ien t cong suat 1

2.8 - HIEN TNG CHUYEN MACH Trong cac phan trc ay, bo chnh lu c phan tch vi gia thiet bo qua

cam khang trong cua nguon ap. He qua la qua trnh chuyen mach gia cac nhanh cha thyristor dien ra tc thi. Trong thc te, nguon co cam khang trong lam dong ien qua no khong the thay oi ot ngot. V the, hien tng chuyen mach gia cac nhanh cha cac thyristor khong dien ra tc thi ma keo dai mot khoang thi gian, hnh thanh trang thai cac nhanh cha thyristor cung dan ien. Hien tng nay c goi la hien tng trung dan (overlapping) hoac hien tng chuyen mach (commutation).

Xet bo chnh lu mach tia ba pha ieu khien (hnh H2.33)

Nguon xoay chieu co cam khang trong Lb. Trang thai V3: Gia thiet dong qua tai lien tuc va V3 dan dong ien. Khi o, dong ien tai

dan qua mach (u3,Lb,V3,RLE ). Phng trnh mo ta mach: uv3 = 0 ; iv3 = id uv1 = u1 - u3 ; iv1 = 0 ; uv2 = u2 - u3 ; iv2 = 0 (2.43) ud = u3 E

dtdiLiRu ddd ++= ..

Trang thai (V1V3): Tai thi iem a xung kch ong cho V1 (goc ), do tac dung ong thi

cua ien ap khoa va xung kch ong, V1 ong . Do V1 mac noi tiep vi Lb nen dong qua no tang lien tuc t gia tr 0. Tng

t , di tac dung cua ien ap chuyen mach (u3-u1) va cam khang Lb, dong ien qua V3 giam mot cach lien tuc t gia tr dong tai. He phng trnh mo ta trang thai mach:

i i iV V d1 3+ =

dt

diLudt

diLu VbVb 3311 +=+

2-35

ien t cong suat 1

dt

diLuu Vbd 11 += (2.44)

EdtdiLiRu ddd ++= ..

uV1 = 0 ; uV3 = 0 Gia s khoang thi gian trung dan nho hn nhieu so vi hang so thi gian

cua tai chnh lu, co the xem dong ien tai khong thay oi o ln. Do o: iv1 + iv3 = Id = const

031 =+dt

didt

di VV (2.45)

T o dan giai:

dXLuudib

V 231

1

= (2.46)

Bieu dien cac he thc trong he truc toa o mi O X vi O dch sang phai

cua iem O va 6

OO = , ta co: XdXUuu m = sin331 (2.47)

T o:

( )

( )dXXsin

L2U3di

X

b

mXi

i 1V1V

1V

=

(2.48)

( ) ( ) ( XcoscosL.2U3iXib

m1V1V

= ) (2.49)

Do iV1()= 0 , nen

( ) ( )'XcoscosL.2U3'Xib

m1V

= (2.50)

1Vd1Vd3V iIiii == (2.51) o th bieu dien cac he thc dong ien cho thay iv1 tang dan t gia tr 0 va

dong iv3 giam dan t gia tr Id . Trang thai V1: Trang thai V1 ,V3 cung dan se keo dai en v tr Xk thoa man: iV1( X k' ) = Id iV3(Xk) = 0 (2.52) Luc o, V3 b ngat v khong cho phep dong qua no oi dau. Hien tng

chuyen mach ket thuc, dong ien tai khep kn qua mach cha (u1,Lb ,V1,RLE), phng trnh mo ta mach co dang:

dtdiLuu

ii;0u

1Vb1d

d1V1V

+=

==

0i;dtdiLuuu

0i;EdtdiLRiu

2V2V

b122V

3Vd

dd

=+=

=++= (2.53)

Tng t nh tren, ta co the xet qua trnh chuyen mach gia V1,V2 va V2,V3. Goi t va X lan lt la thi gian chuyen mach va goc chuyen mach X =.t

2-36

ien t cong suat 1

o ln goc chuyen mach c suy ra t ieu kien ket thuc hien tng chuyen mach:

( ) ( ) +==+= ',' XIiXi dVV 11 (2.54) Ket qua la:

=

m

db

UIL

32cosarccos (2.55)

Goc chuyen mach (va thi gian chuyen mach) phu thuoc vao goc ieu khien , vao o ln dong ien tai va cam khang trong cua nguon ap

Cac he qua: 1. Hien tng chuyen mach lam giam ap tai. V du, trong thi gian chuyen

mach gia V3, va V1, t he phng trnh (2.44) ta suy ra ap tai co dang:

22231 uuuud =

+= (2.56)

Tren hnh ve H2.34, ta thay dang ap chnh lu b mat i mot phan so vi trng hp ap ly tng (Lb = 0). Do o, trong mot chu ky ap chnh lu, tr trung bnh ien ap tai b giam i Udx .

