How do DC motor drives work

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Published Date:25-10-2017
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4 D.C. MOTOR DRIVES INTRODUCTION The thyristor d.c. drive remains an important speed-controlled indus- trialdrive,especiallywherethehighermaintenancecostassociatedwith thed.c.motorbrushes(c.f.inductionmotor)istolerable.Thecontrolled (thyristor) rectiWer provides a low-impedance adjustable ‘d.c.’ voltage for the motor armature, thereby providing speed control. Until the 1960s, the only really satisfactory way of obtaining the variable-voltage d.c. supply needed for speed control of an industrial d.c. motor was to generate it with a d.c. generator. The generator was drivenatWxedspeedbyaninductionmotor,andtheWeldofthegenerator was varied in order to vary the generated voltage. The motor/generator (MG)setcouldbesitedremotefromthed.c.motor,andmulti-drivesites (e.g. steelworks) would have large rooms full of MG sets, one for each variable-speedmotorontheplant.Threemachines(allofthesamepower rating)wererequiredforeachofthese‘WardLeonard’drives,whichwas goodbusinessforthemotormanufacturer.Forabriefperiodinthe1950s theyweresupersededbygrid-controlledmercuryarcrectiWers,butthese were soon replaced by thyristor converters which oVered cheaper Wrst cost, higher eYciency (typically over 95%), smaller size, reduced main- tenance,andfasterresponsetochangesinsetspeed.Thedisadvantagesof rectiWed supplies are that the waveforms are not pure d.c., that the overload capacity of the converter is very limited, and that a single converterisnotcapableofregeneration. Though no longer pre-eminent, study of the d.c. drive is valuable for several reasons: . Thestructureandoperationofthed.c.drivearereXectedinalmostall other drives, and lessons learned from the study of the d.c. drive134 Electric Motors and Drives . Thed.c.drivetendstoremaintheyardstickbywhichotherdrivesare judged. . Under constant-Xux conditions the behaviour is governed by a rela- tively simple set of linear equations, so predicting both steady-state and transient behaviour is not diYcult. When we turn to the succes- sors of the d.c. drive, notably the induction motor drive, we will Wnd thatthingsaremuchmorecomplex,andthatinordertoovercomethe poor transient behaviour, the strategies adopted are based on emula- ting the d.c. drive. TheWrstandmajorpartofthischapterisdevotedtothyristor-feddrives, after which we will look brieXy at chopper-fed drives that are used mainly in medium and small sizes, and Wnally turn attention to small servo-type drives. THYRISTOR D.C. DRIVES – GENERAL For motors up to a few kilowatts the armature converter can be supplied from either single-phase or three-phase mains, but for larger motors three-phase is always used. A separate thyristor or diode rectiWer is used to supply the Weld of the motor: the power is much less than the armature power, so the supply is often single-phase, as shown in Figure 4.1. ThearrangementshowninFigure4.1istypicalofthemajorityofd.c. drives and provides for closed-loop speed control. The function of the twocontrolloopswillbeexploredlater,butreaderswhoarenotfamiliar 3-phase 1-phase Speed Control reference and M Firing Current feedback TG Speed feedback Figure 4.1 Schematic diagram of speed-controlled d.c. motor driveD.C. Motor Drives 135 Plate 4.1 High performance force-ventilated d.c. motor. The motor is of all-laminated construction and designed for use with a thyristor converter. The small blower motor is an induction machine that runs continuously, thereby allowing the main motor to maintain full torque at low speed without overheating. (Photo courtesy of Control Techniques) with the basics of feedback and closed-loop systems may Wnd it helpful to read through the Appendix at this point. The main power circuit consists of a six-thyristor bridge circuit (as discussed in Chapter 2), which rectiWes the incoming a.c. supply to producead.c.supplytothemotorarmature.Theassemblyofthyristors, mounted on a heatsink, is usually referred to as the ‘stack’. By altering the Wring angle of the thyristors the mean value of the rectiWed voltage can be varied, thereby allowing the motor speed to be controlled. We saw in Chapter 2 that the controlled rectiWer produces a crude form of d.c. with apronounced ripple in the outputvoltage. This ripple component gives rise to pulsating currents andXuxes in the motor, and in order to avoid excessive eddy-current losses and commutation prob- lems, the poles and frame should be of laminated construction. It is accepted practice for motors supplied for use with thyristor drives to have laminated construction, but older motors often have solid poles and/or frames, and these will not always work satisfactorily with a rectiWer supply. It is also the norm for drive motors to be supplied with an attached ‘blower’ motor as standard. This provides continuous throughventilationandallowsthemotortooperatecontinuouslyatfull torque even down to the lowest speeds without overheating. Lowpowercontrolcircuitsareusedtomonitortheprincipalvariables ofinterest (usuallymotorcurrentandspeed),andtogenerateappropri- ate Wring pulses so that the motor