Section Thyristor converter driven DC motor drive


 Reynold Marshall Parsons
 2 years ago
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1 Section.3  Thyristor converter driven DC motor drive.3.1 Introduction Controllble ACDC converters using thyristors re perhps the most efficient nd most robust power converters for use in DC motor drives. The thyristor is fourlyer (3junction) semiconductor device which cn be turned ON by smll gte current of few millimps. The voltge (nd current) rtings of thyristors rnge from few tens to severl thousnds of volts (mperes). The thyristor, however, does not hve gte turnoff cpbility, t lest in its conventionl form. (Gte turnoff thyristors exist, however, they require specil gte drive circuits which cn be elborte, lossy, nd hence expensive). Once the thyristor is ON, its nodecthode voltge is bout 1.5 volts nd it cn then be turned off by removing the gte current nd by removing the node current by some uxiliry circuits or by the soclled nturl commuttion. The terms nturl commuttion, refer to thyristor opertion in circuits with AC supply. In these circuits, the node current ttempts to reverse due to the nture of the supply voltge nd lod, either or both of which my reverse bis the conducting thyristor nd provide it with n lternte pth for cureent flow. This turns off the thyristors in the converter nturlly. The thyristor hs very good forwrd nd reverse blocking cpbility when its gte current is kept t zero. In this section, we will only consider conventionl, nturlly commutted thyristor converters for DC motor drives..3. The singlephse hlfwve converter driven seprtely excited DC motor Consider the simple singlephse, hlfwve converter of figure.3.1 in which the thyristor gte is triggered by firing control circuit which is synchronised to the c supply. The supply voltge to the converter is singlephse c t 50 Hz, t voltge which is pproprite for developing the required mximum dc voltge t the motor lod. The pek of the c supply voltge is V mx. The control voltge v c selects the ngle of the trigger pulse (clled the firing ngle, ) in ech c (positive) hlfcycle. L i v c F C C V mx sint v e v E E = 45 Figure.3.1 Thyristor converter driven DC motor 1 F. hmn/july 017
2 In the bove thyristor converter circuit, the gte trigger current pulse is issued t nd the rmture current strts to build up through the L circuit of the rmture ginst its bck emf. The rmture current reches pek nd comes down to zero t ngle, which is well before the next trigger pulse due to the combined effect of the negtive input voltge of the supply nd the bck emf of the rmture, e. The rmture current my sustin beyond ngle due to the inductnce L of the rmture. From to, the output voltge of the converter is the sme s the c input voltge, which is now negtive. This is becuse the thyristor is ON during this time. Therefter, the rmture current remins zero nd the the bck emf of the motor voltge ppers cross the output terminls of the converter. These wveforms re shown in figure.3.1. The vribles V, E nd I re the DC (verge) levels of the respective quntities, but these vry with time, s the control voltge v c (or ) vries. For firing ngle, the DC output voltge of the converter is given by 1 V E V V sintd( t) cos cos mx mx (.3.1) Note tht for this converter, the output voltge wveform cross the motor hs one voltge pulse per input AC cycle nd tht the motor voltge nd current wveforms hve significnt ripples. These re undesirble, since they cuse extr mchine losses, torque ripple, noise nd input AC current distortion Anlysis of rmture current In the following nlysis, we ssume tht the rmture current strts from zero in ech cycle, i.e., discontinuous conduction is ssumed. For t, di vs Vmx sint i L e (.3.) dt At constnt speed of opertion, the bck emf E is constnt. The solution for the rmture current i is V mx L i sint Ae L E t (.3.3) where tn L 1 is the power fctor ngle of the L circuit. We hve ssumed tht i strts to flow t, however, note tht if the bck emf E Vmx sin, 1 E conduction will strt t b sin Vmx. Thus should be replced by b if the bove inequlity pplies. At t =, i = 0, thus Vmx E L 0 sin Ae L (.3.4) Thyristor converter driven DC motor F. hmn/july 017
