Proceedings of the 2008 International Conference on Electrical Machines Paper ID 1455

A New PM Machine Topology for Low-Speed, High-Torque Drives

Kais Atallah!, Stuart Calverley!, Richard ClartC, Jan Rens2, David Howe!

I Dept of Electronic and Electrical Engineering, The University of Sheffield, Mappin St, Sheffield SI 3JD, UK 2Magnomatics Ltd, The Sheffield Bioincubator, 40 Leavygreave Road, Sheffield S3 7RD

Tel: (44) 1142225195, Fax: (44) 1142225196 e-mail: k.atallah@sheffield.ac.uk

Abstract - The paper describes a completely new topology for a low-speed, high-torque permanent brushless magnet machine. Despite being naturally air-cooled, it has a significantly higher torque density than a liquid-cooled transverse-flux machine, whilst its power factor is similar to that of a conventional permanent magnet brushless machine. The high torque capability and low loss density are achieved by combining the actions of a speed reducing magnetic gear and a high speed PM brush less machine within a highly integrated magnetic circuit. In this way, the magnetic limit of the machine is reached before its thermal limit. The principle of operation of such a 'pseudo' direct-drive machine is described, and measured results from a prototype machine are presented.

I. INTRODUCTION

Applications for low-speed, high-torque electrical machines range from wind-turbine powered generators to ship propulsion systems. To date, however, it has usually been weight/size and cost-effective to employ a high-speed machine together with a mechanical gearbox. Nevertheless, because of the disadvantages associated with mechanical gearboxes, such as the need for lubrication, wear between contacting surfaces, noise and vibration, etc, as well as concerns regarding their reliability, direct-drive solutions would generally be more functionally attractive. Of the various competing electrical machine technologies, transverse-flux machines, although having the highest torque density, exhibit a very low power factor so that the required converter VA rating is considerably higher than that required for a conventional PM brushless machine, which results in a significant cost penalty. However, by integrating the speed reducing action of a high performance magnetic gear [1] with the torque producing action of a conventional PM brushless machine, both the torque density and the power factor can be very much higher than for a transverse-flux machine. The operating principle of this patented technology [2] will be described, and the measured performance of a prototype 'pseudo' direct-drive machine will be compared with predictions.

978-1-4244-1736-0/08/$25.00 2008 IEEE

II. MAGNETIC GEAR

Fig. 1 shows a schematic of a magnetic gear, which comprises 3 components, viz. an inner high-speed permanent magnet rotor, an array of soft magnetic pole-pieces and an outer permanent magnet cylinder. Either the array of pole-pieces or the outer cylinder may be mechanically earthed, the other then forming the low-speed rotor. The soft magnetic pole-pieces serve to modulate the field produced by the inner and outer arrays of the magnets, such that each array results in a dominant asynchronous space harmonic field in the airgap adjacent to the other array. In this way, the field produced by the low pole number inner high-speed rotor is modulated into an asynchronously rotating field having the same number of poles as the outer cylinder, and vice-versa to facilitate torque transmission.

Fig. I. Schematic of magnetic gear

The principle of operation is illustrated in Fig. 2 for a magnetic gear in which the inner high-speed rotor has 4 pole-pairs, 27 mechanically earthed soft magnetic pole-pieces and 23 pole pairs on the outer low-speed rotor. The resulting gear ratio is 5.75:1, and the achievable torque transmission capability when rare-earth permanent magnets are employed is -70kNmlm3.

1

Proceedings of the 2008 International Conference on Electrical Machines

.......... illTI1ill "'i " i. U II

:::,..-, ._' ....... _- ':D f'

" ........... "' ...... 01 ...

(a) Field due to 4 pole-pair high-speed rotor alone

(b) field of high-speed rotor modulated by 27 pole-pieces

... ~

. " .'" .. 4:

#

... , ..

4 pole-palr high-speed rotor

5.75:1 gear ratio

(C) 5.75:1 magnetic gear with 23 pole-pair low-speed rotor

Fig. 2. Principle of operation of magnetic gear

The simplest way in which a magnetic gear may be combined with a high speed electrical machine is simply to couple the output shaft of the machine to the high-speed rotor of the magnetic gear, as illustrated in Fig. 3(a). Fig. 3(b) shows how the resultant system torque density varies as a function of the ratio of the magnetic gear, when the torque density of the electrical machine is 10kNmlm3, 20kNmlm3 and 30kNmlm3, these being typical values for naturally air-cooled, forced air-cooled and liquid cooled permanent magnet brushless machines, respectively. As will be seen, system torque densities ranging from -32kNmlm3 to 51kNmlm3 can be achieved for a gear ratio of 5.75:1. However, a significantly higher torque density can be achieved by combining the magnetic gear and the electrical machine, both mechanically and magnetically, into a single 'pseudo' direct-drive machine.

2

Fig. 3. Mechanically couple magnetic gear and electrical machine

III. 'PSEUDO' DIRECT-DRIVE MACHINE

Fig 4 shows a schematic of a 'pseudo' direct-drive machine which is comprised of 3 main components, viz. an inner high-speed permanent magnet rotor, a low-speed rotor equipped with soft magnetic pole-pieces, and a stator in which each tooth carries a coil and on which magnets are mounted at the inner bore.

