Design of PM Synchronous and Brushless DC Motors

8/12/2019 Design of PM Synchronous and Brushless DC Motors

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Design of PM Synchronous and Brushless DC MotorsTJE MillerSPEED Laboratory

University of GlasgowGlasgow G12 8LT UK

Abstract -The fmt part of this paper surveys he design tools required to maintaina competitive design capability in brushless PMmotors ncluding the appropriatesoftware for design drafting, analysis,and simulation;he use of magnetic materialstest facilities; the use of a precision dynamometer; and the use of flexiblecontrollers. The second pan discuse s the design of PM brushless AC machines forflux-weakening capability to give a wide speed range at coostant power. ?he IPMparameter plane concept is used to classify the various classes of PM motor and todefme an optimum combination of reluctance orque and permanent-magnet torqueto maximize the constant-power speed range. Results are presented for an axially-laminated interior-magnet motor wirh a constant-power speed range of 7.5: 1. T b i sis 2-3 times the capability of induction motors nd is achieved at the same time asa slightly improved rated-load efficiency and power-factor.

INTRODUCTIONElectric motors are sometimes perceived as old technology , but thefalsity of that view is clear from the facts: motors are made in largernumbers, and in a wider variety of types and designs, than everbefore; they are controlled by the most advanced forms ofmicroelectronics and power electronic drives, with sophisticatedsensing technology and control theory. Even in the motorsthemselves there is steady development of insulation systems, magnetmaterials, the use of composites and engineering plastics, bearings,lubrication, and electrical steels. Many motors used today canoperate only with electronic control.Apart from a few line-start machines fitted with starting cages, PMsynchronous and brushless DC motors operate only with electroniccontrol. Their place in the hierarchy of motors is well established inhigh-quality, high-efficiency applications such as industrial servodrives which require the ultimate performance- ood dynamics andcontrollability, cool running, and low torque ripple and noise. Incomputers and office equipment these qualities make PM brushless

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PART I

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1 IFig. 3 PC-BDCs winding editor

motors the natural choice over all other motor types; in theseapplications, high-volume manufacturing does an extraordinary jobof keeping the cost down while maintaining uncompromisingly highquality. More recently, the brushless PM motor has found its wayinto residential HVAC applications, where the need is for high-efficiency and low acoustic noise. These three application categoriesreflect the design flexibility that stems from the availabilty of a widerange of different magnet materials from high-energy rare-earthmagnets to low-cost, high-coercivity ceramic magnets. The highcoercivity and remanence of modem magnets, coupled with theability to manufacture a wide i-ange of custom shapes, permits a widerange of geometrical configurations including exterior- and interior-rotor motors and axial- and radial-gap motors.To establish and maintain a competitive design capability in PMbrushless motors, the following elements are essential or highlydesirable:

1.

2.3.4.

In Part I the

Software, for Sour distinct tasks :A. Initial dimensioning of the motor andcontroller to meet customer specifications.Speed is of the essence in this task.B. Drafting, in a format consistent with

manufacturing, inventory control, etc.Accurate refinement of the electromagnetic,thermal, and mechanical design by means ofnumerical analysis techniques.Dynamic simulation including transients andstability analysis.

C.

D.

Magnetic materials testing facilitiesFully instrumented dynamometer.Flexible electronic controller.capabilities and use of these facilities are described,

with the main emphasis on design software.Part I1describes the design of an axially-laminated PM synchronousmotor. This machine has the characteristic of a very wide constant-power speed range under flux-weakening [11, and has been operatedwith sensorless control (i.e., without shaft position feedback), [2].

