Chuong4 Chinh Luu DK Pha

Bi ging tm tt in t cng sut 1

Trang 1 / Chnh lu iu khin pha Hunh Vn Kim

Chng 4 CHNH LU IEU KHIEN PHA IV.1 CHNH LU DIOD ( KHONG IEU KHIEN ):

Cac s o chnh lu c phan loai theo so xung cua ap ra.

1. Chnh lu hai xung: Gom s o cau mot pha va s o mot pha co iem gia.

a. Hoat ong tai R:

v

D1D2

D3D4

io

ovv

io

ov

e

- e

R

D1

D2R

(a) (b)

Hnh 4.1.1 S o cau mot pha (a) va s o mot pha co iem gia (b) va cac dang song dong, ap tai tr (c) (c)

Goi ap nguon ( )wtsinVv 2= < 4.1.1 > V: la tr hieu dung ap nguon. Tr trung bnh ap ra: VdwtwtsinVdtvdtv

TV oT oo ==== 2221211 020 < 4.1.2 >

Vi tai tr: vo = io .R => io = vo/R => Io = Vo/R < 4.1.3 >

e tm ap ngc at len diod, xet trng hp khi D1 dan, D2 phan cc ngc bang ap li, vay ap ngc cc ai cua diod se la tr so nh ap li 2V .

S o hnh 4.1.1(b) s dung bien ap co hai cuon th cap ao pha, ap ngc at vao chnh lu tang gap oi. That vay, khi D1 dan, ap at vao D2 la:

vD2 = e e = 2 e ( qui c ap tren SCR hay Diod luon tnh t A qua K ). Tr hieu dung dong tai : ( )

RVdwtdwtidwtiI R

voooR ==== 0 210 2120 221 ...

vi V la tr hieu dung ap nguon. Tr hieu dung dong tai cung chnh la tr hieu dung IS cua dong qua nguon khi tai la R.

Cong suat tieu thu cua tai R la 2

2. oRVP R IR

= = giong nh noi trc tiep vao li ien, tng ng HSCS cua BB bang 1. Neu ta tnh cong suat mot chieu PDC lam cong suat hu dung, . DC O OP V I P= < , ieu nay co the giai thch de dang khi e y gia tr trung bnh VO be hn gia tr hieu dung V.

b. Hoat ong tai RL:

Mach ien khao sat la s o hnh 4.1.1.a hay .b vi tai la RL v tr cua R. Phng trnh mach khi lay lai goc toa o:

iS

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Trang 2 / Chnh lu iu khin pha http://www.khvt.com

Hnh 4.1.1.d

( ) dtdiLiRwtVvv oo +=== .sin2 ieu kien au : iO(0) = I1 giai ra: ( ) += t1ZV2o eIwtsini . vi R

Ltg 1 = va RL=

Khi wt = , dong ien tr lai gia tr ban au I 1 e lap lai xung dong cho ban ky mi : ( ) 112 IeIsini ZVo =+= Giai phng trnh nay, ta c I1 . Nh vay, bieu thc cho dang dong ra io tng oi phc

tap, tch phan e tnh tr trung bnh dong qua tai = 01 dwtiI oo . rat kho thc hien. Trong thc te, ngi ta thng ch tnh gia tr trung bnh dong ra bang nguyen ly xep chong, Khi gia s he thong la tuyen tnh, cac thanh phan Fourier cua dong tai se c tao ra t cac thanh phan Fourier cua nguon kch thch. Tr trung bnh dong ien la dong ien qua tai khi tai c cung cap ap mot chieu bang tr trung bnh ap ra (cung chnh la thanh phan mot chieu cua khai trien Fourier cua ap ra):

IO = VO/R , co dang v L khong co tac dung oi vi thanh phan mot chieu cua ien ap.

V du: Tnh dong qua mach chnh lu cau diod tai R = 10 ohm, ap nguon 12 V (hieu dung).

Tr trung bnh ap ra Vo = v 8.109.01212 22 == , Tr trung bnh dong ra: A08.110/8.10RVI OO === c. Hoat ong tai RE: ( hnh 4.1.1.e )

Ngc lai vi tai RL co khuynh hng keo dai goc dan ien cua diod, tai co sc phan ien lam cho goc dan thu hep. That vay, t s o mach ien hnh 3.1.f co the nhan xet la cac diod ch dan ien c khi ap nguon ln v hn sc phan ien E cua tai. Goc e diod bat au dan ien: ( )212 VEVEEvwt ==== sinsin th Khi Khi diod dan ien, ( ) wtVvREviEiRvv ooo sin/. 2==+== vi . Dang dong la phan co to tren hnh 4.1.1.f. Khi iO = 0 th wt = v tnh oi xng cua hnh sin. Khi diod tat hay iO = 0 , vO = E. T hnh 3.1.f bieu thc tnh trung bnh ap ra:

+== + + dwtEdwtwtVdwtvV oo ..sin. 211

I 1 I 1



v

D1D2

D3D4

o

ovR

E

Hnh 4.1.1. (e) (f)

va bieu thc tnh trung bnh dong ra: ( ) == dwtdwtiI REvoo .101 . IO co the c tnh theo nguyen ly xep chong khi xet mach tng ng oi vi mach ien mot chieu:

IO = (VO E) /R

e tnh toan cong suat phat nhiet cua ien tr R can tnh toan gia tr hieu dung IOR cua

dong ien ra iO: ( ) == dwtdwtiI REvoOR .. 210 21 . 2. Chnh lu ba xung: S o hnh tia ba pha:

Trong s o tia 3 pha tren hnh 4.1.2.(a), cac diod noi chung catod, tai la thuan tr. Dong ien tai chay t li (nguon), qua diod va ve nguon ien theo day trung tnh N.

Nguyen tac phan tch mach: Co the nhan xet la tai moi thi iem, diod nao co ien ap anod cao nhat se dan va at ap am vao cac diod con lai v cac diod noi chung catod.

V du: tai wt = , vA > vC > vB => D1 dan ien, D2 va D3 b at ap am. Hnh 4.1.2.(b) trnh bay dang ap, dong ngo ra; dang dong, ap tren diod D1. Goc dan cua moi diod la 2/3. Tr trung bnh ap ra:

VdwtwtsinVVo ==

2632

23 6

5

6

< 4.1.5 >

Ap ba pha:

)tsin(Vv

)tsin(Vv

tsinVv

C

B

A

343

2

2

2

2

==

=

< 4.1.4 >

o

v

i

o

A

B

C

N

D1

D2

D3 R

(a)

Hnh 4.1.2 S o chnh lu hnh tia 3 pha (a) va cac dang song tai tr (b)

(b)

Tng t nh a khao sat s o hai xung, v la tai thuan tr, dong ra iO co cung dang vi ap ra vO, va tr so trung bnh IO van tnh theo < 4.1.3 >: IO = VO/R.

Ap ngc cc ai at vao diod la bien o ap day 6V .

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S o tia ba pha co hai nhc iem: phai s dung trung tnh , dong nguon ch co thanh phan mot chieu nen ch c dung khi cong suat tai kha nho so vi nguon ien.

3. Chnh lu sau xung: Khao sat vi tai R.

Cac chnh lu chia lam hai nhom: nhom + gom D1, D2, D3, nhom gom D4, D5, D6. Chnh lu sau xung co dong, ap ra nhap nho 6 lan trong mot chu ky, s dung trong li ien ba pha. Co ba s o thng dung: cau ba pha, tia sau pha va sau pha co khang can bang.

moi luc, dong ien tai phai i qua mot diod cua hai nhom nay. Cung nh s o ba pha tia, co the nhan xet la vi nhom +, diod nao co ien ap anod cao nhat se dan ien va at ap am vao cac diod con lai; vi nhom la ien ap catod thap nhat. Vi th t cac pha la A > B > C > A , trnh t dan ien cua cac diod c trnh bay tren hnh 4.1.4: D1 > D6 > D2 > D4 > D3 > D5 > D1 Tren hnh 4.1.3.(b) khao sat cac dang dong ap cua chnh lu cau 3 pha (hnh 4.1.3.(a)), ien ap v+ va v cua hai au ra so vi trung tnh nguon la ng net am, gom cac phan dng va am nhat cua ap ba pha,

ap ra : vO = (v+) (v) tng ng vi cac khoang dan ien cua cac diod theo trnh t hnh 3.4. Moi diod lam viec 2 / 3, nhng v hai nhom lech pha nen co 6 xung trong mot chu ky. Va ng vi moi cap diod lam viec, ap tren tai se trung vi mot ap day cua li ien.

a. S o cau ba pha:

o

oi

vA

B

C

v+

v-

D2 D3

R

D1

D6D5D4

(a) (b)

Hnh 4.1.3: Chnh lu cau ba pha (a) va cac dang song tai tr (b)

D1 D2 D3

D4D6 D5

2 33

Hnh 4.1.4: Th t dan ien cua cac chnh lu va khoang dan



V du khi D1, D6 dan ien, ap ra: vO = vA vC = vAC khi D3, D5 dan ien, ap ra: vO = vC vB =vCB . . .

Tr trung bnh ap ra V.Vdwt)vv(V BAo === +

34263

26 36

6

< 4.1.7 >

Ap ngc cc ai cua s o cau giong nh trng hp s o tia ba pha, v moi nhom + hay nhom eu hoat ong nh mot s o tia ba pha. b. S o tia sau pha:

Co the xem s o tia 6 pha nh la ba s o mot pha co iem gia lay t ba pha nguon noi chung ngo ra nh hnh 4.1.5.(a) . Bien ap ay co the la ba pha hay ba bien ap mot pha, pha li ien co the noi Y hay .Tren hnh 4.1.5.(a), cac cuon day noi vao ap day ba pha: vAB, vBC, vCA va cac ap pha ngo ra se t le : vab, v-ab, vbc, v-bc, vca, v-ca , tao thanh 6 pha va tng t nh a khao sat trong cac phan trc, trong cac chnh lu noi chung catod, chnh lu nao co ien ap anod cao nhat se dan ien. Dang ap va dong ngo ra c ve hnh 4.1.5.(b) . Ap ra nhap nho sau lan trong mot chu kynh trong s o cau nhng moi diod ch dan ien mot phan sau chu ky, bang

/ 3. Tr trung bnh ap ra

VV

dwtvV abo

==

= +

35123

26

33

3

.

)(< 4.1.8 >

Vi ap pha th cap pha ab la : ( )wtsinVvab 2= < 4.1.9 >. V la hieu dung ap pha th cap. e y la hai cong thc < 4.1.7 > va se giong nhau neu trong < 4.1.7 > ta dung ap day thay cho ap pha. Co the chng minh la ap ngc cc ai at vao chnh lu la hai lan ap pha th cap. So sanh cac iem khong giong gia hai s o cau ba pha va tia sau pha (khi cung ac tnh ngo

A

B

C

o

o

v

iab

- ab

- bcbc

ca

- caD1

D5

D6

D4

D2

D3R

(a)

(b)

Hnh 4.1.5: S o chnh lu sau pha (a) va dang ap ra tai tr (b)

ra) :

S o cau ba pha S o tia sau pha: - Chnh lu dan ien 1/3 chu ky. - Co the noi trc tiep vao li, neu co dung bien ap th kch thc, gia thanh cung be hn. - Thng s dung cho cong suat ln.

