1

CHNG 1 CC PHN T BN DN CNG SUT C BN

Cc phn t bn dn cng sut c s dng trong s cc b bin i nh cc kho in t, gi l cc van bn dn; khi m dn dng th ni ti vo ngun, khi kho th ngt ti ra khi ngun, khng cho dng in chy qua. Cc van bn dn c th ng ct c dng in ln nhng li c iu khin bi cc tn hiu cng sut nh, to bi cc mch in t cng sut nh. Quy lut ni ti vo ngun ph thuc vo s b bin i v ph thuc vo cch thc iu khin cc van trong b bin i. Hiu r nguyn l hot ng v cc c tnh c bn ca cc phn t bn dn l iu v cng quan trng c th s dng ng v pht huy ht hiu qu ca cc phn t bn dn trong cc ng dng c th. Tnh nng k thut ch yu ca cc phn t bn dn cng sut th hin qua kh nng chu in p v cc c tnh lin quan ti qu trnh ng ct cng nh vn iu khin chng. Cc phn t bn dn cng sut u c nhng c tnh c bn chung l:

- Cc van bn dn ch lm vic trong ch kho, khi m cho dng chy qua th c in tr tng ng rt nh, khi kho khng cho dng chy qua th c in tr tng ng rt ln. Nh tn hao cng sut trong qu trnh lm vic s c gi tr rt nh.

- Cc van bn dn ch dn dng theo mt chiu khi phn t c t di in p phn cc thun. Khi in p t ln phn t phn cc ngc, dng qua phn t ch c gi tr rt nh, c mA, gi l dng r. V kh nng iu khin, cc van bn dn c phn loi thnh:

- Van khng iu khin, nh it. - Van c iu khin, trong li phn ra: + iu khin khng hon ton, nh tiristo, triac. + iu khin hon ton, nh bipolar tranzito, MOSFET, IGBT, GTO.

1.1 IT

it l phn t c cu to bi mt lp tip gip bn dn p-n. it c hai cc, ant A l cc ni vi lp bn dn kiu p, catt K l cc ni vi lp bn dn kiu n. Dng in ch chy qua it theo chiu t A n K khi in p UAK dng. Khi UAK m, dng qua it gn nh bng khng. Cu to v k hiu ca it biu din trn hnh 1.1.

Ant

Catt

pn

A

K

D

)a )b Hnh 1.1. it: a) Cu to; b) K hiu

1.1.1 Cu to ca it Tip gip bn dn p-n l b phn c bn trong cu to ca mt it. nhit mi

trng, cc in t t do trong lp bn dn n khi khuch tn sang lp bn dn p s b trung ho bi cc ion dng y. Do cc in tch trong vng tip gip t trung ho ln nhau nn vng ny tr nn ngho in tch, hay l vng c in tr ln. Tuy nhin vng ngho in tch ny ch m rng ra n mt dy nht nh v bn vng n khi cc in t di chuyn i s li cc ion dng, cn bn vng p khi cc in t di chuyn n s nhp vo lp cc in t ho tr ngoi cng, to nn cc ion m. Cc ion ny nm

2

trong cu trc tinh th ca mng tinh th silic nn khng th di chuyn c. Kt qu to thnh mt t in vi cc in tch m pha lp p v cc in tch dng pha lp n. Cc in tch ca t ny to nn mt in trng E c hng t vng n sang vng p, ngn cn s khuch tn tip tc ca cc in t t vng n sang vng p. in trng E cng to nn mt ro cn Uj vi gi tr khng i mt nhit nht nh, khong 0,65V i vi tip gip p-n trn tinh th silic nhit 250C (hnh 1.2). Cc it cng sut c ch to chu c mt gi tr in p ngc nht nh. iu ny t c nh mt lp bn dn n- tip gip vi lp p, c cu to ging nh lp n, nhng t in t t do hn. Khi lp tip gip p-n- c t di tc dng ca in p bn ngoi, nu in trng ngoi cng chiu vi in trng E th vng ngho in tch s m rng sang vng n- in tr tng ng ca it cng ln v dng in khng th chy qua. Ton b in p ngoi s ri trn vng ngho in tch. Ta ni rng it b phn cc ngc (hnh 1.3a).

E

p n

jU

Vng ngho in tch

Hnh1.2. S to thnh in th ro cn

trong tip gip p-n

-n-n

)a)b

Hnh 1.3. S phn cc ca it cng sut: a) Phn cc ngc; b) Phn cc thun

Khi in p bn ngoi to ra in trng ngoi c hng ngc vi in trng trong E, vng ngho in tch s b thu hp li. Nu in p bn ngoi ln hn Uj, c 0,65V, vng ngho in tch s thu hp bng khng v cc in tch c th di chuyn t do qua cu trc tinh th ca it. Dng in chy qua it lc ny s ch b hn ch do in tr ti mch ngoi v mt phn in tr trong it bao gm in tr ca tinh th bn dn gia ant v catt, in tr do phn kim loi lm dy dn ra ngoi v in tr do tip xc gia phn kim loi v bn dn. Ta ni it c phn cc thun (hnh 1.3b). 1.1.2 c tnh vn-ampe ca it Mt s tnh cht ca it trong qu trnh lm vic c th c gii thch thng qua vic xem xt c tnh vn-ampe ca it trn hnh 1.4a. c tnh gm hai phn, c tnh thun nm trong gc phn t I tng ng vi UAK > 0, c tnh ngc nm trong gc phn t III tng ng vi UAK < 0. Trn ng c tnh thun, nu in p ant-catt tng dn t 0 n khi vt qua ngng in p UD0 c 0,6 0,7 V, dng c th chy qua it. Dng in ID c th thay i rt ln nhng in p ri trn dit UAK hu nh t thay i. Nh vy c tnh thun ca it c trng bi tnh cht c in tr tng ng nh.

3

Trn ng c tnh ngc, nu in p UAK tng dn t 0 n gi tr Ung.max, gi l in p ngc ln nht th dng in qua it vn c gi tr rt nh, gi l dng r, ngha l it cn tr dng in theo chiu ngc. Cho n khi UAK t n gi tr Ung.max th xy ra hin tng dng qua it tng t ngt, tnh cht cn tr dng in ngc ca it b ph v. Qu trnh ny khng c tnh o ngc, ngha l nu li gim in p trn ant-catt th dng in vn khng gim. Ta ni it b nh thng. Trong thc t, n gin cho vic tnh ton, ngi ta thng dng c tnh khi dn dng, tuyn tnh ho it nh c biu din trn hnh 1.4b. c tnh ny c th biu din qua cng thc:

DD0.DD I.rUu

trong D

D IUr

l in tr tng ng ca it khi dn dng.

c tnh vn-ampe ca cc it thc t s khc nhau, ph thuc vo dng in cho php chy qua it v in p ngc ln nht m it c th chu c. Tuy nhin phn tch s cc b bin i th mt c tnh l tng cho trn hnh 1.4c c s dng nhiu hn c. Theo c tnh l tng, it c th cho mt dng in bt k chy qua vi st p trn n bng 0. Ngha l, theo c tnh l tng, it c in tr tng ng khi dn bng 0 v khi kho bng .

max.ngU

0.DU 0.DU

Di

0.DU

Di

Hnh 1.4. c tnh vn-ampe ca it:

a) c tnh thc t; b) c tnh tuyn tnh; c) c tnh l tng

1.1.3 Cc thng s c bn ca mt it 1. Gi tr trung bnh ca dng in cho php chy qua it theo chiu thun, ID Trong qu trnh lm vic, dng in chy qua it s pht nhit lm nng tinh th bn dn ca it. Cng sut pht nhit bng tch ca dng in chy qua it vi in p ri trn n. it ch dn dng theo mt chiu t ant n catt, iu ny ngha l cng sut pht nhit t l vi gi tr trung bnh ca dng in. V vy dng in ID l thng s quan trng la chn it cho mt ng dng thc t. 2. Gi tr in p ngc ln nht m it c th chi c, Ung.max Thng s th hai quan trng la chn it l gi tr in p ngc ln nht m it c th chu ng c. Nh c tnh vn-ampe ch ra, qu trnh it b nh thng l qu trnh khng th o ngc c, v vy trong mi ng dng phi lun m bo rng UAK < Ung.max.

4

3. Tn s Qu trnh pht nhit trn it cng ph thuc vo tn s ng ct ca it. Trong qu trnh it m ra hoc kho li, tn hao cng sut tc thi u(t), i(t) c gi tr ln hn lc it dn dng hoc ang b kho. V vy nu tn s ng ct cao hoc thi gian ng ct ca it so snh c vi khong thi gian dn dng th tn tht trn it b quy nh ch yu bi tn s lm vic m khng phi gi tr trung bnh ca dng in. Cc it c ch to vi tn s lm vic khc nhau, do tn s l mt thng s quan trng phi lu khi la chn it. 4. Thi gian phc hi tr v in tch phc hi Qr Cc it khi kho li c dng ngc di chuyn lng in tch Qr ra khi cu trc bn dn, phc hi kh nng kho ca mnh. Thi gian phc hi tr c th b ko di, lm chm li qu trnh chuyn mch gia cc van. Dng in ngc c th tng xung dng trn cc van mi m ra vi bin c th rt ln. Hn na thi gian phc hi cng lm tng tn tht trong qu trnh ng ct cc van. Nhng l do nh trn khin ta phi c bit lu n nh hng ca tr trong nhng trng hp c th. gim thi gian chuyn mch c th phi s dng loi it c tr rt ngn, c s. Tuy nhin khi dng in ngc thay i qu nhanh, c th gy nn in p rt ln trn nhng mch in c in cm. Tm li khng nn ngh rng it l mt phn t rt n gin m b qua qu trnh kho li ca it.

1.2 TIRISTO Tiristo l phn t bn dn cu to t bn lp bn dn p-n-p-n, to ra ba tip gip p-n: J1, J2, J3. Tiristo c ba cc: ant A, catt K, cc iu khin G nh c biu din trn hnh 1.5.

1J

2J3J

1Q

2Q+n

Hnh 1.5. Tiristo: a) Cu trc bn dn; b) K hiu

1.2.1 c tuyn vn-ampe ca tiristo c tnh vn-ampe ca tiristo gm hai phn (hnh 1.6). Phn th nht nm trong gc phn t th I l c tnh thun tng ng vi trng hp in p UAK > 0; phn th hai nm trong gc phn t th III, gi l c tnh ngc, tng ng vi trng hp UAK < 0. 1. Trng hp dng in vo cc iu khin bng khng (IG = 0) Khi dng vo cc iu khin ca tiristo bng 0 hay khi h mch cc iu khin tiristo s cn tr dng in ng vi c hai trng hp phn cc in p gia ant-catt. Khi in p UAK < 0, theo cu to bn dn ca tiristo, hai tip gip J1, J3 u phn cc ngc, lp J2 phn cc thun, nh vy tiristo s ging nh hai it mc ni tip b phn cc ngc. Qua tiristo ch c mt dng in nh chy qua, gi l dng r. Khi UAK tng t n mt gi tr in p ln nht Ung.max s xy ra hin tng tiristo b nh thng, dng in c th tng ln rt ln. Ging nh on c tnh ngc ca it, qu trnh b nh thng l qu trnh khng th o ngc, ngha l nu c gim in p UAK xung di mc Ung.max th dng in cng khng gim c v mc dng r. Tiristo b hng.

5

Khi tng in p ant-catt theo chiu thun, UAK > 0, lc u cng ch c mt dng in rt nh chy qua, gi l dng r. in tr tng ng mch ant-catt vn c gi tr rt ln. Khi tip gip J1, J3 phn cc thun, J2 phn cc ngc. Cho n khi UAK tng t n gi tr in p thun ln nht, Uth.max, s xy ra hin tng in tr tng ng ca mch ant-catt ngt gim, dng in chy qua tiristo s ch b gii hn bi in tr mch ngoi. Nu khi

max.ngU

th.vU max.thU

Vi

dti1GI2GI3GI

Hnh 1.6. c tnh vn-ampe ca tiristo

dng qua tiristo ln hn mc dng ti thiu, gi l dng duy tr Idt, th khi tiristo s dn dng trn c tnh thun, ging nh ng c tnh thun it. on c tnh thun c c trng bi tnh dn dng c th c gi tr ln nhng in p ri trn ant-catt nh v hu nh khng ph thuc vo gi tr ca dng in. 2. Trng hp c dng vo cc iu khin (IG > 0) Nu c dng iu khin a vo gia cc iu khin v catt, qu trnh chuyn im lm vic trn ng c tnh thun s xy ra sm hn, trc khi in p thun t n gi tr ln nht, Uth.max. iu ny c m t trn hnh 1.6 bng nhng ng nt t, ng vi gi tr dng iu khin khc nhau, IG1, IG2, IG3, Ni chung, nu dng iu khin ln hn th im chuyn c tnh lm vic s xy ra vi UAK nh hn. Qu trnh xy ra trn ng c tnh ngc s khng c g khc so vi trng hp dng iu khin bng 0. 1.2.2 M, kho tiristo Tiristo c c tnh ging it, ngha l ch cho php dng chy qua theo mt chiu, t ant n catt, v cn tr dng chy theo chiu ngc li. Tuy nhin khc vi it, tiristo c th dn dng, ngoi iu kin phi c in p UAK > 0 cn cn thm mt s iu kin khc. Do tiristo c gi l phn t bn dn c iu khin phn bit vi it l phn t khng iu khin c. 1. M tiristo Khi c phn cc thun, UAK > 0, tiristo c th m bng hai cch. Th nht, c th tng in p ant-catt cho n khi t n gi tr in p thun ln nht, Uth.max, in tr tng ng trong mch ant-catt s gim t ngt v dng qua tiristo s hon ton do mch ngoi xc nh. Phng php ny trong thc t khng c p dng do nguyn nhn m khng mong mun v khng phi lc no cng c th tng c in p n gi tr Ung.max. V li nh vy s xy ra trng hp tiristo t m ra di tc dng ca cc xung in p ti mt thi im ngu nhin, khng nh trc. Phng php th hai, phng php c p dng thc t, l a mt xung dng in c gi tr nht nh vo gia cc iu khin v catt. Xung dng in iu khin s chuyn trng thi ca tiristo t tr khng cao sang tr khng thp mc in p ant-catt nh. Khi nu dng qua ant-catt ln hn mt gi tr nht nh, gi l dng duy tr (Idt) th tiristo s tip tc trong trng thi m dn dng m khng cn n s tn ti

