Small motor drives expand theirtechnology horizonsIf we consider electrical technology to be divided into the twomain areas of information processing and energy processing,the pervasiveness of drives technology will be immediatelyapparent. In developed economies about one-third of allprimary energy is converted into electricity, and about two-thirds of this electrical energy is subsequently converted inelectric motors and drives

by Tim Miller

Although motor technology appears to bemature, there is undoubtedly moredevelopment activity in drives today than atany time in the past. The two main reasonsare:

• increasing use of computers and electronicsto control mechanical motion

• new 'enabling technology' in powersemiconductors and integrated circuits,leading to the development or revival ofclassical and nonclassical motors such asbrushless DC motors and steppers in a widevariety of designs.

The general structure of a 'motion-controlsystem' or 'drive' is always like that of Fig. 1,but the choice of components is so wide thatthe number of possible configurations isinfinite. In general, the best configuration isone which satisfies the obvious criteria ofefficiency, reliability, performance and low cost.But in any particular application there arealways many other constraints andrequirements. A road map to this technology istherefore essential.

The range of modern motion-controlapplications is unlimited. Any random list of 20of them bristles with new technology:aerospace actuators; washing machines;computer disc and tape drives; printer plattensand print heads; inertial-guidance systems;adjustable-speed pumps, blowers and fans;locomotive and subway traction; automaticmachine tools and machining centres; servodrives and spindle drives; robots; automotiveauxiliaries; refrigeration and air-conditioningdrives; and many others. The European drivesmarket is about $1 billion, the leading usersbeing materials handling; machine tools(including robots); process plants andequipment; and paper, printing and packaging.The growth technologies are AC drives andbrushless DC drives, especially in the powerrange from a few hundred watts up to severalkilowatts.

New technologyBefore focusing on particular drives, it is

perhaps worth reviewing some of thecontributing technological developments in thearea of motion control.

Digital electronicsIt would be hard to overstate the

importance of microelectronics in motioncontrol. At the 'heavy' end of the spectrum arethe multiple drives found in steel rolling mills,paper mills and other heavy process plants,where it is normal to co-ordinate the motion ofall the shafts by means of a computer or anetwork of distributed computers, each ofwhich may be quite large in computing termsand certainly classifiable as a minicomputer. Atthe light end of the spectrum are the drivesfound in office machinery and smallcomputers, where custom integrated circuitsand gate arrays are common. Between theseextremes there are many microprocessor-controlled systems of all levels of complexity.

Historically, the first functions performed bymicroprocessors were low-speed functionssuch as monitoring and diagnostics, but digitalcontrol has steadily pervaded the adjustable-speed drive from the outer position loopsthrough the intermediate velocity loop and,more recently, the innermost torque loop orcurrent regulator. The development of 'field-oriented control' for AC induction andsynchronous motor drives would not havebeen practicable without the microprocessor1.This technique, which is based on thereference-frame transformations that dateback to Park and others, permits the outercontrol loops of AC and DC drives to be thesame, both in hardware and software, and itmarkedly improves the dynamic performanceof the AC drive.

In drives employing torque loops orinstantaneous current regulators to control thechopped or pulse-width-modulated currentwaveforms, it is generally necessary to useanalogue techniques in the current control

POWER ENGINEERING JOURNAL SEPTEMBER 1987 283

Icontrol

1 Structure of anadjustable speed drive.Few motion controlsystems can be made costeffective without carefulconsideration ofalternative componenttechnologies

2 Power circuit for a smallAC or brushless DC drive.The single 'totem-pole'phase leg comprisingsource and sink drivers isbecoming the buildingblock of such circuits.Power integrated circuitsprovide level shifting,lock-out protection, andmany other control andprotective functionswithin this structure.

because of the wide bandwidth required2.Because of the difficulties and costs associatedwith isolated current sensors and analoguecircuits, it is more usual in AC drives to findPWM control applied to the voltage ratherthan to the current, and innumerablealgorithms have been developed to do this.Some use standard microprocessors whileothers use custom ICs or gate arrays. But thereare processors now available that are inprinciple fast enough to perform digital currentregulation.