- Chnh lu dan ien 1/6 chu ky nhng chu ap ngc gap oi. - Phai dung bien ap. - Ch dung cho cong suat nho.

c. S o sau pha co khang can bang: ( hnh 4.1.6 )

Gom co hai s o ba pha hnh tia co ngo ra noi song song qua cuon khang co loi thep KCB. Cac pha ien ap vao cua hai bo chnh lu ngc nhau e chung lam viec hai ban ky cua ien ap

Hc k 2 nm hc 2004-2005


li, lam cho cuon day s cap dan dong hai ban ky, khac phuc nhc iem cua s o hnh tia 3 pha. Cuon khang can bang nay can thiet v mac du thiet ke ien ap trung bnh hai chnh lu la bang nhau nhng ien ap tc thi cua chung khong giong nhau. That vay:

( ) ( )01 02 = v wt v wt

12 2

2 1

11 22

4.1.10 ( ) 4.1.11

o o cb

cb o o

o o o

v v vv v v

v v v

= = < >

= + < >

Nh vay ap ra se la trung bnh cong hai ap ngo vao va se nhap nho 6 xung trong mot chu ky. Mat khac, khi lay trung bnh hai ve cua , ta tm c quan he cua cac gia tr trung bnh: 2 1 1 20 4.1.12 4.1.13 cb o o o o oV V V V V V= = < > = = < >

Khang can bang khong chu ap mot chieu va tr trung bnh ap ra Vo bang vi tr trung bnh cua moi bo chnh lu, moi bo chnh lu se dan mot na dong ien tai.

Viec tnh toan khang can bang c tr lai trong phan khao sat cac bai toan cua chnh lu.

IV.2 CHNH LU IEU KHIEN PHA ( SCR ):

v

io

ov v

io

ov

o

v

i

o

A

B

C

N

o

oi

vA

B

C

(a) (b)

(c) (d)

T?

R

R

T1

T2

T2T1 T3

T4 T5 T6

T1

T2

T3

T2T1

T3 T4

L

L

R

L

R

L

Hnh 4.2.1: Cac s o chnh lu SCR

A

B

C

ov oi

ab

- ab

- bc

bcca

- ca

oI /2

oI /2 o2v

o1v

vcb

KCB

D3

D1

D2

D4

R

D5

D6

Hnh 4.1.6: S o chnh lu diod 6 pha co khang can bang

Vi tnh cach la chnh lu, SCR co the thay the diod trong cac s o a khao sat trong phan IV.1, ket hp vi kha nang ieu khien pha, ta co the thay oi ap ra khi bat chung dan ien cham i so vi diod tng ng nh trong cac khao sat sau.

1. S o chnh lu SCR hai xung - hnh 4.2.1.(a) va (b):

Trng hp tai thuan tr:

Xet trng hp s o cau hnh 4.2.1.(a), cac cap SCR T1, T4 va T2, T3 co cung xung kch khi (tren hnh 4.2.2 la iGT1 va iGT2).

Gia s ong nguon vao luc wt = 0.



iO = 0 => vO = 0 va vT1 + vT4 = v vO > 0 : vay T1 va T4 c phan cc thuan. wt = , co xung ieu khien, T1 va T4 dan ien: vO = v = R.iO => dong tai co cung dang vi ap. Khi wt = , dong ap ra bang 0, SCR tat => goc dan = . Tr trung bnh ap ra:

== dwtwtsinVdwtvVo 211 [ ]12 += cosVoV

RVI

Rvi

oo

oo

=

=

bnhtrung tr

tai ien dong thi tc tr Gia

Khao sat s o chnh lu dung bien ap co iem gia (hnh 4.2.1.b) tng t.

Hnh 4.2.2: Cac dang ap chnh lu 2 xung K pha

Trng hp tai RL: Phhg phap khao sat hoan toan tng t trng hp tai tr, wt = , T1 va T4 se dan ien khi c kch, phng trnh vi phan mo ta mach ien:

wtsinVvLRiv dtdi

ooo 2==+=

ieu kien ban au 0==wtoi Co the nhan xet la phng trnh dong qua mach co dang hoan toan giong nh trng hp bo bien oi ap xoay chieu (chng 3):

=

)(

)sin()sin(t

o etZVi 2

( )

RwLtg

LRZ

1

22

=+=

phagoc va

tai tr tong vi

dong ien tang len t gia tr khong, ch ve khong khi ap ra, bang vi ap nguon, co gia tr am. Goc dan )( > , co the xac nh theo va tnh toan thc hanh theo phu luc 1.

Hnh 4.2.3: Dang dong, ap ra chnh lu 2 xung, tai RL vi dong gian oan

Qua ban ky am, hoat ong cua mach dien ra tng t. Khi wt = + , SCR T2 va T3 dan ien va vO = v, ta van co xung dong dng. Tr trung bnh ap ra ch can tch phan trong ban ky: ( )[ ]+=== + coscos212021 Voo dwtvdwtvV nh vay ap ra phu thuoc goc dan , thay oi theo tai RL. Tr trung bnh va hieu dung dong tai co the nhan c khi tch phan hay tnh toan thc hanh theo phu luc 1. Nhng co the de dang tnh tr trung bnh dong tai t tr trung bnh ap ra khi ap dung nguyen ly xep chong, cong

Hc k 2 nm hc 2004-2005


thc nhan c giong nh trng hp chnh lu diod.

RV

ooI =

Trng hp tai dong lien tuc: Xay ra khi L u ln va goc ieu khien pha be, goc dan tang en khi bang . Khi o SCR c kch khi dong tai cha ve khong. Va nh vay, cac SCR thay phien nhau dan dong tai. Xet chu ky ta xac lap, khi he thong hoat ong u lau e qua trnh qua o cham dt, dong ra lap lai trong moi chu ky li (hnh 4.2.4)

Hnh 4.2.4: Dang dong, ap ra khi dong tai lien tuc

0 < wt < , T2, T3 ang dan dong tai. Ap ra vO < 0 va T1, T4 c phan cc thuan. wt : T1, T4 dan ien khi co dong kch. T2, T3 tat v neu tiep tuc dan th dong qua no se chay ngc t K sang A. Ta co s chuyen mach dong tai t SCR ang dan sang SCR c kch.

Vay khi dong lien tuc, SCR cung catod (hay anod) thay phien nhau dan dong ien tai, goc dan trng hp tong quat cua 1 SCR la 2 / n ,vi n la so SCR noi chung catod (hay anod). Vi s o chnh lu 2 xung, n = 2.

Ket qua la dang ap ra khong oi theo tai, co tr trung bnh :

V bangdiode, lu chnh raap la V vi 22do

+

+

====

do

do11

o

V

cosVdwtwtsin2VdwtvV

Khi L bang vo cung, dong tai tr nen phang, khong con nhap nho. ay la trng hp hay c s dung trong khao sat ly thuyet cac trng hp phc tap hay khi thiet ke, nham n gian bai toan. Ta goi ieu kien nay la gia thuyet dong lien tuc hay tai dong lien tuc.

ieu kien e dong lien tuc tai RL, s o mot pha:

Nh a khao sat, dong tai lien tuc khi goc dan cua mot SCR la 2 / n ,vi n la so SCR noi chung anod (hay catod). e tm ieu kien cho dong tai la lien tuc, co the giai phng trnh e tm ieu kien cho goc dan bang 2 / 2 = :

0 2 wt

vO

iOv

Hnh 4.2.5: Xung dong mot SCR khi =

wt = + => 02 =

= )(

)sin()sin(t

o etZVi .

Vi nhan xet tai moi thi iem, ta cung co tai noi vi li ien xoay chieu qua SCR nh trng hp bo bien oi ap xoay chieu ( chng hai), va ket qua nghien cu cua chng hai se ap dung c cho trng hp nay: Dong ien tai se bat au lien tuc khi goc < ( la goc tai



RL). That vay, khi the = va wt = + vao , ta co ang thc. 2. S o chnh lu SCR ba xung - hnh 4.2.1.(c):

Hnh 4.2.1.(c) thng c goi la s o hnh tia ba pha, bao gom ba SCR noi chung catod (cung co the noi chung anod). Xung kch cong cac SCR lech 2 / 3 , theo th t xoay pha A, B, C :

T1 T2 T3 T1 ( hnh 4.2.6 ) iem chuyen mach t nhien hay = 0 cua cac SCR chnh la iem ma ap pha tng ng bat au cao hn cac pha khac, neu c kch luc o SCR se dan ien nh diod va ap ra se la ln nhat. Xet SCR T1,

= 0 += 2kwt vi 6= Trng hp tai thuan tr:

Hnh 4.2.6: Dang dong, ap ra chnh lu 3 xung, tai thuan tr.

Gia s ong nguon vao luc wt = 0.

iO = 0 => vO = 0 va vT1 = v vO > 0, T1 c phan cc thuan. Khi +=

6wt , co xung ieu khien, T1 dan ien:

vO = vA = R.iO , dong tai co cung dang vi ap. Gia s 6> nh tren hnh ve, tai

wt = , dong ap ra bang 0, T1 tat. Goc dan 6= .

Khi 3

26

++=wt , T2 co xung kch cong, noi pha B vao tai. Co the nhan xet dang ap, dong ra giong nh trng hp T1 dan, nhng cham pha 2 / 3. Trong chu ky li 2 co 3 xung ap, ch can tch phan 1 / 3 chu ky e tnh tr trung bnh ap ra. Co the chia lam hai trng hp:

< / 6 : Ap li cha ve khong trc khi kch SCR ke tiep: ===

+++

cosVdwtwtsinVdwtvV oo 2

6323

23 3

26

632

2

> / 6 : Ap li ve khong trc khi kch SCR ke tiep:

[ ])cos(VdwtwtsinVdwtvV oo 62 232323 1266

+

+ ++===

> 5 / 6 : Ap pha tng ng khi o be hn khong, SCR khong the dan ien khi c kch, suy ra pham vi ieu chnh goc kch SCR khi tai R t 0 en 5 / 6.

Hc k 2 nm hc 2004-2005


Tr trung bnh dong qua tai co the tnh theo tr trung bnh ap ra Vo khi ap dung nguyen ly xep chong.

Trng hp tai dong lien tuc:

Cac SCR thay phien nhau dan dong tai T1 T2 T3 T1 tng ng vi goc dan cho mot SCR la 2 / 3 khong phu thuoc goc ieu pha. Nh vay, dang ap ra khong phu thuoc tai va tr trung bnh ap ra co dang .

Pham vi thay oi goc ieu khien pha theo ly thuyet, la t 0 en . Tr trung bnh ap ra thay oi t Vdo = V2

63 en Vdo. Hnh 4.2.7: Ap, dong ra s o ieu khien pha 3 xung, tai dong lien tuc

3. S o chnh lu SCR sau xung tai dong lien tuc Mach ong lc hnh 4.2.1.(d):

Tng t nh chnh lu diod, hoat ong cua s o cau ba pha ieu khien pha co the phan

tch thanh hoat ong cua hai nhom: nhom dng gom T1, T2, T3 noi chung catod va nhom am gom T4, T5, T6 noi chung anod. iem = 0 ( chyen mach t nhien ) cua cac SCR la cac iem bat au dan ien cua cac diod cung v tr. Th t ieu khien cac SCR cung chnh la th t xoay pha li, cac SCR trong cung mot pha lech na chu ky li:

T1 T2 T3 T1 T6 T4 T5 T6 Nh vay khoang cach xung gia 2 SCR cung nhom ( T1 va T2 ) la 2 / 3 , gia 2 SCR noi tiep ( T1 va T6 ) la / 3 , xem hnh 4.2.8.(b). Lu y moi SCR eu nhan 2 xung: mot e bat au dan ( co to am ) va mot xung phu t SCR c kch ngay sau no ( khong to ), e am bao moi luc co hai SCR lam viec. Vi gia thuyet dong lien tuc, moi luc luon comot SCR cua moi nhom dan, ap ra cua cac nhom v+ va v_ so vi trung tnh li ve bang net am tren hnh 4.2.8.(a) va ap ra tren tai se lan lt la cac ap day tng ng vi cap SCR ang dan.- hnh 4.2.8.(c). Moi chu ky li co 6

Hnh 4.2.8: S o cau 3 pha ieu khien pha: dang ap cac iem tren s o (a), xung kch cac SCR (b), ap ra va cac khoang dan cua SCRs (c)

iO

(a)

(b)

(c)

vAB vAC vBC vBA vCA vBA



xung la mot phan cua hnh sin.

Tr trung ap ra:

==== ++

+

coscos)( doBAoo VVdwtvvdwtvV633

26 6

26

662

vi Vdo la ap ra chnh lu diod.

