A Machine Approach for Field Weakening of Permanent-Magnet Motors 5 Pgs

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OOFCC-30

A Machine Approach for Field Weakening of Permanent-Magnet Motors

John S. Hsu

Oak Ridge National LaboratoryCopyright 1998 Society of Automotive Engineers, Inc

Prepared by the Oak Ridge National Laboratory. Oak Ridge, Tennessee 37631. managed by Lockheed Martin Energy ResearchCorp. forthe U. S. Department of Energy under contract DE-AC05-960R2.2464.The submitted manuscript has been authored by a contractor of the U. S. Government under contract No. DE-AC05-960R22464.

Accordingty. the U. S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of thiscontribution.or allow others to do so, for U. S. Government purposes.

ABSTRACT

The commonly known technology of fieldweakening for permanent-magnet (PM) motors isachieved by controlling the direct-axis currentcomponent through an inverter, without usingmechanical variation of the air gap, a new machineapproach for field weakening of PM machines by directcontrol of air-gap fluxes is introduced. Thedemagnetization situation due to field weakening is notan issue with this new method. In fact, the PMs arestrengthened at field weakening. The field-weakeningratio can reach 1O:i or higher. This technology ;s

particularly useful for the PM generators and electricvehicle drives.

INTRODUCTION

When a vehicle is cruising at high speed, therequired torque is normally low, and the supply voltageto the motor has a certain limit. Therefore, fieldweakening is an essential requirement for motors usedfor electric vehicle drive. The field weakening can beobtained easily for an induction motor when its supplyfrequency is increased without proportionallyincreasing

the motor terminal voltage. On the contrary, from theword permanent, the motor terminal voltage may noteasily be reduced when the supply frequency goes up.The field weakening is not an inherent property of PMmotors. Various relatively complex approaches forobtaining field weakening of PM motors exist. For PMgenerators, if their field can be controlled, the need ofusing a buck-boost converter to control the generatoroutput voltage measured at the direct current (DC) linkand then using the inverter to change back toalternating current could be eliminated.

There are ideas on adjusting the air gap

mechanically for field weakening of PM motors.

However, their mechanical instability may cause acertain concern.

Chan et al. [I] introduced the use oftransformer electromotive force (emf) to oppose therotational emf by purposely producing a slope on thearmature current. Certain torque ripples would begenerated.

Sebastian and Slemon proposed that withoptimum alignment of the stator and magnet fields,maxlmum torque per ampere is achieved up to abreak-point speed [2]. Operation at higher speeds

with reduced torque is achieved by adjustment of thecurrent angle to reduce the effective magnet flux (i.e.,the equivalent of field weakening).

Sozer and Torrey [3] presented an approachfor adaptive control of the surface mount PM motorover its entire speed range. The adaptive fluxweakeningscheme is able to determine the rightamount of direct-axis current at any operatingcondition.

Namuduri and Murty [4] focused theirdiscussion on the pulse-width-modulation (PW)

strategy and the microcontroller system thatimplements the phase-angle control scheme. Theauthors concluded that though the PM motors werenot designed for field weakening by phase-anglecontrol, experiments showed that the torque at no-loadspeed can be increased significantly with phase

advance, at the expense of increased motor losses.Soong and Miller [5] based their study on the

work by Morimoto et al. [6] for the maximum torquefield-weakening control. They concluded that theoptimal high-saliency interior PM motor design is themost promising for applications requiring a widefieldweakeningrange.


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Morimoto et al. again s tated that in the PMmotor, direct control of the magnet flux is not available.However, the direct-axis armature current can weakenthe air-gap flux. In their operation, magnetdemagnetization due to the direct axis armaturereaction must be prevented because the magnet torquedecreases irreversibly if this demagnetization is verylarge.This study suggests a new direct control o f theair-gap flux of PM machines through the magnitude andpolarity of the DC control current fed to a field-weakening control coil [7]. The power electronic controlof direct and quadrature-axis current components,which is demanded by the entire existing fieldweakening technologies, is not required.Subsequently, a position sensor is not necessary forthe inverter control. Under a normal control range, thefield-weakening control coil does not introduce a

situation of demagnetizing the PMs. Contrary to theexisting technology, the PMs are strengthened at fieldweakening. A 1O:l field-weakening ratio can easily beobtained. This new method is robust and particularlyuseful for, but not limited to, PM generators and electricvehicle drives. The same principle can be used foreither axial- or radial-gap PM machines.

