UNIT- III DC CHOPPERS

Transcription

1 UN- D HPPES NDUN A chopper is a saic device which is used o obain a variable dc volage from a consan dc volage source. A chopper is also known as dc-o-dc converer. he hyrisor converer offers greaer efficiency, faser response, lower mainenance, smaller size and smooh conrol. hoppers are widely used in rolley cars, baery operaed vehicles, racion moor conrol, conrol of large number of dc moors, ec.. hey are also used in regeneraive braking of dc moors o reurn energy back o supply and also as dc volage regulaors. hoppers are of wo ypes Sep-down choppers Sep-up choppers. n sep-down choppers, he oupu volage will be less han he inpu volage whereas in sep-up choppers oupu volage will be more han he inpu volage. PNPE F SEP-DWN HPPE hopper i Fig..: Sep-down hopper wih esisive oad Figure. shows a sep-down chopper wih resisive load. he hyrisor in he circui acs as a swich. When hyrisor is N, supply volage appears across he load and when hyrisor is FF, he volage across he load will be zero. he oupu volage and curren waveforms are as shown in figure..

2 v dc N FF i / dc Fig..: Sep-down choppers oupu volage and curren waveforms dc dc N FF N = average value of oupu or load volage = average value of oupu or load curren = ime inerval for which S conducs = ime inerval for which S is FF. FF = period of swiching or chopping period f frequency of chopper swiching or chopping frequency. Average oupu volage N dc N FF.... N dc. d.... bu N d duy cycle....3 Average oupu curren, dc dc....4 dc N d....5

3 MS value of oupu volage Bu during, v N o N v d o herefore MS oupu volage N d N N d upu power P Bu herefore oupu power P P d....8 Effecive inpu resisance of chopper i i d he oupu volage can be varied by varying he duy cycle. dc MEHDS F N he oupu dc volage can be varied by he following mehods. Pulse widh modulaion conrol or consan frequency operaion. ariable frequency conrol. PUSE WDH MDUAN n pulse widh modulaion he pulse widh 3 of he oupu waveform is varied keeping chopping frequency f and hence chopping period consan. herefore oupu volage is varied by varying he N ime, N. Figure.3 shows he oupu volage waveforms for differen N imes. N

4 N FF N FF AABE FEQUENY N Fig..3: Pulse Widh Modulaion onrol n his mehod of conrol, chopping frequency f is varied keeping eiher or consan. his mehod is also known as frequency modulaion. Figure.4 shows he oupu volage waveforms for a consan N and variable chopping period. n frequency modulaion o obain full oupu volage, range frequency has o be varied over a wide range. his mehod produces harmonics in he oupu and for large FF load curren may become disconinuous. v N FF N FF v N FF Fig..4: upu olage Waveforms for ime aio onrol 4

5 SEP-DWN HPPE WH - AD Figure.5 shows a sep-down chopper wih - load and free wheeling diode. When chopper is N, he supply is conneced across he load. urren flows from he supply o he load. When chopper is FF, he load curren i coninues o flow in he same direcion hrough he free-wheeling diode due o he energy sored in he inducor. he load curren can be coninuous or disconinuous depending on he values of and duy cycle, d. For a coninuous curren operaion he load curren is assumed o vary beween wo limis min and max. Figure.6 shows he oupu curren and oupu volage waveforms for a coninuous curren and disconinuous curren operaion. hopper i FWD E Fig..5: Sep Down hopper wih - oad v upu volage i max N FF upu curren min i oninuous curren upu curren Disconinuous curren Fig..6: upu olage and oad urren Waveforms (oninuous urren) When he curren exceeds curren reduces o. min max he chopper is urned-off and i is urned-on when 5

6 EXPESSNS F AD UEN PEAN WHEN HPPE S N i N F NNUUS UEN i E - Fig..5 (a) olage equaion for he circui shown in figure.5(a) is di d i E.... aking aplace ransform E S S. S i.... S S A, iniial curren i min E S S S min S....3 aking nverse aplace ransform E i e mine....4 his expression is valid for N. i.e., during he period chopper is N. A he insan he chopper is urned off, load curren is N max i 6