Vi giar thuyet dong lien tuc, ap ra thay oi t Vdo en - Vdo khi goc ieu khien pha thay oi trong khoang 0 . Trng hp tai thuan tr:

Dong tai iO co cung dang vi ap ra vO , va nh vay khong co gia tr am. Khi > / 3 ap ra co oan bang 0 : dong gian oan . Khi > 2 / 3 mach khong con hoat ong. Viec tnh tr trung bnh ap ra thc hien tng t nh trng hp s o ba pha hnh tia, e y ap tren tai la nhng ap day tng ng vi pha cua SCR dan ien.

IV.3 CAC BAI TOAN CUA CHNH LU IEU KHIEN PHA ( SCR ):

1. Bien luan che o dong ien tai khi tai RL:

- Dong gian oan va lien tuc: V cac ngat ien ban dan ch lam nhiem vu ong ngat mach, mot cach tong quat:

* Khi cac SCR dan ien (ong mach): ap ra se co dang ap cua ap vao, trong cac bo chnh lu la hnh sin.

* Khi SCR khoa (ngat mach): dong qua tai bang khong, ap ra se phu thuoc ac tnh tai, bang 0 neu tai RL, bang sc phan ien khi tai la ong c, accu hay bang ap tren tu khi tai co ien dung song song.

Nh vay ap tren tai thay oi theo trang thai dan ien cua cac SCR, phu thuoc vao goc ieu khien pha va ac tnh phu tai. Vi bo chnh lu m xung, be rong cua moi xung dong trong mot chu ky 2 ch co the m 2 . Dau bang tng ng trng hp dong lien tuc , khi o SCR ke tiep c kch khi cac SCR ang dan cha tat, hay cac SCR thay phien nhau dan dong tai. Dau nho hn cho trng hp dong gian oan co luc dong tai bang khong. Hnh 4.2.3 va 4.2.4 cho ta dang dong trong hai trng hp.

Cung co the s dung phu luc 1 cua chng hai kiem tra che o dong ien cho tai RL. Be rong cua moi xung dong chnh la goc dan cua s o chnh lu 1 SCR vi lu y goc ieu khien pha trong phu luc 1 c tnh vi iem chuyen mach t nhien la wt = 0 . Nh vay, trong chnh lu nhieu pha, iem chuyen mach t nhien so vi goc toa o lech goc , va ta phai s dung goc + thay cho trong khi s dung phu luc 1 nay. Cung theo phu luc, co the bien luan che o dong tai bo chnh lu tai RL khi tnh toan goc dan , ieu kien e dong ien tai lien tuc la: goc dan ( ) m, + 2 la goc tai ( )RwLtg 1= , m la so xung. Vi tai RLE, moi th tng t nhng cac phng trnh co thay oi, trong cac tai lieu tham khao co the tm c cac o th e tra goc dan la mot ham cua goc tai , goc ieu khien pha , va he so tng ng vi sc phan ien E.

Hc k 2 nm hc 2004-2005


- Trung bnh ap ngo ra khi dong gian oan va lien tuc: e y vi s o ieu khien pha, ap nguon la hnh sin co dang xoay chieu. Khi ap giam dan en gia tr am, dong qua SCR giam ve khong neu khong co SCR ke tiep c kch e ap at vao tai tiep tuc dng. Nh a khao sat, nang lng tch tr trong t cam L cua tai RL giup keo dai xung dong, lam tang phan co gia tr am cua ap ngo ra. Ket qua la khi xung dong keo dai, tr trung bnh ap ra giam, at gia tr on nh khi dong lien tuc.

- Bieu thc tong quat e tnh gia tr trung bnh ap ngo ra khi dong lien tuc: Nh vay che o dong lien tuc, dang ap ra khong phu thuoc tai v ch bao gom nhng xung hnh sin, la cac ap li cua SCR dan. T o, co the tnh c bieu thc tong quat cua chnh lu m xung hnh tia c noi vao nguon m pha:

, VsinVcosVV mm

dodoo

==2

Vdo la ap ra chnh lu diod cung s o, V la tr hieu dung ap pha.

Vi nhan xet s o cau, moi luc se co hai chnh lu dan ien va ap ra khi o chnh la ap day, co the dung cho ca s o cau vi m bang hai lan so pha va V la ap day.

- So sanh tr so trung bnh ap ngo ra cac che o dong tai khac nhau: Trng hp tai R va tai dong lien tuc la hai trng hp gii han, thng c s dung trong thiet ke, khi khong ro ac tnh tai hay khi can tnh toan gan ung.

Hnh 4.3.1 cho ta quan he trung bnh ap ra VO cua chnh lu cau mot pha theo goc ieu khien pha vi tai R, RL vi dong gian oan va dong lien tuc. (ac tnh VO() tai RL vi dong gian oan c ve ay ch la mot trng hp tieu bieu, mot cach tong quat no la mot ng cong nam gia ac tnh tai R va dong lien tuc). - Gia thuyet dong tai phang, lien tuc : Khi L tang, s nhap nho cua dong tai giam i, va khong con khi L bang vo cung. Trng hp tai co L bang vo cung con goi la gia thuyet dong tai phang, lien tuc lam cho cac tnh toan dong ien tr nen n gian, c dung trong thiet ke hay khao sat cac trng hp phc tap.

Hnh 4.3.1: quan he vo() vi cac loai tai khac nhau. Vdo: trung bnh ap ra chnh lu diod.

2. Tnh tr trung bnh dong ien tai :

Tr trung bnh dong tai thng tnh theo tr trung bnh ap ra khi ap dung nguyen ly xep chong cac thanh phan Fourier cua ap ra len tai. Thanh phan mot chieu cua dong tai la do thanh phan mot chieu cua ap tren tai tao ra, cong thc co gia tr cho tat ca cac s o chnh lu tai RL.

RV

ooI = hay khi tai RLE : R

EVo

oI = Cung co the s dung phu luc 1 cua chng 2 e tnh gia tr trung bnh cua dong tai, t o suy ra gia tr trung bnh ap ngo ra, khong can tch phan.

3.Dong ien qua SCR va bien ap au vao:



e khao sat dong ien qua cac phan t mach ien, ngi ta thng dung gia thuyet dong tai phang, lien tuc nham n gian cac tnh toan. Vi cung phng phap, cac dang dong thc te cung co the c phan tch vi khoi lng tnh toan ln hn.

Hnh 4.3.2.(b) trnh bay cac dang dong ien qua cac phan t cua bo chnh lu cau ba pha vi tai dong lien tuc, phang. Nh a khao sat trong cac muc trc cua chng, moi SCR dan ien 1/3 chu ky (ghi bang cac ch so t 1 6). Dong ien cuon day th cap bien ap gom dong qua cac SCR cua pha tng ng, v du ia = iT1 iT4 khi quy c dong qua SCR chay t anod sang catod. Dong qua cuon s cap t le vi dong th cap theo t so bien ap K. Ta co:

Tr trung bnh Iav va hieu dung IRMS dong qua SCR :

302

1 32

oIoav dwtII == ( ) 30 221 3

2oI

oRMS dwtII ==

o

oi

vA

B

C

iaAi

iC ci

R

T2T1 T3

T4 T5 T6

L

(a) (b) Hnh 4.3.2

2/3

2

1 12 23 3

4 455 6 6

2

2

wt

wt

wt

ia

Io

Io

Tr hieu dung dong qua cuon th cap bien ap, tnh cho pha a :

( ) ( ) 322 320

221

22

21 /IdwtIdwtiI ooaaR ===

Ket qua nay co c khi e y tch phan trong dau can se gap oi so vi tch phan tng ng cua dong hieu dung qua SCR.

Dong ien qua cuon s cap IAR bien ap t le vi dong cuon th cap IaR qua t so bien ap K cua ap th cap / ap s cap:

Aa

VV

aRAR KI.KI == vi ; Va va VA la tr hieu dung ap pha th va s cap bien ap => Cong suat bieu kien cua bien ap: S = 3. (Va.IaR + VA.IAR)/2 = 3.Va.IaR

the quan he ap pha th cap Va va ap ra VO va cac quan he dong ien dong tai va dong qua bien ap, ta co cong suat bieu kien bien ap:

oooo IVIVS =

=

coscos..

36323

23 Vi s o nay, ay cung la cong

suat bieu kien cua bo chnh lu, suy ra he so cong suat: = cos.3HSCS Bai tap: Tnh tr hieu dung cua song hai bac 1 cua dong nguon, suy ra he so cong suat cua bo chnh lu ieu khien pha.

e n gian cac bieu thc, ta gia s t so bien ap bang 1, bien o dong nguon se la IO va dang dong pha A c ve lai tren hnh 4.3.2 (c). Tnh gia tr hieu dung I1R cua iA khi s dung truc

1

4

Hc k 2 nm hc 2004-2005


tung la truc oi xng cua dang dong:

/ 3

10

1

4 .cos .2

2 2 6sin3

R O

R O O

I I wt dwt

I I I

=

= =

khi tch phan dang song, he so 2 xuat hien do tnh gia tr hieu dung.

Cong suat tac dung cua bo chnh lu tieu thu t nguon AC:

1 163. . .cos 3. . .cosO R OP V I V I = =

3

A

AiT1

T4

6 2wt

oI

02

Hnh 4.3.2.c: Dang dong va ap pha A

v theo hnh 4.3.2.c, goc lenh pha gia ap va dong moi pha la / 36 2 + =

Suy ra HSCS cua mach bang:

63. . .cos 3.cos3 . 23 .

3

OO

RO

V IPHSCSV I

V I

= = =

Co the nhan xet la HSCS cua mach khong the bang 1 ngay ca khi chnh lu diod (dong-ap cung pha) v dong qua mach khong hnh sin. Mot nhan xet khac la gia thuyet t so bien ap k bang 1 khong anh hng en HSCS v ca IR va IR1 eu co k.

4. Bien ap cua bo chnh lu:

Bien ap co 2 nhiem vu trong s o chnh lu: Thay oi ien ap cho thch hp vi tam hoat ong cua BB va cach ly ien li tai nham am bao an toan trong mot so trng hp.

Thong so bien ap gom co: ap, dong cac cuon day s th va cong suat bieu kien SBA. SBA cho phep anh gia o ln cua bien ap va he so s dung (hay o hieu qua s dung) bien ap khi so sanh vi cong suat bieu kien S cua nguon AC. trng hp ly tng, t so nay bang 1.

oI

o2I

oI

3

3

02

02

Hnh 4.3.3: Dang dong th va s cap bien ap noi YY dung cho chnh lu tia 3 pha.

Hoat ong cua bien ap tr nen xau i khi t trng loi thep co thanh phan 1 chieu nh trng hp chnh lu 1 SCR hay s o tia 3 pha vi bien ap thong thng noi YY hay Y. Khi o: - Sc t ong cua mach t khong bang khong nh ta van gia thuyet (bien ap noi YY).

- Loi thep b t hoa khong oi xng (bien ap noi Y) Ket qua la dong t hoa tang cao, loi thep co the b phat nong do bao hoa t cuc bo .

5. S chuyen mach cua SCR trong chnh lu ieu khien pha:



che o dong lien tuc, mot SCR khi c kch se lam tat mot SCR ang dan. Ta noi la co hien tng chuyen mach. Trong cac phan trc, ta cho la s chuyen mach khong co thi gian, dien ra tc thi. Thc te luon luon co t cam noi tiep cac chnh lu, co the la t cam cua ng day hay cua bien ap cap ien va nh vay dong qua chnh lu khong thay oi tc thi. Co khoang thi gian hai chnh lu cung lam viec. V vay, hien tng chuyen mach con goi la s trung dan gia 2 hay nhieu hn chnh lu noi chung anod hay catod.

Iov1

v2

T1

T2

La

La

i1

i2

(a)

( )1

22

2 sin

2 sin m

v V wt

v V wt ==

V1

V22m

V = 2 Vsin mkV

Gian o vect ien ap

Hnh 4.3.4.(a) cho ta mach ien e khao sat s chuyen mach trong he m pha hnh tia, gia SCR 1 cua pha 1 va SCR 2 cua pha 2. Khi kch T2, T1 ang dan dong tai Io, gia s khong oi trong thi gian khao sat. Ta co s chuyen mach dong tai t T1 sang T2, va co cac phng trnh:

(b)

Hnh 4.3.4: Ap ra khi co chuyen mach (trung dan).