I. 1 DESIGN SOFTWAREA . Initial dimensioningprograms (IDPs). Many reasons can be citedfor the use of the computer even for the lowest-level sizingcalculations: consistency of results, good documentation, and thespeed and thoroughness of the process. So great is the need forresponsiveness to customer needs, that one UK company claims aturnaround time for new designs of only one day: the design is donein the morning, the prototype is made during the afternoon, and thecustomer has it the following day. Only with reliable computersoftware is it possible to generate the design so quickly, and in aform where the lamination geometry can be fed electronically to alaser-cutting machine direct from the design program. With theappropriate formats for datafiles, the same design data can be usedby a conventional CAD program to produce the necessary drawingsand documentation for manufacture. Equally, it can be piped tospecialist programs for refinement using such tools aselectromagnetic finite-element analysis, thermal modelling, stressanalysis, etc. A diagram of this overall software environment sshown in Fig. 1, which concentrates on design rather than onmanufacture per se.The initial dimensioning program (IDP) is the p i n t of entry for thecustomers specification. Some IDPs are old favorites written inBASIC or FORTRAN, and some are even written in templates forcommercial spreadsheet programs. The most sophisticated modemIDPs provide outstanding flexibility, accuracy, data links with othersoftware, and good documentation. The development of suchprograms has kept pace with the development of desktop and laptopcomputers, so that design tasks of considerable complexity anddifficulty can routinely be performed on the customers premises, ifnecessary, or even in an airport lounge.The ideal IDP can handle a wide range of motor configurations, withdifferent magnet arrangements, rotor and winding configurations, anddifferent types of controllers. Fig. 2 shows some of theconfigurations available with PC-BDC [3]. All of these motors canbe modelled in PC-BDC with wye, delta, 3-phase unipolar, 2-phase,or 1-phase connections; there are six different control algorithms(four current regulators and two PWM voltage regulators), as wellas pure sinewave operation.PC-BDC also has a winding editor, shown in Fig. 3. This can beused to modify any one of a number of standard windingconfigurations, or the user can build a winding from scratch usingthe cursor. The program can calculate the resistance, self- andmutual inductances, and back-EMF waveforms for any distributionof coils, subsequently using these in the dynamic simulation.Fig. 4 shows an example of the current, EMF, and torquewaveforms calculated by PC-BDC. In addition, the program producesapproximately 200 design parameters including efficiency; abreakdown of losses;peak, mean and Rh4S currents in the motor andthe controller, and many others relating to the winding, the magneticdesign, and the performance. An extract is shown in Fig. 5 . All ofthese facilities are integrated in the one program with file-handling,context-sensitive help, graphics, and several other features. Thispermits the integration the dimensioning of the controller (selectionof power transistors etc) with that of the motor.The importance of the software engineering in producing a tool ofthis kind is very great. Even though the engineering calculationsperformed are not particularly sophisticated, he quantity of data andits organization and reliability require great emphasis to be given tothe useability of the software.

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B . Drafting Modem drafting programs are tending to define the defacto standard for data exchange between the IDP, various advancedanalysis packages (AAP's), and other software-compatible functionsincluding many which are linked to manufacturing operations. Theideal drafting package or IDP c n produce design data in variousformats: Fig. 1shows the design data from the IDP being used in alaser-cutting machine for prototype laminations, in a dynamicsimulation program for setting up controller parameters, and also asinput data for electromagnetic and thermal finite-element analysis.C. Advanced analysis packages (AAP s) Fig. 6a shows a finite-element flux-plot used in the analysis of the back-EMF waveform ofa linear brushless DC motor. The finite-element and boundaq-element methods produce electromagnetic, thermal, and mechanicalanalyses for geometries too complex for manual analysis, and theyaccurately account for non-linear material properties such as the B/Hcurve of electrical steel.Until recently AAP's such as finite-element software were expensive,particularly in terms of the engineering time and skill levels requiredto produce useful results. However, this is rapidly changing for tworeasons: first, the software itself is now available on personalcomputers with a friendly user-interface which makes it mucheasier to set problems up and subsequently extract useful engineeringresults @re-processing and post-processing). Secondly, the linksbetween IDP's and drafting packages and AAP's are being developedso that seamless transfer from an initial dimensioning program ordrafting package into finite-element analysis and back again is

Fig. 6a Finiteelement flux-plot; axially-laminated IPM motor

-s 0.2 -rulz 0 15 -

0.1 AI I0 10 20 30 40 50 60 70 8 90

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Fig. 6b Airgap flux distribution computed by f ~ t elements--.-.100 ,* b--VIc.