( ) +

+

=

= Xd2uu

23Xd.uu

321U 31d1dx (2.57)

T (2.46) ta suy ra:

( )

( )d

bi

i 1VbdxI.

2L3diL

23U 1V

1V

=

= +

Neu at: bcm L23R

=

Ta co: dcmdx IRU = ien ap tren tai thc te co o ln: dcm0ddx IRcos.UU = (2.58)

Vi

=2X.pR bcm (2.59a)

hoac

= bcmX.pR (2.59b)

Cong thc (2.59a) ap dung cho: - bo chnh lu tia mot xung vi diode khong (p=1); - bo chnh lu cau mot pha ieu khien ban phan khi 0> (p=2); - cac bo chnh lu tia m pha ieu khien hoac khong ieu khien, co hoac

khong co diode khong (p=m); - cac bo chnh lu cau ba pha ieu khien hoan toan va khong ieu khien

(p=6); - bo chnh lu cau ba pha ieu khien hoan toan vi mot diode khong khi

3

ien t cong suat 1

- bo chnh lu cau mot pha ieu khien hoan toan va khong ieu khien (p=2);

- bo chnh lu cau mot pha ieu khien ban phan khi 0= (p=2) - bo chnh lu cau ba pha ieu khien hoan toan vi mot diode khong va

3> (p=6). - bo chnh lu cau ba pha ieu khien ban phan (p=3). 2. Hien tng chuyen mach han che pham vi goc ieu khien va do o

han che pham vi ieu khien ien ap chnh lu . Khi phan tch qua trnh ien ap va dong ien cua thyristor trong cac dang

mach chnh lu c ban, vi goc ieu khien va bo qua hien tng chuyen mach (Lb= 0), ta co goc an toan cua thyristor :

== qt. (2.60)

tq la thi gian khoi phuc kha nang khoa cua thyristor. Neu thyristor co gia tr cho trc, gia tr goc ieu khien cc ai cho phep

co o ln bang: max = (2.61) Neu xet ca hien tng chuyen mach vi la o ln goc chuyen mach, o

ln goc an toan con lai cua thyristor bang: (2.62) ( ) = + = Vi gia tr cho trc cua thyristor, goc ieu khien ln nhat cho phep co

gia tr: max = - - (2.63) Ro rang, pham vi goc ieu khien che o nghch lu b han che. Goc

chuyen mach cang ln (v du khi dong tai ln , Lb ln), goc max cang giam. Tren thc te max thng lay gia tr khoang 16001650.

Do max giam nen tr trung bnh ap chnh lu trong che o nghch lu b giam theo:

Udmin = Udo.cosmax - Xcm.Id (2.64) Trong trng hp mach tia ba pha, ta de dang suy ra:

( ) dcmdm

bdd IXI

ULUU ..cos.min

=

32

0 (2.65)

Vi cho trc, ien ap trung bnh nho nhat cua tai phu thuoc vao dong tai. 3. Hien tng chuyen mach lam bien dang ien ap nguon .

Phan tch ien

ap pha nguon tai cac iem A hoac ien ap day gia hai iem AB (hnh H2.35), ta thay no b meo dang, tren o th ien ap pha hnh H2.35 xuat

2-38

ien t cong suat 1

hien cac gai va lom ien ap. Cac lom va gai ien ap co o ln t le vi gia tr cam khang Lb va dong ien tai. Do o, neu mac cac tai tieu thu khac vao mach nguon chung vi bo chnh lu, s bien dang ap nguon gay ra do tac dung chuyen mach cua bo chnh lu co the khong chap nhan c.

e han che o bien dang cua ap nguon, ngi ta khong mac trc tiep bo chnh lu vao li ien ma thong qua may bien ap hoac cuon khang. May bien ap trong hoat ong mach tac dung nh cuon khang loc vi o ln xac nh bi cam khang tan cua cac cuon day. Cau truc mach nguon au vao bo chnh lu co dang thay the ve tren hnh H2.36.