One example is Texas Instruments TMS320series, which was developed for signal-processing applications but which is fastenough (16 bit multiply in 200 ns) to permitentirely new control strategies to beconsidered, in which instantaneous torque isestimated and regulated instead of the averagetorque normally calculated in thesynchronously rotating reference frame. Evenfor such 'mundane' tasks as performing thereference-frame transformations in a'conventional' field-oriented controller, digitalsignal-processing (DSP) chips like the TMS320can far outperform most standard 16 bitprocessors.

There is also a pervasive use of digitalelectronics in axis controllers for CNC machinetools and related motion-control applications,invariably with communications interfaces suchas RS232, IEEE488, Multibus and so on. Such

•

one phase leg

motorwindings

controllers may be used with DC, AC orbrushless drives. Some of them have the abilityto make one type of drive perform like another;forexample, the Galil 'stepping servo controller'that permits a brushless DC motor to becontrolled by a stepper-motor indexer to movein discrete angular increments.

Power integrated circuitsConventional ICs are limited to standard TTL

or CMOS voltage levels, and require interfaceand level-shifting circuitry between them andthe power semiconductor switches theycontrol. Hybrids have widely establishedthemselves in this application, but morerecently there have appeared several 'powerICs' (PIC) with voltage ratings of 40-50V anda few with much higher voltage ratingsranging up to 500V. Some of these chips cansource and sink currents of more than 2 A at40-50V, e.g. Sprague's UDN2936W andUnitrode's UC3620, providing full phase legs orbridges with many additional control andprotective functions (Fig. 2).

National's LM621 brushless motor driverprovides 35 mA outputs at 40V in a full3-phase bridge configuration and includes notonly the decoding (commutation logic) for aHall-effect shaft sensor but also a PWMcurrent-regulating facility, as well as 'lock-out'protection (to prevent shoot-through or short-circuiting the DC supply) and undervoltage andovercurrent protection.

Motorola's MC33034 is similar, with 50 mAoutputs and an integral window detector withfacilities for an elementary speed loop. TheIP3M05, -06 and -13 power ICs fromIntegrated Power Semiconductors Ltd. havethese functions and feature linear currentcontrol. General Electric's high-voltageintegrated circuit provides source and sinkdriver signals at 500V DC for a single 'totem-pole' phase leg3. Other chips use dielectricisolation to achieve comparable voltage ratings.

The power IC permits a massive reduction incomponent-part count and can short-circuitmuch of the design task. It brings savings inconvertor weight and volume, along withimprovements in thermal management, EMIand reliability.

Another circuit function that has been'integrated' recently is current sensing. InMotorola's SENSEFET and InternationalRectifier's HEXsense power MOSFET, a currentmirror is formed from a small number of cellsand provides a current-sensing signal to a user-supplied resistor. Voltage ratings up to 500Vare available at 10 A.

Power-semiconductor devicesA full review of power semiconductors is

beyond the scope of this article, but for smallmotor drives a few exciting developmentsshould be highlighted. While GTO thyristorshave been widely adopted for large drives (andin Europe even down to 1 kW), the steadyprogress in FET and bipolar junction transistors(BJT) has made them the natural choice inmany applications below 100kW. PWMswitching frequencies are now ordinarily above

284 POWER ENGINEERING JOURNAL SEPTEMBER 1987

the audible range, and convertor efficiency andreliability are much improved from even a fewyears ago.

The insulated-gate transistor (IGT) is ofspecial interest for drives4 in the range0-1 -1 OkW. Available from US and Japanesesuppliers, this MOS-gated bipolar device hasvoltage ratings in the 400-500 V range withcurrent capability of up to 25 A, and combinesthe power-handling characteristics of a BJTwith the controllability of a FET. In the futurethere is the potential for the field-controlledthyristor (FCT) to extend these characteristicsup to higher power ranges.

