Another configuration in which the PMs in the inner rotor are tangentially magnetized was proposed by Rasmussen et al. Incorporating the concept of the Halbach PM array and the concept of the magnetic gear, Jian and Chau proposed an improved coaxial Halbach magnetic gear as shown in Figure 4. The key characteristic of this con- figuration is that all PMs are involved in the torque transmission at the same time, hence obtaining a high torque density with 50-150 kNm/m 3. As shown in Figure 3, the fer- romagnetic pole-poles which are sandwiched between the outer rotor and the inner rotor take the charge of modulating the magnetic fields both in inner airgap and outer airgap. In 2001, Howe and Atallah proposed an innovative coaxial magnetic gear with high torque density. Although the topology of parallel-axis magnetic gears is very simple, they have not been widely used for industrial application because of their very low torque density. In order to avoid the complicated structure of mag- netic worm and skew gears, Ikuta proposed the sim- ple parallel-axis magnetic gears which include two basic topologies: radial coupling and axial coupling as shown in Figure 2. However, both of them have the demerits of complexity and poor torque density. In 19, the magnetic worm gear and magnetic skew gear was also developed, respectively. In 1987, Tsurumoto and Kikuchi proposed a new transmission type using an involute SmCo magnetic gear which was shown in Figure 1. Until the emergence of high-energy rare-earth PMs, magnetic gears aroused great interests from researchers. But it also suffers from the low-energy of ferrite used. In 1941, Faus proposed another magnetic gear topol- ogy similar to the traditional mechanical spur gear. However, it attracted no attention at that time for its low efficiency and complicated structure. The basic concept of the magnetic gear can be tracked down to the beginning of the 20th century when a US patent described a device consisting of two rotational shafts with salient electromagnets on their rims. At last, a conclusion will be drawn in Section 5. Section 4 will discuss the application of magnetic gears for EVs. In Section 3, based on the same structure, the magnetic gears installed with Alnico, NdFeB and SmCo PM materials will be analyzed and compared. In Section 2 a comprehensive review of magnetic gears will be conducted. This paper presents the development of magnetic gears, with emphasis on the comparison between the non-rare-earth and rare-earth magnetic gears. Although the rare- earth magnetic gears have better performance, their fluctuant and expensive price and finite reserves will increase the cost of EVs manufacture, hindering fur- ther application in EVs. Recently there is an increasing concern on the price and supply of rare-earth elements. After the advent of high-energy rare-earth PM materials such as the neodymium-iron-born (NdFeB) PM and samarium-cobalt (SmCo) PM, they become the PM materials widely adopted for magnetic gears. Before the invention of rare-earth PMs, alumin- ium-nickel-cobalt (Alnico) which takes the definite merits of high remnant flux density and abundant elements is also used to develop magnetic gears. Ferrite has the advantages of low price, easy manufacture and high coercivity, whereas low remnant flux density is its obvious de- merit. The first PM material applying to the magnetic gear is non-rare-earth ferrite which can only transmit low torque. Magnetic gears offers the merits of physical isolation, silent operation, maintenance free and inher- ent overload protection. It is well known that these characteristics made the replacement of mechanical gears by magnetic gears possible. Obviously the advent of the magnetic gear attributes to the application of permanent magnet (PM) materi- als which can produce a persistent flux and magnetic force and realize contactless torque transmission. However, they suffer from some inherent drawbacks, such as the wear-and-tear, contact friction, annoying noise and regular maintenance. With the help of the mechanical gears and gearboxes, torque and speed can be amplified for various practical applications, such as amplifying the speed (reducing the torque) and amplifying the torque (reducing the speed) to satisfy different driving requirements and road conditions. gears and gearboxes play vital important roles in electric vehicles (EVs).
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