6

ca xung dng iu khin. iu ny c ngha l c th m cc tiristo bng cc xung dng c rng xung nht nh, do cng sut ca mch iu khin c th l rt nh, so vi cng sut ca mch lc m tiristo l phn t ng ct, khng ch dng in. 2. Kho tiristo Mt tiristo ang dn dng s tr v trng thi kho (in tr tng ng mch ant-catt tng cao) nu dng in gim xung nh hn dng duy tr, Idt. Tuy nhin tiristo vn trng thi kho, vi tr khng cao, khi in p ant-catt li dng (UAK > 0), cn phi c mt thi gian nht nh cc lp tip gip phc hi hon ton tnh cht cn tr dng in ca mnh. Khi tiristo dn dng theo chiu thun, UAK > 0, hai lp tip gip J1, J3, phn cc thun, cc in tch i qua hai lp ny d dng v lp y tip gip J2 ang b phn cc ngc. V vy m dng in c th chy qua ba lp tip gip J1, J2, J3. kho tiristo li cn gim dng ant-catt v di mc dng duy tr (Idt) bng cch hoc l i chiu dng in hoc p mt in p ngc ln gia ant v catt ca tiristo. Sau khi dng v bng khng phi t mt in p ngc ln ant-catt (UAK < 0) trong mt khong thi gian ti thiu, gi l thi gian phc hi, tr, ch sau tiristo mi c th cn tr dng in theo c hai chiu. Trong thi gian phc hi c mt dng in ngc chy gia catt v ant. Dng in ngc ny di chuyn cc in tch ra khi tip gip J2 v np in cho t in tng ng ca hai tip gip J1, J3 c phc hi. Thi gian phc hi ph thuc vo lng in tch cn c di chuyn ra ngoi cu trc bn dn ca tiristo v np in cho tip gip J1, J3 n in p ngc ti thi im . Thi gian phc hi l mt trong nhng thng s quan trng ca tiristo. Thi gian phc hi xc nh di tn s lm vic ca tiristo. Thi gian phc hi tr, c gi tr c 5 50 s i vi cc tiristo tn s cao v c 50 200 s i vi cc tiristo tn s thp.

1.2.3 Cc thng s c bn ca tiristo 1. Gi tr dng trung bnh cho php chy qua tiristo, Iv y l gi tr dng trung bnh cho php chy qua tiristo vi iu kin nhit ca cu trc tinh th bn dn ca tiristo khng vt qu gi tr cho php. Trong thc t dng in cho php chy qua tiristo cn ph thuc vo cc iu kin lm mt v nhit mi trng. Tiristo c th c gn ln cc b tn nhit tiu chun v lm mt t nhin. Ngoi ra, tiristo c th phi c lm mt cng bc nh qut gi hoc dng nc ti nhit lng to ra nhanh hn. Vn lm mt van bn dn s c cp n phn sau, tuy nhin c th la chn dng in theo cc iu kin lm mt theo kinh nghim nh sau:

- Lm mt t nhin: dng s dng cho php n mt phn ba dng Iv. - Lm mt cng bc bng qut gi: dng s dng bng hai phn ba dng Iv. - Lm mt cng bc bng nc: c th s dng 100 dng Iv.

2. in p ngc cho php ln nht, Ung.max y l gi tr in p ngc ln nht cho php t ln tiristo. Trong cc ng dng phi m bo rng, ti bt k thi im no in p gia ant-catt UAK lun nh hn hoc bng Ung.max. Ngoi ra phi m bo mt d tr nht nh v in p, ngha l phi c chn t nht l bng 1,2 n 1,5 ln gi tr bin ln nht ca in p trn s . 3. Thi gian phc hi tnh cht kho ca tiristo, tr (s)

7

y l thi gian ti thiu phi t in p m ln gia ant-catt ca tiristo sau khi dng ant-catt v bng khng trc khi li c th c in p dng m tiristo vn kho. Thi gian phc hi tr l mt thng s rt quan trng ca tiristo, nht l trong cc b nghch lu ph thuc hoc l nghch lu c lp, trong phi lun m bo rng thi gian dnh cho qu trnh kho phi bng 1,5 n 2 ln tr.

4. Tc tng in p cho php, )s/V(dtdU

Tiristo c s dng nh mt phn t iu khin, ngha l mc d c phn cc thun (UAK>0) nhng vn phi c tn hiu iu khin th n mi cho php dng in chy qua. Khi tiristo c phn cc thun, phn ln in p ri trn lp tip gip J2 nh c ch ra trn hnh 1.7. Lp tip gip J2 b phn cc ngc nn dy ca n n ra, to ra vng khng gian ngho in tch, cn tr dng in chy qua. Vng khng gian ny c th coi nh mt t in c in dung CJ2. Khi c in p bin thin vi tc ln, dng in ca t c th c gi tr ng k, ng vai tr nh dng iu khin. Kt qu l tiristo c th m ra khi cha c tn hiu iu khin vo cc iu khin G. Tc tng in p l mt thng s phn bit tiristo tn s thp vi cc tiristo tn s cao. tiristo tn s thp

1J

2J

3Jp

p

+n

K

A

G K

-n

n n

Hnh 1.7. Hiu ng dU/dt tc dng nh dng iu khin

dtdU vo khong 50 n 200 sV ; vi cc tiristo tn s cao dtdU c th t 500 n 2000 sV .

5. Tc tng dng cho php, )s/A(dtdI

Khi tiristo bt u m, khng phi mi im trn tit din tinh th bn dn ca n u dn dng ng u. Dng in s chy qua bt u mt s im, gn vi cc iu khin nht, sau s lan to sang cc im khc trn ton b tit din. Nu tc tng dng qu ln c th dn n mt dng in cc im dn ban u qu ln, s pht nhit cc b qu mnh lit c th dn n hng cc b, t dn n hng ton b tit din tinh th bn dn. Tc tng dng cng phn bit tiristo tn s thp, c dtdI c 50-100 /s, vi cc tiristo c tn s cao vi dtdI c 500-2000 /s. Trong cc ng dng phi lun m bo tc tng dng di mc cho php. iu ny t c nh mc ni tip cc van bn dn vi cc cun khng tr s nh. Cun khng c th c li khng kh hoc li ferit. C th dng nhng xuyn ferit lng ln thanh dn to cc in khng gi tr khc nhau tu theo s lng xuyn s dng. Khi dng qua thanh dn nh, in khng s c gi tr ln hn ch tc tng dng; khi dng in ln. cun khng b bo ho, in cm gim gn nh bng khng. Nh vy cun khng kiu ny khng gy st p trong ch dng nh mc qua thanh dn.

8

1.3 TRIAC Triac l phn t bn dn c cu trc bn dn gm nm lp, to nn cu trc p-n-p-n

nh tiristo theo c hai chiu gia cc cc T1 v T2 nh c th hin trn hnh 1.8a. Triac c k hin trn s nh hnh 1.8b, c th dn dng theo c hai chiu T1 v T2. V nguyn tc, triac hon ton c th coi tng ng vi hai tiristo u song song ngc nh trn hnh 1.8c.

)c

2T

1T

G

)b

2T

1T

G

p

p

n

n n

n

)a Hnh 1.8. Triac: a) Cu trc bn dn; b) K hiu;

c) S tng ng vi hia tiristo song song ngc

c tnh vn-ampe ca triac bao gm hai on c tnh gc phn t th I v th III, mi on u ging nh c tnh thun ca mt tiristo nh c biu din trn hnh 1.9a.

2T

1TG

R

)b

i A

dtI

vI

1GI2GI3GI

u

max.thuth.vu0

)a

Hnh 1.9. a) c tnh vn-ampe; b) iu khin triac bng dng iu khin m

Triac c th iu khin m dn dng bng c xung dng dng (dng i vo cc iu khin) hoc bng xung dng m (dng i ra khi cc iu khin). Tuy nhin xung dng m c nhy km hn, tc l dng ch c th chy qua triac khi in p gia T1 v T2 phi ln hn mt gi tr nht nh, ln hn khi dng dng iu khin dng. V vy trong thc t m bo tnh i xng ca dng in qua triac, s dng xung iu khin m l tt hn c. Nguyn l thc hin iu khin bng xugn dng iu khin m c biu din trn hnh 1.9b.

9

Triac c bit hu ch trong cc ng dng iu chnh in p xoay chiu hoc cc cngtct tnh di cng sut va v nh.

1.4 TIRISTO KHO C BNG CC IU KHIN, GTO (GATE TURN-OFF THYISTOR) Tiristo thng, nh c gii thiu mc 1.3, c s dng rng ri trong cc s chnh lu, t cng sut nh vi kW n cng sut cc ln, vi trm MW. l v trong cc s chnh lu, tiristo c th kho li mt cch t nhin di tc dng ca in p li, in p chnh lu c th iu chnh bng cch ch ng thay i thi im m ca cc tiristo. Tuy nhin vi cc ng dng trong cc b bin i xung p mt chiu hoc cc b nghch lu, trong cc van bn dn lun b t di in p mt chiu th iu kin kho t nhin s khng cn na. Khi vic dng cc tiristo thng s cn n cc mch chuyn mch cng bc rt phc tp, gy tn hao ln v cng sut, gim hiu sut ca cc b bin i. Cc GTO, nh tn gi ca n, ngha l kho li c bng cc iu khin, c kh nng v ng ct cc dng in rt ln, chu c in p cao ging nh tiristo, l mt van iu khin hon ton, c th ch ng c thi im kho di tc ng ca tn hiu iu khin. Vic ng dng cc GTO pht huy u im c bn ca cc phn t bn dn, l kh nng ng ct dng in ln nhng li c iu khin bi cc tn hiu in cng sut nh. Cu trc bn dn ca GTO phc tp hn so vi tiristo nh c ch ra trn hnh 1.10. K hiu ca GTO cng ch ra tnh cht iu khin hon ton ca n. l dng in i vo cc iu khin m GTO, cn dng in i ra khi cc iu khin dng di chuyn cc in tch ra khi cu trc bn dn ca n, ngha l kho GTO li.

A

K

G

+p +p +p +p+n +n +n

+n +n +n

1J

2J

3J

n

p

)a

A

G

K

)b

Hnh 1.10. GTO: a) Cu trc bn dn; b) K hiu

Trong cu trc bn dn ca GTO lp p, ant c b sung cc lp n+. Du (+) bn cnh ch ra rng mt cc in tch tng ng, cc l hoc in t, c lm giu thm vi mc ch lm gim in tr khi dn ca cc vng ny. Cc iu khin vn c ni vo lp p th ba nhng c chia nh ra v phn b u so vi lp n+ ca catt. Khi cha c dng iu khin, nu ant c in p dng hn so vi catt th ton b in p s ri trn tip gip J2 gia, ging nh trong cu trc ca tiristo. Tuy nhin nu catt c in p dng hn so vi ant th tip gip p+-n st ant s b nh thng ngay in p rt thp, ngha l GTO khng th chu c in p ngc. GTO c iu khin m bng cch cho dng vo cc iu khin, ging nh tiristo thng. Tuy nhin do cu trc bn dn khc nhau nn dng duy tr GTO cao hn tiristo thng. Do dng iu khin phi c bin ln hn v duy tr trong thi gian di hn dng qua GTO kp vt xa gi tr dng duy tr. Ging nh tiristo thng, sau khi GTO dn th dng iu khin khng cn tc dng. Nh vy c th m GTO bng cc xung ngn, vi cng sut khng ng k.

10

kho GTO, mt xung dng phi c ly ra t cc iu khin. Khi van ang dn dng, tip gip J2 cha mt s lng ln cc in tch sinh ra do tc dng ca hiu ng bn ph v bo to nn vng dn in, cho php cc in t di chuyn t catt, vng n+, n ant, vng p+, to nn dng ant. Bng cch ly i mt s lng ln cc in tch qua cc iu khin, vng dn in s b co hp v b p v pha vng n+ ca ant v vng n+ ca catt. Kt qu l dng ant s b gim cho n khi v n khng. Dng iu khin c suy tr mt thi gian ngn GTO phc hi tnh cht kho.

1.5 TRANZITO CNG SUT, BJT (BIPOLAR JUNCTION TRANZITOR) 1.5.1 Cu to, nguyn l lm vic ca BJT

Tranzito l phn t bn dn c cu trc bn dn gm 3 lp bn dn p-n-p (bng thun) hoc n-p-n (bng ngc), to nn hai tip gip p-n. Cu trc ny thng c gi l Bipolar Junction Tranzitor (BJT) v dng in chy trong cu trc ny bao gm c hai loi in tch m v dng. Tranzito c ba cc: Baz (B), colect (C) v emit (E). BJT cng sut thng l loi bng ngc. Cu trc tiu biu v k hiu trn s ca mt BJT cng

-n

)a )b

Hnh 1.11. BJT: a) Cu trc bn dn; b) K hiu

sut c biu din trn hnh 1.11, trong lp bn dn n xc nh in p nh thng ca tip gip B-C v do ca C-E.