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T I M E [ M I L L I S E C O N D S ]Fig. 6c Back EMF waveform measured (solid) and computed byfinite elements (dotted)possible. AAP's tend to be large and powerful and it makes sense toemploy an AAP in tandem with an IDP or a drafting package, eachprogram being the work of specialists in the respective fields. Theimportance of good data transfer cannot be overemphasized in thisconnection.

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Fig. 7 Controller architecture modelled by PC-BDC

The flux-plot shown in Fig. 6a is useful for visualization of themagnetic field, but more useful engineering information is obtainedby sophisticated post-processing and other features. For example,Fig. 6b shows the distribution of flux-density around the airgap of abrushless AC PM motor, showing the effect of slotting. Fig. 6cshows the back-EMF waveform calculated by means of a series offinite element solutions taken with the rotor at a series of anglescovering 180 electrical degrees of rotation. Modem finite-elementprograms can virtually automate the production of such waveforms,which can subsequently be used in dynamic simulation programssuch as PC-BDC for dynamic performance analysis. In Fig. 6c hemeasured EMF waveform is a few percent lower than the calculatedone. When these graphs were plotted, the discrepancy highlighted aproblem with the magnetization of the magnets that had previouslypassed unnoticed.D. Dynamic simulation sofha re DSS) Once a motor is connectedto an electronic control ler, the performance of the complete systemcan only be calculated by dynamic simulation software. Suchcalculations are essential to the design of the controller ot onlyfor sizing and selecting power transistors and diodes, but also fordetermining the architecture and parameters of the controller. Apopular and well-known example of DSS is PSPICE, but there isnow a wide range of sophisticiated DSS particularly for workstations,and some of these are extremely powerful with facilities formodelling not only analog and digital circuits, but also the non-linearelectromagnetics of the motor and the mechanical equations of theload, all at the same time.. Dynamic simulation can also beincorporated in the most advanced IDPs, and Fig. 7 shows thearchitecture of a brushless I)C motor controller modelled in thedynamic simulation integrated in PC-BDC. Fig. 8shows a trace fromthis simulation.

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Fig. 8 Trace from PC-BDC dynamic

The computing plag?orm. For several years the engineeringworkstation was the natural platform for engineering software, andalthough this remains the case for larger systems, all of the softwarefunctions described here are available on personal computers. Thisbrings the software design environment within reach of smallcompanies, and even in large companies the personal computermakes it possible to equip numbers of engineers with the same tools,or with compatible tools for different functions, with reasonableexpense.134