7 When hopper is FF FF i E Fig..5 (b) olage equaion for he circui shown in figure.5(b) is di i E....5 d aking aplace ransform E S S S i S edefining ime origin we have a, iniial curren i max herefore S max E S S S aking nverse aplace ransform E i maxe e....6 he expression is valid for FF, i.e., during he period chopper is FF. A he insan he chopper is urned N or a he end of he off period, he load curren is FND max AND From equaion (.4), FF min i min A d, i N max 7

8 herefore d d E max e mine....7 From equaion (.6), A, i FF N min d FF herefore d d E min maxe e....8 Subsiuing for min in equaion (.7) we ge, max d e E e....9 Subsiuing for max in equaion (.8) we ge, min d e E e.... max min is known as he seady sae ripple. herefore peak-o-peak ripple curren max min Average oupu volage dc d..... Average oupu curren max min.... dc approx Assuming load curren varies linearly from o insananeous load curren is given by min max. i min for N d d 8

9 i d max min min....3 MS value of load curren MS d d i d d max min MS min d d d MS value of oupu curren d max min min max min MS min d d d d max min min min max min....4 MS 3 MS chopper curren H d i d d max min H min d d max min min min max min H d 3 H d....5 MS Effecive inpu resisance is i S Where S = Average source curren S d dc herefore i d dc

10 PNPE F SEP-UP HPPE D hopper A D Fig..3: Sep-up hopper Figure.3 shows a sep-up chopper o obain a load volage higher han he inpu volage. he values of and are chosen depending upon he requiremen of oupu volage and curren. When he chopper is N, he inducor is conneced across he supply. he inducor curren rises and he inducor sores energy during he N ime of he chopper, N. When he chopper is off, he inducor curren is forced o flow hrough he diode D and load for a period, FF. he curren ends o decrease resuling in reversing he polariy of induced EMF in. herefore volage across load is given by d i. e.,....7 d f a large capacior is conneced across he load hen he capacior will provide a coninuous oupu volage. Diode D prevens any curren flow from capacior o he source. Sep up choppers are used for regeneraive braking of dc moors. EXPESSN F UPU AGE Assume he average inducor curren o be during N and FF ime of hopper. When hopper is N olage across inducor herefore energy sored in inducor =.. N....8, where N N period of chopper. When hopper is FF (energy is supplied by inducor o load) olage across

11 Energy supplied by inducor FF, where FF FF period of hopper. Neglecing losses, energy sored in inducor = energy supplied by inducor herefore N FF N FF FF N Where = hopping period or period of swiching. N FF N herefore d....9 Where d N duy cyle For variaion of duy cycle d in he range of d he oupu volage will vary in he range. PEFMANE PAAMEES he hyrisor requires a cerain minimum ime o urn N and urn FF. Hence duy cycle d can be varied only beween a minimum and a maximum value, limiing he minimum and maximum value of he oupu volage. ipple in he load curren depends inversely on he chopping frequency, f. herefore o reduce he load ripple curren, frequency should be as high as possible. ASSFAN F HPPES hoppers are classified as follows lass A hopper lass B hopper lass hopper lass D hopper

12 lass E hopper ASS A HPPE i v hopper FWD A D v i Fig..4: lass A hopper and v i haracerisic Figure.4 shows a lass A hopper circui wih inducive load and free-wheeling diode. When chopper is N, supply volage is conneced across he load i.e., v and curren i flows as shown in figure. When chopper is FF, v = and he load curren i coninues o flow in he same direcion hrough he free wheeling diode. herefore he average values of oupu volage and curren i.e., v and i are always posiive. Hence, lass A hopper is a firs quadran chopper (or single quadran chopper). Figure.5 shows oupu volage and curren waveforms for a coninuous load curren. i g hyrisor gae pulse i upu curren v H N FWD onducs upu volage N Fig..5: Firs quadran hopper - upu olage and urren Waveforms