maintains constant speed despite136 Electric Motors and Drives variations in the load. The ‘speed reference’ (Figure 4.1) is typically an analogue voltage varying from 0 to 10 V, and obtained from a manual speed-setting potentiometer or from elsewhere in the plant. Thecombinationofpower,control,andprotectivecircuitsconstitutes theconverter.StandardmodularconvertersareavailableasoV-the-shelf itemsinsizesfrom0.5 kWuptoseveralhundredkW,whilelargerdrives willbetailoredtoindividualrequirements.Individualconvertersmaybe mountedin enclosureswith isolators, fuses etc.,or groupsofconverters may be mounted together to form a multi-motor drive. Motor operation with converter supply The basic operation of the rectifying bridge has been discussed in Chapter2,andwenowturntothematterofhowthed.c.motorbehaves when supplied with ‘d.c.’ from a controlled rectiWer. Bynostretchofimaginationcouldthewaveformsofarmaturevoltage lookedatinChapter2(e.g.Figure2.12)bethoughtofasgoodd.c.,and itwouldnotbeunreasonabletoquestionthewisdomoffeedingsuchan unpleasant looking waveform to a d.c. motor. In fact it turns out that themotorworksalmostaswellasitwouldiffedwithpured.c.,fortwo main reasons. Firstly, the armature inductance of the motor causes the waveformofarmaturecurrenttobemuchsmootherthanthewaveform ofarmaturevoltage,whichinturnmeansthatthetorquerippleismuch less than might have been feared. And secondly, the inertia of the armature is suYciently large for the speed to remain almost steady despite the torque ripple. It is indeed fortunate that such a simple arrangement works so well, because any attempt to smooth-out the voltage waveform (perhaps by adding smoothing capacitors) would prove to be prohibitively expensive in the power ranges of interest. Motor current waveforms For the sake of simplicity we will look at operation from a single-phase (2-pulse) converter, but similar conclusions apply to the 6-pulse con- verter. The voltage (V ) applied to the motor armature is typically as a shown in Figure 4.2(a): as we saw in Chapter 2, it consists of rectiWed ‘chunks’ of the incoming mains voltage, the precise shape and average value depending on theWring angle. Thevoltagewaveformcanbeconsideredtoconsistofameand.c.level (V ), and a superimposed pulsating or ripple component which we can dc denotelooselyasV .ThemeanvoltageV canbealteredbyvaryingthe ac dc Wringangle,whichalsoincidentallyalterstheripple(i.e.V ). acD.C. Motor Drives 137 TheripplevoltagecausesaripplecurrenttoXowinthearmature,but because of the armature inductance, the amplitude of the ripple current issmall.In otherwords, thearmaturepresents ahighimpedance to a.c. voltages. This smoothing eVect of the armature inductance is shown in Figure 4.2(b), from which it can be seen that the current ripple is relatively small in comparison with the corresponding voltage ripple. The average value of the ripple current is of course zero, so it has no eVect on the average torque of the motor. There is nevertheless a variation in torque every half-cycle of the mains, but because it is of small amplitude and high frequency the variation in speed (and hence back e.m.f., E) will not usually be noticeable. Thecurrentattheendofeachpulseisthesameasatthebeginning,so itfollowsthattheaveragevoltageacrossthearmatureinductance(L)is zero.Wecanthereforeequatetheaverageappliedvoltagetothesumof the back e.m.f. (assumed pure d.c. because we are ignoring speed Xuc- tuations)andtheaveragevoltageacrossthearmatureresistance,toyield V ¼ EþI R (4:1) dc dc whichis exactly thesame as for operation from a pure d.c. supply.This isvery important, as it underlinesthe factthat we can controlthe mean motor voltage, and hence the speed, simply by varying the converter delay angle. Volts (a) V dc 60˚ Amps High torque (b) I dc Low torque Time Figure 4.2 Armature voltage (a) and armature current (b) waveforms for continuous- current operation of a d.c. motor supplied from a single-phase fully-controlled thyristor converter, withWring angle of 608138 Electric Motors and Drives The smoothing eVect of the armature inductance is important in achieving successful motor operation: the armature acts as a low-pass Wlter,blockingmostoftheripple,andleadingtoamoreorlessconstant armature current. For the smoothing to be eVective, the armature time- constantneedstobelongcomparedwiththepulseduration(halfacycle witha2-pulsedrive,butonlyonesixthofacycleina6-pulsedrive).This conditionismetinall6-pulsedrives,andinmany2-pulseones.Overall, themotorthenbehavesmuchasitwouldifitwassuppliedfromanideal 2 d.c.source (thoughthe I Rlossishigherthan itwouldbe ifthecurrent was perfectly smooth). The no-load speed is determined by the applied voltage (which de- pendsontheWringangleoftheconverter);thereisasmalldropinspeed with load and as we have previously noted, the average current is determined by the load. In Figure 4.2, for example, the voltage wave- formin(a)appliesequallyforthetwoloadconditionsrepresentedin(b), where the upper current waveform corresponds to a high value of load torquewhilethelowerisforamuchlighterload;thespeedbeingalmost the same in both cases. (The small diVerence in speed is due to IR as