Hien tng chuyen mach tac dung lam ngan mach gia cac pha nguon. Cam

khang hoac may bien ap mac noi tiep vi bo chnh lu co tac dung tng ng cam khang LT. Do co LT, ien ap ngan mach tai iem A phu thuoc vao t so cam

khang Lb va LT trong mach, neu LL

T

b rat ln, ien ap li cang t bien dang. Tuy

nhien, viec tang LT lai khong co li ve kha canh s dung may bien ap va ien ap chnh lu giam nhieu do chuyen mach.

o ln ien ap tai cac v tr A luc chuyen mach xac nh theo he thc gan ung :

UA = UTb

TXX

X+

; XT = LT , Xb = Lb (2.66)

Trong thc te, gia tr XT thng chon trong khoang

XT = ( 0,04 0,1).f

f

IU

2

2 (2.67)

U2f , I 2f la tr hieu dung ien ap va dong ien nh mc cua nguon cap cho tai (v du ien ap va dong ien pha th cap may bien ap).

V du 2.20: Bo chnh lu cau mot pha ieu khien hoan toan mac vao tai ong c mot

chieu. Tai co Lu rat ln lam dong tai phang id = 100A. Nguon xoay chieu co tr hieu dung U = 380V, Lb=0,001H, Rb = 0,01, = 314 rad/s. o sut ap tren mot linh kien la 2V.

a/- Phan tch hien tng chuyen mach

2-39

ien t cong suat 1

b/- Tnh ien ap ln nhat do bo chnh lu cung cap cho tai

c/- Tnh o tang max

dtdiV

d/- Tnh o ln goc chuyen mach khi =0 e/- Tnh goc ieu khien max, gia thiet thi gian khoi phuc kha nang khoa cua

SCR la tq = 50S Giai: a/- Gia s V3,V4 ang dan, V1,V2 b ngat. Trong na chu ky dng cua ap

nguon, tai v tr tng ng goc kch , xung kch a vao V1,V2 lam cho chung ong. Do tac dung Lb, hnh thanh trang thai ong dan ien cua V1,V2,V3,V4 vi he phng trnh mo ta sau:

0413141

31

==

==

=

=+

VVd

VVVV

b

dVV

uuuiiiii

dtdiLu

Iii

.

Giai he phng trnh tren ta thu c goc chuyen mach:

=

ULI bd

....cosarccos

22

o tang dong ien qua SCR (v du khi ong V1,V2):

sin..

b

V

LU

dtdi

22

=

b/- o sut ap tren SCR: UV= 2 x 2V = 4[V] o sut ap tren Rb: Urb = 0,01 x 100= 1[V] o sut ap gay ra bi qua trnh chuyen mach :

2-40

ien t cong suat 1

][,.,... VIRU dxxb 91910000103142

===

ien ap trung bnh ln nhat bo chnh lu cap cho tai : Udmax= Ud0 - (UV +URb+Uxb )

( ) ][,,.max VUd 73179191438022

=++=

c/- Tnh o tang dong qua SCR khi ong:

sin..

b

V

LU

dtdi

22

=

o doc at cc ai khi = 2 . Luc o:

]./[,]/[.,,.max

sAsAdtdiV 268701026870

001022380 6 ===

d/- Goc chuyen mach khi = 0

0284880

03802

001031410020

2

2

=

=

=

][,

.,...coscos

....

cosarccos

rad

arcc

U

LI bd

e.-

( )0

06

152653288300

0157000103143802

100290015705010314

==

+=

===

][,,cos

,cos,...

.cos

,][,...

maxmax

max

rad

radtq

2.9 MAY BIEN AP CAP NGUON CHO CAC BO CHNH LU 2.9.1 CHC NANG CUA MAY BIEN AP May bien ap dung lam nguon xoay chieu cho cac bo chnh lu co cac chc

nang sau: 1. Cung cap ien ap nguon co o ln phu hp vi yeu cau cua tai. 2. Cach ly ap nguon cua bo chnh lu vi li ien. Do o, tai co the chay

ngan mach trong thi gian ngan. 3. Tac dung loc cac song hai bac cao. 4. Tao thanh cam khang chuyen mach, do o han che s bien dang gay ra

t qua trnh chuyen mach. 5. Tao he thong nguon xoay chieu nhieu pha e cung cap cho bo chnh lu

nhieu xung.