13 lass A hopper is a sep-down chopper in which power always flows from source o load. is used o conrol he speed of dc moor. he oupu curren equaions obained in sep down chopper wih - load can be used o sudy he performance of lass A hopper. ASS B HPPE D i v v hopper E i Fig..6: lass B hopper Fig..6 shows a lass B hopper circui. When chopper is N, v and E drives a curren i hrough and in a direcion opposie o ha shown in figure.6. During he N period of he chopper, he inducance sores energy. When hopper is FF, diode D conducs, v and par of he energy sored in inducor is reurned o he supply. Also he i curren coninues o flow from he load o source. Hence he average oupu volage is posiive and average oupu curren is negaive. herefore lass B hopper operaes in second quadran. n his chopper, power flows from load o source. lass B hopper is used for regeneraive braking of dc moor. Figure.7 shows he oupu volage and curren waveforms of a lass B hopper. he oupu curren equaions can be obained as follows. During he inerval diode D conducs (chopper is off) volage equaion is given by i D onducing E - di d i E For he iniial condiion i.e., i a. min he soluion of he above equaion is obained along similar lines as in sep-down chopper wih - load 3

14 herefore E i e mine FF A FF i max E FF max e mine FF During he inerval chopper is N volage equaion is given by i hopper N E - di i E d edefining he ime origin, a i. max he soluion for he saed iniial condiion is E i maxe e N A i N min herefore N E min maxe e N 4

15 i g hyrisor gae pulse i FF N max upu curren min v D conducs hopper conducs upu volage Fig..7: lass B hopper - upu olage and urren Waveforms ASS HPPE lass hopper is a combinaion of lass A and lass B hoppers. Figure.8 shows a lass wo quadran hopper circui. For firs quadran operaion, H is N or D conducs and for second quadran operaion, H is N or D conducs. When H is N, he load curren i is posiive. i.e., flows in he direcion as shown in figure.8. he oupu volage is equal o i v and he load receives power from he source. H D i v H D v hopper E i Fig..8: lass hopper When H is urned FF, energy sored in inducance forces curren o flow hrough he diode D and he oupu volage v, bu i coninues o flow in posiive direcion. When H is riggered, he volage E forces i o flow in opposie direcion hrough and 5

16 H. he oupu volage v. n urning FF H, he energy sored in he inducance drives curren hrough diode D and he supply; oupu volage he inpu curren becomes negaive and power flows from load o source. hus he average oupu volage v is posiive bu he average oupu curren can ake boh posiive and negaive values. hoppers H and H should no be urned N simulaneously as i would resul in shor circuiing he supply. lass hopper can be used boh for dc moor conrol and regeneraive braking of dc moor. Figure.9 shows he oupu volage and curren waveforms. i g i g i Gae pulse of H Gae pulse of H upu curren D H D H D H D H N N N N upu volage Fig..9: lass hopper - upu olage and urren Waveforms ASS D HPPE H v D i E v D H i Fig..: lass D hopper Figure. shows a class D wo quadran chopper circui. When boh H and H are riggered simulaneously, he oupu volage v and oupu curren i flows hrough 6

17 he load in he direcion shown in figure.. When H and H are urned FF, he load curren coninues o flow in he same direcion hrough load, and D, due o he energy i D sored in he inducor, bu oupu volage v. he average load volage v is posiive if chopper N-ime is more han heir FF-ime and average oupu volage N becomes negaive if N FF. Hence he direcion of load curren is always posiive bu load volage can be posiive or negaive. Waveforms are shown in figures. and.. i g FF Gae pulse of H i g Gae pulse of H i upu curren v H,H N D,D onducing upu volage Average v Fig..: upu olage and urren Waveforms for N FF 7

18 i g Gae pulse of H i g Gae pulse of H i upu curren v H H D, D upu volage Average v Fig..: upu olage and urren Waveforms for N FF ASS E HPPE H D H 3 D 3 i E H D H 4 D 4 v Fig..3: lass E hopper 8