Fig. 4. Schematic of 'pseudo' direct-drive machine

Fig 5 shows cross-sections of a 'pseudo' direct drive machine having a magnetic gear and a radial-field PM brushless machine that are mechanically and magnetically coupled. It comprises a 2 pole-pair high-speed permanent magnet rotor, a low-speed rotor with 23 soft magnetic pole-pieces, and 21 pole-pair outer permanent magnets at the stator, the stator coils being interconnected to form a balanced 3-phase winding.

Radial

.. .-, ...

-.,

-

z ..... , ... .,....

..... -

a.. ................ ...........

Proceedings of the 2008 International Conference on Electrical Machines

Axial

Fig. 5. Cross-sections of 'pseudo' direct drive machine

Fig 6 shows the radial flux density distribution due to the high-speed rotor permanent magnets, together with the associated harmonic spectra, in the airgap adjacent to the magnets on the stator bore, both with and without the soft magnetic pole-pieces. As will be seen, the pole-pieces on the low-speed rotor result in a dominant 21 pole-pair asynchronous space harmonic field which interacts with the 21 pole-pair stationary permanent magnets to achieve gearing, the gear ratio for the machine being 11.5: I. At the same time, however, a 2 pole-pair fundamental field component exists which enables torque and power to be transmitted from the stator to the high-speed rotor.

0 ..

0 ...

0.40 .

e 0.20

f 0 .. :leO I ......

.....

-0.10 '

......

_ ....... -

0.70 -

0.00

0.50 -" E ':

r" gU.30 -It

020 -

0.10 -

0.00 o 2 4 e a 10 12 M 16 18 ~ ~ ~ a a ~

..........

Fig. 6. Flux density waveforms, and harmonic spectra, due to magnets on high-speed rotor.

3

Similarly, Fig 7 shows the radial flux density waveform, together with the associated harmonic spectra due to the stationary permanent magnets in the airgap adjacent to the high-speed rotor permanent magnets, both with and without the soft pole-pieces. The introduction of the soft magnetic pole-pieces now result in a 2 pole-pair asynchronous field harmonic which interacts with the 2 pole-pair high-speed rotor.

0.04

0.04

0.03

E 0.03 ~ J 0.02

~ 0.02 . 0.01

0.01

Angle (mech ......... ..

o 2 4 e 8 10 12 14 16 18 20 22 24 211 28 30 PoIe ..... I,.

Fig. 7. Flux density waveforms, and harmonic spectra, due to magnets on stator.

In the brushless ac mode of operation, the torque which results from the interaction of the high-speed rotor and the stator winding is similar to that of a conventional surface-mounted permanent magnet brushless machine, and is given by:

(1) k nfi;la B Th = W ------;;:Qrms 1 2,,2

where Ds is the stator bore diameter, BJ is the peak fundamental airgap flux density, la is the active length of the machine, Qrms is the rms electrical loading, and lew is the winding factor. Since the output torque, To of the low-speed rotor is given by:

(2)

Where Gr is the gear ratio, then, from equations (1) and (2);

Proceedings of the 2008 International Conference on Electrical Machines (3)

From equation (3), it can be seen that the effective peak fundamental airgap flux density is now (BjxGr), and for the 'pseudo' direct-drive machine shown in Fig. 5, this is equivalent to 6.4T, which is more than 5 times the remanence of the NdFeB permanent magnets which are used, for which the Br = 1.25T and 11 = 1.05.

IV. PROTOTYPE 'PSEUDO' DIRECT-DRIVE MACHINE

Fig 8 shows a prototype of the 'pseudo' direct-drive machine of Fig 5 on the test-bed, whilst Fig 9 compares the variation of the predicted geared electromagnetic torque and the measured output torque with the rms current density. It can be seen that, although the maximum current density is less than 2Arms/mm2 a torque density in excess of 60kNmlm3 is achieved with natural cooling at a power factor in excess of 0.9. In other words, whilst the stator winding is responsible for torque being developed by the high-speed rotor, since this is amplified by the magnetic gear action, the magnetic limit of the machine is reached before the thermal limit.

Fig. 8. Prototype 'pseudo' direct-drive

4

o Q.2II 0.5 0.75 1.25 1.5 1.75 2

Fig. 9. Variation of output torque and PF with stator winding current density

V. CONCLUSIONS

A unique patented topology for a low-speed, high-torque electrical machine has been developed, and its performance capabilities have been validated experimentally. It exhibits the highest torque density of any known electrical machine technology, whilst at the same time having a high power factor, and has enormous application potential. There is also potential to further enhance the performance capabilities of pseudo direct-drive machines.

ACKNOWLEDGEMENT

The authors would like to acknowledge the support of the UK Engineering and Physical Science Research Council, EPSRC, for the award of a research grant, Reference No. GRlS70685.

1.

2.

REFERENCES

K. Atallah, S. Calverley, and D. Howe, 'Design analysis and realisation of a high-performance magnetic gear', lEE Proc. - Elec. Power Appl., 151, 2004, pp. 135-143 'Pseudo direct drive electrical machines' , Application No. PCT/GB2007/001456.