1 2 1 2 21 2 1 2

1 2

2

2 2

0

vi

ieu kien ban au :

khi chon lai goc toa o la iem ch

o k a a a

k k k

owt wt

di di di di diI i i v v v L L Ldt dt dt dt dt

v V wt V Vm

i I i

= =

= + = = = =

= == =

;

sin , sin

;

1 1 21 2

uyen mach t nhien

suy ra : o adi v vv v Ldt

+= =

trong o vk c goi la ap ngan mach. Nh vay, s chuyen mach lam cho ngo ra b sut ap mot lng :

0

11. ; .2 2 2

o

a ox a a a a

I

mX Idim mU L dwt wL di X w Ldt

+

= = = = Giai e tm i2 , dung ieu kien khi t = + th i2 = Io, ta co phng trnh

xac nh goc chuyen mach :

[ ]2 2

2

1 | 02

2 cos cos2

vi ieu kien au k wta

k

a

i v dt iL

Vi wtwL

== =

=

suy ra ( ) 2

2 + =cos cos a o

k

X IV

va goc chuyen mach (trung dan) tng ng thi gian co cac SCR cung dan ien.

Hc k 2 nm hc 2004-2005


vi Vk la hieu dung ap day gia hai pha tham gia chuyen mach.

5. Chnh Lu tai RLE va che o nghch lu:

v ov

ioT2T1

T3 T4

R

L

E

oI

++

_

_Vo

E

R

(a) (b)

Hnh 4.3.5 (a) Chnh lu cau mot pha tai RLE. (b) mach tng ng /v thanh phan mot chieu khi Vo, E < 0 .

( c)

(c ) dang dong, ap ra khi dong gian oan

Trong trng hp tai co sc phan ien E nh hnh 4.3.5(a), khi dong tai iO bang khong, ap ra vO bang E. Khi T1, T4 c kch, ap ra vO = v, pt vi phan mo ta mach ien la:

0=++== =wtodtdi

oo iELRivv o au kienieu vi

Phng trnh nay la c s e tnh bieu thc dong ra iO va t o bien luan cac che o hoat ong cua bo bien oi tng t nh a thc hien vi tai RL:

Neu E > 0, sc ien ong nay xung oi vi ap ra vO, lam dong ien mau ve khong, ni rong vung dong gian oan. E > 0 cung tao ra kha nang SCR khong the dan ien khi c kch neu luc o E ln hn ap nguon v .

Nh a khao sat 4.1, s o chnh lu diod hai xung khi tai co E, goc kch SCR se nam

trong khoang min- en

= V

E2

1sinmin .

Tr trung bnh ap ra Vo khong thay oi khi dong lien tuc va tang len (E > 0) so vi tai RL khi dong gian oan, v ap ra vO se bang E khi dong bang 0 . Khi ap dung nguyen ly xep chong, mach ien oi vi thanh phan mot chieu hnh 4.3.5.(b) cho ta:

Io = ( Vo E )/ R

Khi tai co sc phan ien, co the co hien tng nghch lu. Hien tng nay xay ra khi Vo , E < 0 va |Vo| < |E|(hnh 4.3.6). Khi o van co dong Io > 0, ap ra bo chnh lu va sc ien ong tai oi dau, tai tr nen cung cap va bo bien oi se nhan nang lng v bieu thc cong suat a oi dau (hnh 4.3.5.b):

P = Vo . Io < 0 ngc vi trng hp

tai thu ong (nh R), tng ng vi HSCS < 0. Hnh 4.3.6

Co the chng minh c la bo bien oi chuyen c nang lng nay ve li khi tnh tch phan cong suat ra.



Tom tat ieu kien xay ra nghch lu:

- Tai co sc phan ien E < 0 (cung chieu vi dong ien).

- Trung bnh ap ra Vo < 0 va |Vo| < |E|.

Goc nghch lu an toan:

Khi tang, ap ra bo chnh lu Vo giam, sau o nho hn zero. Khi gan 180O , co the xay ra hien tng ot bien nghch lu. Hien tng nay xuat hien khi cac SCR chuyen mach that bai: SCR c kch khong chuyen sang trang thai dan ien c va SCR ang dan tiep tuc dan ien ban ky ke tiep. Ap ra vO ang am tr nen dng, dong ien ot ngot tang len rat cao goi la ot bien nghch lu. Goc nghch lu c nh nghia bang . Goc nghch lu an toan la gia tr toi thieu e khong xay ra ot bien nghch lu. Xet trng hp T2, T3 ang dan ien tren hnh 3.19, vi nhan xet la ap v se dng khi wt > , goc nghch lu an toan bang 0O trong trng hp cac SCR ly tng, co chuyen mach tc thi. Trong thc te, sau khi c kch, ta mat mot goc cho chuyen mach (cac SCR trung dan) va goc w.tOFF tng ng thi gian tOFF e SCR b tat phuc hoi kha nang khoa, nh vay goc nghch lu an toan se bang + w.tOFF . 6. Song hai ap va dong ngo ra:

Nh ta a biet chnh lu m xung cho ra ien ap mot chieu nhap nho m lan trong mot chu ky li ien. Khi khai trien Fourier, tn hieu co chu ky nay gom co:

- Thanh phan mot chieu hay trung bnh, la phan hu dung hay mong muon.

- Cac thanh phan hnh sin co tan so k.m.w, con goi la song hai bac n, n = km; vi k la so nguyen, w la tan so li ien tnh bang rad/s. Thanh phan nay khong nhng khong hu ch vi tai mot chieu, ma con gay ra cac tac dung khong mong muon nh: lam tang gia tr hieu dung cua dong ien, dan en tang phat nong; oi vi ong c mot chieu con tao ra mo men phu gay rung. Khi dong tai lien tuc, tr hieu dung VRn cua song hai bac n cua ien ap ra bang: ( )2 2 22 1 sin 1 ( 1)sinmRn mnV V n = +

Bieu thc nay co c khi ta khai trien Fourier cua dang ap ra cho cac song hai bac cao. Trong o, Vdo la tr trung bnh ap ra chnh lu diod cung s o.

Cung nh tnh toan vi thanh phan mot chieu cua ien ap, ta co the tm ra cac thanh phan song hai dong ien bac n khi giai mach ien tng ng bo chnh lu tai RLE cho cac song hai ien ap (hnh 4.3.7):

In R

Vn jnwL

Hnh 4.3.7:

22 )nwL(R/VI RnRn += Theo tnh chat cua cac thanh phan Fourier, tr hieu dung cua dong ien la:

+= noR RnIII 22)( vi IO la tr trung bnh cua dong ien. n 2RnI c goi la hieu dung tong cac song hai, bieu th o nhap nho (khong

phang) cua dong ien. Ch so n c tnh t m en . Trong thc te, v nwL va VRn giam nhanh theo n , ngi ta ch tnh toan vi vai song hai au tien (k = 1 .. 3).

Ngoai viec loc ngo ra, mot bien phap rat hu hieu e giam song hai dong, ap ngo ra la tang so xung cua ap ngo ra, co hai cach:

Hc k 2 nm hc 2004-2005


- Tang so pha: T he 3 pha, ta co the tao ra 6, 12, 24 pha bang cach noi tiep cac cuon day th cap (co ien ap thch hp) cua biep ap 3 pha co nhieu cuon th cap tren 1 pha. Hnh 4.3.8 trnh bay cach thc hien 12 pha t 6 pha A, B, C, -A, -B, -C (ch ve pha 1 va pha 12). Pha 1 c tong hp t pha A va C vi t le thch hp.

Goc Ao1 = 15O, oA1 = 120O cua tam giac oA1 cho ta:

1 1sin120 sin15 sin 45

o A oA= =

Suy ra ien ap cac cuon day oA, A1 theo ap ra o1.

A

B

C - C

- A

- B

o

1

12

Hnh 4.3.8:

Phng an nay con co u iem la thch hp vi cong suat ln. Nhc iem la bien ap co cong suat bieu kien tang cao (tng ng he so s dung bien ap thap) do gia tr hieu dung dong ien qua cuon day tang khi goc dan cua mot chnh lu giam v so xung m tang.

- Noi noi tiep va/hay song song cac bo chnh lu co nguon lech pha. ay la phng ap hieu qua hn oi vi ca ba: nguon, BB va ngo ra. Noi dung se c trnh bay trong phan sau.

V du: Cho bo chnh lu ba pha cau tai R co loc LC ngo ra hnh 4.3.9.(a), ap nguon co hieu dung 100 volt (ap pha), tan so 50 Hz. Goc K pha = 60O , tai R 100 ohm. Mach loc LC bang bao nhieu e bien o nhap nho ap ra ( nh nh) bang 10 volt? Cac gia thiet e n gian tnh toan: - Sau khi qua mac loc LC, cac song hai bac ln hn c ban (k = 1, ng vi tan so nhap nho ngo ra 6 xung, bang 300 Hz) la khong ang ke. Nh vay ch can khao sat song hai c ban. - Dong qua bo chnh lu la lien tuc e co the ap dung .

Hnh 4.3.9: (a) la mach ong lc trong o vO la ap ra bo chnh lu, vC la ap ngo ra bo loc LC; (b) la mach tng /v hai c ban, v1 la song hai c ban - bac k = 1 cua ap ra bo chnh lu, vC1 la song hai bac k = 1 cua ap qua tu C theo gia thiet tren cung chnh la ap ngo ra mach loc; (c) la cac dang song trong mach trong o VO la trung bnh ap ra chnh lu.

vov

vC C1vv1

v

vo

C1

oVL

RC R1/jnwC

jnwL

Hnh 4.3.9: (a) (b) (c)

Ta co ( ) ( )2 6 2 6sin .sin .100 2.34 100 23448.91

v

tnh 4.3.15 n 6 ; 60 V v

mmVdo V

= = = =< > = = =

Hnh 4.3.9.b cho ta: ( )

( ) ( ) ( )222211 11

11

RnwL

C

LCwnjnwLRjnwCRjnwC

VV

+=+= ///

///

Yeu cau cua au bai la bien o nhap nho ap ra ( nh nh) bang 10 volt, suy ra tr so hieu dung song hai c ban qua ien dung C la v 53322101 .)/( ==CV .



Phng trnh tren cho phep tnh toan gia tr Lva C. V ch co mot phng trnh cho hai an so, can phat bieu them mot ieu kien cho L va C trc khi giai.

e y: ( ) ( ) LCwnLCwn RnwL 2222221 + v tan so trong mach se kha ln so vi tan so cong hng cua LC (mach loc ch cho tan so thap qua),

( ) 690310065339481

948533

22221

1 === ELCLCwnVVC .

..

....

Co nhieu phng an e chon LC, ay chon C = 100 uF suy ra L = 0.039 H. Gia tr LC nh vay co the thc hien de dang trong thc te.

Kiem tra lai:

phan tnh toan : nwL/R = 6*100*0.039/100 = 0.74 < mknVdo va

( )( ) ( ) volt 630537

823

1093100121 2100100122622

22 ..

.

.==

+=

VVC . Vay song hai bac

k = 2 la khong ang ke so vi thanh phan c ban bac k = 1, ung nh gia thiet.

Kiem tra dong lien tuc: Cong thc ch ung trong trng hp dong lien tuc, e am bao ket qua can phai kiem tra s lien tuc cua dong ien. Tuy nhien, giai tch mach ien chnh lu ieu khien pha tai RLC trong ieu kien dong gian oan rat phc tap va thng khong can thc hien khi thiet ke v khi dong bang khong, ap ra se bang ap tren tu ln hn ap li trong khoang nay (chnh la ap ra ngo ra khi dong lien tuc). Nh vay nhap nho ap tren tai khi dong gian oan se be hn tnh toan da vao gia thuyet dong lien tuc.

