Performance of split-coil switched reluctance drive

D.W.J. Pulle, MSc, PhD, CEng, MIEE

Indexing terms: Reluctance motors, Power electronics

Abstract: The concept of a split coil switched rel-uctance motor (SSWR) drive is introduced. Thismotor results from the separation of the phasecoils of a conventional switched reluctance motor(SWR) into two halves. Excitation requirementsfor the SSWR motor are met by providing onehalf of the phase winding with a DC current, theother half being connected to a bipolar drivestage. Experimental results in the form of outputpower/speed characteristics are presented for theSSWR drive operating with a range of DC currentand switching angle settings. A comparisonbetween results obtained with a SSWR drive andSWR drive is given. These results indicate that anoverall improvement in performance is achievablefor the SSWR drive when compared to an SWRdrive. Other features of the SSWR drive aimed ateliminating the position sensor and its capabilityfor regenerative braking are also discussed inorder to highlight the potential of the SSWR driveconcept.

1 Introduction

1.1 GeneralThe traditional choice for a variable speed drive has beenbetween a DC or an induction motor connected to anappropriate excitation circuit. Both these drive conceptsare relatively complicated, the DC motor drive requires asimple excitation circuit with a more complicated motorconfiguration; the induction motor has a simple motorconfiguration (as is the case for a squirrel cage motor),but usually requires a more complex excitation circuit.

In switched reluctance type drives both motor configu-ration and excitation circuit are relatively simple [1, 2]and offer a performance comparable with its more tradi-tional counterparts.

In this paper, the concept of a so called split-coilswitched reluctance (SSWR) motor is explored, in whicheach phase winding of a SWR motor is split into twoseparate windings. For this purpose a four phaseswitched reluctance (SWR) motor was modified as indi-cated in Fig. 1.

The <j>x phase winding, originally of N turns, wasreplaced with two separate windings, each of N/2 turns.The other three phases were rewound in the samemanner.

Paper 6248B (PI), first received 12 November 1987 and in revised form25th May 1988The author is with the Department of Electrical & Electronic Engineer-ing, the University of New South Wales, Australian Defence ForceAcademy, Campbell, ACT, Australia 2600

Fig. 1 Winding configuration for SSWR motorOne phase shown

The concept of 'splitting' the phase windings wasintroduced by Acarnley [3] as a means of improving theefficiency of a conventional variable reluctance (VR)motor connected to a forced resistance, contant-voltage-type drive circuit. Experimental work by Acarnley andtheoretical analysis by the author [4] indicated that thehigh speed performance of this drive was also improved.From further work by Hughes and the author [5] it wasconcluded that the high speed performance of a VRmotor, forced resistance constant voltage drive is depen-dent on the magnitude of the DC phase current and fun-damental phase voltage component.

These conclusions have application to stepping motordrives using a chopper circuit to control the current.Such chopper drives exhibit a low-speed chopping mode[2] where current is controlled by phase voltage switch-ing. However, at high speeds the drive acts as a constantvoltage drive when the current rise time to becomes ofthe same order as the duration of the phase excitationinterval.

In the course of this paper, experimental results in theform of output power/speed curves will be presented forthe SSWR drive operating under different conditions.Where possible results obtained with the SSWR drivewill be compared with those found for the SWR drive. Inaddition, attention will be given to other aspects of theSSWR drive aimed at investigating the possibility ofelimination of the position sensor unit and regenerative

318 IEE PROCEEDINGS, Vol. 135, Pt. B, No. 6, NOVEMBER 1988

braking. Both these aspects are under investigation andwill be the subject of future publications.

