## 1 Introduction  
Recently, the weight reduction and floor-lowering of the vehicle-body have attracted much attention in the electric vehicle and railway vehicle market [1]. In addition, as researches for the weight reduction and floor-lowering of the railway vehicles are being actively conducted, the designs for miniaturization of driving devices for railway vehicles were considered prior to other electric equipment [2]. Figure 1 shows the urban railway vehicle and structure of bogies according to the type of railway vehicle. As shown in Figure 1, since the structure and the size of the railway vehicle bottom space are different for each railway vehicle, the structure and the size of the driving devices mounted on the bogie are different. As shown in Figure 1b, a bogie with a parallel-typed reduction gear is installed in general railway vehicles. In the case of a tram in Figure 1c, since the low-floor bogie was applied, there was not enough space to install a drive system inside the low-floor bogie, unlike the conventional railway vehicle bogie. The low-floor bogie for a tram has a complicated structure in which a mechanical reduction gear and a traction motor are vertically engaged with the outer side of the wheel. Therefore, in order to improve the efficiency and maintainability of the driving system for a tram, a structural simplification study of the conventional devices is required. In general, when changing a mechanical gear to a magnetic gear, the efficiency can be improved because there is no power transmission loss in the magnetic gear [3]. However, since it is difficult to achieve structural simplification only by applying magnetic gear, a magnetic-geared permanent magnet synchronous motor (MG-PMSM) is required. MG-PMSM, which has a double rotor that includes the function of a mechanical reduction gear, is a system that can completely replace the drive system consisting of a mechanical reduction gear and a traction motor. Figure 2 shows the structure of MG-PMSM and rotation principle by magnetic flux modulation in the double air gap of MG-PMSM. As shown in Figure 2, the MG-PMSM has two rotors that rotate at different speeds. In the case of the MG-PMSM, where the stator is located on the outside, the outer rotor is a low-speed-high-torque rotor (pole piece rotor), and the inner rotor is a high-speed-low-torque rotor (PM rotor). In general, the driving principle of MG-PMSM can be explained by the magnetic flux modulation principle of magnetic gear. As shown in Figure 2, it is possible to explain the principle of modulation of magnetic flux in two airgaps with the form of a linearly unfolded coaxial magnetic gear. In general magnetic gear, the space harmonic component of the magnetic field generated by the permanent magnet of the inner rotor in the inner air gap appears in the form of a space harmonic modulated in the outer air gap by the number of pole pairs of the inner permanent magnet and the number of pole pieces. Therefore, when a 3-phase power source with a frequency suitable for the modulated space harmonic component in the outer airgap is applied to the stator winding, the outer rotor and inner rotor of MG-PMSM rotate differently with a specific speed-torque ratio. In this research, the design study of the 45 kW-class MG-PMSM was conducted for the application of the MG-PMSM structurally integrated with magnetic gear and PMSM to the driving system for a tram. Figure 3 shows the structure of the bogie and conventional driving system for the low-floor tram. The conventional driving system for low-floor turns consists of a mechanical combination of a single-stage vertical-typed reduction gear and a traction motor. Figure 4 shows an application concept of MG-PMSM to a low-floor tram's driving system [4]. As shown in Figure 4, the outer rotor of MG-PMSM is directly connected to the wheel through a coupler.