explainedinChapter3).Weshouldnotethatthecurrentrippleremains the same – only the average current changes with load. Broadly speak- ing, therefore, we can say that the speed is determined by the converter Wring angle, which represents a very satisfactory state because we can control theWring angle by low-power control circuits and thereby regu- late the speed of the drive. The current waveforms in Figure 4.2(b) are referred to as ‘continu- ous’, because there is never any time during which the current is not Xowing. This ‘continuous current’ condition is the norm in most drives, and it is highly desirable because it is only under continuous current conditions that the average voltage from the converter is determined solely by the Wring angle, and is independent of the load current. We can see why this is so with the aid of Figure 2.7, imagining that the motor is connected to the output terminals and that it is drawing a continuous current. For half of a complete cycle, the current will Xow into the motor from T1 and return to the mains via T4, so the armature is eVectively switched across the supply and the armature voltage is equal to the supply voltage, which is assumed to be ideal, i.e. it is independent of the current drawn. For the other half of the time, themotorcurrentXowsfromT2andreturnstothesupplyviaT3, so the motor is again hooked-up to the supply, but this time the connections are reversed. Hence the average armature voltage – and hence to a Wrst approximation the speed – are deWned once the Wring angle is set.D.C. Motor Drives 139 Discontinuous current WecanseefromFigure4.2thatastheloadtorqueisreduced,therewill come a point where the minima of the current ripple touches the zero- current line, i.e. the current reaches the boundary between continuous and discontinuous current. The load at which this occurs will also depend on the armature inductance, because the higher the inductance thesmootherthecurrent(i.e. thelesstheripple).Discontinuouscurrent mode is therefore most likely to be encountered in small machines with low inductance (particularly when fed from two-pulse converters) and under light-load or no-load conditions. Typicalarmaturevoltageandcurrentwaveformsinthediscontinuous mode are shown in Figure 4.3, the armature current consisting of discrete pulses of current that occur only while the armature is con- nected to the supply, with zero current for the period (represented by u inFigure4.3)whennoneofthethyristorsareconductingandthemotor is coasting free from the supply. The shape of the currentwaveform can be understood by noting that with resistance neglected, equation (3.7) can be rearranged as di 1 ¼ðÞ VE (4:2) dt L which shows that the rate of change of current (i.e. the gradient of the lowergraphinFigure4.3)isdeterminedbytheinstantaneousdiVerence between the applied voltage V and the motional e.m.f. E. Values of (V E) are shown by the vertical hatchings in Figure 4.3, from which itcanbeseenthatifV E,thecurrentisincreasing,whileifV E,the currentisfalling.Thepeakcurrentisthusdeterminedbytheareaofthe upper or lower shaded areas of the upper graph. TheWringangleinFigures4.2and4.3isthesame,at608,buttheload is less in Figure 4.3 and hence the average current is lower (though, for thesakeoftheexplanationoVeredbelowthecurrentaxisinFigure4.3is expanded as compared with that in Figure 4.2). It should be clear by comparing these Wgures that the armature voltage waveforms (solid lines) diVer because, in Figure 4.3, the current falls to zero before the next Wring pulse arrives and during the period shown as u the motor Xoats free, its terminal voltage during this time being simply the mo- tional e.m.f. (E). To simplify Figure 4.3 it has been assumed that the armatureresistanceissmallandthatthecorrespondingvolt-drop(I R ) a a canbeignored.Inthiscase,theaveragearmaturevoltage(V )mustbe dc equal to the motional e.m.f., because there can be no average voltage across the armature inductance when there is no nett change in the140 Electric Motors and Drives 60 V E(=V ) dc θ I dc Figure 4.3 Armature voltage current waveforms for discontinuous-current operation of a d.c. motor supplied from a single-phase fully-controlled thyristor converter, withWring angle of 608 currentoveronepulse:thehatchedareas–representingthevolt-seconds in the inductor – are therefore equal. ThemostimportantdiVerencebetweenFigures4.2and4.3isthatthe average voltage is higher when the current is discontinuous, and hence thespeedcorrespondingtotheconditionsinFigure4.3ishigherthanin 4.2 despite both having the same Wring angle. And whereas in continu- ous mode a load increase can be met by an increased armature current without aVecting the voltage (and hence speed), the situation is very diVerent when the current is discontinuous. In the latter case, the only way that the average current can increase is when speed (and hence E) falls so that the shaded areas in Figure 4.3 become larger. Thismeansthatfromtheuser’sviewpointthebehaviourofthemotor in discontinuous mode is much worse than in the continuous current mode, because as the load torque is increased, there is a serious drop in speed.Theresultingtorque–speedcurvethereforehasaveryunwelcome ‘droopy’ characteristic in the discontinuous current region, as shown in 2 Figure4.4,andinadditiontheI Rlossismuchhigherthanitwouldbe with pure d.c. Under very light or no-load conditions, the pulses of current become virtuallynon-existent, theshadedareasin Figure4.3 becomevery small andthemotorspeedreachesapointatwhichthebacke.m.f.isequalto the peak of the supply voltage. It is easy to see that inherent torque–speed curves with sudden dis- continuities of theform shown in Figure 4.4 are very undesirable. If for example the Wring angle is set to zero and the motor is fully loaded, itsD.C. Motor Drives 141 A Torque a = 60 a = 0 Effect of extra B inductance C 0 Speed Figure 4.4 Torque-speed curves illustrating the undesirable ‘droopy’ characteristic associated with discontinuous current. The improved characteristic (shown dotted) corresponds to operation with continuous current speed will settle at point A, its average armature voltage and current havingtheirfull(rated)values.Astheloadisreduced,currentremaining continuous, there is the expected slight rise in speed, until point B is reached. This is the point at which the current is about to enter the discontinuous phase. Any further reduction in the load torque then produces a wholly disproportionate – not to say frightening – increase inspeed,especiallyiftheloadisreducedtozerowhenthespeedreaches point C. There are two ways by which we can improve these inherently poor characteristics. Firstly, we can add extra inductance in series with the armature to further smooth the current waveform and lessen the likeli- hoodofdiscontinuouscurrent.TheeVectofaddinginductanceisshown by the dotted lines in Figure 4.4. And secondly, we can switch from a single-phase converter to a 3-phase converter which produces smoother voltage and current waveforms, as discussed in Chapter 2. When the converter and motor are incorporated in a closed-loop controltheuser should be unaware of any shortcomingsin theinherent motor/converter characteristics because the control system automatic- ally alters the Wring angle to achieve the target speed at all loads. In relation to Figure 4.4, for example, as far as the user is concerned the control system will conWne operation to the shaded region, and the fact that the motor is theoretically capable of running unloaded at the high speed corresponding to point C is only of academic interest. Converter output impedance: overlap Sofarwehavetacitlyassumedthattheoutputvoltagefromtheconverter wasindependentofthecurrentdrawnbythemotor,anddependedonly142 Electric Motors and Drives onthedelayanglea.Inotherwordswehavetreatedtheconverterasan idealvoltagesource. In practice the a.c. supply has a Wnite impedance, and we must therefore expect a volt-drop which depends on the current being drawn by the motor. Perhaps surprisingly, the supply impedance (which is mainly due to inductive leakage reactances in transformers) manifests itself at the output stage of the converter as a supply resist- ance, so the supply volt-drop (or regulation) is directly proportional to the motor armature current. It is not appropriate to go into more detail here, but we should note that the eVect of the inductive reactance of the supply is to delay the transfer (or commutation) of thecurrent between thyristors;a phenom- enon known as overlap. The consequence of overlap is that instead of the output voltage making an abrupt jump at the start of each pulse, there is a short period when two thyristors are conducting simultan- eously. During this interval the output voltage is the mean of the voltages of the incoming and outgoing voltages, as shown typically in Figure 4.5. It is important for users to be aware that overlap is to be expected, as otherwise they may be alarmed the Wrst time they connect anoscilloscopetothemotorterminals.Whenthedriveisconnectedtoa ‘stiV’(i.e.lowimpedance)industrialsupplytheoverlapwillonlylastfor perhapsafewmicroseconds,sothe‘notch’showninFigure4.5wouldbe barely visible on an oscilloscope. Books always exaggerate the width of the overlap for the sake of clarity, as in Figure 4.5: with a 50 or 60 Hz supply,iftheoverlaplastsformorethansay1 ms,theimplicationisthat the supply system impedance is too high for the size of converter in question, or conversely, the converter is too big for the supply. Returning to the practical consequences of supply impedance, we simply have to allow for the presence of an extra ‘source resistance’ in series with the output voltage of the converter. This source resistance is Overlap Volts V dc Time Figure 4.5 Distortion of converter output voltage waveform caused by rectiWer overlapD.C. Motor Drives 143 in series with the motor armature resistance, and hence the motor torque–speed curves for each value of a have a somewhat steeper droop than they would if the supply impedance was zero. Four-quadrant operation and inversion So far we have looked at the converter as a rectiWer, supplying power from the a.c. mains to a d.c. machine running in the positive direction andactingas amotor. As explainedin Chapter3, thisis knownas one- quadrantoperation,byreferencetoquadrant1ofthecompletetorque– speed plane shown in Figure 3.16. But suppose