7. ac tnh ngoai bo chnh lu:

nh nghia ac tnh ngoai: Vo = f (Io). Co cac ac tnh:

- Ap ra giam khi Io tang v cac sut ap: chuyen mach, qua chnh lu, ien tr day dan va bien ap (neu co).

- Khi dong tai lien tuc, ap ra bo chnh lu khong phu thuoc tai nen ap ra on nh, sut ap theo tai t hn.

- Khi dong gian oan khong co sut ap do chuyen mach, va khong co phan ien ap am nen ap ra cao hn.

Io

vung dong lien tuc

Hnh 4.3.10

- Khi tai co sc phan ien E, o (suat) sut ap tang cao trong vung dong gian oan v khi o E cung la mot bo phan tao ra ien ap ngo ra.

Tnh pham vi ieu chnh goc KP khi cho trc pham vi thay oi phu tai: Goi EB la ap ra BB ly tng, ch phu thuoc va ap nguon. EB = E + V , trong o E la sc ien ong tai, V bao gom tat ca sut ap co the co. Tong quat, EB va E co the > 0 hay < 0 trong khi luon co V > 0. Do o khi thiet ke co the chon min theo Emax + V (vi Emax > 0) e am bao cac sut ap khi dong max va max theo Emin < 0 trong chieu nghch lu khi tnh en trng hp dong bang 0. max can phai kiem tra

Hc k 2 nm hc 2004-2005


them goc nghch lu an toan: max < . IV.4 NOI SONG SONG VA NOI TIEP CHNH LU:

1. Noi song song bo chnh lu:

ieu kien thong thng e noi song song hai bo chnh lu la:

- hai bo chnh lu giong nhau.

- luon luon co ap trung bnh ngo ra bang nhau.

Khi o dong trung bnh qua moi bo chnh lu la dong trung bnh tai.

CHNH LU 1

CHNH LU 2

Ngu

on

Tai

oIIo

oI

2

2

Hnh 4.4.1

e cai thien chat lng BB, hai bo chnh lu se lam viec lech pha nhau (nh hai cau 3 pha tren hnh 4.4.1 la /6) e nhap nho ap tren tai co bien o giam, tan so tang gap oi so vi nhap nho ap ra cua mot bo chnh lu (la m = 12 so vi m = 6 cua cau ba pha). Cuon khang can bang can c them vao e ri phan ap chenh lenh cua hai bo chnh lu. Cung nh s lech pha nay ma dong nguon (la s xep chong cong dong quahai bo chnh lu) se co t song hai bac cao hn khi lam viec thong thng.

Ta rat hay gap bo chnh lu 6 pha co khang can bang ieu khien pha. S o chnh lu 6 pha co khang can bang ieu khien pha co c khi thay cac D trong hnh 4.1.6 bang SCR.

Hnh ve 4.4.2 ben lay ap vAB lam goc pha, qua o co the thay la

( ) ( )01 02 = v wt v wt , cung dang.

Trung bnh ap ra giong nh chnh lu 3 pha tia, la khi gia s dong lien tuc.

e tnh gan ung ap tren cuon khang can bang, ta tm cac thanh phan Fourier cua ien ap:

k = 1, tan so 3w :

( ) ( ) ( )01 02 023 3 3 3= = v wt v wt v wt Hnh 4.4.2 va cho ta vcb = v01 - v02 = 2 v01(3wt) . Suy ra hieu dung thanh phan tan so 3w cua ap qua cuon khang gap oi song hai bac 3 ap ra chnh lu tnh theo

tng t, khi k = 2, co the CM thanh phan tan so 6w ap tren cuon khang bang 0. Cac thanh phan tan so cao hn co the bo qua. Trong mot so tai lieu, ngi ta tch phan tr hieu dung ap tren cuon khang t e lam ap tren cuon khang va thiet ke vi tan so 3 w.

Khang can bang thc ra la mot bien ap t ngau v khong co t trng mot chieu (thanh phan trung bnh cua dong tai qua 2 cuon day kh lan nhau), neu goi i la dong t hoa, can co ieu kien sau e ca hai bo chnh lu luon co dong (am bao hai chnh lu luon lam viec):



1 0 hay 2 2o o

o mI Ii i I = > vi Im la bien o dong t hoa.

Neu t cam LT cuon day be, can tnh gia tr toi thieu e han che dong t hoa theo cong

thc: min

22 2 vi = (1-cos ) 3

LT LTm T LT

T o

Q Q VI L QL I

= = trong o Iomin la dong tai toi thieu,

QLT la tch phan xung (ban ky) ap tren cuon khang can bang. ( Xem tai lieu TCS TS Nguyen van Nh, NXB HQG 2002, ap dung vao trng hp cu the)

2. Noi noi tiep bo chnh lu:

Khac vi song song hai bo chnh lu, viec noi tiep hai bo chnh lu ch can chung co cung kha nang dan dong, ta co the xay dng mot chien lc ieu khien ap ra cua tng bo chnh lu e co ket qua tong cong la tot nhat, v du nh mot bo chnh lu co the dung diod (khong ieu khien) khi pham vi chnh ap ra be, tng ng vi ieu khien bo chnh lu SCR con lai (hnh 4.4.3).

Viec s dung ap nguon lech pha

Tai

Ngu

on

io

CHNH LU 2

CHNH LU 1

o1v

o2v

vo

Hnh 4.4.3: vO = vO1 + vO2

cung cap cho hai bo chnh co tac dung lam tang tan so, giam bien o nhap nho ngo ra cung nh giam nhap nho dong nguon, la tong cua dong qua hai bo chnh lu.

Mot v du kha quen thuoc cua noi tiep hai bo chnh lu la cau 3 pha co the xem la noi tiep hai bo chnh lu 3 pha hnh tia.

IV.5 CHNH LU HON HP SCR VA DIOD:

1. Cac s o:

Viec s dung hon hp SCR va diod trong cung s o chnh lu khong thay oi kha nang c ban cua chnh lu ieu khien pha la ieu khien c ap ra, no co them u iem la lam n gian s o ong lc va ieu khien, va thng c s dung trong cac he thong cong suat trung bnh va nho vi yeu cau chat lng khong cao.

(a)

vov

io

v

o

(b)

v o

i

Av

C

(c)

oB

oi

T2T1 T1

T3

T2 T3T1D2

D5 D6D4D4D4D3

Df Df

Hnh 4.5.1

Hnh 4.5.1 trnh bay 3 s o chnh lu cau hon hp thng gap: mot pha va ba pha.

2. Khao sat s o chnh lu mot pha (hnh 4.5.1.a va .b) tai RL:

S o hnh 4.5.1.a co diod phong ien Df . Df thng c s dung khi tai la cuon day e tao ra ng phong cho dong ien qua cac cuon day khi ngat nguon, tranh qua ien ap trong mach

Hc k 2 nm hc 2004-2005


ien co cam khang khi dong ien giam ve khong ot ngot.

Xet chu ky lam viec au tien. Tai wt = , kch T1: T1 va D4 dan, ap ra vO bang ap vao v nh cac s o cau 1 pha khac.

Tai wt = , v bat au am. Df dan ien v c phan cc thuan: vO = 0, dong ien tai khep mach qua Df, dong qua cau chnh lu bang khong, SCR tat. Dong ien qua tai cuon day luc o suy giam theo ham mu cho en khi wt = + , SCR T2 co xung kch khi, chnh lu ban ky ke. Nh vay vO co dang cua ap ra tai ien tr (hnh 4.5.2.a) ngay khi tai la RL. Tr trung bnh ap ra tnh theo

[ ]12 += cosVoV Hnh 4.5.2.b ch ra cac khoang dan ien cua cac linh kien trong mot chu ky ta xac lap (khi dong ien thay oi co chu ky) cua s o hnh 4.5.1.a co diod phong ien Df. Trong trng hp s o hnh 4.5.1.a khong co diod phong ien Df: dang ap, dong ngo ra khong thay oi. Tai wt = , D3 se dan ien vi T1 trong khoang dan cua Df va nh vay, ap ra vO van bang 0 (e chng minh, co the gia s D4 tiep tuc dan ien => D3 phan cc thuan, tr nen dan ien lam D4 tat). Tng t, cuoi ban ky am, D4 dan ien vi T1 trong khoang dan cua Df. Vi s o hnh 4.5.1.b, khi hoan oi v tr cac chnh lu, ap ra khong thay oi nhng khoang dan cua cac chnh lu thay oi. D2 va D4 ong vai tro cua Df . Phan bo dong qua cac chnh lu ve hnh 4.5.2.c.

S o hnh 4.5.1.a thng c dung vi Df khi can mach ieu khien n gian cho phep mach ieu khien co iem chung vi mach ong lc, trong khi s o hnh 4.5.1.b khong can dung Df vi tai RL. Nhng khi o, cac SCR lai khong the noi chung Kathod, dan en yeu cau cach ly mach ieu khien ong lc.

Hnh 4.5.2 dang ap, dong cua chnh lu cau hon hp SCR Diod (K khong toan phan)

Co the tong quat hoa ket qua khao sat s o mot pha: Co the ieu khien pha bang cach s dung Diod va SCR trong s o cau. V dong ien phai qua hai chnh lu trong s o cau nen co the ieu khien mot chnh lu e thay oi ap ra. Diod se t ong dan ien khi phan cc thuan lam cho ap ra khong the am.

V du: Cho bo chnh lu mot pha hon hp hnh 4.5.1.a co diod phong ien. Ap nguon 220 VAC. Tai RL, R = 10 ohm, L u ln e co the xem dong tai la phang. Tnh tr trung bnh dong qua SCR, Diod goc K pha = 45 O.



Trung bnh ap ra: cho ta:

VO = 0.45*220*(1 + cos 45) = 169 volt

Trung bnh dong ra: IO = 169/10 = 16.9 A

Theo hnh 4.5.3, tr trung bnh dong qua SCR, Diod s o chnh lu:

[ ]

A

IdwtIII ooDT

346360135916

21

2

.. ==

===

0 2 wt = /4

vO

iO O= I

v

Df T1, D4 Df T2, D3

Hnh 4.5.3

Trung bnh dong qua diod phong ien Df:

[ ]

A

IdwtII ooDf

22418045916

120

.. ==

==

Trong cac bai toan thiet ke, nh mc dong ngo ra IO c gi khong oi trong khi goc K pha thay oi, cho nen nh mc dong qua SCR, Diod c chon theo ieu kien lam viec xau nhat cua no, la bang vi dong ngo ra IO.

3. Khao sat s o chnh lu cau ba pha hon hp (hnh 4.5.1.c) tai RL: Giong nh s o cau 3 pha 6 SCR, ta chia cac chnh lu lam hai nhom: nhom + gom cac SCR, co ieu khien pha va nhom gom cac diod. Khao sat s hoat ong cua cac nhom nay cho ta ien ap cac au day ngo ra v+ va v_ la cac net am tren hnh 4.5.4, ap ra vO van la cac ap day nhng gom ca cac xung khong ieu khien pha, nen ngo ra nhap nho 3 lan trong chu ky. Hnh 4.5.4 ve cac dang ien ap khi < / 3. Khi = 0 ap ra tr ve dang chnh lu cau ba pha diod, ngo ra la 6 xung nh nhau. Khi > / 3 ap ra co khoang bang khong do Df hay cac diod va SCR tren cung pha dan (khi khong co Df). V khoang dan ien cua cac diod la cac khoang ien ap pha am nhat khong oi trong khi khoang dan cac SCR thay oi v co ieu khien pha, dong ien ban ky dng va am cua mot pha khong lech 180O. ay la nhc iem ln cua ho s o nay, no han che cong suat tai ap dung v tao ra nhieu hai dong ien cho li. Trung bnh ap ra khi dong lien tuc co the c tnh theo tong hai chnh lu ba pha tia, mot vi goc ieu khien va mot khong ieu khien:

3 6 (1 cos )2o

V V = + Hnh 4.5.4: dong, ap ra khi chnh lu cau 3 pha K

khong hoan toan

V du: Gia s dong tai lien tuc, phang. Ve dang dong qua nguon trong hai trng hp = 36O va = 90O . Tnh he so cong suat bo chnh lu trong hai trng hp.

