AC losses of no-insulation high temperature superconductor (RE)Ba2Cu3Ox
coils induced by ripple magnetic fields in machines
Abstract
No-insulation high temperature superconductor (NI HTS) (RE)Ba2Cu3Ox coil
technology is effective in enhancing the thermal stability of HTS coils.
Applying the NI technique on the rotor windings of HTS machines can
improve the stability and reliability of the machines. However, the NI
HTS rotor windings experience ripple magnetic fields, which leads to
induced eddy currents through the turn-to-turn contacts. The
accompanying turn-to-turn losses will considerably affect the machine
efficiency. In this study, we compared experimentally the losses of NI
HTS coils subjected to external AC magnetic fields with those of
insulated coils. Measurement system based on calibration-free method is
developed for the AC loss measurement on HTS coils exposed to external
AC magnetic fields. The results show that the AC loss of NI HTS coils
can be 20 times higher than that of insulated HTS coils, and lowering
turn-to-turn resistivity can significantly reduce this AC loss. Modeling
analysis shows that most of induced current flows in the outer turns of
the NI HTS coil because of skin effect, and lower turn-to-turn
resistivity leads to higher induced current in superconducting layers
and more significant accumulation of turn-to-turn loss. This will
increase quench risk. Optimization of turn-to-turn resistivity is
required when the NI HTS coil is applied in the machines environments.