3 Vmx E L A sin e L (.3.5) Vmx E Vmx E i sint sin e L L At t, i 0, thus, Vmx E Vmx E 0 sin sin e L L L L t (.3.6) (.3.7) This trnscedentl eqution hs to be solved itertively in order to obtin. The motor voltge (v ) nd current (i ) wveforms re then completely defined. The verge rmture voltge V nd current I cn then be found. From these, the torquespeed (T) chrcteristics of the convertermotor drive for different vlues of the firing ngle cn then be found. Furthermore, from the rmture voltge nd current wveforms, we cn lso find the ripple voltges nd currents of the motor. These nyses re quite complex nd re best crried out by dynmic system simultion on computer. Thyristor converter driven DC motor 3 F. hmn/july 017
4 .3.3 Singlephse, fullycontrolled thyristor bridge converter driven DC motor i p T1 T3 i V mx sint V L T4 T E () v 45 T1&T T3&T4 T1&T (b) = 45 o When conduction of rmture current is continuous, pir of thyristors in ech leg conduct t ll times. With ech tiggering, the rmture currents trnsfers (commuttes) to pir of incoming thyristors. The trnsfer of motor rmture current from one thyristor pir to the next tkes plce immeditely upon triggering if the AC source hs neglible inductnce. With these two ssumtions, the DC voltge cross the motor is given by, 1 V Vmx sintd( t) V mx cos (.3.8) where V mx is the pek lineline voltge of the AC supply to the converter. This converter opertes in two qudrnts, Q1 nd Q4, s indicted in the figure.3.3. With continuous Thyristor converter driven DC motor 4 F. hmn/july 017
5 conduction, this converter is ble to develop negtive voltge cross it but cn not supply negtive current. v Figure.3.(c) = V Q1 V Q4 I Firing ngle, Output ripple Figure.3.3 The ripple in output voltge nd current re responsible for torque ripple nd ddition losses in motor. Additionlly, the input power fctor presented by the converter is lso n issue with the utility supply. The output voltge ripple components re esily found from Fourier nlysis s: b n n Vmx cos n 1 cos n 1 n1 n1 Vmx sin n 1 sin n 1 n1 n1 n n n v b for n =, 4, 6,.. (.3.9) Thyristor converter driven DC motor 5 F. hmn/july 017
6 The mplitude of the ripple voltge flls s the ripple hrmonic number n increses. The lowest ripple order is n = for this converter nd its mplitude is the highest (the dominnt ripple). From the Fourier coefficients of the ripple voltge, it cn be shown tht the ripple voltge cross the motor is the highest when = 90. For ech ripple voltge component, the ripple current is found by dividing the rms ripple voltge by the AC impednce of the rmture. Thus i n v n n L sin n t n 1 n L where n tn (.3.10) i I i (.3.11) n n,4,6,... where I V E V E mx cos (.3.1) I n V n n L (.3.13) where V n Vnmx. The totl MS ripple rmture voltge nd current re: V V V V (.3.14) MS 4 6 I I I I I (.3.15) MS () ipple components of i Figure.3.4 (b) Hrmonic components of i s Thyristor converter driven DC motor 6 F. hmn/july 017
7 IMS The rtio, clled Form Fctor of the rmture current, is n importnt mesure of motor I heting. Idelly, it should be s close to unity s prcticble. Yet nother spect of the this type of converter is the fct tht the input current wveform to the thyristor converter is delyed from the input AC voltge by the firing ngle. This implies drop in the operting power fctor of the converter when the motor is driven t lower thn rted speed. In the limit, t ner zero speed, when the firing ngle is ner 90, the input power fctor is lso nerly zero. This is very undesirble spect of thyristor converters. Note tht input power fctor t which the converter opertes is given somewht differently from purely sinusoidl c circuits for which the Power Fctor is given by the cosine of the ngle of the input current nd voltge. From bsic definition of power fctor, Input Power Fctor = Output Power/Input MS VA Thus, IPF P VI V I cos I out,converter sms 1 1 (.3.16) VsMS IsMS VsMS IsMS VsMS IsMS IsMS cos when converter losses re neglected. Here I 