical power, losses, and harmonics content of the machine were assessed. Key results showed that the motor losses increase due to a high content of harmonics during the soft starting. Consequently, the torque and the mechanical power decreased significantly compared with the direct starting. Nevertheless, the latter may present some risks for the physical integrity of the rotor as a large transient current can flow in the winding accompanied by higher mechanical stresses. For industrial motors of more outstanding power ratings with a wide range of applications requiring one or the other starting methods, it is necessary to further mature the design with machines of increasing rated power if such devices ought to be used reliably in the industry (in the kW range and above).