## 1 Introduction
Given that the electrical resistance of the superconducting layer under the normal operation condition is nearly zero, the current flows only to the superconducting layer. However, when a hot spot caused by local quenching occurs, electrical resistance occurs at the local portion. Therefore, the current flows through the stabilizer layer constituting the superconducting wire, and it is thus bypassed. As a result, NI coils can be protected from quenching without complicated quench detection schemes and/or protection devices, simplifying these HTS coil systems [16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. However, the NI HTS coil is associated with an intrinsic charging/discharging delay phenomenon. The charging/discharging delay phenomenon increases as the coil inductance increases, and the contact resistance between winding turns decreases. Figure 2 shows a circuit model of the NI HTS coil. In this figure, the portion indicated by the light blue background represents the simplified equivalent circuit model of the NI HTS coil. As indicated by this circuit model, when the current is charged or discharged, the current in the NI HTS coil is divided into a spiral direction (winding direction) and a radial direction (contact direction between winding turns). The division ratio of the current is determined by the ratio of the azimuthal impedance value, which is in turn determined by the coil inductance and the current charge/discharge rate to the radial impedance value due to the characteristic resistance. The magnetic field generated by the coil stems from the current flowing in the spiral direction. Therefore, if the coil inductance is large, the current flowing through the contact becomes large, such that the delay phenomenon during the charge and discharge of the magnetic field becomes very large. Therefore, in order to develop a robust electric motor using an NI coil, the effects by these charge and discharge delays should be considered. Various researchers have attempted to apply HTS coils to electrical machines, such as motors and generators [26, 27, 28, 29, 30, 31, 32, 33]. Specifically, these studies are more actively carried out in relation to electrical rotating machines, which require high output power levels, such as those used for wind power generation and in electric vehicle applications. Thus far, most studies on the application of superconducting coils to electrical rotating machines have mainly used an air-core type of structure, which does not use iron cores. In electrical rotating machines with an air-core structure, the superconducting field coils are positioned in the rotor to create a rotating field. Therefore, in the rotating field coil structure, the cooling system and the excitation system are inevitably complicated, and require a highly reliable design [31, 32, 33, 34]. In the rotor excitation machine, the field current must be fed through the slip ring and brushes, and the cooling system requires dynamic seals for cryogenic transfer coupling. On the other hand, electrical rotating machines, such as a flux-switching machine (FSM), a homopolar machine and a Vernier machine, use iron cores, but it is possible to realize a robust machine, because they have stationary field coil structures. The cooling system and excitation system for the field coils in machines with a stationary field coil structure can be made simpler than those in a machine with a rotating field coil structure. For this reason, research on the application of HTS field coils to electrical rotating machines with a stationary field coil structure has been active. In particular, the FSM with stationary HTS field coils is becoming more attractive, given its advantages of a robust rotor and a reliable machine structure [32, 33, 34, 35, 36, 37, 38]. However, while the FSM has a robust rotor structure, the protection issue related to HTS field coils should be considered, as in other HTS rotating machines.