10 Magnews Winter 2007

Introduction

This work describes how segmental rotors can be used to advantage in switched reluctance motors, with signi� cant increases in torque density. The presentation discusses the design of these machines and presents results for a series of working prototypes employing this construction.

Fundamentals of Operation

Conventional SRMs are built with a simple toothed rotor. This arrangement permits a very high air-gap ! ux density, but the ratio of tooth pitch to pole pitch must be maintained well below 0.5 in order to prevent excessive inductance in the unaligned position. Each phase can only be excited for a maximum of one half of a cycle, so consequently only between one sixth and one quarter of the machine air-gap carries magnetic ! ux at any one time. This poor magnetic utilisation of the machine detrimentally affects the torque capability of the machine.

To demonstrate the fundamentals of operation, � gure 1 shows a linearised version of an alternative single phase motor, but instead of a toothed rotor the rotor is composed of a series of separate segments. In the aligned position the rotor segments magnetically short the stator slots, producing a high permeance, whilst in the unaligned position the segments are located over the tooth tips so that the unaligned permeance is limited by the slot opening width. In this example the body of the stator tooth has been increased to 60% of the pole pitch without producing an excessively large unaligned permeance. Consequently, with segmented rotor machines it is possible to magnetically utilise a greater proportion of the air-gap, providing it is possible to maintain the electric loading.

A segmented rotor SRM differs from a toothed rotor SRM in that it only allows short ! ux loops to occur. Any one magnetic ! ux path only ever encloses a single stator slot, and so it is more appropriate to think in terms of the permeance variation per slot, rather than the permeance variation per tooth which is evaluated in toothed SRMs. Because the permeance variation per slot varies most strongly it is sensible to restrict the design so that a single slot only contains the windings of a single phase. By doing so the rate of change of phase self inductance with respect to position is maximised. [1-3]

[This paper was presented at the UKMAG one-day seminar Modelling & Design of Electromagnetic Actuators and Machines, co-sponsored by IET (Institute of Engineering and Technology), held at the University of Newcastle upon Tyne on 18 October 2007]

(a) aligned position

(b) unaligned position

Figure 1 A simple two slot model of a single phase segmental rotor switched reluctance machine, illustrating magnetic � ux plots in the

aligned and unaligned positions.

Three Phase Design Type A

The most obvious approach to a three phase design which meets the above criteria is to employ fully pitched concentrated windings, resulting in a machine of the form illustrated in Figure 2. The design bears a super� cial resemblance to a synchronous reluctance motor. However, the machine is doubly salient, with semi-open stator slots and excited with unidirectional currents. It is therefore very much a switchedreluctance, rather than a synchronous reluctance machine.

Figure 2 12-8 Multi tooth winding 12-8 machine shown with one phase excited in the aligned and unaligned positions

Figure 3 Aligned and unaligned position � ux/MMF curves for both the segmental motor (shown red) and a conventional toothed SRM

of the same size (shown blue)

Because each coil now links the ! ux of two teeth, it now has double the ! ux-linkage of an ordinary SRM, as shown in Figures 2 and 3, but does have long end-windings

Segmented Rotor Switch Reluctance DrivesBarrie Mecrow, University of Newcastle upon Tyne, School of Elec, Elec & Computer Engineering, barrie.mecrow@newcastle.ac.uk

Magnews Winter 2007 11

Segmented Rotor Switch Reluctance Drives contd...

Figure 4 The stator of the 12-8 multi tooth winding design

Figure 5 The 12-8 multi tooth winding machine, showing measured phase voltage, current and the � ux-linkage/current locus whilst de-livering 18.1 Nm at 577 revs per minute. Switch on angle 20 electrical degrees after unaligned position. Conduction angle 120 electrical degrees

This machine was found to be capable of over 40% more torque than a conventional SRM of the same dimensions.

Three Phase Design Type B

Design type B arose in recognition of the disadvantage of fully pitched windings. Short-pitched windings are smaller, simpler to wind and are more suited to fault tolerant applications.