we want to run the machine as a motor in the opposite direction,withnegativespeedandtorque,i.e.inquadrant3;howdowe do it? And what about operating the machine as a generator, so that,theconverterthen‘inverting’power rather than rectifying, and the system operating in quadrant 2 or quad- rant4.Weneedtodothisifwewanttoachieveregenerativebraking.Is it possible, and if so how? The good news is that as we saw in Chapter 3 the d.c. machine is inherently a bidirectional energy converter. If we apply a positive volt- ageVgreaterthanE,acurrentXowsintothearmatureandthemachine runs as a motor. If we reduce V so that it is less than E, the current, torqueandpowerautomaticallyreversedirection, andthemachineacts as a generator, converting mechanical energy (its own kinetic energy in the case of regenerative braking) into electrical energy. And if we want tomotororgeneratewiththereversedirectionofrotation,allwehaveto do is to reverse thepolarity of thearmature supply.The d.c. machine is inherentlyafour-quadrantdevice,butneedsasupplywhichcanprovide positive or negative voltage, and simultaneously handle either positive or negative current. This is where we meet a snag: a single thyristor converter can only handle current in one direction, because the thyristors are unidi- rectional devices. This does not mean that the converter is incapable of returning power to the supply however. The d.c. current can only be positive, but (provided it is a fully controlled converter) the d.c. output voltagecanbeeitherpositiveornegative(seeChapter2).ThepowerXow can therefore be positive (rectiWcation) or negative (inversion). For normal motoring where the output voltage is positive (and as- suming a fully controlled converter), the delay angle (a) will be up to 908. (It is common practice for the Wring angle corresponding to rated d.c. voltage to be around 208 when the incoming a.c. voltage is normal: if the a.c. voltage falls for any reason, theWring angle can then144 Electric Motors and Drives V dc 0 0 90 180 Figure 4.6 Average d.c. output voltage from a fully-controlled thyristor converter as a function of theWring angle delaya be further reduced to compensate and allow full d.c. voltage to be maintained.) Whenaisgreaterthan908,however,theoutputvoltageisnegative,as indicated by equation (2.5), and is shown in Figure 4.6. A single fully controlled converter therefore has the potential for two-quadrant oper- ation, though it has to be admitted that this capability is not easily exploited unless we are prepared to employ reversing switches in the armature or Weld circuits. This is discussed next. Single-converter reversing drives We will consider a fully controlled converter supplying a permanent- magnet motor, and see how the motor can be regeneratively braked fromfullspeedinonedirection,andthenaccelerateduptofullspeedin reverse.WelookedatthisprocedureinprincipleattheendofChapter3, buthereweexplorethepracticalitiesofachievingitwithaconverter-fed drive.Weshouldbeclearfromtheoutsetthatinpractice,alltheuserhas to do is to change the speed reference signal from full forward to full reverse: the control system in the drive converter takes care of matters from then on. What it does, and how, is discussed below. When the motor is running at full speed forward, the converter delay anglewillbesmall,andtheconverteroutputvoltageVandcurrentIwill both be positive. This condition is shown in Figure 4.7(a), and corres- ponds to operation in quadrant 1. In order to brake the motor, the torque has to be reversed. The only way this can be done is by reversing the direction of armature current. The converter can only supply positive current, so to reverse the motor torquewehavetoreversethearmatureconnections,usingamechanicalD.C. Motor Drives 145 Current Current Current V V V EE E (a) Quadrant 1 (b) Quadrant 2 (c) Quadrant 3 Figure 4.7 Stages in motor reversal using a single-converter drive and mechanical reversing switch switch or contactor, as shown in Figure 4.7(b). (Before operating the contactor, the armature current would be reduced to zero by lowering the converter voltage, so that the contactor is not required to interrupt current.) Note that because the motor is still rotating in the positive direction, the back e.m.f. remains in its original sense; but now the motional e.m.f. is seen to be assisting the current and so to keep the current within bounds the converter must produce a negative voltage V which is just a little less than E. This is achieved by setting the delay angleattheappropriatepointbetween908and1808.