19 H - D 4onducs D - D onducs 4 v H - H 4N H - D onducs 4 i H 3- H N H - D onducs 4 D - D3 onducs H 4- D onducs Fig..3(a): Four Quadran peraion Figure.3 shows a class E 4 quadran chopper circui. When H and H4 are riggered, oupu curren flows in posiive direcion as shown in figure.3 hrough H and H, wih oupu volage v. his gives he firs quadran operaion. When boh H 4 and H are FF, he energy sored in he inducor drives hrough and D in he 4 same direcion, bu oupu volage quadran. For fourh quadran operaion he direcion of baery mus be reversed. When H and H are riggered, he load curren i flows in opposie direcion and oupu volage v 3. Since boh i and v are negaive, he chopper operaes in hird quadran. When boh H and H are FF, he load curren i 3 coninues o flow in he same direcion hrough D and D and he oupu volage v. herefore he chopper operaes in second quadran as v 4 is posiive bu differen quadrans. i i i D3 v. herefore he chopper operaes in he fourh is negaive. Figure.3(a) shows he devices which are operaive in EFFE F SUE AND AD NDUANE n choppers, he source inducance should be as small as possible o limi he ransien volage. Usually an inpu filer is used o overcome he problem of source inducance. Also source inducance may cause commuaion problem for he chopper. he load ripple curren is inversely proporional o load inducance and chopping frequency. herefore he peak load curren depends on load inducance. o limi he load ripple curren, a smoohing inducor is conneced in series wih he load. Problem. : For he firs quadran chopper shown in figure.4, express he following variables as funcions of, and duy cycle d in case load is resisive. Average oupu volage and curren upu curren a he insan of commuaion Average and rms free wheeling diode curren. MS value of oupu volage MS and average hyrisor currens. 9

20 i hopper FWD A D v Soluion Fig N Average oupu volage, dc d Average oupu curren, dc dc d he hyrisor is commuaed a he insan N. herefore oupu curren a he insan of commuaion is volage a ha insan., since is he oupu Free wheeling diode (FWD) will never conduc in a resisive load. herefore average and MS free wheeling diode currens are zero. MS value of oupu volage MS N v d Bu v during N MS N d MS N MS d Where duy cycle, N d MS value of hyrisor curren

21 = MS value of load curren MS d Average value of hyrisor curren = Average value of load curren d Problem. : A hopper circui is operaing on a a frequency of khz on a 46 supply. f he load volage is 35 vols, calculae he conducion period of he hyrisor in each cycle. Soluion = 46, dc = 35, f = khz hopping period upu volage f.5 msec 3 N dc onducion period of hyrisor dc N N 46 N.38 msec Problem.3 : npu o he sep up chopper is. he oupu required is 6. f he conducing ime of hyrisor is ssec. ompue hopping frequency, f he pulse widh is halved for consan frequency of operaion, find he new oupu volage. Soluion =, s, 6 N dc dc N

22 Solving for 6 6 hopping frequency f 3s f KHz Pulse widh is halved herefore N 6 s Frequency is consan herefore f 3.33KHz f 3s herefore oupu volage = N ols 6 3 Problem.4: A dc chopper has a resisive load of and inpu volage S. When chopper is N, is volage drop is.5 vols and chopping frequency is khz. f he duy cycle is 8%, deermine he average oupu volage and he chopper on ime. Soluion S,, f = khz d N.8 ch = olage drop across chopper =.5 vols Average oupu volage

23 N dc S ch ols dc hopper N ime, N d hopping period, hopper N ime, f 3 N d 3. secs μsecs N.8. 3 N μsecs Problem.5: n a dc chopper, he average load curren is 3 Amps, chopping frequency is 5 Hz. Supply volage is vols. alculae he N and FF periods of he chopper if he load resisance is ohms. Soluion 3 Amps, f = 5 Hz, =, dc hopping period, f msecs dc dc and dc d herefore dc d dc 3 d.545 hopper N period, N d msecs hopper FF period, FF N FF FF msec 3

24 Problem.6: A dc chopper in figure.5 has a resisive load of and inpu volage of =. When chopper is N, is volage drop is and he chopping frequency is khz. f he duy cycle is 6%, deermine Average oupu volage MS value of oupu volage Effecive inpu resisance of chopper hopper efficiency. hopper i v Soluion Fig..5 =,, hopper volage drop,, d =.6, f = khz. Average oupu volage d MS value of oupu volage d Effecive inpu resisance of chopper is i dc ch ols dc ch ols S dc ch dc dc Amps i S dc upu power is 4