7 2 Drive circuit configuration for the SSWR motorIn Fig. 2a is shown a typical static torque angle curve(obtained by varying the rotor position 9 while one coil

Fig. 2 Static torque/angle curve and MMF waveforms

^•j-phase excited continuouslya static torque/angle curveb MMF waveform for SWR motorc MMF waveform for SSWR motor

of phase (/>! is connected to a DC current source). For a'conventional' SWR motor, positive average torque isobtained when a rectangular unipolar current pulse withamplitude Iref is applied to both </>! coils (connected inseries) of the SSWR motor, which leads to an MMFwaveform as shown in Fig. 2b. The relative position ofthe excitation interval (i.e. when MMF > 0) to the rotorangular position 9 is set by the switching angle 6™. Itsabsolute value will be chosen relative to the rotor polepitch, here 60°. The electrical switching angle, 9S is relatedto the mechanical 9™ by 9S = 6 0™. Ideally, 9S must besmall at low speeds to maximise the average torque.However, as the speed increases the current waveformchanges which means that 9S must increase [6].

An MMF waveform similar to that shown in Figure2b can be realised for a SSWR motor by exciting one coilof each phase with a bipolar current of amplitude Irefwhile the other coil is connected to a constant currentsource set to level ldc. The resultant MMF waveform forthe SSWR motor is shown in Fig. 2c. This MMF wave-form will be identical to that found for the SWR motorwhen Idc is chosen to be equal to Iref.

In implementing the drive circuitry for the SSWRmotor, four MMF waveforms similar to that shown inFig. 2c must be provided, displaced from one another by15°. This means that the bipolar MMF waveforms forphases 1, 3 and 2, 4 are equal but opposite in polarity,

IEE PROCEEDINGS, Vol. 135, Pt. B, No. 6, NOVEMBER 1988

which means that these phases can be connected in seriesor parallel provided that the terminal connections for onephase are reversed. In this case two phase coils whereconnected in series.

The coils required to carry the DC current are con-nected in series, according to a sequence of #4 — <f>2 — <t>$— 0! for reasons to be discussed at a later stage.

The excitation requirements for the SSWR motor cantherefore be met by two bipolar drives stages and a DCcurrent source. An example of such a drive circuit asdeveloped by the author is shown in Fig 3 together with

Fig. 3 Drive circuit for SSWR and SWR motor

a SSWR drive circuitb SWR drive circuit

the circuit used to provide the excitation for the SWRmotor. Observation of the two drive circuit conceptsindicates that the level of complexity has not significantlychanged as a result of using a SSWR instead of a SWRmotor.

The DC current requirements for the SSWR motor aremet by the DC supply source, with a variable resistanceR to control the current setting. Alternatively, a simplechopper circuit could be used in case the power dissi-pation in the variable resistance is unacceptable. Aninductance L, shown in Fig. 3a, is connected in serieswith the resistance R to reduce higher harmonic currentcirculation via the DC source.

Each of the two bipolar drive stages shown in Fig 3autilises two transistor modules as found typically ininverter applications. At any given time, two diamet-rically opposite switches within one stage are activated inaccordance with the required current polarity. Control ofthe two active switches is identical to that required forthe SWR drive (Fig. 3b): the bottom switch is closedduring the selected excitation interval while the topswitch maintains the current at the current referencevalue Iref through chopping action.

2 Power/speed curves of SSWR drive

The 4 kW-rated SWR motor used had a 9 A peakcurrent rating and was modified to comply with the new

319

winding requirements. Access to both sets of windings oneach phase was provided in order to allow easy conver-sion from an SWR to a SSWR motor and vice-versa. Amodular drive circuit system was built and is capable ofimplementing both drive configurations shown in Fig. 3.An eddy- current brake coupled to the shaft of the motorvia an inline torque transducer unit was used to apply avariable load torque, over a 0 to 2000 rpm speed range.

Power for the drive circuits was supplied from a regu-lated 200 V DC source. A current limit Iref level settingof 6 A, chosen on the basis of equipment limitations, wasmaintained throughout the tests.

2.1 Operation with a constant switching angle anddifferent DC current settings

The output power-speed curves for the SSWR drive to bediscussed here, were obtained with a switching anglesetting of 25-8°, the value set by the manufacturer of theSWR motor. Various settings for Idc were selectedaround 6.0 A because this value together with the chosenIref value will provide a low speed MMF waveform iden-tical to that found for a SWR drive.