New magnetic materialsThe sustained success of the permanent-

magnet industry in developing ever-improvingmagnet characteristics is evident from Fig. 3,the latest 'big hit' being neodymium-iron-boron which has been pioneered by Sumitomo,GM and other producers. At room temperatureNdFeB has the highest energy product of allmagnets. Unlike the rare-earth/cobalt magnetsit contains no strategic minerals, and inprinciple it should cost less. The highremanence (1-2T) and coercivity (10-6kOe)permit large reductions in motor frame size forthe same output compared with ferrite motors.However, NdFeB is sensitive to temperature,and present grades cannot be used effectivelyabove 150°C For very high temperatureapplications rare-earth/cobalt magnets mustbe used, particularly 2-17 cobalt-samariumwhich is usable up to 200°C or even 250°C

NdFeB is produced either by a conventionalmill-and-sinter process or by a melt-spincasting process similar to that used foramorphous alloys. Crushed ribbon is processedinto powder which may be bonded or sintered,as in the respective MQI and MQII gradesproduced by the Magnequench Division ofGM. The MQI bonded magnets can be formedin a wide variety of shapes. They are not 100%dense and precautions may be necessaryagainst corrosion. With MQII and othersintered materials a dichromate coating maybe sufficient protection.

For lowest cost, ferrite is the universalchoice. This material has been steadilyimproved and is now available with remanenceof 0-38T and coercivity of 3-7kOe. Thetemperature characteristics of ferrite magnetscan be tailored to the application requirementsso that maximum performance is obtained atthe normal operating temperature, which maybe 100°C or higher.

CAD and numerical analysis in designElectric-motor design has been

computerised since the early days ofcomputers, initially with the coding of wellestablished design procedures and with the'requisitioning' aspects of managing data andperformance characteristics for a range ofmodels that use standard parts, slotgeometries etc. More recently electromagneticfield analysis has emerged from the academicworld into the design office as a tool foroptimising designs, particularly with respect to

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the efficient utilisation of materials andoptimisation of geometry. The most popularmethods for analysis are based on the finite-element method, and there are now fewdesign offices not equipped with it in one formor another. Several commercial packages existfor magnetostatic nonlinear problems in twodimensions (usually in a transverse cross-section of the motor). Three-dimensionalpackages are still rare, but there are severalprograms available to handle eddy-currentproblems in two dimensions.

The finite-element technique suffers fromcertain engineering limitations when applied tomotor design. It is essentially an analytical toolrequiring large amounts of detailed input dataand highly trained operators. It is accurate onlyin idealised situations where the parasiticeffects have been removed. It is too slow to becost effective in a design-synthesis mode, andits output needs a great deal of interpretation.It is also 'current driven' and not easily adaptedto solving 'voltage-driven' problems, which aremore common in electronically controlleddrives. It is most useful in helping tounderstand a theoretical problem that is toodifficult for conventional analysis, and in thisrole it has undoubtedly helped to refine manyexisting motor designs and improve some newones.

The primary problems in motor design arenot simply electromagnetic but require asynthesising approach to materials utilisationand design-for-manufacture. Thismultidisciplinary problem includes not onlyelectromagnetic analysis but heat transfer andmechanical design as well. The situation ismore complicated in adjustable-speed driveswhere the supply waveforms are 'switch mode'rather than pure sine waves or DC. In thesecases simulation may be necessary todetermine the expected performance of agiven design over a wide range of operatingconditions, and there are several softwarepackages available for this type of work,including SIMNON from the University of Lund,Sweden; Control-C; ACSL; the ElectromagneticTransients program (EMTP); and others.Suitably modified and extended, many of these

3 Development ofpermanent-magnetmaterials in terms ofmaximum energy productof commercial grades. Thechoice of magnet dependson many factors includingcost temperaturecharacteristics, and thestraightness of thedemagnetisation curve inthe second quadrant ofthe magnet's B/H loop.Energy product is only arough guide to thesuitability of a magnetmaterial

POWER ENGINEERING JOURNAL SEPTEMBER 1987 285

packages permit the simulation of quitedetailed motor-drive models including controls.They are often used for the development ofcontrol algorithms that are subsequentlyprogrammed in a microprocessor or gate array.