25 P d v d P P d ch d ch d P was npu power, Pi d i d P P d ch d.6 d ch 376 was hopper efficiency, P P i % 376 MPUSE MMUAED HPPE mpulse commuaed choppers are widely used in high power circuis where load flucuaion is no large. his chopper is also known as parallel capacior urn-off chopper or volage commuaed chopper or classical chopper. Fig..8 shows an impulse commuaed chopper wih wo hyrisors and. We shall assume ha he load curren remains consan a a value during he commuaion process. o sar he circui, capacior is iniially charged wih polariy (wih plae a posiive) as shown in he fig..8 by riggering he hyrisor. apacior ges charged hrough S,, and load. As he charging curren decays o zero hyrisor will be urnedoff. Wih capacior charged wih plae a posiive he circui is ready for operaion. For convenience he chopper operaion is divided ino five modes. 5

26 S i S a b _ i FWD A D v _ D _ Fig..8 MDE hyrisor is fired a =. he supply volage comes across he load. oad curren flows hrough and load. A he same ime capacior discharges hrough, D,, and and he capacior reverses is volage. his reverse volage on capacior is held consan by diode D. Fig..9 shows he equivalen circui of Mode. S S D i A D apacior Discharge urren i sin i sin P ; where Fig..9 P Where & apacior olage cos 6

27 MDE hyrisor is now fired o commuae hyrisor. When is N capacior volage reverse biases and urns i off. Now he capacior discharges hrough he load from S o and he discharge ime is known as circui urn-off ime. ircui urn-off ime is given by Where is load curren. Since depends on load curren, i mus be designed for he wors case condiion which occur a he maximum value of load curren and minimum value of capacior volage. hen he capacior recharges back o he supply volage (wih plae a posiive). his ime is called he recharging ime and is given by d S he oal ime required for he capacior o discharge and recharge is called he commuaion ime and i is given by r d A he end of Mode- capacior has recharged o S and he free wheeling diode sars conducing. he equivalen circui for Mode- is shown in fig..3. S _ S _ A D Fig..3. MDE 3 Free wheeling diode FWD sars conducing and he load curren decays. he energy sored in source inducance S is ransferred o capacior. nsananeous curren is i cos Hence capacior charges o a volage higher han supply volage. naurally urns-off. he insananeous capacior volage is 7

28 S S sins Where S S Fig..3 shows he equivalen circui of Mode 3. S _ S S _ FWD A D Fig..3 MDE 4 Since he capacior has been overcharged i.e. is volage is above supply volage i sars discharging in reverse direcion. Hence capacior curren becomes negaive. he capacior discharges hrough S, S, FWD, D and. When his curren reduces o zero D will sop conducing and he capacior volage will be same as he supply volage fig..3 shows in equivalen circui of Mode 4. S S D FWD A D Fig..3 MDE 5 n mode 5 boh hyrisors are off and he load curren flows hrough he free wheeling diode (FWD). his mode will end once hyrisor is fired. he equivalen circui for mode 5 is shown in fig..33 8

29 FWD A D Fig..33 Fig..34 shows he curren and volage waveforms for a volage commuaed chopper. i c p i apacior urren v c p urren hrough v o s c s v c olage across upu olage c - c apacior olage c Fig..34 d hough volage commuaed chopper is a simple circui i has he following disadvanages. A saring circui is required and he saring circui should be such ha i riggers hyrisor firs. oad volage jumps o wice he supply volage when he commuaion is iniiaed. he discharging and charging ime of commuaion capacior are dependen on he load curren and his limis high frequency operaion, especially a low load curren. hopper canno be esed wihou connecing load. hyrisor has o carry load curren as well as resonan curren resuling in increasing is peak curren raing. 9

30 Jone s hopper D FWD Fig..35: Jone s hopper v Figure.35 shows a Jone s hopper circui for an inducive load wih free wheeling diode. Jone s hopper is an example of class D commuaion. wo hyrisors are used, is he main hyrisor and is he auxiliary hyrisor. ommuaing circui for consiss of hyrisor, capacior, diode D and auoransformer ( and ). niially hyrisor is urned N and capacior is charged o a volage wih a polariy as shown in figure.35. As charges, he charging curren hrough hyrisor decays exponenially and when curren falls below holding curren level, hyrisor urns FF by iself. When hyrisor is riggered, load curren flows hrough hyrisor, and load. he capacior discharges hrough hyrisor, and diode D. Due o resonan acion of he auo ransformer inducance and capaciance, he volage across he capacior reverses afer some ime. is o be noed ha he load curren in induces a volage in due o auoransformer acion. Due o his volage in in he reverse direcion, he capacior charges o a volage greaer han he supply volage. (he capacior now ries o discharge in opposie direcion bu i is blocked by diode D and hence capacior mainains he reverse volage across i). When hyrisor is o be commuaed, hyrisor is urned N resuling in connecing capacior direcly across hyrisor. apacior volage reverse biases hyrisor and urns i off. he capacior again begins o charge hrough hyrisor and he load for he nex cycle of operaion. he various waveforms are shown in figure.36 3