Trong ch tuyn tnh, hay cn gi l ch khuch i, tranzito l phn t khuch i dng in vi dng colect IC bng ln dng baz (dng iu khin), trong c gi l h s khuch i dng in. IC = .IB Tuy nhin, trong in t cng sut, tranzito ch c s dng nh mt phn t kho. Khi m dng iu khin phi tho mn iu kin:

CB

II hay

CbhBIkI

trong kbh = 1,2 1,5 gi l h s bo ho. Khi tranzito s trong ch bo ho vi in p gia colect v emit rt nh, c 1 1,5 V, gi l in p bo ho, UCE.bh. Khi kho, dng iu khin IB bng khng, lc dng colect gn bng khng, in p UCE s ln n gi tr in p ngun cung cp cho mch ti ni tip vi tranzito. Tn hao cng sut trn tranzito bng tch dng in colect vi in p ri trn colect-emit, s c gi tr rt nh trong ch kho. Trong cu trc bn dn ca BJT ch kho, c hai tip gip B-E v B-C u b phn cc ngc. in p t gia colect-emit s ri ch yu trn vng tr khng cao ca tip gip p-n-. dy v mt in tch ca lp n- xc nh kh nng chu in p ca cu trc BJT. Tranzito ch tuyn tnh nu tip gip B-E phn cc thun v tip gip B-C phn cc ngc. Trong ch tuyn tnh, s in tch dng a vo cc Baz s kch thch cc in t t tip gip B-C thm nhp vo vng baz, ti y chng c

11

trung ho ht, kt qu l tc trung ho quyt nh dng colect t l vi dng baz, IC = .IB. Tranzito trong ch bo ho nu c hai tip gip B-E v B-C u c phn cc thun. Cc in t s thm nhp vo y vng baz, vng p, t c hai tip gip B-E v B-C, v nu cc in tch dng c a vo cc baz c s lng d tha th cc in tch s khng b trung ho ht, kt qu l vng baz c in tr nh, dng in c th chy qua. Cng do tc trung ho in tch khng kp nn tranzito khng cn kh nng khng ch dng in c na v gi tr dng in s hon ton do mch ngoi quyt nh. l ch m bo ho.

1.5.2 c tnh ng ct ca tranzito

nU+

tRBCC

BEC

C

E

BBR)t(uB

1BU

2BU

)t(iC

)t(iB

)a

)t(uB1BU

2BU

)t(uBE

2BU

V7,0

)t(iB )t(i 1B

)t(i 2B)t(uCE

nU+

bh.CI)t(iC

)b Hnh 1.12. Qu trnh ng ct BJT: a) S ; b) Dng dng in, in p

Ch ng ct ca tranzito ph thuc ch yu vo cc t k sinh gia cc tip gip B-E v B-C, CBE v CBC. Ta phn tch qu trnh ng ct ca mt tranzito qua s kho trn hnh 1.12a, trong tranzito ng ct mt ti thun tr Rt di in p +Un iu khin bi tn hiu in p t -UB2 n +UB1 v ngc li. Dng sng dng in, in p cho trn hnh 1.12b. 1. Qu trnh m Theo th hnh 1.12b, trong khong thi gian (1) BJT ang trong ch kho vi in p ngc UB2 t ln tip gip B-E. Qu trnh m BJT bt u t khi tn hiu iu khin nhy t -UB2 ln mc +UB1. Trong khong (2), t u vo, gi tr tng ng bng Cin = CBE + CBC, np in t in p -UB2 n +UB1. Khi UBE cn nh hn khng, cha c hin tng g xy ra i vi IC v UCE. T Cin ch np n gi tr ngng m U* ca tip gip B-E, c 0,6 0,7V, bng in p ri trn it theo chiu thun, th qu trnh np kt thc. Dng in v in p trn BJT ch bt u thay i khi UBE vt qu gi tr khng u giai on (3). Khong thi gian (2) gi l thi gian tr khi m, td(on) ca BJT.

12

Trong khong (3), cc in t xut pht t emit thm nhp vo vng baz, vt qua tip gip B-C lm xut hin dng colct. Cc in t thot ra khi colct cng lm tng thm cc in t n t emit. Qu trnh tng dng IC, IE tip tc xy ra cho n khi trong baz tch lu lng in tch d tha QB m tc t trung ho ca chng m bo mt dng baz khng i:

B

*1B

1B RU-UI

Ti im cng dng in ti baz trn s hnh 1.12a, ta c:

BBC.CBE.C1B iiiI Trong :

iC.BE l dng np ca t CBE, iC.BC l dng np ca t CBC, iB l dng u vo ca tranzito, iC = .iB.

Dng colect tng dn theo quy lut hm m, n gi tr cui cng l IC() = .IB1. Tuy nhin ch n cui giai on (3) th dng IC t n gi tr bo ho, IC.bh, BJT ra khi ch tuyn tnh v iu kin iC = .iB khng cn tc dng na. Trong ch bo ho c hai tip gip B-E v B-C u c phn cc thun. V kho lm vic vi ti tr trn colect nn in p trn colect - emit UCE cng gim theo cng tc vi s tng ca dng IC. Khong thi gian (3) ph thuc vo ln ca dng IB1, dng ny cng ln th thi gian ny cng ngn. Trong khong (4), in p UCE tip tc gim n gi tr in p bo ho cui cng, xc nh bi biu thc:

UCE.bh = Un IC.bh.Rt Thi gian (4) ph thuc qu trnh suy gim in tr ca vng n- v ph thuc cu to ca BJT. Trong giai on (5), BJT hon ton lm vic trong ch bo ho. 2. Qu trnh kho BJT Trong thi gian BJT trong ch bo ho, in tch tch t khng ch trong lp baz m c trong lp colect. Tuy nhin nhng bin i bn ngoi hu nh khng nh hng n ch lm vic ca kho. Khi in p iu khin thay i t +UB1 xung UB2 u giai on (6), in tch tch lu trong lp bn dn khng th thay i ngay lp tc c. Dng IB ngay lp tc s c gi tr:

B

*2B

2B RUUI

Lc u cc in tch c di chuyn ra ngoi bng dng khng i IB2. Giai on di chuyn kt thc cui giai on (6) khi mt in tch trong tip gip baz - colect gim v bng khng v tip theo tip gip ny bt u b phn cc ngc. Khong thi gian (6) gi l thi gian tr khi kho, td(off). Trong khong (7), dng colect IC bt u gim v khng, in p UCE s tng dn ti gi tr +Un. Trong khong ny BJT lm vic trong ch tuyn tnh, trong dng IC t

13

l vi dng baz. T CBC bt u np ti gi tr in p ngc, bng Un. Lu rng trong giai on ny, ti im cng dng in ti baz trn s hnh 1.12a, ta c:

IB2 = iC.BC - iB trong iC.BC l dng np ca t CBC; iB l ng u vo ca tranzito. T c th thy quy lut iC = .iB vn c thc hin. Tip gip B-E vn c phn cc thun, tip gip B-C b phn cc ngc. n cui khong (7) tranzito mi kho li hon ton. Trong khong (8), t baz - emit tip tc np ti in p ngc UB2. Tranzito ch kho hon ton trong khong (9).

1.6 TRANZITO TRNG, MOSFET (METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANZITOR) 1.6.1 Cu to ca MOSFET

Khc vi cu trc BJT, MOSFET c cu trc bn dn cho php iu khin bng in p vi dng in iu khin cc nh. Hnh 1.13 th hin cu trc bn dn v k hiu ca mt MOSFET knh dn kiu n. Trong G l cc iu khin c cch ly hon ton vi cu trc bn dn cn li bi lp in mi cc mng nhng c cch in cc ln ioxit-silic (SiO2). Hai cc cn li l cc gc (S) v cc mng (D). Cc

n

-n

n n n np p

)a

G

D

S)b

Hnh 1.13. MOSFET (knh dn n): a) Cu t rc bn dn; b) K hiu

mng l cc n cc ht mang in. Nu knh dn l n th cc ht mang in s l cc in t (eletron), do cc tnh in p ca cc mng s l dng so vi cc gc. Trn k hiu phn t, phn chm gch gia D v S ch ra rng trong iu kin bnh thng khng c mt knh dn thc s ni gia D v S. Cu trc bn dn ca MOSFET knh dn kiu p cng tng t nhng cc lp bn dn s c kiu dn in ngc li. Tuy nhin a s cc MOSFET cng sut l loi c knh dn kiu n.

1.6.2. Nguyn l hot ng ca MOSFET Hnh 1.14 m t s to thnh knh dn trong cu trc bn dn ca MOSFET. Trong ch lm vic bnh thng uDS > 0. Gi s in p gia cc iu khin v cc gc bng khng, uDS = 0, khi knh dn s hon ton khng xut hin. Gia cc gc v cc mng s l tip gip p-n- phn cc ngc. in p uDS s hon ton ri trn vng ngho ca tip gip ny (hnh 1.14a).

a)Vng ngho in tch

n

-n

p pn n n n

Knh dn

p pn n n n

n

-n

n

-n

p pn n n n

it trong

b)

c)

Hnh 1.14. S to thnh knh dn trong

cu trc MOSFET

14

Nu in p iu khin m, UGS < 0, th vng b mt gip cc iu khin s tch t cc l (p), do dng in gia cc gc v cc mng s khng th xut hin. Khi in p iu khin l dng, UGS > 0, v ln, b mt tip gip cc iu khin s tch t cc in t, v mt knh dn thc s hnh thnh (hnh 1.14b). Nh vy trong cu trc bn dn ca MOSFET, cc phn t mang in l cc in t, ging nh ca lp n to nn cc mng, nn MOSFET c gi l phn t vi cc ht mang in c bn, khc vi cu trc ca BJT, IGBT, tiristo l cc phn t vi cc ht mang in phi c bn. Dng in gia cc gc v cc mng by gi s ph thuc vo in p UDS. T cu trc bn dn ca MOSFET (hnh 1.14c), c th thy rng gia cc mng v cc gc tn ti mt tip gip p-n-, tng ng vi mt it ngc ni gia D v S. Trong cc s b bin i, trao i nng lng gia ti v ngun thng cn c cc it ngc mc song song vi cc van bn dn. Nh vy u im ca MOSFET l c sn mt it ni ti nh vy.

1.7 TRANZITO C CC IU KHIN CCH LY, IGBT (Insulated Gate Bipolar Tranzitor) IGBT l phn t kt hp kh nng ng ct nhanh ca MOSFET v kh nng chu ti ln ca tranzito thng. V mt iu khin, IGBT gn nh ging hon ton MOSFET, ngha l c iu khin bng in p, do cng sut iu khin yu cu cc nh. Hnh 1.15 gii thiu cu trc bn dn ca mt IGBT. V cu trc bn dn, IGBT rt ging vi MOSFET, im khc nhau l c thm lp p ni vi colect to nn cu trc bn dn p-n-p gia emit (tng t cc gc) vi colect (tng t cc mng), khng phi l n-n nh MOSFET (hnh 1.29b). C th coi IGBT tng ng vi mt tranzito p-n-p vi dng baz c iu khin bi mt MOSFET (hnh 1.15b v c). Di tc dng ca in p iu khin UGE > 0, knh dn vi cc ht mang in l cc in t c hnh thnh, ging nh cu trc MOSFET. Cc in t di chuyn v pha colect vt qua lp tip gip n--p nh cu trc gia baz v colect tranzito thng, to nn dng colect.

+n

pn n

p

pn n

pn n

pn n

+np 1i 2i

Hnh 1.15. IGBT: a) Cu trc bn dn;

b) Cu trc tng ng vi mt tranzito n-p-n v mt MOSFET; c) S thng ng; d) K hiu

1.8 TN HAO CNG SUT TRN CC PHN T BN DN CNG SUT Ngoi tn tht do mch iu khin sinh ra cp n nhng phn t c th ni trn, ta s phn tch cc thnh phn tn tht trong cc ch lm vic ca van sau y.

15

1.8.1 Tn tht trong ch tnh, ang dn dng hoc ang kho Khi phn t ang trong ch dn dng hoc ang kho, tn hao cng sut bng tch ca dng in qua phn t vi in p ri trn n. Khi phn t ang kho, in p trn n c th ln nhng dng r qua van s c gi tr rt nh, v vy tn hao cng sut c th b qua. Tn hao cng sut trong ch tnh ch yu sinh ra khi van dn dng. Vi a s cc phn t bn dn, in p ri trn van thng khng i, t ph thuc vo gi tr dng in chy qua. Nh vy c th d dng xc nh c tn hao cng sut trong trng thi van dn.

1.8.2 Tn tht trong qu trnh ng ct Trong qu trnh ng ct, cng sut tn hao tc thi c th c gi tr ln v dng in v in p ri trn van u c th c gi tr ln ng thi. Ni chung, thi gian ng ct ch chim mt phn nh trong c chu k hot ng ca phn t nn tn hao cng sut trong qu trnh ng ct ch chim mt phn nh trong cng sut tn hao trung bnh. Tuy nhin phn t phi lm vic vi tn s ng ct cao th tn hao do ng ct li chim mt phn chnh trong cng sut pht nhit. Xc nh tn hao trong ch ng ct l mt nhim v khng n gin, v phi phn bit cc yu t nh hng n qu trnh ng ct, do nh hng n tn hao cng sut.

1.9 SO SNH TNG I GIA CC PHN T BN DN CNG SUT

Hnh 1.16. So snh tng i gia cc phn t bn dn

C th so snh mt cch tng i cc phn t bn dn cng sut theo kh nng ng ct v cng sut (in p v dng in) v tn s ng ct thy c phm vi ng dng ca cc phn t khc nhau. Hnh 1.16 m t s so snh tng i ny. Tiristo l nhng phn t c ch to cho kh nng ng ct v cng sut ln nht. Nhng tiristo ln nht c in p chu c n 4500V, dng in ti a n 4000A. Phm vi hot ng v tn s i vi tiristo li l thp nht v thi gian tr ng m ca cu trc p-n-p-n tng i ln, tr khi m c 5s, tr khi kho c 10 n 200 s. V vy cc tiristo c ng dng ch yu trong cc s chnh lu, trong cc kho s chuyn mch t nhin di tc dng ca in p li vi tn s 50 60 Hz.Tiristo l phn t iu khin khng hon ton, c th iu khin m bng cc iu khin nhng khng th kho li c. GTO l bc ci tin ng k v cng ngh ch to ca tiristo. GTO c kh nng ng ct v cng sut thp hn so vi tiristo nhng phm vi hot ng v tn s th li

16

cao hn. Do c kh nng kho li bng cc iu khin nn thi gian tr khi kho c rt ngn li mt cch ng k so vi tiristo. GTO c ng dng trong cc s nghch lu vi cng sut trung bnh v tn s trung bnh. Vic ng dng cc GTO dn n cng sut ca cc b bin tn c ch to ngy cng ln. Tranzito v IGBT l nhng phn t bn dn c ng dng vi nhng phm vi cng sut nh nhng yu cu tn s lm vic cao. c bit l cc IGBT ang c ng dng ngy cng rng ri v thay th dn cc tranzito thng. Vi cng sut iu khin yu cu rt nh vic s dng IGBT lm n gin ng k thit k ca cc b bin i v lm cho kch thc ca h thng iu khin ny ngy cng thu nh. Vi u th tuyt i v thi gian ng ct cc nh (c 0,5 n 1 s) cc MOSFET chim u th tuyt i cho cc ng dng yu cu tn s ng ct rt cao (n vi trm kHz) nhng cng sut tng i nh, v d nh cc b ngun xung cho my tnh PC.