1 is the MS vlue of the fundmentl input current to the converter. The rtio converter. I I 1 MS is known s the distortion fcttor of the input current to the.3.4 Anlysis of rmture current in continuous conduction mode Computtion of ripple components of i from equtions nd dding sufficiently lrge number of terms (n) clcultes the rmture current i. Alterntively, the motor current wveform cn lso be found nlyticlly s shown below: Solving mx sin di V t i L E for t dt Vmx E i sint Ae Z t L (.3.17) 1 L nd tn. (.3.18) where Z L In the stedystte, rmture current flls to its minimum vlue t t =,, nd so on. Thus, by setting Imin i( ) L V e Z L e 1 mx, A sin (.3.19) By setting the expression for A into.3.17, we find the solution for i s Thyristor converter driven DC motor 7 F. hmn/july 017
8 V E L t V e L Z (.3.0) L e 1 mx sin mx i sin t e Z Using i ( ) I, into the finl expression for i, min I min V Z L e 1 mx sin L e 1 E (.3.1) Note tht for this converter, we hve ssumed tht the voltge pplied to rmture circuit for ech hlf of the AC cycle is the sme nd tht the current wveform repets in every hlf cycle. The solution for i for this converter is found by replcing the constnt A in eqution (.3.17) from (.3.0). Expression.3.1 is useful in finding the minimum inductnce L min required in the rmture circuit so s to ensure continuous conduction. Once the solution of i is found, the verge vlue of the rmture current, I, is found from 1 I i( t) dt (.3.) or from (.3.1). The AC components of the rmture current cn be found from Fourier nlysis of the nlyticl solution of i. This will be tedius tsk (refer to pge 199 Shepherd, Hulley nd Ling for Fourier coefficients of i ). It is esier to perform the tsk by using Fourier nlysis of the voltge ripples nd AC circuit nlysis, s given by equtions ( ) Tchrcteristic with continuous conduction V Vmx mx cos I cos T ' L KT m ' ' KE KE Thyristor converter driven DC motor 8 F. hmn/july 017
9 = 0 o 1 I increses v or m V E = 90 o = 180 o Figure Anlysis of i for discontinuous conduction of i 1 I V I or T L decreses everse E regenertive qudrnt with rmture terminls reversed (.3.3) The rmture current cn become discontinuous t light lod nd lso for certin operting conditions of firing ngles nd motor prmeters. The rmture current i flls to zero t which is obtined by equting i to zero when t =. Thus V E 0 sin( ) Z V mx mx Thus, e ( ) L E sin( ) e Vmx Z E cos sin( ) V E cos sin( ) V / L mx / L mx e (.3.4) (.3.5) The extinction ngle is found by solving the bove trnscendentl eqution (.3.4). Once is found, the v wveform is then fully determined nd V cn then be clculted. The boundry between continuous nd discontinuous conduction (the dotted semicircle in the figure below) is found by setting in eqution.3.4. Conduction becomes just discontinuous when i flls to zero t t = = n + nd extinction ngle is then equl to.+. By solving for for given firing ngle nd for given bck emf E (or speed), the output voltge (v ) nd rmture current (i ) wveforms cross the motor terminls re determined. The verge V nd I re then obtined from these wveforms, tking into ccount tht during discontinuous opertion, the motor terminl, nd hence the converter output voltge, is E. Note tht during discontinuous conduction, the voltge t the converter output terminls rises to the bck emf E, nd s consequence V rises (nd hence the speed) with fll of I. Note lso tht when conduction is continuous, the converter output voltge is independent of the lod; the Thyristor converter driven DC motor 9 F. hmn/july 017