31 g Gae pulse of Gae pulse of Gae pulse of apacior olage esonan acion Auo ransformer acion apacior discharge curren urren of olage across 3

32 HYS MMUAN EHNQUES NDUN n pracice i becomes necessary o urn off a conducing hyrisor. (fen hyrisors are used as swiches o urn on and off power o he load). he process of urning off a conducing hyrisor is called commuaion. he principle involved is ha eiher he anode should be made negaive wih respec o cahode (volage commuaion) or he anode curren should be reduced below he holding curren value (curren commuaion). he reverse volage mus be mainained for a ime a leas equal o he urn-off ime of S oherwise a reapplicaion of a posiive volage will cause he hyrisor o conduc even wihou a gae signal. n similar lines he anode curren should be held a a value less han he holding curren a leas for a ime equal o urn-off ime oherwise he S will sar conducing if he curren in he circui increases beyond he holding curren level even wihou a gae signal. ommuaion circuis have been developed o hasen he urn-off process of hyrisors. he sudy of commuaion echniques helps in undersanding he ransien phenomena under swiching condiions. he reverse volage or he small anode curren condiion mus be mainained for a ime a leas equal o he UN FF ime of S; herwise he S may again sar conducing. he echniques o urn off a S can be broadly classified as Naural ommuaion Forced ommuaion. NAUA MMUAN (ASS F) his ype of commuaion akes place when supply volage is A, because a negaive volage will appear across he S in he negaive half cycle of he supply volage and he S urns off by iself. Hence no special circuis are required o urn off he S. ha is he reason ha his ype of commuaion is called Naural or ine ommuaion. Figure. shows he circui where naural commuaion akes place and figure. shows he relaed waveforms. is he ime offered by he circui wihin which he S should urn off compleely. hus should be greaer han q, he urn off ime of he S. herwise, he S will become forward biased before i has urned off compleely and will sar conducing even wihou a gae signal. c c v s ~ v o Fig..: ircui for Naural ommuaion 3

33 Supply volage v s Sinusoidal 3 oad volage v o urn off occurs here 3 olage across S c Fig..: Naural ommuaion Waveforms of Supply and oad olages (esisive oad) his ype of commuaion is applied in ac volage conrollers, phase conrolled recifiers and cyclo converers. FED MMUAN When supply is D, naural commuaion is no possible because he polariy of he supply remains unchanged. Hence special mehods mus be used o reduce he S curren below he holding value or o apply a negaive volage across he S for a ime inerval greaer han he urn off ime of he S. his echnique is called FED MMUAN and is applied in all circuis where he supply volage is D - namely, hoppers (fixed D o variable D), inverers (D o A). Forced commuaion echniques are as follows: Self ommuaion esonan Pulse ommuaion omplemenary ommuaion mpulse ommuaion Exernal Pulse ommuaion. oad Side ommuaion. ine Side ommuaion. 33

34 SEF MMUAN AD MMUAN ASS A MMUAN: (MMUAN BY ESNANG HE AD) n his ype of commuaion he curren hrough he S is reduced below he holding curren value by resonaing he load. i.e., he load circui is so designed ha even hough he supply volage is posiive, an oscillaing curren ends o flow and when he curren hrough he S reaches zero, he device urns off. his is done by including an inducance and a capacior in series wih he load and keeping he circui under-damped. Figure.3 shows he circui. his ype of commuaion is used in Series nverer ircui. i c() - oad Fig..3: ircui for Self ommuaion 34