Experimental results for the SSWR and SWR drive, asshown in Fig. 4, allow a performance comparison to be

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These waveforms indicate that at N = 1300 rpm, theSWR drive is operating in the midspeed range while theSSWR drive is still in the low speed chopping mode.

200 400 600 800 1000 1200 1400 1600 1800 2000N.rpm

Fig. 4 Output power/speed curves for SSWR and SWR driveI,e/ = 6.0A 0, = 25.8°

/dc = 8.0A 6.0A• • • • 4.0A SWR drive

made. Apparent from Fig. 4 is that the peak power levelis well below the rate value of the motor. This is due tohaving to use a Iref setting lower than the rated value forreasons mentioned earlier. Its value according to themeasured flux linkage curves is such that realistic satura-tion levels are encountered. It is emphasised that Fig. 4aims to show the performance of the drives relative toeach other and were measured with identical 9S and Irefsettings.

Readily observable for both drives is the presence of alow speed chopping mode which is characterised by thelinear part of the power/speed curves. As speed increases,the drive enters the mid-speed range where current limit-ing using chopping action becomes ineffective. At highspeeds this chopping action is practically non-existentand the drive is said to be operating in the constantvoltage mode. The presence of these modes of operationcan be readily observed from the current waveforms. Thisis demonstrated in Fig. 5, which shows two representa-tive bipolar (SSWR drive) and unipolar (SWR drive)current waveforms for two selected speeds.

Fig. 59, = 25.8 l,lSa N = 1300 rpmb N = 1800 rpm

SWR —

Typical current waveformsl,lS = 6.0A

SSWR

With the drives operating at N = 1800 rpm, the situationis changed considerably in as much that the SSWR driveis now operating in the constant voltage mode, while theSWR drive is nearing the end of the midspeed range aschopping action is severely limited.

A comparison of the performance of the power/speedcurves for the SWR drive and SSWR drive (with /rx: =6.0 A) indicates that the low speed chopping region isextended by using an SSWR drive. However, the highspeed performance of the SSWR drive (operating with Idc6.0 A) is below that achievable by the SWR drive.

In addition, the results indicate that an increase in theDC current setting improves the overall performance ofthe SSWR drive. Particularly noticeable is the increasedlow speed torque as reflected by the larger gradient of thelow speed power/speed curve.

2.2 Operation with constant DC current setting anddifferent switching angles

In achieving a better high speed performance for theSSWR drive it is important to realise that this concernsoperation where the drive is in the constant voltagemode. Results of earlier work by the author [7] has indi-cated that under those conditions the output power of an

320 IEE PROCEEDINGS, Vol. 135, Pt. B, No. 6, NOVEMBER 1988

SWR drive can be optimised through appropriate selec-tion of the DC current level and switching angle setting.Accordingly it is beneficial to explore whether the highspeed performance of the SSWR drive can be improvedby changing the switching angle. The DC current settingLdc was chosen to be 6.0 A in order to maintain equalMMF waveforms for both drives at low speeds.

The effects of increasing the switching angle 0s from25.8° to 54.6° on the output power/speed characteristicsfor both drives is shown in Fig. 6.

1000

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3: 600

3500 -

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0

-

• J?- JT

r

\_

1

V 8=5A.6

\ s

_̂ ••..40.2^ ^ c — 5A.6

: iT~- "402\ ^ ^ 25.8

•-25.8

200 400 600 800 1000 1200 K00 1600 1800 2000

N, rpm

Fig. 6 Power/speed curves for SWR and SSWR drive, with Q, asparameter

I * = 6.0 A• • • • ) 1

} SSWR drive } SWR driveJ J

Inspection of these results indicates that the highspeed performance for the SSWR drive in particular, issignificantly improved as a result of increasing theswitching angle setting. This improved performance ofthe SSWR drive is reflected in the bipolar current wave-forms. This is demonstrated in Fig. 7, in which a com-

-6. Or

-4.0 -

-6.0L

Fig. 7 Bipolar current waveforms for SSWR drive operating atN = 1800 rpm, with two different switching angleslAc = 6.0 A

0, = 25.8° 6, = 54.6°

parison of the bipolar current waveforms is given for thedrive operating at N = 1800 with 6S = 25.8° (originalsetting) and 9S = 54.6° (highest setting used).