(Thedottedlinein Figure 4.6 indicates that the maximum acceptable negative voltage will generally be somewhat less than the maximum positive voltage: this restriction arises because of the need to preserve a margin for commu- tation of current between thyristors.) Note that the converter current is still positive (i.e. upwards in Figure 4.7(b)), but the converter voltage is negative,andpoweristhusXowingbacktothemains.Inthiscondition the system is operating in quadrant 2, and the motor is decelerating because of the negative torque. As the speed falls, E reduces, and so V must be reduced progressively to keep the current at full value. This is achieved automatically by the action of the current-control loop, which is discussed later. The current (i.e. torque) needs to be kept negative in order to run up to speed in the reverse direction, but after the back e.m.f. changes sign (as the motor reverses), the converter voltage again becomes positive and greater than E, as shown in Figure 4.7(c). The converter is then rectifying, with power being fed into the motor, and the system is operating in quadrant 3. Schemes using reversing contactors are not suitable where the revers- ing time is critical, because of the delay caused by the mechanical reversing switch, which may easily amount to 200–400 msec. Field reversal schemes operate in a similar way, but reverse the Weld current instead of the armature current. They are even slower, because of the relatively long time-constant of theWeld winding.146 Electric Motors and Drives Double-converter reversing drives Wherefullfour-quadrantoperationandrapidreversaliscalledfor,two converters connected in anti-parallel are used, as shown in Figure 4.8. One converter supplies positive current to the motor, while the other supplies negative current. The bridges are operated so that their d.c. voltages are almost equal therebyensuringthat anyd.c.circulatingcurrentissmall,andareactor is placed between the bridges to limit the Xow of ripple currents which result from the unequal ripple voltages of the two converters. Alterna- tively,thereactorcanbedispensedwithbyonlyoperatingoneconverter atatime.Thechangeoverfromoneconvertertotheothercanonlytake place after theWringpulseshave beenremoved fromoneconverter,and the armature current has decayed to zero. Appropriate zero-current detection circuitry is provided as an integral part of the drive, so that asfarastheuserisconcerned,thetwoconvertersbehaveasiftheywere a single ideal bidirectional d.c. source. Prospective users need to be aware of the fact that a basic single converter can only provide for operation in one quadrant. If regenera- tivebrakingisrequired,eitherWeldorarmaturereversingcontactorswill beneeded;andifrapidreversalisessential,adoubleconverterhastobe used. All these extras naturally push up the purchase price. Power factor and supply effects One of the drawbacks of a converter-fed d.c. drive is that the supply powerfactorisverylowwhenthemotorisoperatingathightorque(i.e. high current)and low speed (i.e. low armaturevoltage), andis less than unityevenatbasespeedandfullload.Thisisbecausethesupplycurrent waveform lags the supply voltage waveform by the delay angle a,as shown (for a 3-phase converter) in Figure 4.9, and also the supply current is approximately rectangular (rather than sinusoidal). Figure 4.8 Double-converter reversing driveD.C. Motor Drives 147 Phase voltage Current Figure 4.9 Supply voltage and current waveforms for single-phase converter-fed d.c. motor drive It is important to emphasise that the supply power factor is always lagging, even when the converter is inverting. There is no way of avoid- ingthelowpowerfactor,sousersoflargedrivesneedtobepreparedto augment their existing power factor correcting equipment if necessary. The harmonics in the mains current waveform can give rise to a variety of interference problems, and supply authorities generally im- posestatutorylimits.Forlargedrives(sayhundredsofkilowatts),Wlters may have to be provided to prevent these limits from being exceeded. Sincethesupplyimpedanceisneverzero,thereisalsoinevitablysome distortionofthemainsvoltagewaveform,asshowninFigure4.10which indicates the eVect of a 6-pulse converter on the supply line-to-line voltage waveform. The spikes and notches arise because the mains is momentarilyshort-circuitedeachtimethecurrentcommutatesfromone thyristortothenext,i.e.duringtheoverlapperioddiscussedearlier.For the majority of small and medium drives, connected to stiV industrial supplies, these notches are too small to be noticed (they are greatly exaggerated for the sake of clarity in Figure 4.10); but they can pose a Figure 4.10 Distortion of line voltage waveform caused by overlap in three-phase fully-controlled converter.(The width of the notches has been exaggerated for the sake of clarity.)148 Electric Motors and Drives serious interference problem for other consumers when a large drive is connected to a weak supply. CONTROL ARRANGEMENTS FOR D.C. DRIVES The most common arrangement, which is used with only minor vari- ationsfromsmalldrivesofsay0.5 kWuptothelargestindustrialdrives ofseveralmegawatts,istheso-calledtwo-loopcontrol.Thishasaninner feedback loop to control the current (and hence torque) and an outer loop to control speed. When position control is called for, a further outer position loop is added. A two-loop scheme for a thyristor d.c. drive is discussed Wrst, but the essential features are the same in a chopper-fed drive. Later the simpler arrangements used in low-cost small drives are discussed. The discussion is based on analogue control, and as far as possible is limited tothoseaspects whichtheuserneedstoknowaboutandunder- stand.Inpractice,onceadrivehasbeencommissioned,thereareonlya few potentiometer adjustments (or presets in the case of a digital con- trol)towhichtheuserhasaccess.Whilstmostofthemareself-explana- tory (e.g. max. speed, min. speed, accel. and decel. rates), some are less obvious(e.g.