17

CHNG 2 CHNH LU

2.1 GII THIU CHUNG 2.1.1 Cu trc mch chnh lu Chnh lu l qu trnh bin i nng lng dng xoay chiu thnh nng lng dng mt chiu. Chnh lu l thit b in t cng sut c s dng rng ri nht trong thc t. S cu trc thng gp ca mch chnh lu nh trn hnh 2.1.

~P

~1U

~P

~2U

P

dd I,U

P

dd I,U

Hnh 2.1. S cu trc mch chnh lu

Trong s c my bin p lm hai nhim v chnh l: a) Chuyn t in p quy chun ca li in xoay chiu U1 sang in p U2 thch

hp vi yu cu ca ti. Tu theo yu cu ca ti m my bin p c th l tng p hoc gim p.

b) Bin i s pha ca ngun li sang s pha theo yu cu ca mch van. Thng thng s pha ca li ln nht l 3, song mch van c th cn s pha l 6, 12

Trng hp ti yu cu mc in p ph hp vi li in v mch van i hi s pha nh li in th c th b my bin p. Mch van y l cc van bn dn c mc vi nhau theo cch no c th tin hnh qu trnh chnh lu. Mch lc nhm m bo in p (hoc dng in) mt chiu cp cho ti l bng phng theo yu cu.

2.1.2 Phn loi Chnh lu c phn loi theo mt s cch sau y: 1. Phn loi theo s pha ngun cp cho mch van: mt pha, hai pha, ba pha, 6 pha v.v. 2. Phn loi theo loi van bn dn trong mch van: Hin nay ch yu dng hai loi van l it v tiristo, v th c ba loi mch sau:

- Mch van dng ton it, gi l chnh lu khng iu khin. - Mch van dng ton tiristo, gi l chnh lu iu khin. - Mch chnh lu dng c hai loi it v tiristo, gi l chnh lu bn iu khin.

3. Phn loi theo s mc cc van vi nhau. C hai kiu mc van: a) S hnh tia: s ny s lng van bng s pha ngun cp cho mch van. Tt c cc van u mc chung mt u no vi nhau hoc catt chung, hoc ant chung. b) S cu: s ny s lng van nhiu gp i s pha ngun cp cho mch van. Trong mt na s van mc chung nhau catt, na kia li mc chung nhau ant.

18

Nh vy, khi gi tn mt mch chnh lu, ngi ta dng ba du hiu trn ch c th mch . Th d: chnh lu cu ba pha bn iu khin, c ngha l mch chnh lu ny dng kiu mc van theo s cu, ngun cp cho mch van l ba pha, v dng 6 van c c it v tiristo.

2.1.3 Cc tham s c bn ca mch chnh lu Cc tham s ny dng nh gi cc ch tiu k thut trong phn tch hoc thit k mch chnh lu, gm c ba nhm thng s chnh nh di y: 1. V pha ti Ud gi tr trung bnh ca in p nhn c ngay sau mch van chnh lu:

d)(u21dt)t(u

T1U

2

0d

T

0dd (2.1)

Id gi tr trung bnh ca dng in t mch van cp ra:

d)(i21I

2

0dd (2.2)

Pd = Ud.Id l cng sut mt chiu m ti nhn c t mch chnh lu. 2. V pha van Itbv gi tr trung bnh ca dng in chy qua 1 van ca mch van. Ung.max in p ngc cc i m van phi chu c khi lm vic. y l hai tham s gip vic la chn van ph hp khng hng khi hot ng trong mch. 3. V pha ngun Th hin bng cng sut xoay chiu ly t li in, thng thng s dng theo cng sut biu kin ca bin p:

2SSS 21ba

= ksPd (2.3)

trong :

111 IUS (2.4)

m

1ii2i22 IUS (2.5)

y cc gi tr U1, I1, U2i, I2i l tr s hiu dng ca in p v dng in pha s cp v th cp my bin p. Do pha th cp c th c nhiu cun dy, nn phi tng cng cng sut ca tt c m cun dy. nh gi kh nng bin i ca cng sut xoay chiu thnh mt chiu, cng sut ly t li in Sba c so snh vi cng sut mt chiu Pd m ti nhn c qua h s ks. H s ny cng gn 1 cng chnh t mch c hiu sut bin i tt hn. Ngoi ba nhm tham s trn cn c mt tham s dng nh gi s bng phng ca in p mt chiu nhn c, gi l h s p mch km, c xc nh theo biu thc:

km= 0

m1

UU

19

trong U1m l bin sng hi bc 1 theo khai trin Fourier ca in p chnh lu v U0 l thnh phn c bn cng theo khai trin ny. U0 cng chnh l gi tr in p trung bnh ca in p chnh lu, tc l U0 = Ud .

2.1.4 Lut dn van Mch van thc hin qu trnh chnh lu c kh nhiu, tuy nhin chng u tun theo hai kiu mc vi nhau l mc catt chung v mc ant chung. V th ch cn nhn bit hai quy lut dn ny, ta c th phn tch ton b cc mch chnh lu c trong thc t.

1D

2D

nD

1A

2A

An

KC

1D

2D

nD

1K

2K

Kn

AC

Hnh 2.2: a) Van u catt chung; b) Van u ant chung

1. Nhm van u catt chung Hnh 2.2a l mch van khi tt c cc it c catt u vi nhau. Lut dn ca n c pht biu nh sau: Van c kh nng dn l van c in th ant ca n dng nht trong nhm, tuy nhin n ch dn c nu in th ant ny dng hn in th im catt chung KC. Th d, thi im hin ti ta c:

A1 > A2 > A3 > > An v ng thi A1 > KC th van D1 s dn. Lc nu coi st p trn van bng 0 th khi D1 dn ta thy KC = A1. iu ny dn n in p trn cc van cn li s m:

AK2 = A2 - KC = A2 - A1 < 0 AKn = An - KC = An - A1 < 0

Nh vy cc van cn li s phi kho khng dn c. 2. Nhm van u ant chung nhm van u ant chung (hnh 2.2b) c lut dn van sau: Van c kh nng dn l van c in th catt m nht trong nhm, nhng n ch dn c nu in th ny m hn in th ant chung AC. Trong chng ny s p dng hai lut dn trn phn tch cc mch chnh lu thng dng, trong s coi cc van l l tng, nh vy khi dn st p trn van bng khng (uAK = 0).

2.2 CC MCH CHNH LU C BN S lng cc mch chnh lu kh nhiu, song ch yu l mt s mch chnh c gi l mch c bn. Nhng mch ny c xc nh cc tham s vi mch dng van l it v ti thun tr. 2.2.1 Chnh lu mt pha

20

1. Chnh lu mt pha mt na chu k Mch van ch c mt van duy nht l it D (hnh 2.3). na chu k u (0) khi in p t vo mch van u2 > 0 vi cc tnh dng trn th it D dn. V vi UD = 0 nn c ud u2. na chu k sau ( 2) in p u2 o du (cc tnh trong ngoc trn s ) nn it D kho, v th ud = 0.

2i D

2u1u du

dR

di

)a

Hnh 2.3

2u

du

2

D dn D kho

)b Nh vy in p chnh lu nhn c trn ti l:

22

2

02

2

0dd U45,0U

2dsinU221d)(u

21U

(2.6)

V ti thun tr nn:

RU

I dd (2.7)

Theo mch ta thy dng qua van chnh l dng qua ti v dng chy qua cun th cp bin p, v vy Itbv = Id. in p ngc trn van ch xut hin khi van kho, tc l trong khong ( 2). Theo s lc uAK = u2, do in p ngc trn van Ungmax = 2U2 . Mt s tham s khc ca mch chnh lu ny xem trong bng 2.1. Nhn chung, mch chnh lu ny c cc ch tiu k thut km nn ch thch hp vi ti nh (n mt vi ampe). 2. Chnh lu mt pha hai na chu k c im gia (chnh lu hnh tia hai pha) Bin p c im gia bin ip p s cp u1 thnh hai in p ngc pha nhau 1800 th cp '2u v

''2u .

mch van ny cc it D1, D2 u theo kiu catt chung, v vy chng s lm vic theo lut dn 1 trong ant ca it D1 ni vi '2u , cn ant ca D2 ni vi in p

''2u .

V vy trong khong t (0), it D1 dn do '2u > ''2u ; cn trong khong ( 2) th D2

dn do ''2u > '2u . Do vy in p chnh lu ud s c dng hnh 2.4b vi:

ud = '2u 0

ud = ''2u 2

21

2'u

2''u

)a

Hnh 2.4

2'u 2''u

du

2

)b in p chnh lu nhn c trn ti l:

220

20

dd U9,0U22dsinU21d)(u1U

(2.8)

v:

RUI dd

Do mi it ch dn mt na chu k in p li, trong khi dng in ti tn ti c hai na chu k, do vy dng trung bnh qua it bng mt na dng ti:

2II dtbv

xt in p ngc trn van, ta gi s D1 dn, D2 kho (giai on 0 ). Lc ny ta thy D2 c u song song vi hai cun th cp ni tip nhau, v vy:

sinU22sinU2)180sin(U2uuu 220

2'2

''2D2

nn in p ngc cc i trn it D2 l Ungmax = 2U22 . Mch chnh lu ny c s dng nhiu trong di cng sut nh n vi kW, n thch hp vi chnh lu in p thp v st p trn ng ra ti ch c mt van. Nhc im ca mch l buc phi c bin p i s pha. Hn na mt s thng s khc cng khng tt. 3. Chnh lu cu mt pha Mh chnh lu gm 4 van D1 D4 u thnh hai nhm (hnh 2.5a): D1D3 u catt chung, D2D4 u ant chung. Ngun xoay chiu da vo mch van c th ly trc tip t li hoc thng qua bin p. Trong na chu k u: 0, in p u2 > 0 vi cc tnh trong ngoc trn s . Ta thy vi nhm catt chung D1D3 th ant D1 dng hn ant D3 v vy D1 s dn. Cn nhm ant chung D2D4 th catt D2 m hn catt D4 v vy D2 dn. Nh vy na chu k u D1D2 dn. Trong na chu k sau in p u2 < 0 vi cc tnh o li (trong du ngoc), l lun tng t ta thy it D3D4 dn.

22

u2

ud

2

21 D,D

43 D,D

dn

kho21 D,D kho

43 D,D dn Hnh 2.5

i vi in p ra ti, ta lun thy im a trong c hai na chu k u c ni vi cc tnh dng (+) ca ngun u2, v im b lun c ni vi cc tnh m (-) ca u2. V vy in p ra ti ud ging ca chnh lu hnh tia hai pha, do ta cng c:

RUI

U9,0U22U

dd

22d

(2.9)

Dng in qua mi it cng ch tn ti trong mt n chu k, do 2II dtbv .

Tuy nhin in p ngc trn van ang kho ch bng in p ngun u2:

Ungmax = 2U2 . Chnh lu cu mt pha c s dng kh rng ri trong thc t, nht l vi in p trn 10V, dng ti c th n mt trm ampe. u im ca mch l c th khng cn bin p. Nhc im ca n l c hai it tham gia dn dng: it nhm l dn dng ra ti, nhm it chn dn dng t ti v ngun. Nh vy s c st p do hai it gy ra, chnh l do ny lm cho mch cu khng thch hp vi chnh lu in p thp di 10V khi dng ti ln.

2.2.2 Chnh lu hnh tia ba pha Mch van gm ba it D1, D2, D3 mc thnh mt nhm (hnh 2.6a), y l kiu catt chung, do vy chng s hot ng theo lut dn 1. in p xoay chiu a vo mch van l ngun ba pha i xng ua, ub, uc. Theo s ta thy ant D1 u vi ua; ant D2 u vi ub; ant D3 u vi uc. V th: Trong khong 1 2 (tc t 300 1500), ua > ub, uc nn it D1 dn, ud = ua. Trong khong 2 3 (tc t 1500 2700), ub > ua, uc nn it D1 dn, ud = ub. Trong khong 3 4 (tc t 2700 3900), uc > ua, ub nn it D1 dn, ud = uc. Nh vy in p ra ti ud lun ly cc in p pha dng nht ca ngun, theo th hnh 2.6b ca ud, ta c:

23

d

dd

22

150

302d

RUI

U17,1U2

63dsinU23/2

1U0

0 (2.10)

Dng in qua mi van ch tn ti trong 1/3 ca chu k in p ngun, v vy:

3II dtbv

in p ngc trn van: ungc van = udy ngun. in p ngc cc i trn van l in p dy cc i:

Ungmax = Udy max = 22 U6U23 (2.11) Chnh lu hnh tia ba pha c c im tng t chnh lu hnh tia hai pha. mch hot ng cn c bin p a im trung tnh N ra ti. V mch dng ngun ba pha nn cng sut c th tng ln nhiu, dng in ti n vi trm ampe.