2.9.2 TNH TOAN MAY BIEN AP

2-41

Phan tch dang ien ap va dong ien may bien ap

ien t cong suat 1

Phan tch chnh xac qua trnh ien ap va dong ien mach may bien ap co the at c bang cach giai bai toan mach ien t. Tuy nhien, viec giai toan tng oi phc tap. Trong thc te, ngi ta thng giai bai toan di dang c n gian hoa. Trong vai trng hp n gian, dong ien t hoa c bo qua.

ien ap pha s cap va th cap c xem co dang sin va t so cac ien ap nay bang t so vong day cuon s cap va th cap.

Dong ien qua cuon th cap c xac nh phu thuoc vao cau hnh bo chnh lu. Dong ien cuon th cap c xac nh phu thuoc vao thanh phan xoay chieu cua dong qua cuon s cap. Thanh phan dong ien mot chieu qua cuon th cap co tac dung a trang thai mach t vao che o bao hoa nen can c han che.

V du, xet may bien ap ba pha mac vao bo chnh lu tia ba xung nh tren hnh H2.39. Bo qua dong ien t hoa, sc t ong F1, F2 va F3 (hnh H2.39) tren cac tru may bien ap at gia tr nh nhau:

F1=F2=F3 (2.66) Gia thiet cac cuon day c quan cung chieu, ta co:

ppssppssppss iNiNiNiNiNiN 332211 ...... == (2.67)

Np.. tong so vong day tren mot pha pha s cap Ns.. tong so vong day tren mot pha cuon th cap e n gian, gia thiet Ns=Np=N, ta thu c:

psps iiii 3322ps ii 11 == (2.68)

nh luat Kirchoff cho nut dong ien cho ta:

0321 =++ ppp iii (2.69)

Dong ien qua cuon s cap:

32 321 sss

piii

=.

1i ;

32 132 sss

piii

=.

2i ; (2.70)

32 213 sss

piii

=.

3i

Qua trnh dong ien qua cac pha cua cuon s cap va cuon th cap c ve tren hnh H2.40.

Tr hieu dung dong ien qua cuon th cap va cuon s cap:

2-42

ien t cong suat 1

3IdXi

21I d

2

0

2s1s =

=

(2.71)

d2

0

2p1p I.3

2.kdXi21I =

=

(2.72)

vi k la t so may bien ap. nh mc ien ap pha th cap may bien ap: Goi : - Utmax la ien ap ln nhat cua tai - UdT la tong cac sut ap tao nen bi cac linh kien - Udrmax la tong cac sut ap cc ai tao tren ien tr day dan va

ien tr nguon - Udxmax o sut ap cc ai gay ra bi qua trnh chuyen mach Taco: - UdT = n. U1T - n: la so linh kien trong mach mac noi vi tai trang thai dan ien

(khong xet trang thai chuyen mach ) - U1T: o sut ap theo chieu thuan tren mot linh kien - Udrmax = R.Idmax - R : la tong tr cua day dan, cua nguon trong mach trang thai

dan ien - Udxmax = Xcm.Idmax - Xcm : tr khang chuyen mach, phu thuoc vao cau tao mach - Idmax : dong ien tai cc ai cho phep. ien ap chnh lu phai thoa man ieu kien: Udmax = Ud0.cosmin Utmax +UdT + Udxmax + Udrmax (2.73) oi vi mach chnh lu tia ba pha:

minmax cos. UUd 2

63= (2.74)

Chon:

UUd

2630 == maxmin (2.75)

T o, ien ap nguon pha th cap may bien ap c chon sao cho

maxmaxmax drdxdTt UUUUU +++>263 (2.76)

hay U >

263

maxmaxmax drdxdTt UUUU +++ (2.77)

Neu ien ap nguon c phep giam xuong thap nhat bang b.U ( 0 < b < 1 ), ta can co:

b. U >

263

maxmaxmax drdxdTt UUUU +++ (2.78)

2-43

ien t cong suat 1

hay U > b

UUUU drdxdTt

263

maxmaxmax +++ (2.79)

Trong trng hp s dung bo chnh lu kep va ieu khien no theo phng phap ong thi oi hoi min > 0. Khi o, thay v cosmin = 1, ta can gi nguyen bieu thc cosmin trong he thc xac nh U.

Neu tai phai lam viec che o nghch lu va co gia tr thap nhat Utmin ( Utmin < 0 ), bo chnh lu hoat ong vi goc ieu khien tng ng max . Nguon phai co kha nang nhan nang lng t tai a ve. Khi o, ieu kien thiet lap gia ap chnh lu va pha tai la:

maxmaxminmaxcos.. drdxdTtd UUUUUb +++0 (2.80) Do cac bieu thc trong dau tr tuyet oi eu am nen ta suy ra: maxmaxminmaxcos.. drdxdTtd UUUUUb +++0 (2.81) Ta chon nguon sao cho: b.Ud0.cosmax < Utmin (2.82) Bo chnh lu mach tia ba pha:

minmaxcos... tUUb (2.84)

nh mc dong ien pha th cap may bien ap Dong ien nh mc cuon th cap c tnh theo tr nh mc cua dong tai.