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Waveformgenerator

Poweramplifier h c i t i n gcoil

DATAACQUISITION

PROCESSING

Fig. 9 Test system for magnetic materials

1.2 MAGNETICATERIALS A N D TESTING FACILITIES

IbLYRIVE

Fie. 10 Precision dvnamometer

This paper is too short for a satisfactory review of permanent magnetmaterials and electrical steels, so these reviews are left for morespecialist papers. It should be mentioned that the most importantrecent development in permanent magnets is the manufacturingtechnology for NdFeB magnets, permitting great flexibility in thedesign of brushless motors. This stems from the wide range ofmagnet grades and the equally wide range of production methods fordifferent shapes and sizes and different methods of magnetization.Induction motor manufacturers have long maintained core-loss testingfacilities for electrical steels, an essential element in quality-assurance on incoming materials. However, the testing of hardmagnetic materials appears to be less widely practised, wmemanufacturers being content with property data from the magnetsupplier. Test equipment can be purchased ready for use for testingboth AC and DC properties of both soft and hard magnetic materials,and an example of a test system configuration is shown in Fig. 9.Because of the wide variation in magnet properties, test data shouldalways cover the expected temperature range and the test equipmentshould have the facility for heating or cooling the test samples.During the design process the core losses in the motor must becalculated. Traditional core-loss estimation formluas for inductionmotors are not always applicable in brushless PM motors,particularly in squarewave motors because of the non-sinusoidalnature of the flux waveform. Unfortunately, manufacturers data forcore loss of electrical steels is generally based on sinewave data, andthe adaptation of this data for calculation of losses under moregeneral conditions is rather an uncertain process. Nevertheless, goodresults have been obtained and published [4] by separating thehysteresis and eddy-current loss components and modifying theircoefficients based on actual flux-waveform parameters.

1.3PRECISION DYNAMOMETERA production-line test might include phase resistance, back-EMFconstant (generated volts when the motor is spun at IO00 rpm), andshort-circuitcurrent. All of these are easy to measure on a more-or-less automatic test station.

Fig. 11 Special dynamometer for measuring no-load core losses andwindage losses

The comprehensive testing of brushless DC motor prototypes, fordesign validation and the calibration of design equations, requires aprecision dynamometer and data-acquisition system, with transducersfor speed torque, phase and line currents, and temperatures atvarious points in the motor. It is also essential to measure rotor shaftposition if any meaningful tests are to be conducted in relation to thecommutation or sinewave control.Fig. 10 shows a schematic of the precision dynamometer used in theSPEED Laboratory at the University of Glasgow. The test stand hasa range of in-line torque transducers to accommodate different sizesof motor. The NICOLET data acquisition system c n sample fourchannels with 12-bit resolution at 10 Ms/s with a further four 12-bitchannels capable of 1 Ms/s sampling. Waveforms captured by thissystem are stored for off-line processing and documentation, whichc n be automated by means of the system control software.

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In addition, it is desirable to build prototypes with search coils (atleast one full-pitch search coil and at least one single-tooth searchcoil). These permit the measurement of the flux waveform, fromwhich the back-EMF waveform of the whole winding is derived.When there is any departure from the ideal sinusoidal or trapezoidalphase EMF waveform, the flux waveform is always the first placeto start looking, and these search coil waveforms produce valuableinsight that is often not available from the entire winding EMF.A problem that arises with brushless PM motors is the accuratedetermination of the core lossesand the windage-and-friction losses.Since the magnets are permanently magnetized, these losscomponents cannot be differentiated by any rotating test. In theSPEED Laboratory it is the practice to make a dummy rotor ofexactly the Same shape and dimensions as the actual rotor, but withsteel or aluminium instead of permanent magnets. This rotor is usedwith the Same shaft and bearings to measure the windage and frictionlosses.

M i C r O -controllerPC

097, LM628

Because these loss components are very small, typically less than 5%of the rated output, they cannot be measured accurately on adynamometer whose full-scale reading is comparable to the ratedpower of the motor. For this reason, the windage-and-friction testand the no-load core loss test are performed on a specialdynamometer with a gimballed drive machine and a very sensitivetorque-measuring mechanism based on a lever arm with a forcegauge. Fig. 11shows this dynamometer.1.4 FLEXBLEONTROLLER

3-phaseMOSFET rIGBTinverter

It is often the case with brushless PM motors - ndeed with allmotors intended for variable-speed drives - hat the motor andcontroller are not designed or built together in the same facility.Consequently the motor designer is often unable to test hidherdesigns under the control of the electronic controller hat will be usedin production. This is not an easy problem to deal with in general,because there may be a different controller for every different motordesign. What can help is afiexible controller:that is, a black boxwhich any brushless motor c n be plugged into, and whose controlalgorithms and parameters c n be dialled up or set up via a seriallink to a personal computer. Fig. 12shows the schematic diagram ofsuch a controller.The controller is modular, so that different inverters (or squarewavecontrollers) can be controlled from the same micro-controller. Thelinke between the gate drives and the microcontroller is optical. Themicrocontroller is designed together with a specially written PCinterface program that pemuts the user to set up the controlparameters on-line. The control parameters include speed, current set-points, and others such as those which appear in Fig. 7.The powerinverter includes wide-bandwidth current and voltage sensors forinstrumentation.

