a novel ‘‘pseudo’’ directdrive brushless permanent magnet machine

7/24/2019 A Novel Pseudo Directdrive Brushless Permanent Magnet Machine

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008 4349

A Novel Pseudo Direct-Drive Brushless Permanent Magnet Machine

Kais Atallah , Jan Rens , Smail Mezani , and David Howe

Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 3JD, U.K.

Magnomatics Ltd, The Sheffield Bioincubator, Sheffield S3 7RD, U.K.

GREEN-UHP, Facult des Sciences, 54506 Vandoeuvre ls Nancy, France

For low-speed electrical machine applications, it is usually weight/size and cost effective to employ a high-speed machine together witha mechanical gearbox. However, the disadvantages associated with magnetic gearboxes can be overcome by mechanically and magnet-ically integrating a magnetic gear and a permanent magnet brushless machine, to create a pseudo direct-drive machine. It is shownthat a torque density in excess of 60 kNm/m can then be achieved, at a power factor in excess of 0.9.

Index TermsElectric machines, magnetic gears, permanent magnet.

I. INTRODUCTION

FOR LOW-SPEED electrical machine applications, it

is usually weight/size and cost effective to employ a

high-speed machine together with a mechanical gearbox.

However, in many instances, the disadvantages associatedwith mechanical gearboxes, such as acoustic noise and

mechanical vibration, the need for lubrication, concerns

regarding reliability and maintenance requirements, make

direct-drive solutions more functionally and/or economically

attractive. Liquid-cooled permanent magnet (PM) brushless

machines exhibit relatively high torque densities, typically

being 30 kNm/m for radial-field and 50 kNm/m for

transverse-field topologies, respectively [1][3]. However,

although transverse-field machines exhibit the highest torque

density, since their power factor is very low [4], typically

ranging from 0.3 to 0.5, the required inverter/converter VA

rating is a factor of 2 3 times higher than that for an equivalent

conventional brushless machine. This results in a significant

cost penalty, which is limiting the take-up of transverse-field

machine technology.

Recent advances in magnetic gears have led to their torque

transmission capability becoming competitive to that of me-

chanical gears, whilst they offer significant operational advan-

tages [5]. Further, there are various ways in which a magnetic

gear may be combined with an electrical machine to realize a

high torque density pseudo direct-drive. Irrespective of the

machine technology, the simplest method is simply to mechani-

cally couple the output shaft of the machine to the input shaft of

the magnetic gear, as illustrated in Fig. 1, or to incorporate the

electrical machine within the bore of a magnetic gear [6].This paper describes a radically different approach to creating

a pseudo direct-drive machine by combining a magnetic gear

and electrical machine both mechanically and magnetically [7].

It will be shown that a torque density in excess of 60 kNm/m

can then be achieved from an air-cooled machine, while the

power factor can be larger than 0.9.

Digital Object Identifier 10.1109/TMAG.2008.2001509

Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.

Fig. 1. Mechanically coupled magnetic gear and electrical machine.

II. PSEUDO DIRECT-DRIVEPERMANENTMAGNET

BRUSHLESS MACHINE

A. Principle of Operation

Fig. 2 shows a schematic of the proposed magnetically and

mechanically coupled magnetic gear and permanent magnet

brushless machine, while Fig. 3 shows the radial flux density

waveform due to the pole-pair high-speed permanent

magnet rotor in the airgap adjacent to the stationary permanent

magnets, both with and without the ferromagnetic pole-pieces.

Fig. 4 shows the associated harmonic spectra. It can be seen that

the introduction of the ferromagnetic pole-pieces re-sults in a dominant 21 pole-pair asynchronous space harmonic

field which interacts with the pole-pair stationary

permanent magnets to transmit torque from the high-speed

rotor to the low-speed rotor, the magnetic gear ratio being

, and vice versa, while the 2 pole-pair

fundamental component interacts with the stator winding to

produce electromagnetic torque.

Similarly, Fig. 5 shows the radial flux density waveform due

to the stationary permanent magnets in the airgap adjacent to

the high-speed rotor permanent magnets, both with and without

the ferromagnetic pole-pieces, while Fig. 6 shows the associ-

ated harmonic spectra. It can be seen that the introduction of

0018-9464/$25.00 2008 IEEE


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4350 IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008

Fig. 2. Magnetically and mechanically coupled magnetic gear and permanent

magnet brushless machine or pseudo direct-drive machine. (Active diameter:178mm; active length: 75 mm).(a) Radial cross-section.(b) Axial cross-section.