Theo ket qua phan tch dong ien qua may bien ap, ta co cho mach chnh lu tia ba pha:

Is =3dI (2.85)

nh mc cong suat bieu kien cua may bien ap V du xet bo chnh lu mach tia ba pha. Cong suat bieu kien may bien ap:

dtps

t P.k2SS

S =+

= (2.86)

- kt la he so s dung may bien ap - Sp, Ss la cong suat bieu kien pha s cap va th cap may bien ap - Pd la cong suat tai - Sp = mp.Up.Ip - Ss = ms.Us.Is - ms : la so pha - U ,I : tr hieu dung ien ap va dong ien T ket qua phan tch dong ien qua may bien ap, ta co:

Is =3dI

ds I32.kI = (2.87)

2-44

ien t cong suat 1

Tr hieu dung ap nguon:

0ds U632UU == (2.88)

kU.

632U.

k1U.

k1U 0dsp

=== (2.89)

T o:

d0dppp I32k.

kU.

632.3IU3S == (2.90)

dd0dp P332IU

332S ==

dd

0dsss P.33

3I.U

632.3I.U.3S === (2.91)

dddt P35,1P..33P

332

21Sva =

+

= (2.92)

V the, he so s dung may bien ap kt = 1,35 V du 2.21:May bien ap ba pha au dang Y-Y co tham so: Cong suat bieu kien St = 200 kVA ien ap ngan mach unm = 5% Cong suat ton hao P j = 5,14 [kW] ien ap day pha th cap Ud = 880 [V] Dong ien nh mc pha th cap Itcdm = 131 [A] Xac lap cac tham so mach nguon tng tng qui oi sang pha th cap. Giai:Thanh phan ien ap ngan mach tren ien tr trong may bien ap ur:

0257,010.20010.14,5

SP

u 33

t

jr ==

=

ien tr va tr khang trong may bien ap quy oi sang pha th cap:

][0997,0131.3

880.0257,0I.3U.uRtcdm

drb ===

][194,0131.3

880.05,0I.3u.uL.Xtcdm

dnmbb ====

2-45

ien t cong suat 1

V du 2.22: Cho bo chnh lu cau ba pha ieu khien hoan toan. Nguon ien ap xoay chieu

lay t pha th cap U = 220 V, tan so = 314 rad/s. Tai R = 0,1 , L rat ln dung lam

dong tai lien tuc va phang, E = 200 V, goc ieu khien

=3

[rad].

a/- Tnh tr trung bnh ap Ud va dong Id . b/- Tr trung bnh va tr hieu dung dong qua SCR. c/- Tnh tr hieu dung dong ien qua nguon xoay chieu. d/- Gia s trong qua trnh ieu khien do tai thay oi (E), thay oi trong pham

vi (0,). Dong tai c ieu chnh gia tr xac nh cau a/- . Tnh cong suat may bien ap.

Hng dan; a/-

][

,

][cos..cos..

AR

EUI

VUU

dd

d

57210200257

2573

2206363

=

=

=

=

==

b/-

][..

][,

AII

AII

dTRMS

dATV

33057231

31

61903

===

==

c/-

][.. AII dRMS 46757232

32

===

d/-

2SS

S sp+

=

Is = dI32

2-46

ien t cong suat 1

( )0;U.63U

I.32.U.3I.U3S

dmaxd

dssss

=

=

==

sp

ss

p

sp

ss

pp

I.NNI.

NNI

U.NN

U

==

=

T cac he thc tren, ta co the suy ra c: S = 1,05.Udmax .Id

][

][,.max

AI

VU

d

d

572

551422063

=

==

S = 1,05. 514,5 . 572 = 309068 VA S = 309 kVA V du 2.23 Tnh toan nh mc may bien ap, mach chnh lu. Cho biet bo chnh lu mach

cau ba pha cap nguon cho ong c mot chieu kch t oc lap. ong c co tham so: udm = 400 V Idm = 188 A R = 0,051 nm = 470 v/ph o sut ap cho phep cua li ien la 5% o sut ap tren moi SCR khi ong la 2 V. Dong ien cc ai cho phep cua ong c bang 1,5 Idm. Giai:Viec nh mc ien ap cua nguon ( may bien ap) phai thoa man yeu cau ap

