Short Circuit Tests on a Coreless HTS Synchronous Generator

A series of short circuit tests have been performed on a coreless (air-core) high-temperature superconducting (HTS) rotor, installed inside a conventional stator. The machine was driven like a generator using an 11 kW drive motor, to a maximum frequency of 20 Hz. The short circuit was activated at the stator terminals using a three-phase contactor switch and a remote trigger. Tests were conducted with a range of values for each of the following parameters: i) initial frequency, between 4 and 20 Hz; ii) operating field current, between 5 and 80 A; and iii) external resistance, between 14 and 160 mΩ. For each investigation, the other parameters were fixed. Large current surges were observed in both the stator (3 times its nominal current) and in the rotor (1/4 of the initial operating current). Some localized thermal measurements of a coil in the HTS winding were also recorded but, during the transient events, these measurements are invalidated by inductive voltages. The temperature is believed to remain steady during the short circuit event. The rotational kinetic energy of this rotor was almost half of its counterpart, but it was the power supply and the series resistor that absorbed most of the magnetomotive energy after the short-circuit. These components are essential parts of the HTS generators' overall system and their operating limits must also be considered in new designs.