IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014 8001804

A Novel Magnetic Gear With Intersecting AxesYulong Liu, S. L. Ho, and W. N. Fu

Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong

Compared with mechanical gears, a magnetic gear (MG) has many merits by virtue of its contactless feature, high efficiencyand inherent overload protection. In this paper, a novel MG with intersecting shaft axes based on the flux modulation effect isproposed. Compared with previous MGs, the transmission direction of the novel MG can be designed very flexibly and it also hasgood competitive edge in relation to the use of permanent magnets, which enables it to have a relatively high-torque density. Theworking principle and performance are validated by an example design using finite element method. The effects of the dimensionalparameters on the torque performance are analyzed. It is also verified that the torque density can be further improved by adoptinga flux concentration configuration in its rotor design. It is concluded from the torque per unit volume density performance that thenovel MG is very promising as a substitute to conventional gears.

Index Terms— Finite element method (FEM), flux-modulation, magnetic gear (MG), permanent magnet (PM).

I. INTRODUCTION

MAGNETIC gear (MG) is a very promising concept asit has the merits of having high efficiency, low main-

tenance, no audible noise, and inherent overload protectionfeature when compared with mechanical gears. The history ofMG can be traced back to the early 20th century. Some MGtopologies similar to mechanical spur gears or worm gearswere proposed in the early decades [1], [2]. However, thepoor performance of permanent magnets (PMs) at that timeundermines the full potential of MGs in practical use. Sincethe 1980s, the availability of high-performance NdFeB magnetlargely revives the development of MGs. Some magnetic spurgear topologies are proposed in [3]–[5]. The worm gears, skewgears, and bevel gears with PMs are also analyzed [6]–[8].A magnetic bevel gear with a topology similar to that of themechanical bevel gears is shown in Fig. 1(a). However, allthese MGs have a poor utilization of PMs because only asmall fraction of PMs contribute to torque transmission at anytime, which is quite similar to the situation of mechanicalgears in that only a few teeth mesh at any one time. Thisresults in a low torque per unit volume density, which is onlyaround 20 kNm/m3 for magnetic spur gears, 2 kNm/m3 formagnetic worm- or bevel-gears and even lower for magneticskew gears [9].

Recently, a coaxial MG in which some iron segments areplanted between two PM rotors breaks the limitation of havinga constant 1:1 gear ratio for coaxial gears [10]. Fig. 1(b)shows the structure of a coaxial MG. The rotors have differentmagnetic pole-pair numbers. Due to the introduction of theiron segments, PMs on the two rotors could interact via acommon space harmonic component by virtue of the fluxmodulation principle. The two rotors rotate at different speedswith different torques and thus a gearing effect is achieved.This coaxial MG has a better utilization of PMs and it hasa significantly higher torque density than those in previousdesigns. However, the coaxial input and output shafts design islargely limiting its application range. In [11], an MG arrange-ment is proposed to extend this coaxial MG to offset such

Manuscript received March 7, 2014; revised May 7, 2014; accepted May 15,2014. Date of current version November 18, 2014. Corresponding author:W. N. Fu (e-mail: eewnfu@polyu.edu.hk).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2014.2325608

Fig. 1. (a) Magnetic bevel gear. (b) Coaxial MG.

shortcomings, but the iron segments which have the shape ofsimple broken lines in that design are not very good flux paths.

In this paper, a novel MG with intersecting axes (IAMG)is proposed. The basic idea is using segmented iron totransmit the PM flux as well as generating an asynchronousharmonic field for the interaction of two rotors. By changingthe shape of the iron segments, the input and output shaftaxes can intersect at any angle or even stagger in space.By designing a combination of PM pole-pair numbers andthe iron segment number, various gear ratios can be realized.Compared with previous MGs, the proposed IAMG has themerits of being very flexible in the transmission direction aswell as having a high-torque density due to relatively goodutilization of PMs. It is expected to become a promisingsubstitute to mechanical gears, such as skew gear, bevel gear,and hypoid gear, especially in applications where there isa high requirement on efficiency or quietness. As an example,an IAMG with perpendicular shaft axes is analyzed usingfinite element method (FEM). A parametric analysis is doneto study the effects of the geometric parameters on the torqueperformance of the proposed IAMG. The performance of theconfiguration with flux concentration is also investigated.

II. STRUCTURE AND WORKING PRINCIPLE

The proposed IAMG topology is shown in Fig. 2. Thestructure of the rotors is similar to that of a conventionalsurface-mounted PM machine. The PMs are all magnetizedalong the radial direction relative to their rotation axes. Theiron segments surrounding the rotors function to transmit andmodulate the PM flux. In this paper, solid steel is used for theiron segments because the maximum speed of the prime mover

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8001804 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014

Fig. 2. Configuration of an IAMG with perpendicular shaft axes. (a) Totalassembly. (b) Active part.

Fig. 3. Flux density vector diagram in the iron segments.

is assumed to be 30 r/min and hence the eddy current inside itis small. Soft magnetic composite material or laminated siliconsteel may be used when high-operation speed is needed.