cung cap ln nhat do pha tai yeu cau: Ta co: U max = RIu max + c ktm.mHang so kch t nh mc

dm

udmuudmktdm

IRUc

52,=

vi udm = 440 V R = 0,051 Im = 188 A

]s/rad[218,4960n2 dmdm ==

2,5 [V] la ien ap tiep xuc choi quet vi co gop. ay bo qua anh hng nhiet o len ien tr mach phan ng

cktdm = 8,69

2-47

ien t cong suat 1

T o : umax = R.Iumax + cktm . m + 2,5 = 0,051 . 188. 1,5 + 8,69 . 49,218 + 2,5 = 444,6 V o sut ap tren SCR: Uv = 2 x 2 [V] = 4 [V] Do cac tham so cua may bien ap cha co nen o sut ap cua chung khong xac

nh c. e chon may bien ap ta phai phong oan o sut ap do qua trnh chuyen mach. T o ta chon may bien ap va thc hien kiem chng ien ap, neu khong phu hp (cao qua hoac thap qua so vi yeu cau ) ta lan lt chon va kiem tra lai.

Chon o sut ap tong khoang 10% so vi ap nh mc. Ta co ap chnh lu ln nhat:

Ud msx =440 + 0,1.440 = 484 V

Ud max = U

63

Xet ca sut ap li ien ( 5% ), ta co

95063 ,..max UUd =

T o US= ][,.,.

V821748495063

=

ien ap day pha th cap may bien ap : ULS = ]V[376217.3U3 S == Cong suat bieu kien may bien ap: St = kt.Pd = 1,05. 440. 188 = 86856 VA =86,8 kVA kt = 1,05 la he so s dung may bien ap ap dung cho bo chnh lu cau ba pha. Tra bang ta chon may bien ap vi: St = 125 kVA, Yy 0 ien ap pha cuon s cap (ap day) ULP=380 V Dong s cap IP = 190 A ien ap th cap (ap day) ULS = 510 / 410 / 320 / 220 [V] Dong ien mach th cap IS =141 / 176 / 255 / 298 [A] ien ap ngan mach 4% Gia s bo qua ton hao may bien ap Rb = 0 Chon mc ap th cap 410 V:

== 0537901763

410040 ,.

.,bX

o sut ap do qua trnh chuyen mach :

maxuxx IRU =

=

=

= 0513,0205379,0.6

2X.6R bx

]V[487,145,1.188.0513,0U x == Kiem chng ien ap: Ud max =Uu max +Uv +Ux = 444,6 +4 +14,487 = 463,08 [V]

2-48

ien t cong suat 1

ien ap chnh lu do may bien ap a chon cung cap:

95,0.U.2395,0.U.63U LSSmaxd =

=

Udmax ]V[52695,0.410.23

=

=

Nh vay may bien ap a chon at yeu cau ve ien ap. nh mc dong ien cho may bien ap da tren c s dong tai nh mc. Tr

hieu dung dong ien qua cuon th cap:

]A[5,153188.32I.

32I.

32I udmdP ====

Dong ien nh mc qua cuon s cap:

]A[6,1655,153.380410I.

UUI SLP

LSP ===

nh mc ien ap va dong ien cho linh kien da tren c s o ln cc ai xuat hien cua chung.

ien ap ln nhat xuat hien tren SCR: ]V[8,579410.2U.2UU tLSDWMRWM ====

Dong ien ln nhat qua SCR co tr trung bnh :

][,,.maxmaxmax AIII udTAV 66623

5118833

====

T o, cac SCR c chon vi tham so: URRM = UDRM = Ku . URWM ; IATV = Ki . ITAVmaxvi Ku =(2,5 -> 3,5) ; va Ki =(1.2 1,5)

V du 2.24Cho s o mach tren hnh H2.41. Tnh cho may bien ap va cac linh kien bo chnh lu. Cho biet rang tai co R=1 , L . Ap nguon xoay chieu u co tr hieu dung 220V, 50Hz. Bo qua sut ap tren may bien ap.

Hng dan: Cong suat bieu kien pha th cap (gom 2 cuon th cap):

2I.U.2S dss =

Cong suat bieu kien pha s cap:

dP

Sp I.U

UI = ; PPP I.US =

2-49

ien t cong suat 1

Cong suat may bien ap:

)SS(21S SPt +=

2-50