In order to meet the requirement that only the conductors of a single phase lie in any one slot then only every other tooth is wound. Figure 6 shows magnetic ! ux plots for a 12-10 prototype machine with one phase conducting, illustrating the aligned and unaligned positions. Current ! owing in a single phase excites two adjacent slots, so to maximise torque production it is necessary for the permeance of these two slots to vary in phase. This is accomplished by ensuring that the slot pitch of the wound teeth is equal to one complete rotor pole pitch, ie 36 mechanical degrees in the 12-10 machine.

Figure 6 12-10 Single tooth winding machine shown with one phase excited in the aligned and unaligned positions

Figure 7 shows the stator of such a machine, whilst Figure 8 shows the rotor and Figure 9 static torque characteristics.

As with machine Type A, this machine has been run from a series of asymmetric half bridges, with example results shown in Figure 10. Torque density is similar to design A, without the long endwindings.

Figure 4 shows a demonstrator of this type, with Figure 5 showing examples of measured voltage, current and ! ux-linkage curves of the rotating machine.

Figure 7 The stator of the 12-8 Multi tooth winding design

Figure 8 The segmental rotor of the 12-10 single tooth winding design

Figure 9 Static torque characteristics for the 12-10 machine

12 Magnews Winter 2007

Segmented Rotor Switch Reluctance Drives contd...

Figure 10 The 12-10 single tooth winding machine, showing meas-ured phase voltage, current and � ux-linkage locus whilst delivering 33 Nm at 900 revs per minute. Switch on angle 9 electrical degrees after unaligned position. Conduction angle 167 electrical degrees

Design Type C

The design of the previous section is particularly suitable for use in fault tolerant applications because there is no electrical or magnetic coupling between phases.

The concept has been applied to an aeroengine starter-generator, with production of a demonstrator for a shaft-line-embedded high-pressure shaft electrical machine. As well as providing a generating function it would perform the starting function for the engine, replacoing the current air starting system.

The machine is an outer rotor construction for mechanical reasons and is shown in Figs 11-13.

Figure 11 18-15 outer rotating segmented geometry

Figure 12 Wound stator

Figure 13 Rotor segments and support structure

(a) aligned position

(b) unaligned position

Figure 14 Magnetic � ux distribution in the prototype machine

Figures 15 and 16 show the machine under construction. For running tests the machine was connected to a brake via a belt drive to permit operation at speeds up to 17,000 revs per minute. The � eld was separately excited via a low voltage dc power supply and the armature via a single phase voltage fed inverter, fed from 240V ac, as shown in Figure 17.

Conclusions

It is possible to replace the simple toothed rotor design of a switched reluctance motor with one of segmental construction. Although the rotor is more complex to construct, there can be major advantages in terms of torque density. A range of different prototypes have been designed, built and tested successfully.

Figure 15 The stator assembly

Design Type D � 2 Phase Flux Switching

The concept of segmental rotors is not constrained to three phase machines. When used in a two phase motor with single tooth windings the two phases are mutually coupled and the result is a � ux switching motor [4] without any need for overlapping windings.

The machine has an even number of teeth, with adjacent teeth wound with a dc � eld winding and an ac armature winding. Figure 14 shows a � ux plot for a prototype machine in the aligned position with both � eld and armature excited. As the rotor rotates the segments bridge the gaps between adjacent teeth and give an ac � ux linking each armature coil, inducing a back emf.

Magnews Winter 2007 13

Segmented Rotor Switch Reluctance Drives contd...

Figure 16 The rotor of the 2 phase machine under construction

References

[1] Mecrow, BC, Finch, JW, El-Kharashi, EA and Jack, AG, The Design of Switched Reluctance Motors with Segmental Rotors, 15th International Conference on Electrical Machines, Brugge, Belgium, August 25-28, 2002, paper 336

[2] Mecrow, BC, El-Kharashi, EA, Finch, JW, and Jack, AG, Segmental Rotor Switched Reluctance Motors with Single Tooth Windings, IEE Proceedings, Electric Power Applications, vol 150, no 5, September 2003, pp 591-599

