IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 3, MARCH 2008 403 Development of a Magnetic Planetary Gearbox Cheng-Chi Huang , Mi-Ching Tsai , David G. Dorrell , and Bor-Jeng Lin Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan Department of Electronics and Electrical Engineering, The University of Glasgow, Glasgow G12 8LT, U.K. Institute of Mechanical & Electro-Mechanical Engineering, National Formosa University, Huwei Township, Yunlin 632, Taiwan In this paper, we describe a new design for a magnetic planetary gearbox. We discuss the theory of operation and a simulated design. We constructed and verified the simulation by measuring the transmitted torque and cogging torque. A magnetic planetary gearbox operates like a mechanical planetary gearbox, except that it is contact-free and needs no gear lubrication. Hence, it has the same char- acteristics of three transmission modes, a high-speed-reduction ratio, and high durability. The starting point for the design procedure is to avoid possible sliding (i.e., pole-slipping), and we propose three steps to obtain the maximum number of magnetic planet gears. We show that using more planetary gears is a way to increase the transmission torque. Cogging torque can be high in this design. We assessed this potential by using finite-element analysis and then measuring performance of the fabricated gearbox. While the simulation overestimates the cogging torque (for various reasons), we propose a method to reduce the cogging torque to a very low value. We present a literature review to illustrate the development of magnetic gearing and highlight the innovation of this design. Index Terms—Analysis, finite element, magnetic gears, magnetic planetary gearbox, speed reduction ratio. I. INTRODUCTION M ECHANICAL gears and gearboxes are obviously widely used in various applications for power transmission and speed change, and play an important role in industrial machines. However, they have some inherent problems such as contact friction, noise, and heat. Therefore, they require regular ser- vicing and lubrication changes. To avoid these issues, magnetic gears and gearboxes (using high-performance Nd–Fe–B mag- nets) have been proposed by several authors (reviewed below). Magnetic gears were first developed using the low-energy fer- rite material which can only transmit low torque. High-energy Nd–Fe–B magnets now allow the transmission of much higher torques. The combination of an electrical machine with mag- netic gears is suitable for implementation in conveyers in clean environments, where noncontact operation, low noise, and high durability are required (such as in semiconductor and TFT-LCD manufacturing plants, as well as many other applications). Ref- erences [1]–[3] show that simple magnetic gears are possible but they have poor torque density, lower velocity ratio, and just one transmission mode. This type of simple gearing is shown in Fig. 1(a). Therefore, several novel magnetic gears have been proposed to try to overcome these drawbacks. The magnetic gearing described in [4] was a new transmis- sion type at the time. This gearbox is shown in Fig. 1(b) and had a transmitted torque of 5.5 Nm, a velocity ratio of 3:1, and a torque-per-volume of 1.7 kNm/m . Later, magnetic worms and skew gears were proposed in [5] and [6]—the arrangements for these are illustrated in Fig. 1(c) (worm) and Fig. 1(d) (skew gear). The magnetic worm gear in Fig. 1(c) was fabricated from expensive SmCo magnet material. This gearbox had a velocity ratio of 33:1, with a maximum transmitted torque of 11.5 Nm. Using the sum volume of the worm and worm wheel, the worm gear in [5] had a torque density of approximately 0.1 kNm/m . In Fig. 1(d), the skew gear replaced the worm of [5]. With this configuration, the torque density and velocity ratio were lower but it was possible to obtain more transmission modes. In 2001, Digital Object Identifier 10.1109/TMAG.2007.914665 a novel magnetic gear was proposed in [7]. This produced a transmitted torque density exceeding 100 kNm/m ; with a ve- locity ratio of 5.5:1. This was analyzed and manufactured, and reported in [8] [as shown in Fig. 1(e)]. It is really a special design for electrical power generation applications, where high- and low-speed rotors are coupled through several stationary steel segments. The design in [7] can be further modified by replacing the surface-mounted magnets of the high-speed internal rotor with interior-spoke magnets as shown in Fig. 1(f). This gearbox has high torque density and the same velocity ratio; it was described in [9]. Later, Okano et al. [10] proposed a superconducting ver- sion of the gearbox shown Fig. 1(b). It is interesting to note that combining super-conduction with magnets can improve the transmission torque. However, this adds considerable volume due to the required cooling system and power source. The various improved designs described above are aimed at overcoming the issues related to poor torque density, lower velocity ratio, and single transmission mode; and they all have their advantages. However, this paper proposes a magnetic planetary gear arrangement (MPG) with Nd–Fe–B magnets as shown in Fig. 2. The gear combines the structure of a mechan- ical planetary gear system with the transmission principle of magnetic spur gearing [Fig. 1(a)]. The MPG has three transmis- sion modes and a high-speed reduction ratio. It should also be mentioned that the number of magnetic planet gears is the key to the improvement of the MPG transmission torque. In Fig. 2, three planetary gears are shown, but it is possible to increase this number to six in order to increase the transmission torque by approximately two. The coupling between the planetary gears will be studied later in the paper. The MPG system has the advantages of the magnetic spur gearbox (i.e., simple arrangement and contact-free) and it can be fabricated in full automatic assembly line, where produc- tion costs are lower. It offers some advantages for several new applications, such as wind power generation, electric propul- sion, etc., where it can be used as either a straightforward single input and single output gearbox, or as a differential gearbox, al- lowing the transmission and conversion of variable speed to fix 0018-9464/$25.00 © 2008 IEEE