1 2 3 4

1 2 3 4

dnD1 dnD3dnD2

)b

)240-sin(U2u

)120-sin(U2u

sinU2u

02c

02b

2a

Hnh 2.6

2.2.3 Chnh lu cu ba pha Mch van gm 2 nhm, cc it D1, D3, D5 u kiu catt chung (hnh 2.7), nn hot ng theo lut : D1 dn trong khong 1 3 khi ua dng nht; D3 dn trong khong 3 5 khi ub dng nht; D5 dn trong khong 5 7 khi uc dng nht; Cc it D2, D4, D6 u kiu ant chung, nn:

24

D2 dn trong khong 2 4 khi uc m nht; D4 dn trong khong 4 6 khi ua m nht; D6 dn trong khong 6 8 khi ub m nht; i chiu theo th dn cc van trn hnh 2.7c ta thy, bt k thi im no cng c mt it nhm trn dn vi mt it nhm di. Th d trong khong 1 2 l it D1D6 dn. Lc theo s thay th hnh 2.7b ta thy in p ra ti ud chnh l in p dy ca ngun xoay chiu uab. Lm tng t nh vy ta s thy rng, trong mt chu k ca in p xoay chiu, in p ud s hnh thnh t 6 on in p ca ngun xoay chiu theo th t uab uac ubc - uba uca - ucb. in p trung bnh nhn c trn ti l:

d)120-sin(U-sinU26d)u-u(

6/21U

0

0

0

0

90

30

0m2m2

90

30bad

22 U34,2U63

(2.12)

1 2 3 4 5 6 7

Hnh 2.7

So snh gi tr ny vi chnh lu ba pha hnh tia, ta thy n c tr s gp 2 ln. iu ny c th thy trn s 2.7a, s cu ba pha dng nh l hai s hnh tia mc ni tip nhau, nhm it l chnh lu ly in p dng, nhm it chn chnh lu ly nt phn in p m cn li, v vy tng qut ta c hai chnh lu ba pha hnh tia ni tip nhau. in p ud ca cc mch chnh lu c dng gn sng, khng phng, gi l p mch. S ln p mch (k hiu mm) trong mt chu k ca ngun xoay chiu 2 ph thuc vo s chnh lu. S p mch mm cng cao th dng ud cng phng, tc l h s p mch k m nh hn. Tham s ca mch chnh lu c bn xem bng 2.1

25

Tham s S

Ud0 Itbv Ungmax I2 I1 Sba U m m km

Mt pha mt na chu k 0,45U2 Id 1,41U2 1,57Id 1,21Idkba 3,09Pd - 1 1,57

Mt pha c im gia 0,9U2 Id/2 2,83U2 0,58Id 1,11Idkba 1,48Pd da IX

1

2 0,67

Mt pha s cu 0,9U2 Id/2 1,41U2 1,11Id 1,11Idkba 1,23Pd da IX2

2 0,67

Ba pha hnh tia 1,17U2 Id/3 2,45U2 0,58Id 0,47Idkba 1,35Pd da IX23

3 0,25

Ba pha s cu 2,34U2 Id/3 2,45U2 0,816Id 0,816Idkba 1,05Pd da IX3

6 0,057

Su pha hnh tia 1,35U2 Id/6 2,83U2 0,29Id 0,58Idkba 1,56Pd da IX23

6 0,057

Su pha c cun khng cn bng 1,17U2 Id/6 2,45U2 0,29Id 0,41Idkba 1,26Pd da IX4

3

6 0,057

Bng 2.1. Tham s chnh ca cc mch chnh lu c bn Ghi ch: Ud0 - tr s trung bnh ca in p chnh lu; U2 - tr s hiu dng ca in p pha cun th cp bin p ngun; Itbv - tr s trung bnh ca dng in qua van; Ungmax - in p ngc ln nht van phi chu khi lm vic; I2, I1 - tr s hiu dng dng in cun th cp v cun s cp bin p ngun; Id - tr s trung bnh dng in ra ti; kba - h s my bin p ngun; Sba cng sut tnh ton my bin p ngun; Pd cng sut mt chiu trn ti Pd = Ud0.Id; U - st p do hin

tng trng dn gy ra (khi La 0); k m - h s p mch ca in p chnh lu: k m = 0d

m1

UU

,

trong U1m l bin sng hi c bn ca in p chnh lu theo trin khai Fourier.

2.2.4 Chnh lu m pha tng qut T cc mch xt trn ta thy, vi mt mch chnh lu tng qu m pha, in p ud c dng nh hnh 2.11. N l ng bao theo cc in p pha ngun xoay chiu vi h s p mch l m m, trong : Chnh lu hnh tia: m m = mpha Chnh lu cu: nu m chn: m m = mpha nu m l: m m = 2mpha Bin in p chnh lu Um cng ph thuc vo s u van: Chnh lu hnh tia: Um = U pha max = U2m Chnh lu cu: nu m chn: Um = 2U2m

nu m l: m2

cosU2U m2m

Gi tr trung bnh in p chnh lu:

26

2

m=U md

m

m

m dcosU m

mm

msinU

m

(2.13)

2.3 CHNH LU IU KHIN DNG TIRISTOR

2.3.1 Khi nim v gc iu khin Mch chnh lu dng van l it tuy n gin nhng ch cp ra ti mt in p xc nh Ud = ksU2, ch ph thuc vo mch van v in p ngun U2, khng cho php thay i hoc gi n nh theo yu cu cng ngh ca ti. iu ny do it lun t dn di tc ng ca chnh in p ngun xoay chiu gi l m t nhin. Nu thay it bng tiristo s iu khin c im dn ca van theo mun, v m cn c ng thi hai iu kin: Th nht, in p trn van phi dng, UAK > 0; th hai, c dng iu khin mnh tc ng vo cc iu khin ca n. Nh vy s dng iu kin th hai ta khng ch c im m tiristor theo mun. thc hin trong mch iu kin ny ngi ta s dng khi nim gc iu khin (cn gi l gc m) c k hiu bng . Quy c v gc ny nh sau: Gc iu khin l gc tnh t thi im m t nhin n thi im tiristo c pht xung vo cc iu khin m van. Thi im m t nhin l thi im m nu van l it th n bt u dn. Vic tnh ton gc m van trong mch chnh lu tiristor theo yu cu cng ngh do khi iu khin m nhim v c cp chi tit chng 5. Ti chng ny ch xem xt nh hng ca gc iu khin n tham s Ud ca chnh lu. 2.3.2 Chnh lu iu khin mt pha 1. Chnh lu iu khin mt pha mt na chu k ti thun tr so snh chnh lu khng iu khin v chnh lu iu khin, trn s hnh 2.8 dng cc th ud hai trng hp ny. Hnh 2.8b l in p chnh lu nhn c khi dng it. Hnh 2.8c l chnh lu c iu khin. Trong s ny giai on (0 ) mc d in p trn tiristo T dng, song phi n thi im th tiristo mi nhn c tn hiu iu khin IG t khu pht xung (FX). Do : Trong giai on (0 ) tiristo kho: ud = 0. Trong giai on ( ) tiristo kho: ud = u2(). Trong giai on ( 0) tiristo kho: ud = 0. Nh vy in p ud by gi khng cn l ton b na hnh sin dng ca in p ngun xoay chiu u2, m ch l mt phn ca n vi ln tu thuc gc . Ta c:

2cos1U2dsinU2

21d)(U

21U 22

2

02d

(2.14)

Khi iu khin vi = 0 c gi tr Ud0:

27

Hnh 2.8

220d U45,0U2U

y chnh l biu thc (2.6) tng ng vi chnh lu khng iu khin dng it. V vy c th coi rng chnh lu it l trng hp ring ca chnh lu dng tiristo vi = 0. Biu thc (2.14) c th vit li thnh:

)(fU2cos1UU 0d0dd

(2.15)

Biu thc ny cho thy in p chnh lu Ud l mt hm ph thuc vo gc iu khin . Nh vy mun iu chnh in p ra ti ch cn tc ng vo tham s duy nht l . mch chnh lu ny, bng cch thay i t 0 n 1800 ta iu chnh c in p Ud t gi tr ln nht Ud0 n gi tr nh nht (bng 0). Cc tham s ca chnh lu dng tiristo u ly t chnh lu dng it (bng 2.1), vi l do n gin l khi = 0 (tng ng vi chnh lu khng iu khin) th in p chnh lu ln nht v mch cng mang ti nng nht. 2. Chnh lu mt pha mt na chu k vi ti RdLd Tng t nh trng hp ti thun tr, tiristo T ch c kh nng dn na chu k khi in p u2 dng. Tuy nhin van ch dn thi im pht xung tng ng gc 1 = . iu ny v phng tin vt l, n l mt bi ton qu trnh qu (hnh 2.9b) vi thi im ng mch , v tiristo ng vai tr kho chuyn mch T. Khi tiristo dn, tng ng vi kho T ng, ta c phng trnh mch:

sinU2uuu 22RL hay:

sinU2Ridt

diL 2dd

dd (2.16)

Dng in id() gm hai thnh phn: dng cng bc icb v dng t do itd: id = icb + itd

Dng cng bc chu s tc ng ca ngun u2 theo quy lut:

)sin(XR

U2i2d

2d

2cb

(2.17)

28

y Xd = .Ld; = arctagXd/Rd.

u1 u2 udid

T

Rd

Ld

u2 ud

idT

Rd

Ld

a)

b)

1 3

2

Hnh 2.9 Hnh 2.10

Thnh phn t do l hm tt dn theo thi gian:

Qtt

td AeAeAei

(2.18)

y d

d

RL

v d

d

d

d

RX

RLQ

Vy dng ti l:

Q2d

2d

2d Ae)sin(

XR

U2i

(2.19)

H s A xc nh t iu kin ng mch c in cm id( = ) = 0. a vo (2.19) ta rt ra:

Q2d

2d

2 e).sin(XR

U2A

Cui cng ta c:

Q2d

2d

2d e)sin()sin(

XR

U2i (2.20)

V bn cht, sau thi im , di tc dng ca ngun u2, dng in id s tng dn t 0, m khng tng t bin do tc ng ca in cm Ld. Sc in ng eL ca in cm ny lun chng li s bin thin ca dng in i qua n. V vy cng id bin thin chm pha hn so vi in p ngun u2. iu ny dn n thi im = , mc d ngun bng 0, song dng id > 0, v vy tiristo cha kho li c. Ch n thi im 2 > , khi dng id v n 0, tiristo mi bt u kho.

29

th hnh 2.10 m t dng dng id v tc ng c s.. t cm eL ln mch. Nh vy, nu trng hp ti thun tr khi cc van lun kho thi im , v gc dn ca van lun l = ( ), th khi c in cm Ld, dng in ko di qua im , v gc dn ca van = 3 1 > . Gi tr xc nh t biu thc (2.20) vi iu kin i( = + ) = 0, v ta c phng trnh sau:

Q-

)e-sin()-sin(

(2.21) y l phng trnh siu vit v ch c th gii gn ng. Tuy nhin s ph thuc vo gi tr Q v nh hn 2. Dng in p ud cng khc so vi trng hp ti thun tr. Do chng no tiristo cn dn, th vn c ud = u2, nn in p ud bm theo u2 n ht im 3. Nh vy in p ud c on m. y lu rng, tuy ud c th coi l xoay chiu (c hai du (+) v (-)), song dng in id ch c mt chiu duy nht. Theo th ud ta tnh c tr s trung bnh:

2)cos(-cosU2dsinU2

21U 22d

2)cos(-cosU 0d

(2.22)

Gi tr trung bnh ca dng in ti Id cng c th tnh theo biu thc (2.20). Song v Id l thnh phn khng i nn n khng gy st p trn cun cm Ld, do vn c:

RUI dd

Khi mch dng van it, ta c cc quy lut rt ra t cc biu thc (2.20); (2.21); (2.22) vi = 0, c:

Q2d

2d

2d e)sin()sin(

XR

U2i (2.23)

Qe.sin)sin(

( xc nh gc dn ) (2.24)

2cos1UU 0dd

(2.25)

2.3.3 Chnh lu iu khin hnh tia 1. Chnh lu hnh tia hai pha Hnh 2.11b l th minh ho chnh lu iu khin ny. Lu rng trong mch chnh lu nhiu pha, gc iu khin ca cc tiristo phi bng nhau: 1 = 2 = . S sai lch gia chng c nh gi bng mt i xng. Mch iu khin c nhim v m bo mt i xng khng vt qu 10 n 20 in. Theo th ta nhn c:

02

2

0dd dsinU2

1d)(u21U

2cos1U

2cos1U22 0d2

(2.26)

30

2u

2u 1u

du

a)

Hnh 2.11

2u 2u 2u

1GI

2GI

du

1 12

b) vi Ud0 = 0,9U2. Vi ti thun tr, dng dng in id tng t dng in p Ud, v ta thy dng in s c on bng 0 (id = 0) trong ton di iu chnh . Do vy dng in ny c gi l dng in gin on. 2. Chnh lu hnh tia ba pha dng tiristo th in p ud ca mch chnh lu ny th hin trn hnh 2.12b vi gc iu khin = 300. y l gc c bit.

dubu

cu

au

dR

a)

Hnh 2.12

2u

1GI

2GI

3GI

du

au bu cu

b)

a. Nu 300, in p ud s c on bng 0, v vy khi ti thun tr, dng in id s gin on, tc l c nhng on id = 0, v dng in qua van lun kt thc khi in p pha v 0. th ud c dng hnh 2.13a, theo c:

)30cos(1U2

23dsinU223d)(u

21U 02

02

2

0dd

3)30cos(1U

3)30cos(1U

263 0

0d

0

2

(2.27)

31

b. Nu < 300, dng in p ud hnh 2.13b. Ta thy in p ud lun ln hn 0. Nh vy ti thun tr, dng in id s lun tn ti v chy lin tc qua ti, v vy dng dng in ny gi l dng in lin tc. y quy lut in p ud khc i, khng tun theo biu thc (2.27) va c. Vi lu rng ba van s thay nhau dn trong mt chu k, nn mi van dn mt khong 23, do :

cosUcosU2

63dsinU223U 0d2

12030

302d

00

0

(2.28)

Nh vy, vi mch chnh lu ba pha hnh tia, quy lut in p Ud ph thuc vo ch dng: nu dng gin on theo quy lut (2.27); nu dng lin tc li theo (2.28).

au bucu

du

030>)a

au bucu

du

030

32

y phi m bo gc iu khin cc van phi nh nhau: 1 = 2 = = 6 = . Theo th ud() ta thy gc gii hn gia dng lin tc v dng gin on bng 600. Vy: Nu 600 ta s c quy lut d nh l:

Ud = Ud0cos = 2,34U2cos (2.29) Nu > 600 th dng in s gin on. in p chnh lu nhn c l:

2)60cos(1U63dsinU323U

0

260

2d0

2)60cos(1U

0

0d

(2.30)

du

1GI

2GI

3GI

4GI

5GI

6GI

Hnh 2.16

2.4 QU TRNH CHUYN MCH VAN V NH HNG CA IN CM XOAY CHIU LA 2.4.1 Cc qu trnh chuyn mch mc trc ta xem xt nh hng ca in cm pha mt chiu (ti RL) n s lm vic ca mch chnh lu. Mc ny ta xt n tc ng ca in cm pha xoay chiu n chnh lu. Khi phn tch mch chnh lu cc mc trc y, ta lun coi ngun xoay chiu cp cho mch van l l tng, tc l c tng tr trong bng khng. Tuy nhin cc ngun thc t u c ni tr, nht l chnh lu c dng bin p. Vi di cng sut ln thng in tr

33

ngun nh hn in khng ngun, v vy ta ch xem xt ngun vi in khng ngun c in cm l La. 1. Khi ngun l tng (La = 0) Ti lm vic ch dng lin tc, ta xt hai van cnh nhau hnh 2.17. Gi s T1 ang dn dng ti Id, n thi im 1 l lc van T2 c pht xung m. Do thi im ny u2 > u1 nn T2 dn s lm T1 kho ngay. V vy dng ti Id cng chuyn ngay t van T1 sang van T2. Nh vy ngay 1: dng qua T1 gim t bin t gi tr Id v 0, cn dng qua T2 tng t bin t 0 n Id. y l hin tng chuyn mch van mt cch tc thi. Trong sut chu k lm vic, in th im catt chung lun gn vi mt ngun no c van dn: udN = upha.