‘currentstability’,‘speedstability’,‘IRcomp’.)sotheseare explained. To appreciate the overall operation of a two-loop scheme we can consider what we would do if we were controlling the motor manually. For example, if we found by observing the tachogenerator that the speed was below target, we would want to provide more current (and hence torque) in order to produce acceleration, so we would raise the armature voltage. We would have to do this gingerly however, being mindful of the danger of creating an excessive current because of the delicate balance that exists between the back e.m.f., E and applied voltage, V. We would doubtless wish to keep our eye on the ammeter at all times to avoid blowing-up the thyristor stack, and as the speed approachedthetarget,wewouldtrimbackthecurrent(byloweringthe applied voltage) so as to avoid overshooting the set speed. Actions of thissortarecarriedoutautomaticallybythedrivesystem,whichwewill now explore. A standardd.c. drive system with speed andcurrent controlis shown in Figure 4.11. The primary purpose of the control system is to provide speedcontrol,sothe‘input’tothesystemisthespeedreferencesignalon the left, and the output is the speed of the motor (as measured by the tachogenerator TG) on the right. As with any closed-loop system, theD.C. Motor Drives 149 overallperformanceisheavilydependenton thequalityofthefeedback signal, in this case the speed-proportional voltage provided by the tachogenerator. It is therefore important to ensure that the tacho is of high quality (so that its output voltage does not vary with ambient temperature, and is ripple-free) and as a result the cost of the tacho often represents a signiWcant fraction of the total cost. We will take an overview of how the scheme operates Wrst, and then examine the function of the two loops in more detail. To get an idea of the operation of the system we will consider what will happen if, with the motor running light at a set speed, the speed reference signal is suddenly increased. Because the set (reference) speed isnowgreaterthantheactualspeedtherewillbeaspeederrorsignal(see also Figure 4.12), represented by the output of the left-hand summing junction in Figure 4.11. A speed error indicates that acceleration is required, which in turn means torque, i.e. more current. The speed error is ampliWed by the speed controller (which is more accurately described as a speed-error ampliWer) and the output serves as the refer- enceorinputsignaltotheinnercontrolsystem.Theinnerfeedbackloop is a current-control loop, so when the current reference increases, so does the motor armature current, thereby providing extra torque and initiating acceleration. As the speed rises the speed error reduces, and thecurrentandtorquethereforereducetoobtainasmoothapproachto the target speed. Wewillnowlookinmoredetailattheinner(current-control)loop,as its correct operation is vital to ensure that the thyristors are protected against excessive overcurrents. Speed reference Current reference, I ref Current Speed Controller Controller TG M _ _ Speed Current error error Current feedback Speed feedback Figure4.11 Schematicdiagramofanaloguecontrolled-speeddrivewithcurrentandspeed feedback control loops150 Electric Motors and Drives Current control Theclosed-loopcurrentcontroller,orcurrentloop,isattheheartofthe drive system and is indicated by the shaded region in Figure 4.11. The purpose of the current loop is to make the actual motor current follow the current reference signal (I ) shown in Figure 4.11. It does this by ref comparing a feedback signal of actual motor current with the current reference signal, amplifying the diVerence (or current error), and using the resulting ampliWed current error signal (an analogue voltage) to control the Wring angle a – and hence the output voltage – of the converter. The current feedback signal is obtained either from a d.c. current transformer (which gives an isolated analogue voltage output), or from a.c current transformer/rectiWers in the mains supply lines. The job of comparing the reference (demand) and actual current signals and amplifying the error signal is carried out by the current- error ampliWer. By giving the current error ampliWer a high gain, the actual motor current will always correspond closely to the current reference signal, i.e. the current error will be small, regardless of motor speed.In otherwords, wecanexpecttheactualmotorcurrenttofollow the ‘current reference’ signal at all times, the armature voltage being automatically adjusted by the controller so that, regardless of the speed of the motor, the current has the correct value. Of course no control system can be perfect, but it is usual for the current-error ampliWer to be of the proportional plus integral (PI) type (see below), in which case the actual and demanded currents will be exactly equal under steady-state conditions. TheimportanceofpreventingexcessiveconvertercurrentsfromXow- ing has been emphasised previously, and the current control loop