Fig. 3. Radialflux density waveformsdue to permanentmagnetson high-speedrotor in airgap adjacent to stationary permanent magnets.

the ferromagnetic pole-pieces now results in a 2 pole-pair asyn-

chronous field harmonic which interacts with the 2 pole-pairs

high-speed rotor.

B. Electromagnetic Torque

In the brushless ac mode of operation, the electromagnetictorque which results from the interaction of the high-speed rotor

Fig. 4. Harmonic spectra of radial flux density waveforms due to permanentmagnets on high-speed rotorin airgap adjacent to stationary permanent magnets.

Fig. 5. Radial flux density waveforms due to stationary permanent magnets inairgap adjacent to permanent magnets of high-speed rotor.

Fig. 6. Harmonicspectraof radialflux density waveforms dueto stationary per-manent magnets in airgap adjacent to permanent magnets of high-speed rotor.

and the stator winding is similar to that of a conventional sur-

face-mounted permanent magnet machine, and is given by

(1)

where isthestator borediameter, isthepeak fundamentalairgap flux density, is the active length of the machine,


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ATALLAHet al.: A NOVEL PSEUDO DIRECT-DRIVE BRUSHLESS PERMANENT MAGNET MACHINE 4351

Fig. 7. Variation of output torque with angular rotor position.

is the rms electric loading, and is the winding factor. Since

the output torque, , of the low-speed rotor is given by

(2)

then, from (1) and (2)

(3)

From (3), it can be seen that the effective peak fundamental

airgap flux density is now , and for the pseudo di-

rect-drive machine shown in Fig. 2, this is equivalent to 6.4 T,

which is more than 5 times the remanence of the NdFeB perma-

nent magnets which are used, for which the remanence1.25 T and the recoil permeability .

C. Cogging Torque

The cogging torque which manifests itself as torque ripple

on the output rotor results from interactions between the ferro-

magnetic pole-pieces and both the high-speed rotor permanent

magnets and the stationary permanent magnets. Qualitatively,

the cogging torque is determined by the cogging torque factor

[8], , which has been shown to be applicable to mag-

netic gears [5], where is the smallest common multiple be-

tween the number of poles ( or ) and the number of

ferromagnetic pole-pieces .For the pseudo direct-drive shown in Fig. 2, ,

, , and the cogging torque factor is 1. Hence, the

cogging torque is inherently small. Fig. 7 shows the variation

of the output torque with the angular rotor position when the

machine is on full-load. It can be seen that the torque ripple is

less than 0.7% of the rated full load torque.

D. Electromechanical Modeling

For a conventional permanent magnet brushless machine, the

motion of the rotor is governed by

(4)

Fig. 8. Variation of output speed with time.

where is the electromagnetic torque, resulting from the inter-

action of the stator winding and the rotor, is the load torque,

is the angular position of the load, and is the combined

inertia of the rotor and the load.

For the pseudo direct-drive machine shown in Fig. 2, since

the torque is transmitted magnetically from the high-speed rotor

to the output rotor (low-speed), the equations which govern the

motion of the high-speed and low-speed rotors are

(5)

and

(6)

where is the angular position of the high-speed rotor,

is the maximum torque which can be produced by the magnetic

gear, and is the inertia of the high-speed rotor.

Fig. 8 compares the speed of a load having a total inertia

when driven by the pseudo direct-drive machine shown in

Fig. 2 with that when driven by a conventional brushless ma-

chine having a similar torque rating. For both machines, the

electromagnetic torque exhibits a ripple of 140% peak-peak,

and the load torque has viscous, windage and constant com-ponents which are equal at the load speed of 25 rad/s. It can

be seen that, in contrast to the conventional machine, the in-

fluence of the electromagnetic torque ripple in the pseudo di-

rect-drive machine, is almost totally filtered out, and is not trans-

mitted to the load. It is also worth noting that a torque ripple

of 140% peak-peak will correspond to the torque ripple caused

by a short-circuited phase in a 3-phase fault-tolerant machine.