Hc k 2 nm hc 2004-2005


Dang ap hai au bo chnh lu va dong ien pha A trong hai trng hp nh sau:

Khi a = 36O dong qua pha A gom hai xung bien o 2/3 va bang = /2 khi = /2 > /6 (xem hnh ve).

- Khi a = 36O dong hieu dung qua pha A bang: 2 2 / 3 2

2 3R o oI I I= =

i . Cong suat tai PO =VO .IO vi VO tnh theo va he so cong

suat: 3 62 5 5

23

(1 cos ) 3(1 cos )23

oo

o

V IPHSCSS V I

+ += = =ii i = 0.863

Hnh 4.5.5: Dong qua nguon cua chnh lu cau 3 pha K khong hoan toan

- Khi a = 90O dong hieu dung qua pha A bang: 2 ( ) ( )

2 2o

R o oII I I

= = =i va he

so con g suat 3 62 2

12

(1 cos ). 33. .

oo

o

V IPHSCSS V I

+= = = = 0.551 Vay HSCS luon luon cao hn chnh lu ieu khien hoan toan (ch dung SCR).

IV.6 BO CHNH LU AO CHIEU: 1. Cac s o: Chnh lu ieu khien pha du co the cung cap ap ra > 0 va < 0 nhng dong ra ch cho phep > 0 (lam viec phan t I va IV cua mat phang tai VO, IO) hnh 4.6.1 . e co the ao chieu dong ien tai, co hai s o chnh: - S dung cac tiep iem ao chieu (hnh 4.6.2.a): Hnh ve ang co tiep iem T ong, cung cap 1 chieu dong tai, neu T ngat va N ong dong tai se c phep ao chieu.

oPhan t I

Phan t IV

I

Vo

Hnh 4.6.1

Li

Li_

I

Tai

o

_Li

Tai

_

++

+

T

T

N

N

Hnh 4.6.2 S o nguyen ly: (a) ao chieu dung tiep iem (b) BB ao chieu



- BB ao chieu: Gom hai bo chnh lu cung cap hai chieu dong tai, hnh 4.6.2.(b) la s o nguyen ly va hnh 4.6.3 la s o cu the vi cac bo chnh lu hnh tia.

2. Nguyen ly ieu khien BB ao chieu: e hai BB cung cap cung gia tr VO cho tai, cac goc k pha cua hai BB se co quan he nh sau khi gia s dong tai la lien tuc:

BB 1 cung cap ap trung bnh VO1 vi goc 1, BB 2 cung cap ap VO2 va 2.

VO =VO1 = Vdo.cos 1 = VO2 = -Vdo.cos 2 => cos 1 = - cos 2 hay 1 + 2 = neu 1 > 0 : BB 1 la chnh lu => 2 < 0 : BB 2 la nghch lu.

o

ov

i

CBA

N

KC

B

Taivo2

vo1

T4 T6

T2T1

T5

T3

hnh 4.6.3 Du cac tr trung bnh hai BB la bang nhau, gia tr tc thi cua chung khong bang nhau

lam xuat hien dong ien can bang (con goi la tuan hoan circulation) ch chay qua hai bo chnh lu khi chung cung lam viec. Dong can bang co the rat ln neu ta khong co tong tr han che chung. Ngi ta co cac cach ieu khien sau:

- ieu khien rieng: Moi luc ch cho mot bo chnh lu lam viec tng ng vi chieu dong ien hoat ong hay mong muon. Nh vay khong co dong can bang. Vi cung ien ap VO tren tai, khi ao chieu dong th mot BB la chnh lu, bo con lai la nghch lu va ngc lai.

Can co thi gian ca hai BB khong lam viec khi chuyen BB lam viec e tranh trng hp co the ca hai BB cung dan ien.

- ieu khien chung (ong thi): Hai BB cung co xung ieu khien nhng ch co mot bo co dong tai, dong can bang c han che bang cuon khang KCB va qui luat ieu khien thch hp. Co hai cach phoi hp : tuyen tnh va phi tuyen.

Phoi hp tuyen tnh: Goi 1, 2 la goc ieu khien pha hai bo chnh lu. Cac ap trung bnh VO1 = VO2 = VO cho ta 1 = - 2, tng t nh a khao sat ieu khien rieng. Ap tren cuon khang can bang vcb = vo1 vo2 khong co thanh phan mot chieu (tr trung bnh bang 0) co the tnh tng t nh khang can bang cua bo chnh lu sau pha co khang can bang (muc IV.4.1). Dong can bang co tac dung lam dong qua cac BB luon lien tuc.

Phoi hp phi tuyen: e giam nho kch thc cuon khang can bang trong khi van han che dong can bang gia tr mong muon, ngi ta ieu khien cho ap ra nghch lu ln hn ap ra chnh lu:

2 = + - 1 hay 1 + 2 > IV.7 S DUNG NGAT IEN CHUYEN MACH CNG BC CHO CHNH LU:

Viec s dung ngat ien ong ngat theo ieu khien nh GTO hay to hp transistor + diod trong cac s o chnh lu cho phep:

- nang cao he so cong suat cua BB nh cac s o chnh lu ieu khien goc oi xng (symmetrical angle control) hay ieu rong xung, cai thien dang (giam bien o song hai bac cao) dong nguon.

- phat tra cong suat phan khang ve nguon nh s o ieu khien goc tat (extinction angle control).

Hc k 2 nm hc 2004-2005


1. Chnh lu ieu khien goc tat: (Hnh 4.7.1)

- S o lam viec mat phang tai.

Trung bnh ap ra s o cau mot pha khi goc dan = :

0

1 22 sin . (1 cos )oVV V wt dwt

= =

vov

oi

i

D4D3

Df

GTO2GTO1

Hnh 4.7.1.a: Chnh lu cau 1 pha GT0

- Khi gia s dong tai phang, lien tuc: io = Io khong oi dong nguon la xung vuong, sm pha vi ap nguon v goc ( ) / 2 Vay ht cung cap c cong suat phan khang cho nguon. Tuy nhien, do dong khong sin, hieu qua thc s cua phng an nay khong cao.

wt

2

GT02 DfGT01 Df

ov

io

v

Dong qua nguon i

goc sm pha

hnh 4.7.1b: Ap ra chnh lu cau 1 pha ieu khien goc tat

2. Chnh lu ieu khien goc oi xng hay ieu rong xung: (Hnh 4.7.2)

- Khi s dung mach ien hnh 4.7.1.a , co the ong ngat cac GTO theo luat phc tap hn e cho ra cac bo chnh lu co he so cong suat tien en 1 nh dang ap hnh 4.7.2. Tnh toan tung t nh s o ieu khien goc khoa, HSCS khong bang 1 mac du dong nguon cung pha vi ap v khong hnh sin. Ta co the tang so xung trong mot ban ky e ieu che dong hnh sin (xem chng 6).

o

2

v

wt

DfDf

i

GT02o

v

GT01

( )

Hnh 4.7.2: Ap ra s o ieu khien goc oi xng



u iem ln nhat cua s o nay la HSCS khong giam nhieu khi giam ap ngo ra ve 0 nh khi s dung SCR. 3. Chnh lu lam viec hai phan t I va II: Vi viec s dung ngat ien chuyen mach cng bc, ta co the cho dong ien tai ao chieu khi ap mot chieu tren tai ln hn gia tr

oi

Tai tch cc

A

C

Bov

L

C

Q6

Q3Q2

Q5

Q1

Q4

LL

Hnh 4.7.3: BB lam viec hai phan t (dung IGBT) ap nguon chnh lu diod nh hnh 4.7.3. Moi ngat ien bao gom mot IGBT co diod song song ngc. Khi ap tren tai ln hn ap chnh lu diod, dong qua diod ve 0 va ta co the ieu khien khoang dan cua cac IGBT e tra nang lng ve nguon. Vay s o hnh 4.7.3 co Vo Vdo va Io co the > 0 hay < 0

IV.8 MACH KCH SCR IEU KHIEN PHA:

1. Nguyen ly ieu khien pha:

Hieu mot cach n gian nhat, mach kch cac SCR trong s o ieu khien pha se cung cap cho cc cong mot dong ien cung tan so ( ong bo) vi li ien nhng pha thay oi c theo tn hieu ieu khien. Theo nh nghia, goc lech pha nay se bang 0 trong trng hp ap ra la cc ai vi tai R, tng ng vi viec thay SCR

Hc k 2 nm hc 2004-2005


Phat xung ieu khien pha theo nguyen ly so sanh co hat nhan la mach so sanh vi ngo vao la hai tn hieu: Uk la tn hieu ieu khien la tn hieu mot chieu, Ub la tn hieu ngo ra mach ong bo, la tn hieu cung tan so li, co o doc khong oi dau trong khoang = min en = max (thng la 0 va ) la khoang thay oi cua goc ieu khien pha . Khi Uk = Ub, ngo ra bo so sanh thay oi trang thai, anh dau thi iem kch SCR. Thay oi ap Uk se ieu khien goc ieu khien pha . Hnh 4.9.2.(b) ve cac dang song vi ap ong bo rang ca tang.

Khi e y goc KP xac nh khi so sanh ub va Uk tng ng vi so sanh hieu so ub Uk va 0, do o nguyen ly nay con goi la nguyen ly thang ng (thay oi khi ta nang ha theo chieu

so sanhU

uTao apong bo

b

k

LiKch SCR

(a)

(b)

Hnh 4.9.2 thang ng ap ieu khien). T dang ub co the tm c quan he (ub,Uk) va suy ra quan he trung bnh ap ra Vo va Uk.

2. Ham truyen BB:

Cac BB cua TCS eu co ham truyen gan ung theo dang:

( )1

s BB

KH s K es

= +

vi KB= Vo/Uk va la thi gian tre cua BB, c lay bang thi gian trung bnh gia 2 lan phat xung ieu khien, la T/ 2m oi vi bo chnh lu SCR, T la chu ky li, m: so xung. Trong mot so tai lieu khac c lay bang T/ m. V du: Tm ham truyen cua bo chnh lu KP, s o cau 3 pha, ap li 220 V (ap pha), s dung ap ong bo nh hnh 3.28.b , bien o ap ong bo Ub = 10 volt.

Gia s dong tai lien tuc, trung bnh ap ra Vo = 2.34 Vcos . T hnh 3.28.b ta co hai tam giac ong dang: = k

b

UU

vi ap ong bo co min = 0 va max =

suy ra 2.34 220cos 514.8cos10 10

k kO

U UV = = i va KB = Vo / Uk co dang cosin, la mot ham phi tuyen.

Thi hang = T/2m = 0.02 / (2*6) = 0.00167 giay. Vi dang ap ong bo rang ca tang nh hnh , khi Uk tang , tang lam ap ra giam khong phu hp vi ieu khien theo sai lech. Trong thc te, ngi ta thay the Uk bang (Ub Uk) nh



vao mach di mc nh trong v du cuoi chng.

3. Mach kch chnh lu ieu khien pha theo nguyen ly so sanh:

Nguyen ly so sanh c s dung rong rai khi thiet ke bang vi mach tng t (analog).

a. S o khoi:

Hnh 4.8.3 cho ta s o khoi ay u mach kch chnh lu ieu khien pha. Khoi lech pha hieu chnh o lech pha cua ap li sao cho mach kham pha zero cho ra cac xung goc pha = 0. Tn hieu zero nay se ong bo cac mach phat xung theo nguyen ly lam tre hay kch khi mach tao ap ong bo. Nh ta a biet, mach so sanh xac nh thi iem kch SCR khi Ub bang Uk . Mach n on ngo ra bo so sanh xac nh be rong xung kch SCR.

Cac mach va c ke thuoc vao khoi phat xung kch SCR, khac nhau moi pha. Trong s o chnh lu nhieu pha, moi pha se co t nhat mot khoi phat xung kch SCR, cac ngo ra cua chung c a va khoi logic e phoi hp, am bao s o nhieu pha lam viec ung. Khoi khuech ai va ghep nang mc cong suat xung va noi vao cc cong SCR, co the phai am bao cac ieu kien cach ly ien gia cac SCR vi nhau, SCR va mach ieu khien.