Design programs are usually proprietary andthere are very few available for commercialsale, even for common motor types like the DCcommutator motor or the brushless DC motor.However, the proliferation of new designs andthe increasing use of PCs and single-userworkstations may encourage the developmentof commercial software in this area.

Other contributing technologiesPlastics and composite materials find many

applications in motors. Fans, slot liners andwedges, endbells and covers, and windingsupports are the commonest, but moulded slotinsulation and encapsulation of rotors arenewer possibilities. In brushless motorsdesigned for high peripheral speeds, Kevlar orglass banding can be used to retain surface-mounted magnets.

Motor drives often require transducers forcontrol and protection, and there has beenexcellent progress in current-sensor and shaft-position sensor technology. In particular thelinearity and temperature independence ofHall-effect current sensors has improvedgreatly, and it is common to mount thesedevices in the same package, or on the sameprinted-circuit card, as the driver stage of thepower electronics in small drives. For largerdrives flux-nulling current sensors can be usedwith bandwidths of up to several kilohertz andisolation at least as good as that of a currenttransformer.

In brushless drives the commutation signalsare often derived from three Hall sensors in themotor. These are activated either by the rotormagnet or by a separate magnet ring.Alternatively, optical interrupters may be usedwith a slotted disc mounted on the shaft.These devices only provide commutationsignals, although at sufficiently high speedsthey can be used to generate a speed signalvia a frequency/voltage conversion. For motioncontrol systems and servo-quality drivesseparate velocity and position transducersusually have to be used. For such systems theresolver is gaining in popularity because of itsruggedness and its capability to provideabsolute position and velocity with highresolution (16bits) and accuracy in one sensor.The brushless resolver is particularly reliableand there are available several types of R/Dconvertors with tracking capability up to morethan 40000 rev/min. Low-cost versions arealso available.

The ease with which the R/D convertor canbe interfaced to digital controls is anotherfactor in its favour. It can, of course, be.used toprovide the commutation signals, and it is arelatively simple matter with a digital controlsystem to modulate these as a function ofspeed and load to improve performance. Intracking R/D convertors the velocity signal is atrue analogue signal usable down to very lowspeeds.

Which motor?Today's technology

The traditional DC commutator motor withelectronically adjustable voltage has held on toa prominent place in the world of motioncontrol right up to the present time. There aregood reasons for this. It is easy to control,stable and requires relatively fewsemiconductor devices. It has benefitted fromthe developments that have taken place inelectronics, and this has helped to keep itcompetitive. However, the relative cost of theelectronics is decreasing steadily, tending tobalance the cost equation in favour of ACinduction motor and brushless DC drives. Andalthough brushgear is reliable, it generallyrequires costly maintenance.

AC drives are indeed taking over from DC athigher power levels in many applications (butnot all). In the fractional and low integral-horsepower range the complexity of the ACdrive is an impediment, especially when thedrive must be designed for good dynamicperformance, high efficiency, or a wide rangeof operating speeds. These design goals -cannot be met adequately with series- or triac-controlled induction motors, which aretherefore restricted to applications where lowcost is the only criterion.

The efficiency and power factor of inductionmotors falls off markedly in small sizes becauseof the natural laws of scaling, particularly atpart load. It is helpful to understand thefundamental reason why this is so. As a motorof given geometry is scaled down in dimension,if all dimensions are scaled at the same rate,the MMF required to produce a given fluxdensity decreases in proportion to the lineardimension. But the cross-section available forconductors decreases with the square of thelinear dimension, as does the area available forheat transfer. This continues down to the sizeat which the mechanical air gap reaches alower limit determined by manufacturingtolerances. Further scaling down results in amore-or-less constant MMF requirement, whilethe areas continue to decrease with the squareof the linear dimension. There is thus an'excitation penalty' or 'magnetisation penalty'which becomes rapidly more severe as thescale is reduced. It is for this reason thatpermanent magnets are so necessary in smallmotors. By providing flux without copperlosses, they directly alleviate the excitationpenalty.