The working principle of the novel MG is similar to thatof the coaxial MG in [10] except that the iron segmentsare stationary. The flux density vector diagram in the ironsegments is shown in Fig. 3. It should be noted that a fractionof the magnetic fluxes are appearing as leakage at the endsof the IAMG. This implies that there are end effects in thisstructure and it should be reduced by properly designing thedimensional parameters. The radial flux density in the twoairgaps and the corresponding spatial harmonics are shown inFigs. 4 and 5. The 10th and 18th harmonics are dominant inthe two airgaps and the rotors could thus interact via thesetwo harmonic components.

III. PARAMETRIC ANALYSIS

The main geometric parameters used to study of the IAMGare shown in Fig. 6. The fixed parameters as well as thematerial properties are given in Table I.

To investigate the effects of the geometric parameters onthe performance of the IAMG to realize a reasonable design,a parametric analysis aiming at maximizing the torque densityis done using finite element analysis. For the calculation oftorque density, the torque of rotor 2, and the volume of theactive part (the total volume surrounded by the iron segments)are considered. The outer radius Rs as well as Rc and Ri isfixed. The variables L1, L2, Hpm, and Hs are analyzed one byone and those parametric values giving the best performance

Fig. 4. (a) Radial flux density in the airgap adjacent to rotor 1 due to thePMs of rotor 2 and (b) corresponding spatial harmonics.

Fig. 5. (a) Radial flux density in the airgap adjacent to rotor 2 due to thePMs of rotor 1 and (b) corresponding spatial harmonics.

Fig. 6. Geometric parameters.

during the independent analysis are selected in the overalldesign.

The analysis results are shown in Fig. 7. As can be expected,the torques increase with L1. However, the torque densityachieves its peak value when L1 is around 160 mm. This isbecause the MG suffers heavily from the end effect when the

LIU et al.: NOVEL MG WITH INTERSECTING AXES 8001804

TABLE I

FIXED GEOMETRIC PARAMETERS AND MATERIAL PROPERTIES

TABLE II

COMPARISON OF INITIAL AND FINAL PARAMETERS

rotor length is small, but then the saturation problem in the ironsegments gradually outweighs the end effect influence whenthe rotor length gets larger. The effect of L2 on the torqueperformance is quite similar to that of L1. An increase in Hpmcauses the torques to increase until Hpm reaches about 7 mm,after which the torques are nearly constant, which is quitesimilar to what happens in a conventional PM machine. Thetorques increase with Hs when Hs is small because saturationin the iron segments is alleviated. However, the compressionof rotor diameter outweighs the saturation problem whenHs is large enough and thus the torque decreases.

A comparison between the initial and the final designs islisted in Table II. The torque-angle curve of the IAMG withits final parameters is shown in Fig. 8. The pull-out torquewaveform with rotor 1 rotating at 10 r/min and rotor 2 at18 r/min is shown in Fig. 9. From the graph, the torque ratiois 1.78, which is quite similar to the theoretical gear ratioof 1.8.

IV. IAMG WITH FLUX CONCENTRATION

Similarly to the PM synchronous machines, the torquedensity of IAMG can also be further improved by arrangingthe PMs with a flux concentration configuration. As shownin Fig. 10, a flux-concentrated IAMG is obtained by onlychanging the rotors of the proposed IAMG with its finalparameters. The PMs are all magnetized along the azimuthaldirection and every two nearby PMs are magnetized in theopposite direction. Thus, the flux is concentrated and forcedto pass through the steel poles in the radial direction. Theradial thickness of PMs is extended to 30 mm.

Fig. 7. Torque or volume torque density as functions of (a) length of rotor 1,(b) length of rotor 2, (c) thickness of PM, and (d) thickness of iron segments.

Fig. 8. Torque-angle curve.

The torque-angle curve of the flux-concentrated IAMG isanalyzed using FEM and shown in Fig. 11. As a largerPM surface area contributes to increases in the total fluxin the airgap, the pull-out torque of the proposed IAMGis significantly improved when compared with its originalsurface-mounted configuration.

8001804 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014

Fig. 9. Pull-out torque waveform.

Fig. 10. IAMG with flux concentration.

Fig. 11. Torque-angle curve of the IAMG with flux concentration.

Table III shows the key parameters of the proposed IAMG,a magnetic bevel gear proposed in [8] and a mechanicalbevel gear. Although the gear ratios are different, the torquedensity of the proposed IAMG is expected to be higher thanthat of the conventional magnetic bevel gear. Considering thatits torque density can be further improved by decreasing theairgap length, using PM with higher remanence and shorteningthe length of the curved iron segments, the IAMG should becomparable to the mechanical bevel gear in the table.

TABLE III

COMPARISON OF IAMGS AND BEVEL GEARS

V. CONCLUSION

By exploiting the flux modulation effect, a novel MG withintersecting axes is proposed. Using FEM, the effects ofdimensional parameters on the torque performance are inves-tigated based on parametric analysis. The configuration withflux modulation is also studied. A torque density comparablewith a mechanical bevel gear can be achieved. The IAMGis expected to be a promising substitute to correspondingconventional gears like skew gear, bevel gear, or hypoid gear.It can also be conveniently combined with an electric machinefor some special applications.

ACKNOWLEDGMENT

This work was supported by the Hong Kong PolytechnicUniversity, Hong Kong, under Grant G-YM14 and GrantG-YM15.

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