Lech pha Kham pha ZERO

Tao apong bo

n on so sanh

LOGICK. ai&ghep

cac SCR cuachnh lu

Phali

Xung t cac pha khac

= 0 u

U

Mach phat xung K pha

b

k Hnh 4.8.3 c. Mach phat xung ieu khien pha dung khuech ai thuat toan (KTT):

12V

_12V

_12V

Kham pha zero Tao xung ong boSo sanh

Mach n on

C200.1u

C210.1u

D19

D22

D20

D18

R2510k

R342k2

R32100k

R273k3

R283k3

R333k3

R29

1k

R2615k

R30

10k

U5B

5

67

U5C

10

98

U5D12

1314

U5ATL0843

21

411

DB

DK

SCR

Hnh 4.8.4.(a)

Mach kham pha zero U5A so sanh tn hieu DB la sin li chnh lu va ngng mot chieu, cho ra xung dng khi DB qua zero [dang song (1) tren hnh 4.8.4(b)].

U 5D la mach tch phan e tao ap ong bo Ub. Khi ngo ra U5A cao, U 5D tch phan xuong vi thi hang gan bang R27*C20 co tr so be tao canh xuong cua ap ong bo. Khi ngo ra U5A thap, U 5D tch phan len vi thi hang bang R32*C20 co tr so ln cho ta canh len cua rang ca, la o doc lam viec. Diod D18 co anod noi ngo vao cua KTT gi cho gia tr Ub trong khoang t V en Ubmax. U 5C la bo so sanh dung KTT, cho ra xung dng [dang song (4)] khi Ub > Uk. Canh len cua ngo ra U5C la thi iem kch SCR.

U5B la bo so sanh cua mach n on khong t gi, xac nh be rong xung kch SCR, thng lay bang 1 mili giay cho cac s o chnh lu. Be rong nay thay oi theo thi hang C21*R26 cua mach vi phan [dang song (3)]. Diod D19 khong cho ngo vao U 5B co gia tr am,

Hc k 2 nm hc 2004-2005


bao ve ngo vao KTT va xa nhanh tu C26, gi khong oi be rong xung trong suot khoang thay oi cua . S o hnh 4.8.4.a cho ra xung kch SCR hai ban ky[dang song (5)], co thong so ch phu thuoc hai tu ien (C20, C21) va hai ien tr (R32, R26); dung ap mot chieu e ieu khien goc cho thay kha nang che tao vi mach ieu khien pha. Trong cong nghiep, nhng vi mach ieu khien pha eu co s o khoi tng t, nhng ky thuat mach thay oi e co the dung mot nguon, cac tr tu thng noi xuong iem chung e giam so chan s dung.

3. Ap ong bo dang cosin va khoi lech pha:

Nh a trnh bay trong phan nguyen ly, ap ong bo cua mach phat xung ieu khien pha can co o doc khong oi dau trong khoang bang 0 en max. Hnh 4.8.4.(b)

Nh vay co the co cac dang rang ca vi (a) o doc dng, (b) o doc am va (c ) dang cosin [tren hnh 4.8.5].

Dang (a) rat de thc hien nhng co bat li la khi Uk tang, goc kch tang tng ng ap ra Vo giam. Dang (b) co ac tnh ngc lai, rat kho thc hien. Ap ong bo rang ca cho ta goc ieu khien pha thay oi tuyen tnh vi ap ieu khien Uk. ieu nay lam cho quan he trung bnh ien ap ngo ra va Uk khong the tuyen tnh, v quan he Vo() co cac ham sin, cos. ay la mot bat li cho he thong ieu khien t ong v kho hieu chnh he thong phi tuyen.

ong bo cosin cho ta quan he (Uk) co dang Arccos, ham nay se b kh bo khi dong qua chnh lu la lien tuc, khi o, Vo() co dang cos (). Thc vay, neu goi bien o hnh cosin la Ubmax, khi Uk = Ub ta co:

Uk = Ub = Ubmax. Cos hay ( )maxUbUkcos 1= va viet lai sin vi , cos VVVV m

mdodoo

2==

(a)

(b)

(c)

0

0

0

2

2

2

Hnh 4.8.5

Hnh 4.8.6

Cac bat li co the ke ra la: mot dang cos ch co the kch cho mot SCR thay v mot pha nh ong bo rang ca (hnh 4.8.5), pham vi thay oi goc kch hep v khong the giam ve khong va ap



ong bo thng co nguon goc li nen bien o khong co nh va de b nhieu e tao ham cosin, ngui ta thng lay ap li qua bien ap giam va cho lech pha. Co the s dung mach lech pha dungRC, RC va KTT, mach xoay pha bang bien ap va RC hay chon pha thch hp. Hnh 4.8.6 cho ta cac vector ap pha va day cua li ien ba pha. Cac ap pha va day cua li ba pha co cac o lech pha 30O, 60O, 90O, 120O . Co the chon trong o cac thanh phan thch hp e co ham cosin cho mach ieu khien pha.

V du e kch SCR cho mach 1 pha, ta can lech 90O . T hnh 4.8.6, nhan xet pha A va ap day BC lech 90O . Mot v du khac, s o ba pha tia (hnh 4.2.1 va 4.2.6) oi vi SCR pha C la T3, = 0 khi ap day CB bang khong. Ham cosin kch SCR T3 la ao cua pha A (pha A). Cac phng phap lam lech pha trnh bay tren cung ng dung vao khoi lech pha trong s o khoi tong quat cua mach kch SCR ieu khien pha (hnh 4.8.4.a). Khoi lech pha can thiet cho viec s dung mach kham pha zero tm ra iem = 0 khi ap li khong qua zero = 0 (hnh 3.28).

V du s o ba pha tia (hnh 4.2.1) e kch SCR pha A la T1, co the lam cham ap pha A goc 30O hay dung ap day AC a vao mach kham pha zero.

4. Mach khuech ai xung va ghep vi khoi ong lc:

Cac muc tren ay ch trnh bay cach tao ra xung ieu khien pha SCR, cac tn hieu nay can c khuech ai e co u nang lng (dong) kch cac SCR. Mach khuech ai xung chu yeu la khuech ai dong, v ap at vao cc cong SCR kha be, khoang vai volt trong khi dong kch cong co the en 5 ampe cho SCR co dong anod vai tram ampe. Khi ghep trc tiep mach kch vao cc cong, mach khuech ai thng ch la cac mach theo phat (tai cc phat E). Co the s dung them cac tang khuech ai transistor ghep trc tiep va tu gia toc (xem chng 4, phan mach lai transistor).

Hnh 4.8.7 (a)

VCC

D2

D1

3k3

3k3

3k3

Q1

BAX

3.3 ohm

D247n

100SCR

(b)

Mot bai toan hay gap cua thiet b ien t cong suat la yeu cau cach ly mach ieu khien va mach ong lc. Viec cach ly mach ieu khien co mc nang lng be va mach ong lc cong suat ln am bao an toan cho ngi van hanh va thiet b, tang cng kha nang chong nhieu va trong trng hp mach ien t cong suat con la s bat buoc khi cac ngat ien khong co iem chung (common hay mass) nh cac s o cau.

Va nh vay can phng tien e ghep khong ien xung ieu khien vao cc cong cua SCR hay cac cc ieu khien transistor. Co hai moi trng thong dung: t bien ap xung, va quang cac bo ghep LED quang ien t vi ten thng mai OPTRON.

Hc k 2 nm hc 2004-2005


Hnh 4.8.7.(a) va (b) trnh bay cac dang song va mach khuech ai xung va ghep vi SCR cho bo chnh lu cau ba pha. Cac diod D1, D2 , transistor Darlington Q1 thc hien ham OR va khuech ai dong. Bien ap xung BAX cc thu C cua transistor qua ien tr han dong 3.3 ohm. Nguon VCC co the la 24V va t so giam ap cua BAX la 4 :1 e giam dong qua transitor va nguon cap ien. Tu ien 47 nF va ien tr 100 ohm noi cang gan SCR cang tot e chong nhieu khi day noi mach ieu khien va SCR dai. Cac thong so tren co the dung cho SCR co nh mc trung bnh dong en 250 A.

Khi Q1 bao hoa, ap nguon VCC co the xem nh at vao s cap bien ap. ien ap nay cam ng qua th cap tao nen dong kch cho SCR. Dong qua cuon s cap bien ap gom dong t hoa va dong phan anh t th cap. Khi be rong xung u be, loi thep bien ap cha bao hoa va ta co ien the va dong ien cam ng th cap. Khi Q1 tat, dong t hoa bien ap phong qua diod D3 va giam ve khong. Nh vay bien ap xung chu t hoa mot cc tnh va can phai thiet ke sao cho khong bao hoa. Khi xung u rong, dong kch t tang cao, loi thep b bao hoa va t thong khong thay oi, ap cam ng giam en bang khong. Hien tng nay cung xay ra khi ta co chuoi xung va dong t hoa cha ve khong th a co ke tiep. Be rong xung kch SCR (xung hep) khoang 1 mili giay

IV.9 NG DUNG CHNH LU IEU KHIEN PHA:

La thiet b bien oi nang lng ien t xoay chieu mot chieu, bo chnh lu c s dung rong rai trong cong nghiep va cac nganh ky thuat khac can ien mot chieu, v nguon ien la xoay chieu.

Trong cong nghiep, ta quan tam en hai nhom ng dung: Truyen ong ien ong c mot chieu va cac bo nguon mot chieu cho cac qua trnh cong nghe khac nhau.

Trong a so trng hp, du thong so ac trng la ien ap nhng ai lng anh hng trc tiep en chat lng phu tai la dong ien. Trong ong c, dong ien tao ra momen chuyen ong. cac qua trnh ien hoa, dong ien quyet nh so lng va chat lng san pham va s phat nhiet. Do o, cac bo chnh lu ieu khien pha c s dung rong rai, mach loc co the khong can thiet hay ch la cuon khang co loi thep e san phang dong ien. Trong mot so trng hp ac biet, loc LC mi c s dung.

Tuy nhien trong cac bo nguon cap ien cho mach ien t, chat lng ien ap ngo ra c yeu cau rat cao. Ngi ta s dung hai dang bo bien oi:

- Chnh lu diod co loc C hay LC va mach on ap transistor (on ap tuyen tnh) a c khao sat tng oi ay u trong cac tai lieu ve mach ien t, khong c e cap en trong giao trnh nay. Khi cong suat ln va rat ln, co the s dung chnh lu SCR va loc LC e gi on nh ap ra thay cho on ap tuyen tnh. Nhc iem cua phng an nay la o bang phang cua ap ra kem.

- Ngay nay tat ca cac c cong suat, bo nguon xung c s dung tng oi pho bien v cac u iem ve kinh te va ky thuat cua no. Bo bien oi nay se c khao sat trong cac chng ke tiep.

1. Cac bo nguon mot chieu ieu khien pha:

Nh a phan tch tren, cac bo nguon mot chieu cong nghiep co ai lng tac ong trc tiep la dong ien. Co the ke cac ng dung: nap accu, ien phan, xi ma, may han ien mot chieu S o khoi cac thiet b chnh lu co ieu khien nh sau:



Li TaiBien ap Chnh lu Loc (co the khong can)

Li TaiChnh luBien apK pha ap AC

(a) (b) Hnh 4.9.1: Bo nguon DC dung chnh lu ieu khien pha

Trong o:

- Loc: Mach loc thng la cuon khang e loc dong ien, co the khong can.

- Chnh lu diod hay SCR bien oi AC DC, la s o nhieu pha khi cong suat ln e giam o nhap nho (song hai) va phan eu tai tren cac pha li, khai thac tot nguon ien.

- Bien ap: Giam, tang ap nguon en gia tr thch hp; cach ly li va tai, am bao an toan cho ngi van hanh may san xuat.