10 converter essentilly behving s n idel DC source (this lso ssumes instntneous trnsfer of current (commuttion) from one pir of thyristors to the next pir t ech triggering). Thus, when rmture current becomes discontinuous t light lod, the converter voltge (nd hence motor speed) rises shrply with fll of rmture current (or lod), or conversely, speed flls shrply with rise of rmture current (or lod), s shown in figure.3.6. = 0 m = 60 0 T, Nm = 150 = 180 Figure.3.6 From the solution of i, the verge I cn be found for the give firing ngle nd speed (or beck emf E ). thus giving the developed torque T. The coordintes of the operting point for the drive for the chosen (or E ) nd re thus found. By repeting the bove clcultions for given firing ngle nd rnge of speeds, the torquespeed chrctersitic of the drive for given firing ngle cn be found. The T  chrcteristic of seprtely excited DC motor for rnge of firing ngles re indicted in the figure.3.6. Note tht during continuous conduction in qudrnt 1, the bck emf of the rmture E is lower thn V, by the I drop which is smll. This voltge drop, divided by K E, is the smll droop in speed, which depends on I nd hence lod torque. During discontinuous conduction, the chnge of speed with chnge of lod toque cn be rther high, implying poor speed regultion with lod. The rise in V with discontinuous conduction cn be considered to be loss of converter voltge gin dv /d or rther dv /de c, where E c is the control voltge to the firing control circuit. This loss of gin (usully it is severe loss of gin) mkes the converter response time to chnge of operting conditions uncceptbly slow. High inductnce in the rmture circuit (or lrge ) reduces the likelyhood of discontinuous conduction, so tht the minimum required inductnce to ensure continuous conduction for the minimum expected lod my hve to be found. Thyristor converter driven DC motor 10 F. hmn/july 017
11 .3.4. The criticl rmture inductnce The minimum inductnce, L min, required in the rmtude circuit to prevent discontinuous conduction t given speed nd lod cn be found by equting I min = 0 in eqution.3.1. For just discontinuous conduction,. The condition for minimum L min is given by sin L L L e 1 E V e 1 mx (.3.6).3.5 Effect of source inductnce i p i L s T1 T3 I V mx sint v si V L T4 T + E Figure.3.7() The flt V I chrcteristics of the converter with continuous conduction t ny firing ngle, re lso idelised. In relity, the DC output voltge of phsecontrolled thyristor converter flls linely with lod current becuse of the source inductnce in the AC supply nd the lekge inductnce of the supply trnsformer. A phenomenon clled commuttion overlp is responsible for this droop. Anlysis of this spect of thyristor converter opertion will not be covered in this course. For converter with given source inductnce, the droop of the converter output DC voltge with lod current my simply be represented by n equivlent DC source resistnce L x. Thus, for the singlephse fullycontrolled converter, the DC voltge to the motor for firing ngle my be given by V V cos L I (.3.7) mx s where L x represents the voltge droop fctor of the converter due to the commuttion overlp mentioned bove, is the AC supply source frequency (50 Hz for Austrli), nd L x is the equivlent source inductnce which lso depends on the converter circuit. The effect of source inductnce is therefore responsible for further droop in speed thn wht is indicted in the figure With AC side source inductnce nd commuttion overlp, the speed of the motor is thus given by Thyristor converter driven DC motor 11 F. hmn/july 017
12 V mx cos LI s I rd/sec when conduction is continuous. (.3.8) K E Figure.3.7(b) Thyristor converter driven DC motor 1 F. hmn/july 017
13 .3.6 Singlephse hlfcontrolled thyristor bridge converter I T1 T L V mx sint D f D1 D E Figure.3.8 A hlfcontrolled bridge converter hs two thyristors nd two diodes in the bridge rms nd ddtionlly commutting diode cross the motor terminl to llow the thryristor current to commutte into when the AC supply voltge reverses polrity. The commutting, or freewheeling diode is required to prevent unconrolled opertion of bridge rm. The presence of the freewheeling diode circultes (freewheels) the rmture current nd prevents the lod voltge from becoming negtive. 1 V Vmx sin t d t 0 1 V mx mx V sin t d t = 1 cos with continuous conduction of i. (.3.9) v = Figure.3.9 Thyristor converter driven DC motor 13 F. hmn/july 017