35 urren i / apacior volage Gae pulse olage across S Fig..5: Self ommuaion Wave forms of urren and apaciors olage ESNAN PUSE MMUAN (ASS B MMUAN) he circui for resonan pulse commuaion is shown in figure.. a b i FWD oad 35

36 Fig..: ircui for esonan Pulse ommuaion his is a ype of commuaion in which a series circui is conneced across he S. Since he commuaion circui has negligible resisance i is always under-damped i.e., he curren in circui ends o oscillae whenever he S is on. niially he S is off and he capacior is charged o vols wih plae a being posiive. eferring o figure.3 a he S is urned N by giving a gae pulse. A curren flows hrough he load and his is assumed o be consan. A he same ime S shor circuis he combinaion which sars oscillaing. A curren i sars flowing in he direcion shown in figure. As i reaches is maximum value, he capacior volage reduces o zero and hen he polariy of he capacior volage reverses b becomes posiive). When i falls o zero his reverse volage becomes maximum, and hen direcion of i reverses i.e., hrough S he load curren and i flow in opposie direcion. When he insananeous value of i becomes equal o, he S curren becomes zero and he S urns off. Now he capacior sars charging and is volage reaches he supply volage wih plae a being posiive. he relaed waveforms are shown in figure.3. Gae pulse of S apacior volage v ab p i S olage across S Fig..3: esonan Pulse ommuaion arious Waveforms 36

37 AENAE U F ESNAN PUSE MMUAN he working of he circui can be explained as follows. he capacior is assumed o be charged o wih polariy as shown, is conducing and he load curren is a consan. o urn off, is riggered.,, and forms a resonan circui. A resonan curren ic flows in he direcion shown, i.e., in a direcion opposie o ha of load curren. ic = p sin (refer o he previous circui descripion). Where p & and he capacior volage is given by vc i. d vc sin. d. v cos c i () a b () 3 i () FWD A D Fig..6: esonan Pulse ommuaion An Alernae ircui When ic becomes equal o (he load curren), he curren hrough becomes zero and urns off. his happens a ime such ha p sin p 37

38 hrough and he corresponding capacior volage is nce he hyrisor sin v cos c urns off, he capacior sars charging owards he supply volage and load. As he capacior charges hrough he load capacior curren is same as load curren, which is consan. When he capacior volage reaches, he supply volage, he FWD sars conducing and he energy sored in charges o a sill higher volage. he riggering of reverses he polariy of he capacior volage and he circui is ready for anoher 3 riggering of. he waveforms are shown in figure.7. EXPESSN F c Assuming a consan load curren which charges he capacior c seconds Normally For reliable commuaion should be greaer han, he urn off ime of S. is c q o be noed ha depends upon and becomes smaller for higher values of load curren. c 38

39 urren i () apacior volage v ab () Fig..7: esonan Pulse ommuaion Alernae ircui arious Waveforms ESNAN PUSE MMUAN WH AEEANG DDE D i () i () - () 3 FWD A D Fig..7(a) 39

40 i () Fig..7(b) A diode D is conneced as shown in he figure.7(a) o accelerae he discharging of he capacior. When hyrisor is fired a resonan curren i flows hrough he capacior and hyrisor. A ime, he capacior curren i equals he load curren and hence curren hrough is reduced o zero resuling in urning off of. Now he capacior curren i coninues o flow hrough he diode D unil i reduces o load curren level a ime. hus he presence of D has acceleraed he discharge of capacior. Now he capacior ges charged hrough he load and he charging curren is consan. nce capacior is fully charged urns off by iself. Bu once curren of hyrisor reduces o zero he reverse volage appearing across is he forward volage drop of D which is very small. his makes he hyrisor recovery process very slow and i becomes necessary o provide longer reverse bias ime. From figure.7(b) cos ircui urn-off ime MPEMENAY MMUAN (ASS MMUAN, PAAE APA MMUAN) n complemenary commuaion he curren can be ransferred beween wo loads. wo Ss are used and firing of one S urns off he oher. he circui is shown in figure.. 4

