A HTS synchronous motor at different operating temperatures

A four pole synchronous machine comprising HTS racetrack field windings has been designed and constructed. The device has been investigated at 4.2 K, 77 K and at the nominal operating temperature 20 K. The nominal power of 1500 W was achieved at a speed of 1500 rpm and an excitation current of 205 A. The field coils provided expected performance. The transition between the superconducting and the normal state took place gradually both at 77 K and 4.2 K, operation remained stable although the voltage was clearly rising. Stability performance of HTS magnets differs strongly from their LTS counterparts. The normal zone propagation velocity is roughly 20 times higher at liquid helium temperatures than at 77 K. These questions are briefly addressed in this paper, along with a discussion of test result.