(a) Normal situation Demagnetizing force~fromarmature d-axis current

neting

(b) Conventional field weakeningFig. I, Conventional field weakening.

CONVENTIONAL FIELD WEAKENING BY DIRECT-AXIS CURRENT COMPONENTFig. la shows the conventional situation of acoil in a PM machine where the air-gap fluxes ofopposite polarities are acting on the coil edges 1 and 2.A back emf is induced when there is a relative

movement between the coil and the air-gap fluxes.Fig. 1 b shows that for a conventional PM machine fieldweakening, the direct-axis current component weakensthe field. The magnitudes of the air-gap fluxes at bothcoil edges are equally weakened in opposite polarities.Subsequently, a smaller back emf is induced. Thedirect-axis current component is obtained through apower electronic circuit that is capable of handling afull-toad rating.NEW PRINCIPLE OF EQUALIZATION OF AIR-GAPFLUX DENSITIES FOR FIELD WEAKENING

The principle of equalization of air-gap fluxdensities for field weakening of the new type of PMmachines is explained in Fig. 2.~-------

coil edge Ifly1

(a) Normal situation ,F~~mw~~k$~n~l fL,i

(b) Field weakeningFig. 2. Direct control of air-gap flux of PM machine.


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Fig. 2 shows an example that explains the newtechnology. Compared to Fig. 1, the PMon coil edge 2 is removed and added to theof the PM acting on coil edge 1. The totalof PM per pole pair remains unchanged. Aniron piece that has no polarity on its own is placed in theposition. Fig. 2a shows that when all the flux isrestricted to go from one pole to the other, the air-gapdensity remains the same as that shown in Fig. la.Fig. 2b shows that when a DC control field acts on thePM and the iron, the air-gap flux densities acting on thecoil edges tend to be equalized. Subsequently, theinduced emf is reduced. The PM is strengthened underfield-weakening situation.

The waveform of the back emf of a coil edge is areflection of the air-gap flux distribution of a pole that thecoil edge is under. The properly shaped poles and airgaps and the distribution and pitch factors can be usedfor harmonic reduction.

PROTOTYPE MACHINEThe upper drawing of Fig. 3 shows a cut view ofa four-pole prototype machine, and the lower drawingshows the rotor. The rotor of this particular prototypemachine is sandwiched between two axial-gap stators.Two of the rotor poles are made of mild steel. The othertwo poles are made of PMs. These two PM poles havethe same polarity with a thickness sufficient for pushingthe air-gap fluxes through the PMs and the mildsteelpoles to form a comple te magnetic path. The poles aremounted in an aluminum rotor structure that is attachedto a nonmagnetic shaft made of stainless steel. Theround shape of the PMs and the ferromagnetic polesresults from the use of available round PMs to fill anormal pole shape shown in dotted lines. It is expectedthat the use of two round PMs per pole may causeharmonics in the back emf. However, a pole cap thatfollows the dotted line of a rotor pole shown in Fig. 3 isplaced on top of the round PMs to reduce the air-gap-flux harmonics. More appropriate magnet geometryshould be used in a real design.Two field-weakening control coils wound intoroidal shapes are placed between the stator windingand the steel supporting frame of the machine. They areapproximately 100 turns each. Like a field coil of a DCmachine, more turns of the field-weakening control coils

require less excitation current. Normally. the field coilpower is a very small percentage of the total power of aDC machine. The prototype stator has 12 slots, and thecoils are of full pitch with one coil per pole per phase.The steel support frame of the machine is madeof % in. mild steel. It is expected that the thickness of around steel frame would be significantly reducedbecause the peripheral of a frame is about three timesthe outer diameter of a machine.