14 50 V V or m Q Firing ngle, I or T Figure.3.10 The torquespeed chrcteristic of seprtely excited DC motor driven by the converter of Figure.3.7 is given by Vmx 1cos Ls T / KT m rd/sec (.3.30) K where continuous conduction hs been ssumed. E The hlfcontrolled converter hs the desirble ttribute tht it presents higher input power fctor thn the fully controlled converter. This is becuse the lgging component of the AC source current is constrined to loclly loclly through the motor circuit by the freewheeling diode. Its output voltge nd current ripples re lso lower nd the rmture current is less likely to become disconitnuos, so tht the crticl inductnce required for continuous conduction is not s lrge. The gte drive nd converter circuit costs re lso somewht cheper. It however opertes in one qudrnt only, so regenertive brking of the drive is not possible with this converter. Thyristor converter driven DC motor 14 F. hmn/july 017
15 .3.7 Threephse fully controlled converter driven SE DC motor v n i i T 1 T 3 T 5 v bn v cn i b i c T 4 T 6 T V L + E Figure.3.11 For lrger DC motor drives, threephse thyristor converter circuits re preferred due their lower output voltge ripple nd input current hrmonic performnce. Assuming continuous conduction, 3 mxll 3 1 V V sin td( t) /3 3V mx ll cos (.3.31) This converter opertes in qudrnts 1 nd 4, developing both positive nd negtive polrity dc output voltge. For firing ngles 0 o 90 o, the converter opertes in qudrnt 1 nd o o for , the opertion is in qudrnt 4. Opertion in qudrnt 4 is of course possible only when the lod includes n ctive DC source, ble to source power into the c circuit V Q1 V Firing ngle, Q4 I () (b) Figure.3.1. () Converter V vs firing ngle. (b) converter qudrnt chrcteristic Thyristor converter driven DC motor 15 F. hmn/july 017
16 Figure.3.13 Output voltge nd input current wveforms for = Hz Figure () ipple content of v ; f h = 6kf s Thyristor converter driven DC motor 16 F. hmn/july 017
17 Figure (b) Hrmonic content of i s ; f h = (k 1)f s Figure.3.14 Converter output voltge nd input current wveforms for = 145 From Fourier nlysis, the output voltge hrmonics for continuous conduction re given by n Vmxl l cos n t d t Vmx sin t 30 cosn td t 0 30 Thyristor converter driven DC motor 17 F. hmn/july 017
18 sinn1 cosn1 sinn1 cosn1 n1 n1 3Vmx l l 6 6 (.3.3) Similrly, b n sinn1 sinn1 sinn1 sinn1 n1 n1 3Vmx ll 6 6 where n = 6, 1, 18 etc,. (.3.33) The pek ripple voltge of hrmonic number 6n is given by 3Vmx ll 1 1 cos cn n bn n1 n1 n 1n 1 (.3.34) Threephse bridge converter circuits hve the following ttributes, compred to the singlephse bridge converter: 1. The output voltge wveform of the converter is smoother, hving the lowest hrmonic order of six, compred to two for the singlephse bridge converter. The ripple voltge t the motor hs hrmonic order or 6k where k is ny positive integer. The rmture current ripple hs the sme hrmonic order. The ripple voltge nd currents re lso of lower mgnitude. The highest output ripple occurs for = 90.. The lower ripple in the rmture current nd the smoother voltge wveform clls for smller rmture inductnce which my be required to ensure continuous conduction or limit the rmture current ripple ( i ). 3. The effective converter switching frequency is six times the supply frequency (300 Hz, compred to 100 Hz for singlephse bridge converter). 4. The input current wveform to the converter is closer to being sinusoid (i.e., better distortion fctor), compred to the input current wveform for the singlephse bridge converter. The hrmonic order of the AC input line current is given by 6k ± 1, compred to k ± 1 for the singlephse converter, where k is ny positive integer. This clls for reduced filter requirement t the input AC side. Thyristor converter driven DC motor 18 F. hmn/july 017