- ieu chnh ap/dong ra thc hien qua ieu khien pha chnh lu trong s o hnh 3.34.(a) va bo bien oi ap xoay chieu (b). Bo nguon mot chieu co s o hnh (b) co ngo ra hoan toan giong nh s o hnh (a) v ien ap ieu khien pha s cap se c chnh lu th cap. S o nay se co hieu qua kinh te ln trong hai trng hp:

+ Ap ra be va dong rat ln: Trong cac thiet b co dong ra en vai ngan ampe ien ap vai chuc volt, viec thay the chnh lu SCR bang diod giam c gia thanh v SCR at tien hn diod, tang hieu suat thiet b nh diod co sut ap be hn SCR. Cac li ch nay rat ln khi so sanh vi viec them bo ieu khien pha ap xoay chieu.

+ Ap ra rat ln va dong be: Trong cac bo nguon cho thiet b tao tnh ien tan so cong nghiep, ngi ta s dung ap rat ln, t vai chuc en hang tram kV vi dong ien khoang di 10 A. ieu chnh ap pha s cap bien ap va dung chnh lu diod th cap, ngi ta tranh c viec phai ieu khien nhieu SCR mac noi tiep e co the chu ien ap cao, rat phc tap va ton kem.

Hc k 2 nm hc 2004-2005


Viec chon s o cho bo nguon mot chieu thong thng c quyet nh bi gia thanh bo chnh lu va bien ap giam ap, trong o s o cau 3 pha c dung cho cac mach cong suat ln va va sau pha co khang can bang s dung bo chnh lu hieu qua nhat.

2. ieu khien ong c mot chieu:

a. S o khoi va ac tnh c ong c:

Hnh 4.9.2.(a) cho ta s o khoi cua he thong truyen ong ien mot chieu dung bo chnh lu ieu khien pha . Truyen ong ien con c goi la dan ong ien (electric drive) hay cung cap sc keo bang ien bao gom ong c ien va thiet b ieu khien no (controller). Bo chnh lu ieu khien pha cho phep cung cap va ieu khien dong nang lng cho ong c mot chieu t li ien xoay chieu cong nghiep. Cuon kch t ong c co the cung cap t chnh lu diod hay SCR.

Trong hnh 4.9.2.(a), bo chnh lu bao gom ca bien ap (neu can bien oi ap li ra gia tr thch hp), cuon khang loc c s dung

LiChnh lu Loc (co the khong can)

Kch t

(a)

w

I, M

wo

0 U

(b)

Hnh 4.9.2

e san phang dong, va ni rong khoang lam viec vi dong lien tuc, gan ay thng khong dung v kch thc to, gia thanh cao. ong c mot chieu la loai kch t oc lap hay hon hp.

Cac quan he ien t cua ong c che o xac lap:

U

R

L

E

Hnh 4.9.3

===

=++=

CeEIkICeM

kCeIREU

suy ra: ( )IRUw Ce = 1

( khi bo qua phan ng phan ng )

U: ien ap phan ng ong c. E, I: sc ien ong, dong ien phan ng cua ong c. R: ien tr tng ng cua cac sut ap tren phan ng. : Phan ng phan ng cua ong c, luon c bo qua khi khao sat truyen ong ien. Ce: Hang so ien t cua ong c mot chieu, t le (he so k) vao t thong cua cuon kch t. M: momen ong c, che o xac lap bang momen can tren truc. : toc o, tnh bang rad/giay.

cho thay khi ieu chnh ap cung cap, quan he toc o dong ien hay quan he toc o momen (con goi la ac tnh c ong c) la nhng ng thang song song nh hnh 4.9.2.(b). Ngi ta ch ieu chnh ap di gia tr nh mc giam ap va toc o luc o nho hon gia tr nh mc, goi la ieu chnh di toc o c ban. e co momen ln, t thong khi o la nh mc. Viec ieu chnh tren toc o c ban c thc hien bang cach giam t thong vi ap at vao ong c bang gia tr nh mc. Co the chng minh la cong suat c nhan c tren truc e ong c khong b h hong van xap x cong suat nh mc khi o. Viec ieu chnh t thong thc hien bang mot bo chnh lu SCR th hai, se cho phep giam ap cung cap cho cuon kch t khi ap at vao phan ng



at gia tr nh mc.

b. Cac bai toan cua he thong ieu khien ong c:

Co cac bai toan sau cua he thong ieu khien ong c:

- ieu khien toc o: La bai toan gi toc o khong oi theo tai, la yeu cau quen thuoc nhat.

- ieu khien dong ien va momen: Theo , momen va dong ien cua ong c t le, ieu khien dong va momen he truyen ong s dung mot c cau vi hai muc ch:

* Bao ve ong c va bo bien oi trong che o xac lap va qua o. * Theo yeu cau cua may moc truyen ong.

- ieu khien v tr va he tuy ong.

- ieu khien gia toc va giam toc chuyen ong:

(khi ong, ham ong c)

IV.10 TOM TAT CHNG:

Chng ba khao sat cac s o chnh lu ieu khien pha. III.1 trnh bay chnh lu diod vi muc ch lam quen, v cac cong thc cho chnh lu diod la trng hp rieng cua chnh lu ieu khien pha, goc ieu khien pha = 0 O. Cac khao sat chnh lu ieu khien pha trong chng nay a c n gian hoa, gii han cac ieu kien ly tng: tai R va tai dong lien tuc va phang. Mot so van e cua bo chnh lu thc te hay lien qua en thiet ke cung c gii thieu. Cac s o ieu khien pha c gii thieu kha chi tiet nhng gii han mot so s o thng gap. Phan ng dung c trnh bay dang tng oi tong quat, nhng s o ieu khien dong, ap c gii thieu kha ky v chien lc ieu khien nay a c s dung het sc rong rai trong ien t cong suat.

BAI TAP VA CAU HOI:

1. Tnh trung bnh ap ra Vo ap ra cua bo chnh lu cau 1 pha ieu khien pha, tai dong lien tuc khi goc ieu khien pha bang 0, 45, 90 o. Biet ap nguon la 120 volt. Tnh ap ngc cc ai at vao SCR, khong e y en sut ap qua SCR.

ap so: 0 o 45 o 90 o . Ap ngc cc ai tren SCR la 169.7 volt. Vo 108V 76.4V 0V

* Lu y : Khi e y sut ap tren SCR, lay bang 1.5V , ket qua se la:

ap so: 0 o 45 o 90 o Vo 105V 73.4V 0V

2. Lam lai bai 1 khi s dung cau hon hp SCR va diod, co diod phong ien.

ap so: 0 o 45 o 90 o . Ap ngc cc ai SCR va diod van la 169.7 volt. Vo 108V 92.2V 54V

3. Tnh chon dong ien cho SCR va diod cua bai 2. khi dong tai khong oi, bang 25A trong suot khoang thay oi cua t 0 en 180 o. Tnh vi s o hnh 3.23.a. Hng dan: Khi dong tai phang - lien tuc, 25A cung chnh la bien o cua dong qua DCR, diod.

T hnh 3.24VD, ta nhan xet c cac ket qua sau: - SCR va diod cua cau chnh lu se co goc dan ln nhat, bang 180 O khi goc kch bang 0O khi o, dong trung bnh qua chung la 12.5 A ( hay dong hieu dung la 17.7A ).

Hc k 2 nm hc 2004-2005


- Diod phong ien se co goc dan ln nhat tien en 360 O khi goc kch bang 180 O tng ng dong trung bnh la 25 A.

4: a. Ve mach ong lc bo chnh lu ieu khien pha, s o mot pha ieu khien khong hoan toan ( hon hp SCR + diod). Mo ta ngan gon hoat ong.

b.Ve va chu thch ay u dang xung kch SCR va dang ap, dong qua tai vi goc kch = 30O. Cho biet tai RL ( tai dong lien tuc ), nguon hnh sin: wtsinVe 2= . c. Vi ieu kien cau b, tnh tr trung bnh ap, dong ngo ra. Cho biet tr so hieu dung ap nguon la 220 volt, tan so nguon 50 Hz, R =10 ohm va L = 0.01 henry. Chu y: Gia tr cua L trong cau c. la khong can thiet cho viec xac nh dang ap ra hay tr trung bnh dong ien, ch can thiet cho viec ve dang dong nhng trong e khong yeu cau.

5. Cho chnh lu tia 3 pha ieu khien pha [hnh 3.7.( c)], ap nguon 220/380V. Tai (10 +j10) . S dung phu luc 1 cua chng II e tnh tr trung bnh ap, dong ra bang 60O va 90O . Hng dan: Goc tai = tg-1 (10/10) = 45O. Goc kch quy ve trng hp s o 1 SCR = 30O 1. = 60O : Tra phu luc 1, o th hnh PL1.2 cho ta (+ = 60 + 30, = 45 ) 130 > 2/3 .suy ra dong tai lien tuc (hnh 3.13), tr trung bnh ap ra la: cho ta Vo = 128.7 volt Tr trung bnh dong Io = 12.87 A 2. = 90O . Kiem tra tng t, (+ = 90 + 30, = 45 ) 90 < 2/3 : dong ien gian oan, can tch phan e

tnh tr trung bnh ap ra: [ ]

( ) ( )[ ] volt.coscoswtcosVdwt.wtsinVVo

8539090309030

22

222203

23

23

=+++===

+++

+++

Ttr trung bnh dong ra khi o: Io = 5.38 A

6. Cho chnh lu cau 3 pha ieu khien pha, cap ien t nguon 220/380V qua bien ap ba pha noi Y/Y. Tai ong c (RLE), R =0.2 , L = 0.02 H. Gia s dong tai lien tuc, tnh: a. Tnh t so bien ap k e trung bnh ap ra la 240 V goc kch be nhat ttnh toan = 15 O. b. Vi t so bien ap cua cau a, tnh ap ra khi = 45 O.va = 90 O. c. Trong ieu kien cau b, khi gan them diod phong ien Df, cac tr trung bnh nay tr nen bao nhieu. d. Trong ieu kien cau b, gia s cac song hai dong ien cao hn bac 1 bang khong, tnh hieu dung song hai dong khi = 15 O, = 45 O, = 90 O. Trong cac tnh toan khong e y sut ap do chuyen mach, bien ap va qua cac chnh lu.

Hng dan: a. Ap pha th cap la 102.5 V suy ra k = 0.47 . b. Vo (45 O) = 169.7 V, Vo (90 O) = 0 V. c. Ve dang ap ra, Df se dan ien khi vo 0, suy ra Vo (45 O) = 169.7 V, nhng ( ) = 3/2 2/26oo dwtwtsin238090V > 0 V d. Tnh bien o hai bac 1 ( w = 300Hz), suy ra dong. CAU HOI:

1. Hay trnh bay cac dang dong ra cua chnh lu ieu khien pha tai RL khi L thay oi t 0 ( tai thuan tr ) en khi L bang vo cung ( tai dong lien luc va phang ). Ve dang ap ra cua chnh lu ieu khien pha, s o 1 pha tai RL goc kch bang 60O khi L thay oi t 0 en khi L bang vo cung.

2. Chuyen mach la g? Tai sao goi chnh lu ieu khien pha la bo bien oi chuyen mach li.

3. Pham vi thay oi cua goc ieu khien pha bang bao nhieu.



Hng dan: Tai RL, [ 0, 180 O ]. Khi tai co sc phan ien, pham vi ieu khien giam xuong 4. Tac dung cua diod phong ien oi vi tai cam khang.

5. Ve dang xung kch SCR, ap ra cua s o chnh lu ieu khieu pha, s o 3 pha cau 6 SCR = 60O . Giai thch yeu cau can xung kep, co the s dung dang xung nao khac khong.

6. Trnh bay hai nguyen ly c ban e thc hien s o kch SCR cho BB ieu khien pha. Giai thch tai sao tan so nguon ien co anh hng ac tnh cua cac mach kch nay.

7. Hay cho biet ngan gon cac ng dung cua chnh lu va chnh lu ieu khien pha va ac iem cua chung.

8. Giai thch s o ieu khien mach kn hnh 3.39, tong quat hoa hai yeu cau ieu khien ap va dong cua bo bien oi .

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