RS232 Opticalfiberlinks

11.1 AXIALLY-LAMINATEDNTERIOR PM MOTORFlux-weakening One of the recognized limitations of surface-magnetbrushless DC motors is the limited speed range at constant power,which stems from the low inductance. In fact it can be shown that ifthe inductance is zero, the torque/speed characteristic is rectangular,meaning that constant-torquecan be maintained up to the rated speedand voltage, but no higher speeds c nbe reached because the torquefalls rapidly to zero when the back-EMF exceeds the supply voltage.In order to achieve any speed range with a constant powercharacteristic, flux-weakening is necessary (corresponding to field-weakening in the DC separately excited motor). In an ACsynchronous machine, flux-weakening capability implies the presenceof inductance.

1 1I I

J

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0 0 5 1NORMALISED MAGNET FLUX-LINKAGE SPM)

Fig. 13 IPM parameter plane, showing motor cross-sections

1 1

z - -

FINITE >NORMALISED MAGNET FLUX-LINKAGE

Fig. 14 IPM parameter plane, showing the five motorclasses

Fig. 12 Flexible controller configuration736


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With this background, there has been considerable research in recentyears on hybrid AC PM motors which combine permanent-magnetalignment torque with reluctance torque, notably the interiorpermanent-magnet motor described by Jahns [ 5 ] and the inset magnetmotor described by Sebastian and Slemon [6].A comprehensiveanalysis of the relationship between the reluctance nature and thepermanent-magnet nature of AC synchronous motors has recentlybeen presented in [7]. This paper introduces the concept of the IPMparameter plane, Figs. 13and 14. The axes of the IPM parameterplane are the normalised magnet flux-linkage (which is equal to theratio of the back-EMF at rated speed to the rated voltage), and thesaliency ratio X d X d ratio). A pure permanent-magnet motor has nosaliency and is represented by a point on the x-axis. A puresynchronous reluctance motor has s saliency but no magnet-alignmenttorque and is represented by a point on the y-axis. Fig. 13showsgraphically the cross-sections of various brushless synchronous ACmotors according to their relative positions in the IPM parameterplane. Fig. 14 classifies the drives in to five classes, the maindivision being between those which have a theoretical maximumspeed (limited by available voltage, with limited flux-weakeningcapability), and those with no such maximum speed. The morepermanent-magnet a machine is, the more likely it is to have afinite maximum speed and limited flux-weakening capability.For a given amount of magnetization, there is an optimum saliencyratio that is just sufficient to produce an infinite maximum speedcapability, representing the maximum effect of flux-weakeningpossible with that degree of magnetization. Designs on the optimumIPM design line have this characteristic, but they also have thelimitation that the base-speed power factor is limited to 0.707.The axially-laminated IPMs shown in Fig. 15are a 50W machine anda 7.5 kW machine. This machine has been built in two versions, oneas a pure synchronous reluctance (SYNCHREL)otor, and the other asa hybrid designed on the optimum IPM line by using rubber bondedpermanent magnet material for the flux-barrier material. Performancedata for these machines, together with equivalent parameters for aninduction motor in the same stator, are summarised in Table 1. Inparticular it can be seen that the constant-power speed range (CPSR),measured at 7.5:1 greatly exceeds that of either the induction motoror the pure synchronous reluctance motor. Interestingly, the power-factor and efficiency are both significantly better than those of theinduction motor at rated load. The power vs. speed characteristics forall three motors are shown in Fig. 16.

