a novel high-performance magnetic gear

2844IEEE TRANSACTIONS ON MAGNETICS,VOL.37,NO.4,JULY2001

A Novel High-Performance Magnetic Gear

K.Atallah and D.Howe

Abstract—Mechanical gearboxes are used extensively to match

the operating speed of prime-movers to the requirements of their

loads.Although,high system torque densities can be achieved,

gear lubrication and cooling are often required,whilst noise,

vibration and reliability can be significant issues.The paper

describes the design and performance of a magnetic gear,which

employs rare-earth magnets,which simulation studies have shown

to have a transmitted torque density exceeding100kNm/m3.

Index Terms—Gearboxes,gears,magnetic circuits.

I.I NTRODUCTION

M ECHANICAL gearboxes are used extensively to match

the operating speed of prime-movers to the require-

ments of their loads,both for increasing the rotational speed

(e.g.,windmill-powered generators)and decreasing the speed

(e.g.,electric-ship propulsion),since it is usually more cost and

weight effective to employ a high-speed electrical machine to-

gether with a gearbox to transform speed and torque.However,

although,high system torque densities can then be achieved,

gear lubrication and cooling are often required,whilst noise,

vibration and reliability can be significant issues.In contrast,

magnetic gearing has received relatively little attention,prob-

ably because of the relative complexity and poor torque density

of the magnetic circuits which have been proposed[1],[2].

For applications in which mechanical gearing cannot be ac-

commodated,direct electrical drives must be employed.Of the

various competing direct drive machine topologies,permanent

magnet transverse-field machines offer potentially the highest

torque densities[3],[4],typically in the range40–80kNm/m

is the number of pole-pairs on permanent magnet rotor,

and

,is different to the velocity of the rotor which carries the

permanent magnets.Therefore,in order to transmit torque at a

different speed,the number of pole-pairs of the other permanent

magnet rotor must be equal to the number of pole-pairs of a

space harmonic for which

ATALLAH AND HOWE:A NOVEL HIGH-PERFORMANCE MAGNETIC GEAR2845 TABLE I

P ARAMETERS OF M AGNETIC G EAR

Fig.2.V ariation of radial flux density,due to high speed permanent magnet

rotor,in airgap adjacent to low speed rotor.

III.S IMULATION S TUDIES

Two-dimensional magnetostatic finite element method was

employed for the simulation studies.Table I gives the param-

eters which have been assumed for the magnetic gear,shown in

Fig.1.Both the high and low speed rotors being equipped with

sintered NdFeB permanent magnets,and the steel pole-pieces

and the rotors cores being made of silicon steel laminations.

A.Flux Density Distributions

Fig.2shows the variation of the radial component of flux

density due to the high speed rotor permanent magnets in the

airgap adjacent to the low speed rotor,whilst Fig.3shows the

corresponding space harmonic spectrum.It can be seen that the

presence of the steel pole-pieces results in a number of asyn-

chronous,viz.,space harmonics,the largest of which is

the22pole-pair space harmonic,

2846IEEE TRANSACTIONS ON MAGNETICS,VOL.37,NO.4,JULY

2001

Fig. 6.Variation of radial component of resultant fluxdensity in airgap adjacent to high speed

rotor.

Fig.7.Variation of radial component of resultant flux density in airgap adjacent to low speed

rotor.

Fig.8.Variation of maximum torque on low and high speed rotors.

Figs.6and 7show the variation of the radial component of the resultant flux density,due to both permanent magnet ro-tors,in the airgaps adjacent to the high and low speed rotors,respectively.

B.Torque Transmission

Fig.8shows the variation of the maximum torque which is exerted on the high and low speed rotors as they rotate.As can be seen,a transmitted torque density exceeding 100

kNm/m

,20

kNm/m being

typical

Fig.9.Variation of system torque density with gear ratio for different electrical machine torque densities (magnetic gear torque density is 100kNm/m

can be achieved.It has also been shown that the system (i.e.,magnetic gear and electrical machine)torque density can be comparable or better than that of a transverse-field machine.

R EFERENCES

[1] D.E.Hesmondhalgh and D.Tipping,“A multielement magnetic gear,”

IEE Proceedings ,pt.B,vol.127,pp.129–138,1980.

[2]K.Tsurumoto and S.Kikushi,“A new magnetic gear using permanent

magnet,”IEEE Transactions on Magnetics ,vol.MAG-23,no.5,pp.3622–3624,1987.

[3]M.R.Harris,G.H.Pajooman,and S.M.Abu Sharkh,“Performance and

design optimization of electric motors with heteropolar surface magnets and homopolar windings,”IEE Proceedings ,pt.B,vol.143,no.6,pp.429–436,1996.

[4]H.Weh,“Transverse flux (TF)machines in drive and generator applica-tions,”in Proceedings of IEEE/KTH Stockholm Power Technology Con-ference ,Stockholm,Sweden,1995,pp.75–80.