Together these factors favour the use ofbrushless PM drives in the low power range.The smaller the motor, the more sense it makesto use permanent magnets for excitation. It isnot practical to be precise about the'breakpoint' below which PM brushless motorsoutperform induction motors, but it is certainlyin the range between 5 and 10kW. Above thissize the induction motor improves rapidly,while the cost of magnets works against thePM motor.

Evolution of motorsA better appreciation of the choice between

motor types can be derived from an attemptto trace their evolution. In Fig. 4 the top row

286 POWER ENGINEERING JOURNAL SEPTEMBER 1987

contains the three 'classical' motors: DCcommutator (with wound field); ACsynchronous; and AC induction. The term'classical' is used because of the long pedigreeof these machines, together with the fact thatthey all produce essentially constantinstantaneous torque (i.e. no torque ripple)when operating from pure DC or AC sinewavesupplies. All these motors, of course, can runwithout electronic converters.

The second row shows how the PM DCcommutator motor is derived from the wound-field DC machine by replacing the field windingand pole structure with permanent magnets.Generally speaking this permits a largereduction in frame size, because of the efficientuse of radial space by the magnet and theelimination of field losses. Armature reaction isusually reduced and commutation is improved.The facility for varying the flux is sacrificed, butin smaller drives the need for field weakening israre anyway. The PM DC motor is usually fedfrom an adjustable voltage supply, either linearor pulse-width modulated.

The second machine in row 2 is thesynchronous machine with its field windingreplaced by permanent magnets. The facilityfor varying the flux is again sacrificed, but theadvantage is the elimination of brushes,slip-rings and rotor copper losses. This motor isstill a synchronous AC motor and can thereforerun from a sinewave supply without electroniccommutation. If a cage winding is included, itcan self-start 'across-the-line'5.

The AC PM synchronous motor can also bebuilt without the cage winding, which makesconsiderably more space for magnets andeases the problem of optimising the rotorgeometry; but in this case synchronism can beguaranteed only by controlling the supplyfrequency to be phase-locked to the rotorposition at all times6. The most direct way toachieve this is through field-oriented current-controlled pulse-width modulation using ashaft-position transducer. Since the field isideally a pure rotating wave, the shaft-positiontransducer should provide a continuousposition signal like a resolver. Simplertechniques not requiring a shaft-positiontransducer are possible, but they require somecontrol means of stabilisation and there maybe a compromise in torque margin andpossibly in output; dynamic performance willcertainly be poorer.

The configuration of the magnets in the ACPM synchronous motor is important. Torque isproduced by the tendency of the magnets toalign with the axis of stator MMF, but there isalso a reluctance torque because of the 2-axissymmetry. The magnet torque variessinusoidally with the angle between the statorMMF axis and the direct axis of the rotor, the'period' being equal to two pole pitches. Thereluctance torque also varies sinusoidally withthis angle, but with half the period. There isonly one angle at which the sum of thesetorques is maximised for a given stator current.For smooth torque production this angle mustbe kept constant, which is the main reasonwhy such motors cannot readily be operated

3-phase AC 3-phaseAC

06DC

3-phaseAC

3-phase square wave

PM

from square-wave brushless DC controllers,even though the 3-phase power circuit is thesame.

The AC motor works better with 180°conduction and three transistors conducting atany time (not two). Its control is thus a littlemore complicated than that of the brushlessDC motor (see below), but it is lesscomplicated than that of the induction motor,and there are other important advantages. Oneis that, because of the reluctance torque, themagnet weight is much smaller than that ofthe conventional brushless DC motor. A secondis the considerable potential for fieldweakening by changing the control anglediscussed above. This permits a constant-power locus on the torque/speed characteristicat high speeds. The PM AC motor is muchmore efficient than the induction motor andhas the important advantage of thesynchronous machine in having very low rotorlosses.