pro- videsthemeanstothisend.Aslongasthecurrentcontrolloopfunctions properly,themotorcurrentcanneverexceedthereferencevalue.Hence by limiting themagnitudeof thecurrentreferencesignal(bymeansofa clamping circuit), the motor current can never exceed the speciWed value. This is shown in Figure 4.12, which represents a small portion of Figure 4.11. The characteristics of the speed controller are shown in the shaded panel, from which we can see that for small errors in speed, the current reference increases in proportion to the speed, thereby ensuring‘linearsystem’behaviourwithasmoothapproachtothetarget speed. However, once the speed error exceeds a limit, the output of the speed-error ampliWer saturates and there is thus no further increase in the current reference. By arranging for this maximum current reference to correspond to the full (rated) current of the system there is no possibility of the current in the motor and converter exceeding itsD.C. Motor Drives 151 Current reference I max Speed error -I max Speed error Speed Current Speed Controller reference reference _ Speed feedback Figure 4.12 Detail showing characteristic of speed error ampliWer rated value, no matter how large the speed error becomes. This point is explored further in Section 4.3.3. This‘electroniccurrentlimiting’isbyfarthemostimportantprotect- ivefeatureofanydrive.Itmeansthatifforexamplethemotorsuddenly stalls because the load seizes (so that the back e.m.f. falls dramatically), the armature voltage will automatically reduce to a very low value, thereby limiting the current to its maximum allowable level. The Wrst thing we should aim at when setting up a drive is a good current loop. In this context, ‘good’ means that the steady-state motor current should correspond exactly with the current reference, and the transientresponsetostepchangesinthecurrentreferenceshouldbefast and well damped. The Wrst of these requirements is satisWed by the integralterminthecurrent-errorampliWer,whilethesecondisobtained by judicious choice of the ampliWer proportional gain and time- constant. As far as the user is concerned, the ‘current stability’ adjust- ment is provided to allow him to optimise the transient response of the current loop. Onapointofjargon,itshouldperhapsbementionedthatthecurrent- error ampliWer is more often than not called either the ‘current control- ler’(as in Figure4.11) or the‘currentampliWer’. TheWrstof these terms isquite sensible,butthesecondcanbeverymisleading: thereisafterall no question of the motor current itself being ampliWed.152 Electric Motors and Drives Torque control For applications requiring the motor to operate with a speciWed torque regardless of speed (e.g. in line tensioning), we can dispense with the outer(speed)loop,andsimplyfeedacurrentreferencesignaldirectlyto thecurrentcontroller(usuallyviathe‘torqueref’terminalonthecontrol board).Thisisbecausetorqueisdirectlyproportionaltocurrent,sothe current controller is in eVect also a torque controller. We may have to make an allowance for accelerating torque by means of a transient ‘inertia compensating’ signal, but this is usually provided for via a potentiometer adjustment or digital preset. In the current-control mode, the current remains constant at the set value, and the steady running speed is determined by the load. If the torque reference signal was set at 50%, for example, and the motor was initially at rest, it would accelerate with a constant current of half rated value until the load torque was equal to the motor torque. Of course, if themotorwasrunningwithoutanyload,itwouldacceleratequickly,the applied voltage ramping up so that it always remained higher than the back e.m.f. by the amount needed to drive the speciWed currentinto the armature. Eventually the motor would reach a speed (a little above normal ‘full’ speed) at which the converter output voltage had reached its upper limit, and it is therefore no longer possible to maintain the set current: thereafter, the motor speed would remain steady. Speed control TheouterloopinFigure4.11providesspeedcontrol.Speedfeedbackis providedbyad.c.tachogeneratorandtheactualandrequiredspeedsare fed into the speed-error ampliWer (often known simply as the speed ampliWer or the speed controller). Any diVerencebetween theactualand desiredspeed isampliWed, and the output serves as the input to the current loop. Hence if for example theactualmotorspeedislessthanthedesiredspeed,thespeedampliWer willdemandcurrentinproportiontothespeederror,andthemotorwill therefore accelerate in an attempt to minimise the speed error. When the load increases, there is an immediate deceleration and the speed-error signal increases, thereby calling on the inner loop for more current. The increased torque results in acceleration and a progressive reduction of the speed error until equilibrium is reached at the point where the current reference (I ) produces a motor current that gives a ref torque equal and opposite to the load torque. Looking at Figure 4.12, where the speed controller is shown as simple proportional ampliWer (P control), it will be readily appreciated that in order for there to be a

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