Fig. 15 Axially-laminated IPM motors, 7.5 kW and 50 W

ParameterAirgap [mm]Stator Inner Dia. [mm]Stack Length [mm]PolesLamination Thick. [mm]Ins./Magnet Thick. [mm]Rotor LayersPole Arc [elec deg]Pole PiecesMagnet Flux [Vs rms]Lnsat. (Sat .Rated Line Voltage Lc [VIRated Current I [A]Ym [deglKnee Speed WI; rpm]Rated Torque Tk [xm]Rated Output Power pk [kW]Efficiency 7 [%]Power Factor cos QInverter Ctilisation K =CPSR

cos o

3.5

3

2.5-3E *zw3 1.s0

I

0.5

C

Fig. I C

IM S YNCH REL I PM0.48 0.517 0.917127 127 127202 202 202

4 4 40.50 0.500.50 0.5062 62131 131

brass iron0 0 0.174

11.5 6.7.9.6 6.3

415 415 41515 15 15

64.1 48.11460 1442 139650 49.6 53.1

7.48 7.7687.5 85.5 89.50.72 0.813 0.8040.63 0.696 0.7202.5 2.5 >> 7.5

5

Table 1

500 1m 1500 zoo0 2500 3000 IS P E E D [ R P M ]

Power vs. speed curves of AC PM brushless motors

CONCLUSIONSA competitive design capability in PM synchronous and brushless DCmotors includes the use of several software tools for initialdimensioning, drafting, advanced analysis, and dynamic simulation,These tools should be supplemented with magnetic materials testingfacilities, a precision dynamometer, and a flexible controller .As an example of the development of a brushless PM AC machinefor the maximum possible speed range at constant power, the axially-laminated IPM motor is described. With a constant-power speedrange of 7.5:1, and superior efficiency and power factor, this machineoffers outstanding performance advantages for applications such asmachine tool spindles or electric vehicle traction.

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ACKNOWLEDGMENTThe author thanks his colleagues Calum Cossar, Dave Staton,Malcolm McGilp, Wen Liang Soong, Rolf Lagerquist and JimmyKelly, who contributed most of the substance of this paper. Supportis acknowledged from the following organizations: the SPEEDLaboratory and subscribing companies, Glasgow University; theScience and Engineering Research Council; the Association ofCommon wealth Universities; the Committee of Vice Chancellors andPrincipals; and Brook Crompton.

REFERENCES

3. D.A. Staton, M.I. McGilp, TJE Miller and G. Gray,. High-speedPC-based CAD for motor drives , presented at the European PowerElectronics Conference, Brighton, UK September 13-16, 1993.4. R. Rabinovici and TJE Miller, Back-EMF waveforms and corelosses in brushless DC motors , in 1993 SPEED Report, alsosubmitted for publication in IEE Proceedings, Part B.5. T.M. Jahns, Flux-weakening regime operation of interiorpermanent magnet synchronous motor drive , IEEE Trans. Ind. Appl.,vol. 23, pp. 681-689, JUlJAUg. 1987

1 . W.L. Soong, D.A. Staton and TJE Miller, Design of a newaxially-laminated permanent magnet motor , presented at the IEEEIndustry Applications Society .AnnualMeeting, Toronto, October 4-8,1993.2. R. Lagerquist, I. Boldea and TJE Miller, Sensorless control of thesynchronous reluctance motor , presented at the IEEE IndustryApplications Society Annual Meeting, Toronto, October 4-8, 1993.

6. T. Sebastian and G.R. Slemon, Operating limits of inverterdrivenpermanent magnet motor drives , IEEE Trans. Ind . Appl., vol. 23, pp.327-333, MarJApr. 19877. W.L. Soong and TJE Miller, Theoretical limitations to the field-weakening performance of the five classes of brushless synchronousAC motor drive , presented at the IEE Intemational Conference onElectric Machines and Drives, Oxford, UK September 8-10, 1993.

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Documents

Design of PM Synchronous and Brushless DC Motors