These waveforms indicate that the current rise anddecay times for the SSWR drive are now smaller becausethe effective inductance phase inductance is significantly

reduced. This means that the average resultant MMF(due to the DC and bipolar current) is increased withinthe designated excitation interval which explains thepower output improvement obtained with the SSWRdrive as shown in Fig. 6. It is important to note this theoutput power improvement at high speeds is gained atthe loss of low speed performance corresponding to areduced starting torque, as can readily be observed fromFig. 6 by the lower gradient of the low speed power/speed curves. For an SSWR drive, this reduction in thelow speed torque can be partly offset by increasing theDC current setting. However for an SSWR drive it wouldappear to be particularly beneficial to be able to controlboth DC current setting and switching angle setting withspeed. Control schemes incorporating a speed dependentswitching angle have been used successfully for a SWRdrive [8, 9] and can be applied for the SSWR drive.

3 Further possibilities of SSWR drive

In the context of this first presentation on the SSWRdrive concept, it is appropriate to discuss two otheraspects of the drive which are currently being investi-gated by the author. The first relates to the possibility ofeliminating the position sensor unit as the voltage acrossthe 'DC coils' must invariably reflect the position of therotor at any given time. Secondly, the prospect of regen-erative braking should also be conceivable by reversingthe DC current. Both these aspects will be briefly exam-ined in the following two subsections to underline thepotential of the SSWR drive.

3.1 Eliminating the position sensor unitAt present, the SWR and SSWR motor utilise a positiontransducer for either phase on/off switching (SWR drive)or current reversal (SSWR drive). Typically, such a sensorunit contains four individual sensors [9] of which two areused for single quadrant operation. Methods to eliminatethe sensing unit by deriving the coil switching informationfrom for example the current waveform [10] are beingpursued, this is complicated by the phase switchingaction of the drive stage.

In the SSWR drive, half of each phase winding is nowexcited by a DC current, accordingly the flux linked withthis coil must be a function of the rotor position.However the flux in each of these coils is also influencedby the rate of current change in the bipolar current carry-ing coils. The effect of the bipolar flux contrbution in theDC coil can be reduced by measuring the DC voltageacross two coils corresponding to those phases whichhave been connected in series, i.e. phases 1-3 and 2-4. Inthis case the voltage is measured across the DC currentcarrying coils of phases 1 and 3. Ideally, the voltageacross both coils as a result of the bipolar current tran-sitions will now be zero, as both bipolar phase coilprovide an equal but opposite voltage contribution. Inreality, the effects of the bipolar current transitions isapparent as is indicated in Fig. 8a.

The waveform shown in Fig. 8b represents the voltageacross both DC current carrying coils in case the bipolarcurrent units are deactivated. Inspection of the two wave-forms suggests that active filtering of the voltage acrossthe DC coils (Fig. 8a) will lead to the voltage waveformshown in Fig. Sb, which can be used to control bothbipolar drive stage switches.

The method proposed cannot be used when the motoris either stationary or running at very low speeds. Under

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these circumstances, operation is similar to that normallyused for conventional stepping motors.

32 Regenerative brakingIn a SWR drive, regenerative braking will require accessto a position transducer with four sensors. Under normal

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4 Discussion

Experimental results for a split-coil switched reluctance(SSWR) drive have been presented and compared with aconventional switched reluctance (SWR) drive. Results

2.0

1-0

i. o

-1.0

-2.0

t,ms 130.0

4 0 , -

- 4 0 L

,130.0

Fig. 8 Voltage across DC current coils of phases 1 and 3N = 1000 rpm /dc = 6.0Aa Normal operation b Without bipolar voltage switching

operation, two are required for controlling the phase-switching activities. If at a given speed, regenerativebraking is required than it will be necessary to switchcontrol to the other two sensors in order to reverse thetorque direction.