1

dNu

1u

2u

1T

2T

1i

2i

di

Hnh 2.17

2. Khi ngun khng l tng (La 0) Ta c s nh hnh 2.18:

Hnh 2.18

i1

i2

T1

T2

)m90( 0

1u 2u

2uu 21

1

Vn coi rng van T1 ang dn dng ti Id: i1 = Id. n thi im 1 pht xung m T2. Lc ny mc d u2 > u1, song ant ca hai van khng mc thng vo ngun nh trng hp trn m b cch ly bng cc in cm La. Cc in cm, vi tnh cht chng s t bin dng in qua n, s khng cho php dng in qua cc van T1, T2 bin thin t ngt nh trn na. Cn phi c mt khong thi gian dng i1 gim dn t Id v 0, cng nh dng i2 tng dn t 0 n Id. Nh vy trong giai on ny c hai van cng dn dng, v th qu trnh chuyn mch van ny gi l hin tng trng dn. Gc tng ng giai on ny c k hiu l . Trong khong , r rng in th im catt khng gn theo mt ngun na: udN upha.

34

2.4.2 Quy lut ca chuyn mch trng dn 1. Quy lut in p udN Khi hai van T1, T2 dn, ta c s thay th mch in nh trn hnh 2.19. Theo vit c hai phng trnh cn bng in p sau:

dtdiL-uu 1a1dN (2.31)

dtdiL-uu 2a2dN (2.32)

Cng hai phng trnh trn c:

)dt

didtdi(L-uuu2 21a21dN (2.33)

1u

2u

1i

2i

diaL

aL

dNu

N

Hnh 2.19

Ti im nt catt chung: d21 Iii , nu coi dng id bng phng hon ton trong khong , o hm hai v biu thc ny c:

0dt

didtdi 21

T y a vo biu thc (2.33) cho php rt ra quy lut:

2uuu 21dN

(2.34)

Nh vy: trong giai on chuyn mch trng dn, in p bin thin theo quy lut bnh qun cc in p pha ngun c van tham gia chuyn mch. 2. Quy lut dng in xc nh cc dng i1, i2 trong khong , ta xt dng i trn s mch hnh 2.19. Ta c phng trnh:

12a uudtdiL2 (2.35)

Vi ngun m pha, nu ly u2 lm gc th:

sinUsinU2u m222

v )m2sin(Uu m21

Tnh hiu ca chng v sau khi bin i ta c:

m

90sinm

sinU2uu 0m212

V qu trnh bt u 1, ta dch trc to sang im ny c:

sinm

sinU2uu m212

Quay li (2.35):

35

sinL

msinU

dtdi

sinm

sinU2dtdiL2

a

m2

m2a

C)cos(X

msinU

-)(ia

m2

= 0 (gc to mi) c i(0) = 0, tnh c hng s C:

cosX

msinU

Ca

m2

Cui cng:

)cos(-cosm

sinX

U)(ia

m2

(2.36)

Quy lut dng i2 (van mi m) chnh l i(). Quy lut dng i1 l: i1() = Id i().

3. Gc trng dn Xc nh t iu kin i2( = ) = Id. Thay vo (2.36) rt ra:

msinU

IX)cos(cosm2

da

(2.37)

2.4.3 nh hng ca chuyn mch trng dn So snh vi in p udN khng c trng dn ta thy, khi trng dn in p udN b mt mt on. Tc l in p chnh lu b nh i mt lng U:

0

212

0dN2 d2

uu-u2mdu-u

2mU

0m2

0

12 d)sin(m

sinU2md

2u-u

2m

cos-cosm

sinU2m

m2

Thay (2.37) vo biu thc ny ta c:

m/2IXU aa

(2.38)

Vy in p chnh lu ch cn:

U-cosUU 0dd (2.39)

36

Lu : a. Cc phn tch trn dnh cho qu trnh chuyn mch ti mt nhm van, v vy n hon ton chnh xc cho chnh lu hnh tia. Vi chnh lu cu, v mch c hai nhm van nn qu trnh chuyn mch tn ti c hai nhm, do ch ng cho tng nhm mt, v vy cc biu thc cng ch ng cho tng nhm. S khc bit th hin hai biu thc chnh.

1. Vi udN: Chnh lu hnh tia udN = ud. Chnh lu cu: udN ud v c hai nhm

udN1 = ud catt chung udN2 = ud ant chung ud = udN1 udN2

2. Vi U: Chnh lu hnh tia tnh theo (2.38). Chnh lu cu phi thm h s 2 vo (2.38) do hai nhm cng gy st p U.

b. Chnh lu it vi La 0 ng nhin cng c hin tng chuyn mch vi . Cc biu thc tnh ton s c thay bi iu kin =0.

2.4.4 Qu trnh chuyn mch mt s mch chnh lu c bn. 1. Chnh lu hnh tia hai pha Quy lut in p theo (2.34):

02

uuu 21dN

(2.40)

Quy lut dng theo (2.36): - dng qua van bt u dn:

im() )cos(cosmsinXU

a

m2

- dng qua van bt u kho: )(i-I=)(i mdkho

Gc trng dn theo (2.37):

m2

da

UIX)cos(-cos

St p do chuyn mch:

da IXU (2.41)

do :

U-cosUU 0dd

da2

IX-cosU9,0 (2.42)

du

Ti

2'u2''u

1Ti 2Ti2Ti

Hnh 2.20

37

2. Chnh lu cu mt pha y l trng hp c bit v ngun xoay chiu a ti mch van ch c mt pha. Theo nguyn l hot ng van phi dn theo cp T1T2 v T3T4. V vy thi im 1, khi van T3T4 ang dn ta cho m T1T2 th ng thi c hai nhm van chuyn mch: T3 chuyn cho T1; T4 chuyn cho T2.

Nh vy trong giai on c bn van u dn dng in. Theo s ta thy mch ti b ngn mch bi hai ng do cc van cng m to thnh l T1T4 v T2T3, do in p trn ti cng s bng 0:

ud() = 0. Cc quy lut khc: St p do trng dn gp i s tia hai pha:

da IX2U (2.43)

du

Ti

Ti

1Ti

2Ti

3Ti 3Ti

4Ti 4Ti

1 2

1 2

21TT 43TT

Hnh 2.21

Gc trng dn:

m2

da

UIX2)cos(-cos (2.44)

v:

da

2dI2X

-cosU9,0U (2.45)

2.5 CH NGHCH LU PH THUC TRONG THIT B CHNH LU Thit b chnh lu c xem xt n mc ny lun hot ng ch m dng nng lng chuyn t pha ngun xoay chiu sang pha mt chiu - ch chnh lu. Tuy nhin, khi ti c cha sc in ng Ed, c th lm cho nng lng chuyn t pha mt chiu sang pha xoay chiu - gi l ch nghch lu. Thc t iu ny thng xy ra vi ti l ng c in mt chiu khi n lm vic ch my pht, v nng lng ny phi chuyn tr v li in xoay chiu. Lc thit b chnh lu chuyn sang chy ch nghch lu. V mch van hot ng phi da theo ngun xoay chiu, ph thuc ngun ny, nn c gi l nghch lu ph thuc. 2.5.1 Bn cht nghch lu ph thuc v cc iu kin thc hin. Hnh 2.22a th hin cu trc mch chnh lu bit. Ta c th coi Ud nh ngun mt chiu thay th tng ng cho mch van v ngun xoay chiu pha trc, v ta c s mch thay th nh hnh 2.22b.

Ta bit rng mt ngun c th l pht nng lng hoc tiu th nng lng, tu thuc chiu ca dng in v chiu s..:

- Nu chiu s.. v dng in qua n trng nhau, n l ngun pht nng lng. - Nu chiu s.. v chiu dng in tri nhau, n l ti tiu th nng lng. T nguyn tc ny ta thy rng, mun c nghch lu cn hai iu kin:

38

1. pha mt chiu l pht nng lng, cn m bo dng id v s.. Ed cng chiu nhau.

2. pha xoay chiu nhn nng lng, tng ng l ngun Ud phi nhn nng lng, th chiu id phi ngc chiu vi ngun Ud.

Song do mch van ch cho php dng id i theo mt chiu xc nh: a) Khi gi nguyn mch van chnh lu, tc l gi

chiu dng id chnh lu, ta buc phi: - Thc hin 1 bng cch o chiu s.. Ed. - Thc hin 2 bng cch o chiu Ud v c quy

lut Ud = Ud0cos, nn iu kin ny c ngha cn lm cho Ud < 0, suy ra phi iu khin vi > 900. Phng php ny th hin hnh 2.23a.

b) Khi gi nguyn chiu s.. Ed cn: - o chiu dng id bng cch a vo mt mch

van th II ngc vi mch chnh lu I. Nh vy khi cn chy ch nghch lu s a b II vo hot ng l thc hin c iu kin 1.

- iu kin 2 cng ch m bo khi iu khin b II theo > 900. Phng php ny th hin hnh 2.23b.

=

dR di

dL

dEdU~U

I

II

Ii

IIi

)adR di

dL

dEdU)b

Hnh 2.22

Trong c hai phng php lun cn iu khin mch van vi > 900 m bo Ud o du so vi ch chnh lu. Nh vy ch c cc mch c quy lut Ud = Ud0cos mi cho php chy ch nghch lu. Mch chnh lu bn iu khin vi quy lut

2cos1UU 0dd

, nn khng th o du

Ud, v vy khng thc hin c ch nghch lu. Xt theo kha cnh l thuyt mch, ch nghch lu i vi mch van ch l cc van c iu khin vi gc > 900. V th tt cc biu thc tnh ton khng thay i so vi ch nghch lu.

dR di

dL

dEdU

~U

II

b)-

+

dR di

dL

dEdU

~U

I

a)

-

+

Hnh 2.23

2.5.2 Mch chnh lu mt pha ch nghch lu ph thuc Xt trn th d s cu mt pha (hnh 2.24a). 1. Ch chnh lu: < 900, Ud > 0 ngun xoay chiu pht nng lng; Ed > 0 nhn nng lng. Dng nng lng chuyn t pha xoay chiu sang mt chiu. th hnh 2.24b cho thy in p ud ch yu l cc on ln hn khng. 2. Ch nghch lu: > 900, Ud < 0 v nhn nng lng; Ed < 0 v c chiu trng chiu dng id tr thnh ngun pht nng lng. th ca ch ny hnh 2.24c.

39

in p ud ch yu l giai on m. Cc biu thc tnh ton vn theo (2.43), (2.44),

(2.45). C mt lu l cc biu thc tnh dng in tuy vn l: d

ddd R

E-UI , nhng do

Ed< 0 nn thc cht c:

d

ddd R

EUI

(2.46)

phn bit cho d nhn bit ch nghch lu, ngi ta a vo gc b ca gc iu khin:

- (2.47) Lc cc biu thc (2.44), (2.45) tr thnh:

da2d

IX2-cosU9,0-U (2.48)

m2

da

UIXcos-)-cos( (2.49)

id

T1 T3

T4 T6

dL

dE

2u

a) s

du

Ti1Ti

1Tu

21TT 43TT

du

Ti

1Tu

Hnh 2.24

3. Gc kho v ch sp nghch lu ch nghch lu ph thuc, cn c bit lu thi gian phc hi tnh cht kho cho tiristo, l khong thi gian in p trn tiristo m sau khi van kho. Trn th (hnh 2.24c) c c trng bi gc kho . Theo th thy rng:

= - ( + ) ch chnh lu do nh nn gc ln v lun m bo kho tt cho van. ch nghch lu do ln nn gc nh, do vy nu iu khin van vi qu ln c th gc khng m bo kho chc van na. Lc tiristo s dn tr li ngay im in p va dng ln, tng ng = 0, tc l mch b mt iu khin. Dng id s tng vt, do ud > 0 nn trong mch cng mt lc c hai ngun pht nng lng v dn c ln in tr Rd, gy s c mch.

40

Hin tng mt iu khin do < phc hi ca van lm mch s c gi l sp nghch lu.

Nh vy khi hot ng ch nghch lu cn lun khng ch gc trong phm vi cho php, thng > 50.

