# Efficiency Improvement Strategy for Multiple Operating Points in Doubly Fed Magnetic Geared Motor
## 1 Introduction  
The magnetic geared motor, which is devised from the magnetic gear, has been widely reported in direct-drive system and low-speed, high-torque applications. As shown in Figure 1a, unlike the general synchronous machine, the magnetic geared motor has modulating pieces which modulate the winding magnetic flux and the permanent magnets (PMs) magnetic flux, respectively. A modulating part consists of a pole piece with ferromagnetic material to modulate the magnetic flux and a supporting body with nonmagnetic material to hold each piece which can play a role as a fixed part or output rotor. In a case in which a modulating part is used as the rotor, the gear ratio is larger than that of the PM rotor case. The winding magnetic flux is changed into the magnetic field with a high number of poles by the magnetic gearing effect, and it is synchronized with the PM magnetic flux [1]. Thus, the speed of the PM rotor is reduced by the ratio of the pole numbers of the winding and the PMs, and the output torque is inversely increased by the same ratio. Due to these characteristics, many studies on the magnetic geared motor have focused on low-speed, high-torque applications, and there are continued efforts to replace the power transmission of an electric vehicle (EV) consisting of thesynchronous machine and a mechanical gear box with a magnetic geared motor [2; 3; 4; 5; 6; 7; 8; 9]. However, the fact that the range of the field weakening operation gets narrower as the ratio of PM MMF to winding MMF increases, and the unstable price and supply with respect to rare earth materials are regarded as the shortcomings in the traction applications including the EVs.  
Meanwhile, as shown in Figure 1b, recently, the doubly fed magnetic geared motor (DFMGM), with three-phase windings and alternating current (AC) in both inner and outer stators, has been researched [10; 11; 12; 13; 14; 15]. In a DFMGM, the modulating part is used as the output rotor, and the speed of the output rotor is determined by the frequencies of the inner and outer windings. Furthermore, the number of pole pairs in each winding and the number of modulating pieces are determined by the magnetic gearing effect, and thanks to this operating principle, the base speed of the DFMGM can be increased as compared with the case of only one three-phase AC winding stator. Otherwise, if it is not necessary to increase the output speed, the base frequency is decreased because it is designed as the summation of rated frequencies of the inner and outer windings. Namely, the base frequency is divided up into two low frequencies which means that the inner and outer parts share the whole operating region. Hence, the one high power inverter, which is required to reach the base speed, can be split into two low capacity inverters. However, in the motor which is operated by the principle of magnetic field modulation, undesirable space harmonic components are produced at the air gaps, and their effect is severe when the operating frequencies of the inner and outer windings are different from each other. Especially, the unmodulated space harmonic, which does not synchronize with the working harmonic at the air gap, significantly affects the magnetic loss. Furthermore, these space harmonics have different influences on the iron losses at each stator and the modulating pieces, respectively, and the resultant iron losses at each part and the efficiency are varied by how the frequency of each winding is designed. Accordingly, it is very important to determine the frequency ranges of the inner and outer windings considering iron loss depending on the space harmonics. Thus, this paper presents the characteristics of space harmonics and iron loss according to the frequencies of the inner and outer windings and proposes a concept for efficiency improvement by using individual frequency control and frequency combination selection.