19 Anlysis of rmture current for continuous conduction The motor current cn be exctly determined from the DC nd hrmonic currents, using DC nd AC circuit nlysis techniques nd Fourier nlysis of the motor voltge wveform, nd then o o dding the currents for ech voltge component. Alterntively, for intervl: 30 t 90, o di Vmxll sint30 i L e (.3.35) dt As before, we ssume tht the motor bck emf e (hence speed) is constnt t E. Note tht the output voltge wveform repets every 60. Solving for i, t mxl l E L 6 V i t sint Ae Z 6 (.3.36) In writing the solution for i, the phse shift ngle of, or 30, of the lineline voltge from the 6 phseneutrl voltge hs been tken into ccount. The constnt of integrtion, A, is found from noting tht i is minimum t n t,, where n = 1,, 3,. nd so on Using the fct tht i is the sme t t, nd t, Vmxll sin A L 3 Z e 1 The minimum nd mximum rmture current vlues re given by: I min V E V sin Z 3 Z mx l l mx l l sin L 3 e 1 (.3.37) (.3.38) I mx V E V sin Z Z mx l l mx l l sin e L 6 L 3 e 1 (.3.39) From the complete solution of the rmture current, the verge nd MS rmture currents cn be determined. For the verge rmture current, the motor developed torque is found, for the speed for which E ws used in the clcultion. Note tht when conduction is continuous, which is redily indicted by the positive sign of Thyristor converter driven DC motor 19 F. hmn/july 017
20 I 3Vmxll cos E (.3.40) the verge rmture current is esily found from eqution Torquespeed chrcteristic with continuous conduction For given firing ngle, the T chrcteristics for seprtely excited DC motor re given by, m 3Vmxll 3 cos Ls I I (.3.41) K E where L 3x is the equivlent source rectnce of the threephse AC source. The T chrcteristics for few firing ngles re s shown in figure = 0 m rd/sec = 60 0 T, Nm = 150 = 170 Figure.3.15 Note tht opertion in qudrnt 1 is for forwrd driving nd opertion in qudrnt 4 is for reverse (regenertive) brking. The droop in speed with lod with continuous conduction is prtly due to the voltge drop in rmture resistnce nd prtly due to the voltge regultion chrcteristic of the converter s consequence of the AC side source inductnce. The steeper droop in speed with lod indicted by the grphs to the left of the dotted semicircle for given firing ngle is when discontinuous conduction occurs, i.e., the instntneous rmture current is not more thn zero t ll times Criticl inductnce for continuous conduction As for the singlephse converter, the rmture current cn become discontinuous, depending on the lod, firing ngle nd motor prmeters. The boundry between continuous nd discontinuous conduction is found by equting the expression for I min (eqn.3.38) to zero. This is indicted by the dotted semicircle in the T plne in figure The criticl inductnce for just discontinuous conduction for this converter is given by Thyristor converter driven DC motor 0 F. hmn/july 017
21 sin E sin Z 3 V L 3 e 1 mxll (.3.4).3.8 Hlfcontrolled threephse thyristor bridge converter driven DC motor +V D / v v n i T 1 T 3 T 5 i L n v bn v cn i b i c D 4 D 6 D D f V d L e V D / Figure.3.16 Threephse hlf controlled converter driven DC drive. Figure.3.17 Output voltge nd input current wveforms of threephse hlf controlled thyristor converter Assuming continuous conduction, 1 V Vmxll sintdt / 3 3V mx l l = 1 cos (.3.51) Thyristor converter driven DC motor 1 F. hmn/july 017
22 Note tht this converter only opertes in qudrnt 1, becuse negtive voltge cross the dc rils will initite freewheeling of the lod current through D f. It should be noted tht freewheeling is present when the firing ngle is greter thn 60. It should lso be noted tht the input current wveform is not symmetricl, becuse the commuttion of the lod current through the freewheeling diode, thereby relieving the input source of the lgging component of current. [The converter input current wveform will include some even order hrmonics]. As result, the input power fctor of this converter is higher thn the fully controlled converter nd its output voltge ripples re lso smller. V 300 V V Q I Firing ngle, Figure.3.18 V dc vs firing ngle nd qudrnt chrcteristic of the hlfcontrolled converter. As for singlephse, hlfcontrolled converter, the output voltge nd current ripple re smller for this converter, compred to the fully controlled converter. It lso opertes with better input displcement fctor..3.9 Converter Voltge Gin Firing control circuits of phsecontrolled ACDC converters normlly include conversion such tht the firing ngle mde equivlent to cos 1 (v c ) where v c is the control voltge to the firing ngle controller. This is esily incorported with the firing control circuit. For the threephse fully controlled bridge converter, the output DC voltge to the motor is then given by 3Vmx ll 1 3Vmx ll V cos cos vc vc (.3.5) Between the firing controller nd motor terminls, the converter thus behves s voltge gin 3V of mx l l. cos 1 Firing v c Control Circuit Figure.3.19 Thyristor converter driven DC motor F. hmn/july 017