Field0.50 weakeningcontrol coils1

thick, Fe 13.75 Dia.

2.0625 Dia.m(Ref)Fig. 3. A prototype PM machine for testing back emf ofthe direct control of air-gap flux of the new PMmachine.

Fig. 4 shows the picture of an actual prototypemotor driven by a low-speed motor for back emfmeasurement. In order to measure the back emf of thestator armature winding, the rotor of the prototypemachine is driven by a small motor of roughly 1200 rpm.

Fig. 4. A prototype PM motor w ith direct control of air-gap flux.


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motor is shown on the left-hand side ofand 4. The two axial-gap stators werey designed for a 50,000~rpm high-speedPM motor. It is understandable that a low voltagegenerated when a low-speed motor

I I I-1010 -55 0 5 100

Field weakening control current [Alield weakening control current [AlFig. 5. Tested back emf values versus control currentsof the field-weakening coil.

Fig. 5 shows the tested back emf values versusthe control currents o f the field-weakening coil. Asmentioned before, because the driving motor runs at lowspeed, the back emf is expectedly low. Attention shouldbe drawn to the ratio of the emf values that are changedfrom the maximum value to the minimum value. Testresults show that a field-weakening ratio of 1O:l andhigher can be obtained.

Fig. 6 shows the back emf of the prototypemachine traces with pole caps under various fieldstrengths. A significant field control capability is clearlyobserved.

CONCLUSIONS

l A new field-weakening and adjustment


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CONTACTJohn S. Hsu (Htsui) received a B.S. degree fromTsing-Hua University, Beijing, China, and a Ph.D.degree from Bristol University, England.He worked in research and development areasfor Newman Industry of England, Emerson ElectricCompany, and Westinghouse Electric Corporation. He

served as head of the Rotating Machines and PowerElectronics Program, Center for Energy Studies, theUniversity of Texas at Austin for four years.Currently, he is a senior staff scientist of thePower Electronics and Electric Machine Group, OakRidge National Laboratory. Dr. Hsu is the author or co-author of more than one hundred technical papers andreports.

REFERENCES[I] C. C. Chan, J. Z. Jiang, W. Xia, and K. T. Chau,Novel Wde Range Speed Control of PermanentMagnet Brushless Motor Drives, pp. 539-546, /EEETransactions on Power Electronics, Vol. 10, No. 5, Sept.1995.[Z] T. Sebastian and G. R. Slemon, Operating Limits ofInverter-Driven Permanent Magnet Motor Drives,pp. 800-805, CH2272-3/86. IEEE.

[3] Y. Sozer and D. A. Torrey, Adaptive Flux WeakeningControl of Permanent Magnet Synchronous Motors,pp. 475482, Conference Record, 1998 IEEE IndustryApplications Conference, Vol. 1, 12-15, October 1998,St. Louis, Missouri.[4] C. S. Namuduri and B. V. Murty, H igh Power DensityElectric Drive for an Hybrid E lectric Vehicle, ConferenceProceedings, APEC 98, Vol. 1, pp 3440, 98CH36154,February 15-19, 1998, The Disneyland Hotel, Anaheim,California.[5] W. L. Soong and T. J. E. Miller, Field-WeakeningPerformance of Brushless Synchronous AC MotorDrives, pp. 331-340, IEE Proc.-Electr. Power Appl.,Vol. 141. No. 6, November 1994.[6] S. Morimoto, Y. Takeda, T. Hirasa, and K. Taniguchi,Expansion of Operating Limits for Permanent Magnetby Current Vector Control Considering lnverterCapac ity, pp. 866-871, /EEE Transactions on IndustryApplications, Vol. 26, No. 5, September/October 1990.[7] John S. Hsu, U.S. Patent Application, EqualizedField Weakening of Permanent Magnet Motors andGenerators, Ned January 21. 1999, in the U.S. Patentand Trademark Office at Reel 9724 and Frame 0131.

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A Machine Approach for Field Weakening of Permanent-Magnet Motors 5 Pgs