41 a b i Fig..: omplemenary ommuaion he working of he circui can be explained as follows. niially boh and are off; Now, is fired. oad curren flows hrough. A he same ime, he capacior ges charged o vols hrough and ( b becomes posiive wih respec o a ). When he capacior ges fully charged, he capacior curren becomes zero. o urn off, is fired; he volage across comes across and reverse biases i, hence urns off. A he same ime, he load curren flows hrough and. he capacior charges owards hrough and and is finally charged o vols wih a plae posiive. When he capacior is fully charged, he capacior curren becomes zero. o urn off, is riggered, he capacior volage (wih a posiive) comes across and urns off. he relaed waveforms are shown in figure.. Gae pulse of p urren hrough urren hrough Gae pulse of i c olage across capacior v ab urren hrough - olage across 4

42 MPUSE MMUAN (ASS D MMUAN) he circui for impulse commuaion is as shown in figure.5. 3 () FWD A D Fig..5: ircui for mpulse ommuaion he working of he circui can be explained as follows. is assumed ha iniially he capacior is charged o a volage wih polariy as shown. e he hyrisor be conducing and carry a load curren. f he hyrisor is o be urned off, is fired. he capacior volage comes across, is reverse biased and i urns off. Now he capacior sars charging hrough and he load. he capacior volage reaches wih op plae being posiive. By his ime he capacior charging curren (curren hrough ) would have reduced o zero and auomaically urns off. Now and are boh off. Before firing again, 3 he capacior volage should be reversed. his is done by urning on 3, discharges hrough and and he capacior volage reverses. he waveforms are shown in figure Gae pulse of Gae pulse of 3 Gae pulse of S apacior volage olage across Fig..6: mpulse ommuaion Waveforms of apacior olage, olage across. 4

43 AN AENAE U F MPUSE MMUAN s shown in figure.7. () i _ D Fig..7: mpulse ommuaion An Alernae ircui he working of he circui can be explained as follows: niially le he volage across he capacior be wih he op plae posiive. Now is riggered. oad curren flows hrough and load. A he same ime, discharges hrough, and D (he curren is i ) and he volage across reverses i.e., he boom plae becomes posiive. he diode D ensures ha he boom plae of he capacior remains posiive. o urn off, is riggered; he volage across he capacior comes across. is reverse biased and i urns off (volage commuaion). he capacior now sars charging hrough and load. When i charges o vols (wih he op plae posiive), he curren hrough becomes zero and auomaically urns off. he relaed waveforms are shown in figure.8. 43

44 Gae pulse of Gae pulse of apacior volage his is due o i urren hrough S oad curren olage across Fig..8: mpulse ommuaion (Alernae ircui) arious Waveforms EXENA PUSE MMUAN (ASS E MMUAN) 3 S AUX AUX Fig..34: Exernal Pulse ommuaion 44

45 n his ype of commuaion an addiional source is required o urn-off he conducing hyrisor. Figure.34 shows a circui for exernal pulse commuaion. is he main volage source and AUX is he auxiliary supply. Assume hyrisor is conducing and load is conneced across supply S. When hyrisor 3 is urned N a, AUX, 3, and from an oscillaory circui. Assuming capacior is iniially uncharged, capacior is now charged o a volage wih upper plae posiive a. When curren hrough falls o zero, subjeced o a reverse volage equal o. his resuls in hyrisor being urned- AUX 3 3 S AUX off. nce is off capacior discharges hrough he load ges commuaed. o urn-off he main hyrisor, hyrisor is urned N. hen is AD SDE MMUAN n load side commuaion he discharging and recharging of capacior akes place hrough he load. Hence o es he commuaion circui he load has o be conneced. Examples of load side commuaion are esonan Pulse ommuaion and mpulse ommuaion. NE SDE MMUAN n his ype of commuaion he discharging and recharging of capacior akes place hrough he supply. S S 3 r _ FWD A D _ Fig.:.35 ine Side ommuaion ircui Figure.35 shows line side commuaion circui. hyrisor he capacior. When charges o a volage of, is fired o charge is self commuaed. o reverse he volage of capacior o -, hyrisor is fired and commuaes by iself. Assuming ha is conducing and carries a load curren hyrisor is fired o urn off. he urning N of will resul in forward biasing he diode (FWD) and applying a reverse volage of across. his urns off, hus he discharging and recharging of capacior is done hrough he supply and he commuaion circui can be esed wihou load