23 .3.10 FourQudrnt Converter Single nd threephse fullycontrolled thyristor converters described so fr operte in two qudrnts, 1 nd 4. In order to drive the motor in ll four qudrnts, two such converters re needed, to be connected bck to bck s shown in figure C1 I L C E = 180 Figure.3.0 In one mode of control, clled suppressedhlf control, either converters C1 or C, but not both, is enbled with firing pulses if opertion in qudrnt 1 nd 4 or 3 nd, respectively, is required. Firing pulses to one of the converter is lwys suppressed. For forwrd motoring opertion in qudrnt 1 with positive rmture voltge nd current, converter C1 is enbled with the required firing ngle 1. For forwrd brking opertion in qudrnt (with positive rmture voltge (speed) nd negtive (brking) current, converter C is enbled with firing ngle, such tht its output DC voltge is smller thn the bck emf E by mrgin required mintining the desired level of brking current, s the speed drops. For reverse motoring opertion in qudrnt 3 with negtive rmture voltge nd current, converter C is enbled with the required firing ngle. For reverse brking opertion in qudrnt 4 with negtive rmture voltge (negtive speed) nd positive (brking) current, converter C1 is enbled with firing ngle 1 such tht its output voltge is smller thn the bck emf by mrgin required for mintining the desired level of brking current, s the speed drops. In this mode of control, firing pulses to both converters re relesed simultneously, otherwise short cross the DC bus will occur. It should be obvious tht smooth trnsfer of current from motoring to brking nd vice vers will incur short crossover dely. This dely must llow the outgoing converter to regin forwrd blocking cpbility before the incoming conferter is fired. In nother mode of control, clled circultingcurrent control, centertpped inductor is plced between the two converters t the point where the motor rmture is connected to the DC bus between the two converters. Both converters re operted together, with firing ngles determined from (.3.53) Both converters C1 nd C now produce the sme DC output voltge, one with firing ngle suitble for motoring nd the other with firing ngle suitble for brking. The verge DC voltge produced by both converters is now nerly the sme. The difference between the two Thyristor converter driven DC motor 3 F. hmn/july 017
24 voltges is such tht the converter which does motor the drive sinks smll but continuous regenertive current. The dvntge of this scheme is tht the dely ssocited with converter trnsfer, such s in the suppressed hlf scheme, is now reduced to zero, so tht quicker (higher bndwidth) control of the rmture cn be chieved Motor derting due to ripple current. For DC motor, its continuous current rting is given by the mximum smooth DC current it cn crry continuously. (Note tht the MS vlue of ripplefree DC vlue is the DC vlue itself). When the motor is supplied from converter in the stedy stte, the rmture current includes DC vlue, which is responsible for the developed torque, nd some ripple which produces ripple torque nd extr heting due to the I rms (or copper) loss. Becuse of these fctors, the DC motor hs to be derted in the rtio of the DC to MS current. For exmple, if the rtio of DC to MS current of 50 kw DC t rted lod is 0.8, this motor is to be regrded s 40 kw mchine becuse of the ripple content of its rmture current. In other words, 50kW motor would need to be selected for 40kW lod when the DC to MS rtio of the rmture current is 0.8. Thyristor converter driven DC motor 4 F. hmn/july 017
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