The AC PM synchronous motor operates asa synchronous reluctance motor if themagnets are left out or demagnetised. Thisprovides a measure of fault tolerance in theevent of partial or total demagnetisationthrough abnormal operating conditions. It alsopermits the motor to be built as a magnet-freereluctance motor. This may be desirable for

4 Evolution of motortypes for small drivesin the range 01—10kW.Top row (left to right):wound-field DCcommutator; wound-fieldsynchronous; induction.Middle row (left to right):PM DC commutator; PMreluctance synchronous.Bottom row: PM DCbrushless

POWER ENGINEERING JOURNAL SEPTEMBER 1987 287

5 Cross-sections of switched reluctance motors: (a) 3-phase with 6 stator poles and4 rotor poles; (b) 4-phase with 8 stator poles and 6 rotor poles; (c) 3-phase with 6stator poles and 2 rotor poles

certain applications, e.g. in a high-temperatureapplication.

Returning to the evolution of Fig. 4, there isno second-row equivalent of the inductionmotor. Permanent magnets cannot be used ininduction motors because the rotating field isnot fixed relative to either the stator or therotor. Slip is essential for torque production.Because of this it is impossible, even in theory,to achieve zero rotor losses. This is one of thechief limitations of the induction motor, sincerotor losses are more difficult to remove thanstator losses. The induction motor is indeed'brushless' and can operate with simplecontrols not requiring a shaft-positiontransducer. However, for the best dynamicperformance and efficiency it requires suchcontrol sophistications as slip control, fluxoptimisation, or full field orientation withcompensation for variations in motorparameters with temperature and load.

The final transformation of the PM DCmotor in Fig. 4 is to eliminate the commutatorand brushes; this requires the inversion of thestator and the rotor. The disposition ofmagnets and windings, and the currentwaveforms, must now be determined in such away as to reproduce the ripple-free torque ofthe commutator motor. The commutatormotor really has a polyphase AC rotor, thenumber of phases being related to the numberof commutator segments. The commutatorconnects the supply to the phases in such away as to maximise the collected EMF, whilesharing the current between the phases at alltimes. With sufficient commutator segmentsthe resulting overlap between 'phases'produces very smooth torque and current, andthere is a high utilisation of materials. Thebrushless motor cannot employ the sameprinciples because a large number of phasestranslates into a large number of powersemiconductors. In practice there is noeconomic option other than to employ threeor four phases (occasionally one or two), andto use more or less concentrated windingswith 120° conduction intervals in an attemptto achieve smooth torque. Inevitably thisresults in ripple associated with thecommutations, of which there are typically sixevery 360° electrical.

Brushless PM motors have the advantagethat the power winding is on the stator whereits heat can be removed more effectively, whilethe rotor losses are theoretically zero. Thisprovides a high efficiency or power density (orboth), with a high torque/inertia ratio.

An interesting point about all the motors onthe diagonal of Fig. 4 is that they all share thesame power-circuit topology (three 'totem-pole' phase legs with the motor windingsconnected in star or delta to the midpoints), asin Fig. 2. This gives rise to the concept of afamily of motor drives providing a wide varietyof motors and motor characteristics, but with ahigh degree of commonality and uniformity inthe control and power electronics and all theassociated transducers. It is now common topurchase complete phase legs or evencomplete 3-phase bridges with freewheeling

288 POWER ENGINEERING JOURNAL SEPTEMBER 1987

diodes in a single package, making thisconcept even more attractive. It is interestingto note that the switched reluctance motorcannot use this circuit topology without havingblocking diodes in series with the windings.However, the synchronous reluctance motor,derived from the PM AC motor, arguably hasall of the advantages of the switchedreluctance motor (with the additionaladvantages of low noise and torque ripple).This family of drives therefore covers almost allrequirements, the main types being theconventional brushless DC; the 'magnet-free'brushless DC or AC drive; the induction-motordrive; and the PM synchronous drive with itswide speed range and low magnet weight.

A major class of motors not included inFig. 4 is the stepper motor. Steppers are alwaysbrushless and almost always operate withoutshaft-position sensing. Although they havemany properties in common with synchronousand brushless DC motors they cannot naturallybe evolved from the motors in Fig. 4. Bydefinition they are pulsed-torque machinesincapable of achieving ripple-free torque bynormal means. Variable-reluctance (VR) andhybrid steppers can achieve an internal torquemultiplication through the use of multipleteeth per stator pole and through the 'vernier'effect of having different numbers of rotor andstator poles. Both these effects work byincreasing the number of torque impulses perrevolution, and the price to be paid is anincrease in commutation frequency and ironlosses. Steppers therefore have hightorque/weight and high torque/inertia ratios,but are limited in top speed and power/weightratio. The fine tooth structure requires a smallair gap, which adds to the manufacturing cost.Beyond a certain number of teeth per pole thetorque gain is 'washed out' by the effect ofMMF drop in the iron, and such motors requireexpensive lamination steels to get the best outof them.

