## 1 Introduction  
Many research groups have studied quench detection methods for low-temperature superconductor (LTS) systems using voltage taps to measure abnormal voltage signals. However, quench detection in high-temperature superconductor (HTS) coils is more difficult to detect than that of LTS coils. The normal zone propagation velocity (NZPV) of an HTS is about 1000 times slower than for an LTS coil. Therefore, thermal runaway can occur in a hot spot and damage the HTS coil due to stored magnetic energy before the resistance generated in the normal zone can be measured. For these reasons, various quench detection methods that do not use a traditional voltage tap have been studied [1; 2; 3; 4; 5]. [PERSON] et al. proposed a quench detection method based on the change in stray capacitance between magnet elements when quenching occurred [1]. This method comprehensively determines whether or not quenching occurs due to factors such as cryogenic fluid boil-off, variation of the dielectric properties of the insulation material, and thermal expansion, which changes the local distance between a coil and metal parts. However, because there is drift for a long time when the quench signal is measured that corresponds to the change in capacitance, the reliability of the quenching detection is decreased. In addition, the capacitance monitoring system is temporarily interrupted by circuit transients, which is a drawback in quench detection. Other quench detection methods that do not rely on voltage taps have also been studied, including such as acoustic emission [2; 3], telemetry [4], and strain gauges [5; 6]. A universal quench detection technique was proposed by [PERSON] et al. with active power detection using a central voltage tap [7; 8]. With this method, a central voltage tap measures the voltage imbalance with a reduced noise signal achieved using a low-pass filter (LPF). However, this method has the limitation that it can cause a short circuit to a large magnet when high voltage is applied. More studies to improve the reliability of quench detection by reducing the electromagnetic noise continued to be conducted by [PERSON] and [PERSON] [9]. This technique is related to noise reduction through inductive voltage cancellation of the HTS coil, and can increase quench detection reliability through the cancel coil and LPF. However, in order to cancel the inductive voltage, an air-core solenoid coil must be installed. [PERSON] et al. proposed a new method using LTS wire as a quench detector [10]. That is, when an LTS is installed in the stabilization layer using epoxy, it is attached to the REBCO coil again, and when a normal zone is generated in the REBCO coil, current is passed through the stabilizer layer, which causes quenching in the LTS wire. Finally, the abnormal signal of the REBCO coil is indirectly detected by the LTS wire. However, to utilize this method, the REBCO coil must be impregnated with the epoxy surrounding the LTS, which may limit increase in the coil volume. In addition, LTS quench detection by heating the epoxy should be preceded by quench detection for the REBCO coil. However, extension of the quench detection time causes fatal damage to an HTS coil because of its slow NZPV. Furthermore, quench detection in HTS rotating machinery is even more difficult. This is because the monitoring signal of the HTS rotating machinery contains noise due to interaction with the industrial environment. For these reasons, in this paper, it is proposed that a new method for sensitive quench detection in HTS rotating machinery using an RLC resonant circuit is utilized. The RLC resonant circuit acts as a band-pass filter and reduces the influence of noise in the detection of quenching [11; 12]. The feasibility of the proposed quench detection method is confirmed by simulation results and equivalent circuit analysis. The simulation results are also verified through experiments.