For an SSWR drive not utilising a sensor system asdiscussed in Section 3.1, only two transducers will berequired, as regenerative braking can now be implement-ed by reversing the DC current. Under these conditionseither the DC current or Iref will control the brakingtorque.

The prospects for regenerative braking are illustratedusing Fig. 9, which shows the input power waveform forphases 1-3 (connected in series) together with the corre-sponding current through these phases, in case the SSWRmotor is driven by a DC motor.

Observation of these waveforms indicates that thebipolar phases current becomes larger than the (6.0) refer-ence current setting in the time interval where the currentchopping switch is turned off. In a time interval imme-diately following the timing mark defining the end ofexcitation in a SWR drive and current reversal in aSSWR drive energy is returned to the supply. Theamount of energy returned to the supply is now increasedbecause the current increases to a value exceeding the Irefsetting during a time interval preceding current reversalas indicated in Fig. 9b. One immediate consequence forthe bipolar transistor drive ratings is that these must bedimensioned to accommodate currents in access of thechosen Iref value if regenerative braking is to be used.

Fig. 9 Input power and phase current under regenerative braking con-ditions

N = 770 rpm 7dc = - 1.5A 6, = 54.6

have indicated that the high speed performance of theSSWR drive is significantly better than that obtainablewith the SWR drive provided an appropriate choice forthe DC current setting and switching angle is made.

The SSWR motor requires access to two windings oneach phase, such that one is connected to a bipolarsource, and the other to a DC current source. This doesnot lead to a more complex drive configuration as com-pared to that required for a SWR drive because twophase windings can now be connected in series while usecan be made of switching modules commonly used ininverter applications.

In addition to an improved high speed performance ithas been demonstrated that the SSWR drive has thecapability to operate without a position transducer,because access to a voltage representing the rotor posi-tion in now readily available.

Furthermore the SWR drive offers the capability forregenerative braking by simply reversing the DC current.

5 References

1 RAY, W., LAWRENSON, P.J., DAVIS, R.M., STEPHENSON,J.M., FUTLON, N.N. and BLAKE, R.J.: 'High-performanceswitched reluctance brushless drives', IEEE Trans. Ind. Appi, 1986,IA-22, (4)

2 DAVIS, R.M., RAY, W.F., BLAKE, R.J.: 'Inverter drive forswitched reluctance motor. Circuits and component ratings', IEEProc. B, 1981,128, (2)

3 ACARNLEY, P.P.: 'Analysis and improvement of the steady-stateperformance of variable-relucance stepping motors' Ph.D. Thesis,University of Leeds, 1977

4 PULLE, D.W.J.: 'Prediction and analysis of variable reluctancestep-motor drive systems'. Ph.D. Thesis, University of Leeds, 1982

322 IEE PROCEEDINGS, Vol. 135, Pt. B, No. 6, NOVEMBER 1988

5 PULLE, D.W.J, and HUGHES, A.: 'High speed performance ofvariable-reluctance stepmotors'. IEEE Trans. Ind. Electron, 1987,35, (1), pp. 80

6 LAWRENSON, P.J., STEPHENSON, J.M., BLENKINSOP, P.T.,CORDA, J. and FULTON, N.N. 'Variable-speed switched reluc-tance motors', IEE Proc. B, 127, pp. 253-265

7 PULLE, D.W.J.: 'Power limits in stepping motors and the Blondelcircle digram'. International drives and machines conference, Adel-aide, 1987

8 CHAPPEL, P.H., RAY, W.F. and BLAKE, R.J., 'Microprocessorcontrol of a variable-reluctance motor', IEE Proc. B, 1984, 131, (2),pp. 51-60

9 BOSE, B.K., MILLER, T.J.E., SZCZESNY, P.M., and BICKNELL,W.H.: 'Microcomputer control of a switched reluctance motor',IEEE Trans. Ind. Appl., 1986, IA-22, (4)

10 ACARNLEY, P.P., HILL, RJ. and HOOPER, C.W.: 'Detection ofrotor position in stepping and switched motors by monitoring ofcurrent waveforms', IEEE Trans. Ind. Electron. 1985, IE-32, (3)

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