The switched reluctance motor or variable-reluctance motor is a direct derivative of thesingle-stack VR stepper, in which the phasecurrent is carefully phased relative to the rotorposition in order to optimise operation in the'slewing' (continuous rotation) mode (see Figs.5 and 6). This is easiest to implement with ashaft-position transducer similar to that whichis required for the brushless DC motor, andindeed the resulting drive is very much like abrushless DC drive without magnets. With thisform of control the switched reluctance motoris not strictly a 'pulsed-torque' motor becauseit can produce continuous torque at any rotorposition and any speed. There is still aninherent torque ripple, but this can beminimised by several methods and in practice itis not necessarily worse than that of thebrushless DC motor.

In relation to the earlier comments madeabout scaling effects, the switched reluctancemotor suffers the same excitation penalty asthe induction motor and therefore it cannot beexpected to equal the efficiency or powerdensity of the PM motor in the same framesize, provided that the speed, copper weight,

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lamination material and temperature rise arethe same. This qualification is, of course, wherethe interest lies. For equal cost, the switchedreluctance motor may well be the best ofthem all.

ConclusionThe brief survey of new technology in small

motor drives shows a proliferation of newideas, materials and components that obviouslygenerates many opportunities but alsopresents a confusing prospect: what is the bestdrive for a particular job? Later articles will dealin more detail with particular drives, but a starthas been made in this article by attempting totrace the evolution of the different motor typesin such a way as to bring out their mostimportant advantages and disadvantages. Themotor is the primary determinant of thecharacteristics of the drive as seen by the user,and it also determines the requirements on thepower semiconductors, the convertor circuit,and the control. So, far from being 'maturetechnology', the electric motor is goingthrough major evolutionary changes, especiallyin the low-integral-horsepower and fractional-horsepower ranges for motion-controlapplications.

References1 BOSE, B. K.: 'Power electronics and AC drives'

(Prentice-Hall, 1986)2 LEONHARD, W.: 'Microcomputer control of high

dynamic performance AC drives', Automatica,1986, p. 1

3 OWYANG, K., et a/.: 'High voltage switchingmodule uses power IC technology',Powertechnics, Jan. 1987, pp.25-30

4 BALIGA, B. J.: 'Modern power devices' (JohnWiley, 1987)

5 MILLER, T. J. E.: 'Synchronisation of line-startpermanent-magnet AC motors' IEEE Trans.,1984, PAS-103, pp.1822-1828

6 JAHNS, T. M., et a/.: 'Interior magnetsynchronous motors for adjustable-speed drives',ibid, 1986, IA-22, pp.738-747

© IEE: 1987

Tim Miller is Titular Professor in PowerElectronics, The University, Glasgow G12 8QQ,UK. He is an IEE Member

6 Convertor circuit forswitched reluctancemotor. This is the onlypractical circuit that givesquiet PWM and fullcontrol. Unipolar andother circuits employingfewer than 2n switchesfor an n-phase motor allsuffer from extracomponents or controllimitations

POWER ENGINEERING JOURNAL SEPTEMBER 1987 289

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Prices include postage within the UK. Outside the UK customers should add10% of the total price to cover postage by Bulk Air Mail to Europe. OutsideEurope 15% should be added to the price to cover postage by AcceleratedSurface Post. Airmail rates are available on request. Credit card orders(Access/Mastercard and Visa) are considered prepaid and will be accepted bytelephone on 0462 53331. Invoices for orders that are not prepaid will include ahandling and package charge of £1.50 per book (maximum £6.00).

IEE

290 POWER ENGINEERING JOURNAL SEPTEMBER 1987