[3] Mecrow BC, El-Kharashi EA, Finch JW, Jack AG, Performance Evaluation of Switched Reluctance Motors with Segmental Rotors, IEEE Trans on Energy Conversion, Vol 19, No 4, Dec 2004, pp 679-686

[4] Pollock, C, Wallace, M, The Flux Switching Motor, a DC Motor without Magnets or Brushes, IEEE Industrial Applications Conference, vol.3, pp 1980-1987, Oct 1999

barrie.mecrow@newcastle.ac.uk Figure 17 Electrical connection

This is a follow-on article to �Changing Colour by Magnetic Field�, which appeared in the autumn 2007 issue of Magnews

A liquid that changes colour when exposed to a magnetic � eld could cheaply replace the colour components in

conventional LCD monitors, claim US researchers.

The liquid contains tiny iron oxide particles coated with plastic. It is cheap and easy to make, and could also be used in � exible, rewritable, electronic paper, the researchers say. Yadong Yin and colleagues at the Department of Chemistry at University of California, Riverside, US, created the liquid by coating particles of iron oxide � each about 100 nanometres in diameter � with a polymer and suspending the mixture in water. The plastic coating means that each particle has a highly charged surface. Because the individual particles have the same charge, they repel each other in the solution. However, since iron oxide is also magnetic the particles will come together when exposed to a magnetic � eld.

The opposing forces of electrostatic repulsion and magnetic attraction result in the particles arranging themselves into an ordered structure, known as a colloidal �photonic crystal�. The colloidal crystal re� ects light because the spacing between neighbouring particles in the structure is equivalent to the wavelength of light. Also, tuning the spacing slightly alters the exact wavelength, or colour, of light that is re� ected. This can easily be done by varying the strength of the magnetic � eld applied to the crystal. The researchers did this in experiments simply by moving a magnet further away from, or closer to, the liquid. The crystal re� ects brilliant colours from red to violet as the magnetic � eld strength increases (see image, right). But, when the � eld is switched off, the crystal reverts back to its original brownish colour.

�This is the � rst report of a photonic crystal that is fully tuneable in the visible range of the electromagnetic spectrum,� says Yin. �We see applications in various areas, including sensors, optical switches and � exible colour displays,� he told New Scientist. �For example, the system can be used to make extra-large displays or posters to replace expensive LCD monitors. And, because the colour is based on re� ection, it is better for outdoor applications than current LCD displays that perform poorly in direct sunlight.� A colour display would contain millions of small pixels made from the photonic crystals,

Chameleon l iquid could outshine LCDs

explains Yin, with each pixel being assigned a different colour using a distinct magnetic � eld. The crystals could also be sandwiched between two plastic sheets to form rewritable paper with a magnetic � eld acting as the �pen�, he adds.

This article � rst appeared on the New Scientist website

As part of its role in securing the future vitality of the magnet-ics community, the UK Magnetics Society has introduced a competitive bursary scheme to assist postgraduate students to attend international conferences. A total of 5 x £100 bur-saries per year are available to support attendance at confer-ences of international standing in the UK or abroad, in subject areas which re� ect the interests of the UK Magnetics Society membership. The award of a bursary is intended to acknowl-edge the student�s contribution to the magnetics community and act as a catalyst for attracting additional support.

The following eligibility criteria will be applied in selecting students:

� restricted to full-time postgraduate students, ie not con-tract re search staff

� restricted to students from institutions which are members of the UK Magnetics Society

� students should be presenting a paper, either poster or oral

� applications from students presenting a paper on collabora-tive work between two or more members of the UK Magnet-ics Society (academic or industrial partners) will be particu-larly welcome

� successful recipients of student bursaries are requested to provide a brief review of the conference to appear in a subsequent issue of Magnews

STUDENT BURSARY SCHEME

DENNIS HADFIELD MEMORIAL PRIZEIn addition to the Student Bursaries, the annual Dennis Had! eld Memorial Prize of £100 will be awarded to the best Student Bursary conference report published in Magnews, and will be given out at the annual Ewing Event, held each December. The prize winner will be invited to attend the Ewing Event free of charge.