A study of the electromagnetic characteristics of no-insulation GdBCO racetrack coils under an external magnetic ripple field

Abstract

Here we report the effect of an external magnetic ripple field on the electromagnetic characteristics of GdBCO racetrack coils being operated with a constant DC current. Two types of GdBCO racetrack coils, one wound without turn-to-turn insulation (NI) and the other wound with Kapton tape (INS), were examined under external ripple fields generated by a permanent magnet mounted on a rotor, which was driven by a separate AC motor. The voltage fluctuations and magnetic field variations were measured with respect to the external ripple field intensity (BERF), rotating speed, and the operating condition. When the INS and NI coils were exposed to an external ripple field (herein, Iop = 80 A, BERF = 2 mT, and 5 rpm), a voltage fluctuation occurred because a time-varying magnetic field interacted with an electric circuit creating an electromotive force. The peak-to-peak voltage (Vpp = 0.29 mV) of the NI coil was ∼1.86 times lower than that (0.54 mV) of the INS coil, because the voltage response of the NI coil lagged behind dB/dt due to the existence of turn-to-turn contact. Furthermore, the Vpp of the INS coil increased with increasing BERF and rotating speed, while those of the NI coil were barely affected due to the delay of electromagnetic induction. In excessive current and ripple field conditions (Iop = 1.125 Ic, BERF = 8 mT, and 50 rpm) the INS coil eventually quenched while the NI coil did not, implying that the electromagnetic stability of the NI coil in excessive time-varying field conditions was superior to that of the INS coil.

Calculation and measurement of coupling loss in a no-insulation ReBCO racetrack coil exposed to AC magnetic field

“…The temporal variation of the magnetic field generated by the coils themselves was investigated in a number of studies, although AC loss was not analyzed in detail [7,[19][20][21][22][23]. Kim et al measured the AC loss with an applied AC current [24] and found that the no-insulation coils had lower AC loss than their insulated counterparts because the turn-to-turn contacts create a bypass for the applied current.…”

Study of the demagnetization behavior of no-insulation persistent-current mode HTS coils under external AC fields by 3D FEM simulation

“…The no-insulation (NI) winding technique [24] provides HTS coils with better thermal stability [25,26], higher current density [27,28], defect-irrelevance properties [29,30] and better mechanical integrity [31][32][33][34][35]. In recent years, the NI winding technique has facilitated quite a few breakthroughs in HTS magnet technology [28,36].…”

Time-variant magnetic field, voltage, and loss of no-insulation (NI) HTS magnet induced by dynamic resistance generation from external AC fields

“…Researchers have studied the inductive voltages of NI coils caused by their electromagnetic induction with external AC magnets [20][21][22], resistive voltages of NI coils with the observation of a local thermal quench under time-varying fields [5,23,24] and the critical current degradation of NI coils caused by an axial AC field [5,25]. However, the unique behaviors of NI magnets affected by the dynamic resistance have not yet been clearly observed.…”