Therefore, since a pseudo direct-drive machine can also be de-

signed to be fault-tolerant, the influence of a phase open-circuit

or a phase short-circuit on the torque ripple seen by the load will

not be significant, and torque ripple minimization techniques [9]

which may be required for conventional fault-tolerant machines,

may not be necessary for a pseudo direct-drive fault-tolerantmachine.


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4352 IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008

Fig. 9. Pseudo direct-drive machine on test-bed.

Fig. 10. Variation of low-speed torque with average rms stator current density.

III. DEMONSTRATORPSEUDO DIRECT-DRIVEBRUSHLESS

PERMANENTMAGNETMACHINE

The pseudo direct-drive brushless permanent magnet ma-

chine shown in Fig. 2 has been prototyped and tested. Fig. 9

shows the machine on the test-bed, while Fig. 10 compares the

variation of the predicted geared electromagnetic output torque

(at 20 C) and the measured output torque with the average rms

current density in the stator slots. It can be seen that due to fric-

tion in the bearings, windage and temperature rise of the perma-

nent magnets, the output torque is about 5% lower than the pre-

dicted electromagnetic torque. It can also be seen that a torquedensity in excess of 60 kNm/m can be achieved with an av-

erage current density which is less than 2 Arms/mm (which is

significantly lower than the current density which conventional

machine designs would employ), while the power factor can

be in excess of 0.9 (which is significantly higher than could be

achieved with transverse-field machines). Thus, the volumetric

torque density of a naturally air-cooled pseudo direct-drive ma-

chine is comparable with, or even higher, than that of a trans-

verse-field machine, while the volt-ampere rating of the power

electronic converter is almost identical to that for a similarly

rated conventional permanent magnet brushless machine.

IV. CONCLUSION

A novel method of coupling a magnetic gear and a perma-

nent magnet brushless machine, both mechanically and mag-

netically, to realize a pseudo direct-drive machine has been

presented. It has been shown that a torque density in excess of

60 kNm/m can be achieved from a naturally air-cooled ma-

chine, at a power factor of 0.9 or higher.

ACKNOWLEDGMENT

This work was supported by the U.K. Engineering and

Physical Science Research Council, EPSRC, under Grant

GR/S70685.

REFERENCES

[1] M. R. Harris, G. H. Pajooman, and S. M. Abu Sharkh, Compar-ison of alternative topologies for VRPM (transverse-flux) electricalmachines, in Dig, IEE Coll. New Topologies PM Mach., 1997, pp.2/12/7.

[2] M. R. Harris, G. H. Pajooman, and S. M. Abu Sharkh, Performanceand design optimisation of electric motors with heteropolar surfacemagnets and homopolar windings, IEE Proc. Part. B, vol. 143, no.6, pp. 429436, 1997.

[3] A. J. Mitcham, Transverse-flux motors for electric propulsion ofships, inDig. IEE Coll. New Topologies PM Mach., 1997,pp. 3/12/6.

[4] M. R. Harris, G. H. Pajooman, and S. M. Abu Sharkh, The problemof power factor in VRPM (transverse-flux) machines, inProc. 8th IEE

EMD Conf., 1997, pp. 386390.[5] K. Atallah, S. D. Calverley, and D. Howe, Design, analysis and real-

isation of a high-performance magnetic gear, IEE Proc. Elec. PowerAppl., vol. 151, pp. 135143, 2004.

[6] K. T. Chau,Z. Dong, J. Z. Jiang,L. Chunhua,and Z. Yuejin,Design ofa magnetic-geared outer-rotor permanent-magnet brushless motor forelectric vehicles,IEEE Trans. Magn., vol. 43, no. 6, pp. 25042506,Jun. 2007.

[7] Univ. Sheffield, Sheffield, U.K., Electrical machines, PCT/GB2007/001456.

[8] Z. Q. Zhu and D. Howe, Influence of design parameters on coggingtorque in permanent magnet machines, IEEE Trans. Magn., vol. 15,no. 5, pp. 407412, May 2000.

[9] K. Atallah, J. B. Wang, and D. Howe, Torque-ripple minimisationin modular permanent magnet brushless machines, IEEE Trans. Ind.

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Manuscript received February 27, 2008. Current version published December17, 2008. Corresponding author: K. Atallah (e-mail: [email protected]).

a novel ‘‘pseudo’’ directdrive brushless permanent magnet machine

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