2.6 B LC MT CHIU H s p mch ca mch chnh lu ph thuc vo s p mch m m v gc iu khin . Vi mt mch chnh lu xc nh m m = const, h s k m tt nht khi = 0 (van it), trong qu trnh iu khin gc h s ny lun km hn. Trng hp km() khng tho mn yu cu ca ti, cn a thm mch lc mt chiu nhm ci thin h s p mch. Hiu qu ca khu lc c nh gi bng h s san bng:

mra

mvosb k

k=k (2.50)

u vo ca mch lc l mch van, do h s p mch vo chnh l h s p mch ca chnh lu. H s p mch nhn c sau khi lc kmra ng nhin phi nh hn kmvo. Nh vy theo (2.50) h s san bng phi ln hn 1, v cng ln th lc cng tt.

mvk mrak

Hnh 2.25

Cc b lc thng dng trnh by hnh 2.26:

vu rau

L

a)

rauvuC

b)

vu rauC

c)

rauCC

vu

d)

Hnh 2.26. a) Lc in cm; b) Lc in dung; c) Lc LC; d) Lc (lc CLC)

2.7 CHNH LU BN IU KHIN V CHNH LU C IT M Chnh lu bn iu khin ch s dng khi mch van l s cu, lc mt na s van l it, mt na s van cn li l tiristo. 2.7.1 Chnh lu cu mt pha bn iu khin tiristo mc catt chung Nhm catt chung l cc tiristo nn chng c m cc thi im ca n. Nhm ant chung l van it nn chng lun m t nhin theo in p ngun: D1 m khi u2 bt u m; D2 m khi u2 bt u dng. Do vy s dn ca cc van trong chu k li l:

Trong khong : T1D2 dn. Trong khong ( + ): T1D1 dn do , D1 m t nhin lm D2 kho. Trong khong ( + ) 2: T2D1 dn, T2 c pht xung m im ( + ) v

dn lm cho T1 kho. Trong khong 2 (2 + ): T2D2 dn, D2 m t nhin im 2.

Qua y ta thy c hai on c hin tng dn thng hng ca hai van: T1D1 v T2D2, do nhng on ny ti b ngn mch nn ud = 0 (cc on cn li ud bm theo ngun). Nh vy dng id vn lin tc, song dng i2 li t on do dng ti id chy qun

41

qua hai van thng hng m khng v ngun. iu ny c li v kha cnh nng lng, v nng lng khng b tr v ngun m gi li trong ti. Dng in p ud tr li ging nh chnh lu iu khin vi ti thun tr (hnh 2.6b), do vy quy lut ud l:

2cos1U9,0

2cos1UU 20dd

(2.51)

Dng ti: d

dd R

UI

Cc van dn u nhau mt khong l , do vy tr s trung bnh ca dng qua van vn l Id/2.

Rd

id

T1 T2 dL

2u

D1 D2

Hnh 2.27

du

2

2 2

1Ti

2Ti

2Di

1Di

2i

12i

2.7.2 Chnh lu cu ba pha bn iu khin Khi lm vic, cc it chuyn mch t nhin cn cc tiristo chuyn mch ti cc gc iu khin . Khi < 600, in p ud lun ln hn 0. Nhng khi > 600 (hnh 2.28b l = 900) s xut hin cc giai on hai van mc thng hng dn ng thi:

1 2: T3D3 dn; 3 4: T1D1 dn; 5 6: T2D2 dn.

Do vy trong cc on ny in p ud = 0, v dng in id chy qun trong ti m khng chy v ngun nn nng lng c gi li ti, khng b tr v ngun. Quy lut in p Ud c th suy ra t l lun mch cu tng ng hai mch chnh lu hnh tia ni tip:

42

Chnh lu hnh tia ba pha iu khin gm T1, T2, T3 cho in p: cosU17,1cos.UU 2tia0dd

Chnh lu hnh tia ba pha khng iu khin gm D1, D2, D3 cho in p: 2tia0dd U17,1UU

Vy tng li: )cos1(U17,1cos.UUU 2tia0dtia0dd

aubucu

a)

Hnh 2.28

du

1Ti

2Ti

3Ti

1Di

2Di

3Di

au bu cu

1T 2T 3T

1D2D 3D 2D

1 2 34 5 6

b)

V chnh lu cu c 2tia0d0d U34,2U2U nn quy i biu thc trn sang dng cu ta c:

2

cos+1U34,2=

2

cos+1U=U 2cu0dd

(2.52)

Mch cu ba pha bn iu khin c u im l iu khin n gin hn, tit kim nng lng hn. Song cng c nhc im l s p mch trong ton di iu chnh bng 3: m m = 3; ch = 0 mi c m m = 6 nh s cu iu khin.

2.7.3 H s cng sut cos ca chnh lu iu khin v chnh lu bn iu khin Ta bit rng h s cng sut tiu th ca mt h tiu th in l cos, trong l gc lch pha gia in p v dng in ca ph ti . Hnh 2.29 l th ch ra quan h gia in p ngun u2 v dng in chnh lu nhn t ngun i2 cho hai trng hp. Theo th dng in i2 v sng hi bc 1 ca n i2(1). Vi tn s bng tn s ngun xoay chiu, v so snh vi in p ngun u2, rt ra: Chnh lu iu khin c : =

Chnh lu bn iu khin c: 2

Nh vy h s cos cu chnh lu bn iu khin tt hn chnh lu iu khin. Qua y ta cn lu rng, v kha cnh h tiu th in th chnh lu l ph ti c tnh in cm do dng in chm pha hn in p mt gc . im c bit cn ch gc ny t l thun vi gc iu khin , v v khi tng h s cng sut xu i r rt.

43

du

2u

2i

12i

du

2u

2i

12i

a) b) Hnh 2.29 a) Chnh lu iu khin; b) Chnh lu bn iu khin

2.7.4 Chnh lu iu khin c it m So snh s chnh lu iu khin v chnh lu bn iu khin thy rng h s cos ca chnh lu bn iu khin tt hn. Mch chnh lu iu khin cng c th ci thin c cos bng cch mc u ra ca mch van mt it m D0 (hnh 2.30). Lc ny, mi khi in p ud bt u m th it ny lp tc dn dng ti chy vng qua n, dn ti ud on ny tr thnh bng 0. Nh vy dng in p ud tr thnh ging nh chnh lu bn iu khin, it D0 ng vai tr nh hai van dn thng hng. Kt qu t c l = /2 nh chnh lu bn iu khin. Lu : i khi trong s chnh lu bn iu khin ngi ta vn mc thm it D0, nht l s nhiu pha. Lc ny it m c nhim v bo m kho tt cho cc tiristo khi ngt xung iu khin. iu ny cho php chng hin tng mt iu khin ca tiristo (khng c xung iu khin m van vn dn). MV

Hnh 2.30

2.8 GHP CC B CHNH LU Thc t c nhng ph ti i hi in p cao, hoc dng in ln m van c cng nghip ch to cha t ti. Trong nhng trng hp ny ngi ta c th dng cc bin php khc phc sau: u ni tip nhiu van hoc u ni tip cc mch chnh lu vi nhau chu c

in p cao. u song song nhiu van hoc song song nhiu mch chnh lu vi nhau chu

c dng ti ln. y cp n phng php u ghp cc mch chnh lu vi nhau.

44

2.8.1 Ghp ni tip cc mch chnh lu Hnh 2.31 l th d v u ni tip hai mch chnh lu s cu ba pha. Nu mch c dng bin p th hai cun th cp c pha lch nhau 300. iu ny cho php ud = ud1 + ud2 c p mch gp i s cu thng thng, c ngha y m m = 12 v in p ra s bng phng hn.

2.8.2 Ghp song song mch chnh lu vi nhau Khi ghp song song cc mch chnh lu vi

nhau, phi m bo tng mch hot ng c lp.

1du

2du

du

Hnh 2.31

t c iu ny buc phi s dng mt in cm cch ly cc mch chnh lu, c gi l cun khng cn bng Lcb. Mch hay dng u song song l hnh tia ba pha v cu ba pha. Trong cp in p thp v dng ln rt thng dng u

a

cb

a

cbdu

2Id2Id

cbL

Hnh 2.32

song song hai mch hnh tia ba pha v c tn gi ring l chnh lu 6 pha c cun khng cn bng.

Hnh 2.32 l s u song song hai mch chnh lu. 2.8.3 Chnh lu 6 pha c cun khng cn bng My bin p c hai b cun dy th cp to thnh h thng ngun i xng 6 pha: a, b, c v a, b, c. Hai nhm van u theo s hnh tia ba pha, v lm vic c lp nh in cm cn bng Lcb. Thy ngay rng, nu khng c in cm cn bng Lcb th mch tr thnh chnh lu hnh tia 6 pha v 6 van u thnh mt nhm duy nht kiu catt chung.

Theo s mch in ta c: udI = ud + u1 (2.53) udII = ud - u2 (2.54)

trong udI, udII l in p ca hai mch chnh lu ba pha hnh tia; u1 v u2 l st p trn mi na cun khng cn bng, do u1 = u2. T hai biu thc trn ta c:

2uuu dIIdId

(2.55)

Bit dng udI, udII ta dng c th ud. Hnh 2.34 l th cc in p chnh lu vi = 0. Ta thy in p chnh lu c s p mch gp i s tia, tc l mm = 6. Tuy nhin gi tr trung bnh Ud vn ch bng in p chnh lu tia ba pha, v hai mch u song song:

220d U17,1U263U

(2.56)

(trong khi nu l chnh lu 6 pha hnh tia th 220d U35,1U23U

).

45

a

b c

a

bc

Hnh 2.33

du

2u 'bu au 'cu bu 'au cu

Lcbu

Lcbi

dIu dIIu du

Hnh 2.34

Mi mch chnh lu chu mt na dng ti Id nn mi van chu mt dng trung bnh l:

6I

32I

I ddtbv (2.57)

Cng sut bin p ngun: Sba = 1,26Pd (2.58) Cun khng cn bng:

Ly hiu ca (2.53) v (2.54) ta nhn c in p trn cun khng cn bng: u1 + u2 = uLcb = udI udII (2.59)

Nh vy chnh lch in p gia hai b chnh lu ri trn Lcb, v c dng nh trn hnh 2.34 vi tn s gp ba ln tn s ngun xoay chiu. Dng t ho cun khng Lcb c cng tn s v chm pha hn in p 900. V dng ny l xoay chiu v chy xuyn qua hai mch chnh lu nn n nm trong dng idI, idII. Do vy nu tr s cc dng IdI, IdII qu nh s khng cp dng t ho cho cun khng hot ng. Lc Lcb mt tc dng

46

cch ly hai mch chnh lu, v mch tr thnh chnh lu 6 pha hnh tia, khng cn l hai mch chnh lu ba pha hnh tia u song song. Cng sut cun khng cn bng:

Scb = 0,071Pd (2.60) Khi iu khin vi 0, ta c quy lut quen bit:

Ud = Ud0cos - U (2.61) St p do trng dn tnh theo biu thc s tia ba pha, vi lu rng dng qua mt mch chnh lu l Id/2:

dada IX.

43

2I.

2X3U

(2.62)

Gc trng dn:

m2

da

U3IX)cos(cos (2.63)

Lu : Trong thc t cun khng cn bng khng mc gia hai nhm van m mc gia hai im trung tnh N1, N2. iu ny thun tin cho vic lp rp thit b.

2.9 C TNH NGOI CHNH LU c tnh ngoi l quan h gia in p chnh lu v dng in ra ti:

Ud = f(Id) (2.64) Gi tr Ud0 m cc biu thc tnh ton ch l in p chnh lu khng ti. Khi mang ti din p nhn c gim i so vi Ud0. Tng qut c:

Ud = Ud - U (2.65) trong : U l tng st p khi chnh lu mang ti, bao gm:

1. St p thc trn cc van, l gi tr ph thuc vo dng qua n: UV = U0 + rId (2.66)

(U0 v r l cc gi tr tra cu theo loi van c chn). 2. St p trn cc in tr pha ngun, nh in tr ca cun th cp my bin p, ca

dy dn in, ca cun khng Ld UR = R.Id (2.67)

3. St p do in cm pha xoay chiu La m ta gi l st p do chuyn mch trng dn :

U = k.Xa.Id (2.68) Vy c tnh ngoi c dng:

Ud = Ud0cos - UV (R +k.Xa)Id (2.69) v l hm tuyn tnh theo Id khi gi khng i. Hnh 2.35 l c tnh ngoi ca chnh lu mt pha hai na chu k. Thc t, c tnh ngoi dng trong h trc n v tng i:

0d

d*d U

UU v dnm

d*d I

II

47

y: Idm l dng in Id nh mc; Idnm l dng Id ngn mch, thng c tnh theo:

a

2dnm X

U2I

Hnh 2.36 trnh by c tnh ngoi ca chnh lu cu ba pha.

00=045=075=

0900120

0150

dU

dI

Hnh 2.35

0dd*d U/U=U

a

2*d X

U2=I

030=

060=

090=

01200150

Hnh 2.36

2.10 B CHNH LU O CHIU

Khi ti i hi in p mt chiu cp cho n c hai cc tnh, ta cn s dng b chnh lu o chiu. Ti thng gp ca dng ny l ng c in mt chiu, khi hot ng cn quay c c chiu thun v chiu ngc.

a

c

b

a

c

b

Hnh 2.37

B chnh lu o chiu thc cht l hai mch chnh lu cng loi u song song, ngc nhau so vi ti (hnh 2.37). Mi b chnh lu m nhn mt du (mt chiu) ca in p ti. Tuy nhin mch lm vic bnh thng, cn phi hp s hot ng ca hai b chnh lu vi nhau. C hai phng php iu khin thc hin iu ny. 2.10.1 Phng php iu khin chung c im ca phng php ny l hai mch chnh lu cng hot ng, tc l cng c pht xung iu khin. Tuy nhin mt b lm vic ch chnh lu, l b xc nh du ca in p mt chiu hoc chiu quay ca ng c, cn b kia chy ch nghch lu v lun sn sng chuyn sang ch chnh lu. Hnh 2.38a l th d v b chnh lu o chiu s dng s cu ba pha. Do hai b chnh lu cng u vo mt ti nn gi tr trung bnh ca chng phi bng nhau; theo quy c trn s hnh 2.38 ca in p udI v udII, iu ngha l:

UdI = - UdII (2.70) hay: Ud0cosI = - Ud0cosII

48

suy ra: cosI + cosII = 0 Phng trnh ny cho ta quan h, hay lut phi hp iu khin hai mch chnh lu:

I + II = (2.71) Hnh 2.38b l th in p udI v udII khi b CLI ch chnh lu, b CLII ch nghch lu.

du

Lcbu

1IT

1II 'T II

1 2

34TT 45TT 56TT 61TT 12TT 23TT

Hnh 2.38

Theo (2.71) ta c: II = - I

V b II nghch lu nn theo (2.47): II = - II

suy ra: II = I. Lu rng, im gc xc nh gc cho hai b l trng nhau i vi van c cng s th t; th d van T1 v T1 c cng im mc tnh gc iu khin

1T v 1T .

49

Lut phi hp iu khin theo (2.71) mi ch m bo tr s trung bnh ca hai b chnh lu bng nhau theo (2.70). Song gi tr tc thi ca chng l khc nhau, tc l:

udI udII (2.72) iu ny dn n buc phi dng cun khng cn bng chng dng ngn mch chy xuyn qua hai b chnh lu. Th d, trong khong 1 2, b CLI c T1T6 dn, th b CLII tng ng l:

on u T5T4 dn to thnh ng ngn mch: pha c T5 T6 pha b. in cm Lcb2 l chng ngn mch theo ng dy pha di ni gia hai b CLI v CLII.

on sau n lt T6T5 dn, to thnh ng ngn mch: pha a T1 T6 pha b. in cm Lcb1 l chng ngn machcj theo ng pha trn ca mch.

Nh vy mch phi dng hai cun khng cn bng. Dng in p trn c hai cun ny hnh 2.38b, dng theo biu thc:

udI + udII = uLcb = u1 + u2 By gi ta xem xt qu trnh o chiu in p ti Ud. Do b I ang ch chnh lu nn dng in ti l dng ca b CLI: Id = IdI, b II khng c dng, IdII = 0, v chiu dng ny ngc chiu id nn khng th chy c (tuy nhin vn tn ti dng cn bng). Khi o chiu, phi iu khin tng gc iu khin I, tng ng gim dn II theo (2.71). Do I tng nn UdI gim, trong khi sc in ng Ed khng gim nhanh bng, dn n Ed > UdI, do :

0R

EUI ddd

tc l dng ti s o chiu. Nhng b CLI khng th cho dng idI o chiu, nn dng id s chuyn sang chy qua b CLII. Mch vng gia CLII v Ed l ng cc iu kin chy ch nghch lu, nn lc ny CLII thc hin vic tr nng lng ca sc in ng Ed v ngun lm Ed gim. Khi I tng n bng 900, II cng gim v gi tr 900, in p UdI = -UdII = Ud0cos = 0, qu trnh nghch lu ca CLII kt thc. Sau II tip tc gim nh hn 900 v chuyn sang ch chnh lu, in p i du. B CLI chuyn sang ch nghch lu ph thuc, qu trnh o chiu kt thc. Phng php iu khin chung cho php tin hnh o chiu nhanh do hai b chnh lu lun ng thi hot ng. Tuy nhin phi tun th nghim ngt (2.71) l iu kh thc hin chnh xc. ng thi buc phi c cc cun khng cn bng, lm tng kch thc, gi thnh v gim hiu sut ca thit b. V th phng php ny thng ch ng dng khi cn c tc ng nhanh hoc phi o chiu thng xuyn vi tn sut ln.

2.10.2 Phng php iu khin ring c im ca phng php ny l cc b chnh lu lm vic khng ng thi. Vi mi chiu ca in p ra ch c mt b chnh lu c pht xung v chy ch chnh lu; cn b kia c ngh, khng c pht xung iu khin. Nh vy khng th c dng in chy xuyn thng gia hai mch, do hon ton khng cn cc cun cm cn bng v hai b chnh lu c u song song ngc nhau mt cch trc tip. Tuy nhin iu ny dn n buc phi loi tr kh nng hai b cng hot ng, v lp tc s xut hin

50

dng ngn mch xuyn thng gy s c cho thit b. Do qu trnh o chiu phi thc hin theo trnh t cht ch. Th d, cn chuyn s hot ng t CLI dnag CLII, phi lm nh sau: 1. Ngt xung iu khin b ang chy, y l b CLI. Do ti c tnh cht in cm v tiristo l phn t bn iu khin nn n vn tip tc dn mc d ngt xung m van. Lc ny khng th pht xung ngay cho CLII, v s xy ra ngn mch xuyn thng do b CLI vn cng ang dn dng id. 2. Theo di dng in id xc nh thi im id = 0. Lc c ngha van ca CLI kho li. 3. ch mt khong thi gian cho van ca CLI ph hi tnh cht kho, m bo tiristo kho chc chn. Khong thi gian ny c gi l thi gian cht do mch ti khng cn dng chy id= 0. 4. Bt u pht xung m cho CLII ch nghch lu II > 900 ri gim dn chuyn sang ch chnh lu II < 900. S d c yu cu ny v thng thng sc in ng Ed khng gim nhanh v vn gi chiu nh c sau khi id = 0. Nu pht xung ngay vi II < 900 s lm sp nghch lu v c hai ngun Ed v li u pht nng lng. Ch nghch lu s lm tiu tn nhanh chng nng lng ca Ed (nu l ng c in n chnh l c nng trn trc ng c). Tc gim gc II c khng ch bng cch o dng ti, sao cho dng ny khng vt qu tr s cho php. Quy trnh 4 bc m bo chiu thng do mt mch iu khin logic c tin cy cao m nhim. Quy trnh ny cho thy phng php iu khin ring c tc o chiu thp hn phng php iu khin chung, song b li khng cn m bo yu cu (2.71) nn d thc hin hn. V vy khi khng c yu cu v tc ng nhanh hoc tn sut o chiu thp, trong thc t u dng phng php iu khin ring.

51

CHNG 3: B BIN I XUNG P

B bin i xung p (BBXA) l b bin i m in p ngun c ng, ct vo ph ti mt cch c chu k. Do in p trn ti l nhng xung p mt chiu (BBXA mt chiu) hoc xoay chiu (BBXA xoay chiu) tu thuc vo in p ngun l in p mt chiu hoc in p xoay chiu.

3.1 B BIN I XUNG P MT CHIU (IU P MT CHIU) 3.1.1 Gii thiu chung 1. Khi qut ng ct in p ngun ngi ta thng dng cc kho in t cng sut v chng c c tnh tng ng vi kho l tng, tc l khi kho dn in (ng) in tr ca n khng ng k; cn khi kho ngt (m ra) in tr ca n ln v cng (in p trn ti s bng khng). Nguyn l c bn ca b bin i xung p mt chiu c m t trn hnh 3.1.

Ru

1t

T

RU

t0

Ru

)a)b

Hnh 3.1. a) S nguyn l; v b) th ca b bin i xung p

Trong khong thi gian 0 t1, kho K ng li, in p trn ti UR s c gi tr bng in p ngun (UR = E); cn trong khong t1 T, kho K m ra v UR = 0. Nh vy gi tr trung bnh ca in p trn ti s l:

ET

EEdtT1U

0R (3.1)

trong : - thi gian kho K ng. - h s iu chnh. T chu k ng ct ca kho K.

Biu thc (3.1) cho thy, thay i in p trn ti c hai cch: 1. Thay i thi gian ng kho K, khi gi chu k ng ct khng i (phng php iu ch rng xung). 2. Thay i tn s ng ct (f = 1/T) v gi thi gian ng kho K khng i ( = const). Nh vy b bin i xung p c kh nng iu chnh v n nh in p ra trn ph ti. N c nhng u im c bn sau: - Hiu sut cao v tn hao cng sut trn b bin i khng ng k so vi cc b bin

i lin tc.

52

- chnh xc cao cng nh t chu nh hng ca nhit mi trng, v yu t iu chnh l thi gian ng kho K m khng phi gi tr in tr ca cc phn t iu chnh thng gp trong cc b iu chnh lin tc.

- Cht lng in p tt hn so vi cc b iu chnh lin tc. - Kch thc gn nh.

Nhc im c bn ca cc b bin i xung p l: - Cn c b lc u ra, do lm tng qun tnh ca b bin i khi lm vic trong h

thng kn. - Tn s ng ct ln s to ra nhiu cho ngun cng nh cc thit b iu khin.

Tuy nhin b bin i xung p vn c s dng rng ri, nht l khi cc yu t v tin cy, d iu chnh, n nh cng nh kch thc l nhng tiu ch t ln hng u. i vi cc b bin i cng sut trung bnh (hng trc kW) v nh (vi kW), ngi ta dng cc kho in t l cc bng bn dn lng cc IGBT. Trong trng hp cng sut ln (vi trm kW tr ln) ngi ta s dng GTO hoc tiristo. 2. Kho bng tiristo Nh chng ta bit, tiristo l van dn bn iu khin. Mun kho tiristo cn gim dng qua tiristo nh hn gi tr nht nh no bng cch t in p ngc ln tiristo.

tR R

CT fT

C

)a

tR

CT+

CfT

DL

)b Hnh 3.2. Mch kho tiristo

Khi s dng tiristo l kho in t trong b bin i xung p mt chiu, kho tiristo ngi ta thng dng cc tiristo ph v ngun nng lng tch tr trong t in kho tiristo chnh.

Mt s mch kho tiristo c gii thiu trn hnh 3.2. Trn s hnh 3.2, tiristo TC l tiristo chnh (kho in t). Tiristo ph (Tf) cng vi cc phn t C, R, D, L lm thnh mch chuyn mch kho tiristo chnh.

i vi s hnh 3.2a, khi TC m, t C c np in thng qua in tr R ti gi tr in p ngun, mun kho TC ngi ta m Tf. Nh vy trn TC s c in p ngc bng gi tr in p trn t v n lm cho dng qua TC gim v khng v kho li. i vi s hnh 3.2b, khi m Tf, t C c np in vi du dng pha trn v m pha di. Khi cho tiristo chnh (TC) lm vic, t C s phng in qua mch D, L v do hin tng cng hng n s np in theo chiu ngc li (du in p trong ngoc hnh 3.2b). Du in p ny ph hp to in p ngc cho TC. Mun kho TC ta li m Tf v in p ngc ca t C lc ny c tc dng kho TC li.

53

Nh vy bng cc mch ph tr ta bin b TC v Tf tr thnh mt kho in t c th ng m tu . Phng php chuyn mch nh vy gi l chuyn mch cng bc. 3.1.2 B bin i xung p ni tip Xt trng hp ti tr cm: S bao gm t lc u vo (C), kho in t dng van iu khin hon ton (GTO) T, cun khng L, it m D v ph ti R (hnh3.3a).

+

E C D R

LT A

B

1i

2i

tiDi

1t T T2t

t

t

t

t

t

t

t

GU

ABU

timaxti

minti

GTOi

Di

ti

GTOi

Di

b)

c)

d)

e)

f)

g)

h)

i)

a)

Hnh 3.3. B bin i xung p

Kho in t s c ng ct vi chu k T theo lut iu khin nh hnh 3.3b, c. Van T s dn in khi UG > 0 v van s kho khi UG < 0. Trong khong t 0 n t1, khi T dn in, nng lng ca ngun s c cp cho ph ti, nu coi van l l tng, thi gian ng ct ca van bng khng th UAB = E. Khi van kho (t1 T), do nng lng tch tr trong in cm, dng in vn theo chiu c v khp mch qua it m D, lc ny UAB = UD = 0. Gi tr trung bnh ca in p ti hai im A, B l:

54

.ETtEEdt

T1U 1

t

0AB

1

(3.2)

Tt1

l h s iu chnh in p. tm dng ti ta dng phng php ton t Laplace: Ta tm c nh ca UAB:

PT

Pt

AB e-1e-1

PE)P(U

1

(3.3)

nh ca dng ti l:

)PLR)(e1(e1

PE

)P(Z)P(U)P(I PT

PtAB

1

(3.4)

Dng in trong khong t t1 l:

t

1

111 ea1

ba11REi (3.5)

trong khong t1 T l:

1

1

11

2 ea1b11

REi , (3.6)

= L/R l hng s thi gian ca mch ti.

T-

1

T-1 eb;ea

T (3.5) v (3.6) ta xy dng c th dng in trong b bin i nh hnh 3.3d v tng ng l dng qua kho in t hnh 3.3e v qua it hnh 3.3f. Cho cc gi tr t = t0 v t = T t0 ta tm c:

1

1-1

max a1b-1

REI (3.7)

1

11min a-1

a)1-b(REI (3.8)

Dng in trung bnh qua van T s l:

1

111-

1T a-1

)ba-1)(b-1(T

-(REI (3.9)

Dng in trung bnh qua it:

1

111-

1D a-1

)ba-1)(b-1(TR

EI (3.10)

55

Dng in trung bnh qua ti:

REIII DTt (3.11)

Biu thc (3.11) cho thy dng ti khng ph thuc vo tn s ng ct ca van cng nh khng ph thuc vo hng s thi gian ca mch ti. Bin p mch ca dng ti:

1

111-

1minmaxmax a-1

)ba-1)(b-1(REI-II (3.12)

Khi = 0,5 th Imax t gi tr cc i. Vic tnh chn van nhiu khi mang tnh c l v khi chn bao gi cng c d tr nht nh no . Do trong cc tnh ton khng nht thit phi tnh chnh xc dng ti it xc nh gi tr ca dng qua van. n gin php tnh ngi ta thng chn:

t.REI1 ; 0 t t1 vi = max (3.13)

v thay th biu thc (3.13) cho (3.5) tnh dng trung bnh qua cc van. Vic tnh ny cho kt qu sai s khng qu 10% v c th chp nhn c khi chn van vi mt h s d tr no . T cc gi tr ca it ngi ta dng c th ca dng ti it, dng qua T v qua dit (hnh 3.3d, e, f) cho trng hp dng ti l lin tc v khi dng gin on (hnh 3.3 g, h, i). H s p mch ca in p ra:

km =

1

111

1AB

a1ba1b1

EU

(3.14)

Khi nng lng tch tr trong in cm L l hu hn, s xy ra ch d dng in gin on (hnh 3.3 g, h, i). Dng qua it m s gim v khng trc khi kho in t T c ng li. Ch dng in gin on s lm tng h s p mch trn ti v lm gim cht lng c tnh ngoi ca b bin i. 3.1.3. B bin i xung p mt chiu c o chiu

B bin i xung p mt chiu dng van iu khi