kỹ thuật điện - ts lưu thế vinh

7/27/2019 K thut in - TS Lu Th Vinh

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BO GIAO DUC VA AO TAO

TRNG I HC A LAT

TS. LU THE VINH

KY THUAT IEN

a Lat 2006


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LI NOI AUGiao trnh ky thuat ien nham trang b cho ngi hoc nhng kien

thc c ban ve ng dung nang lng ien trong san xuat va i song. Sovi cac dang nang lng khac nang lng ien co nhng u iem het scto ln sau ay:

ien nang c san xuat tap trung vi nguon cong suat rat lnDe dang bien oi va truyen tai i xa nh may bien theDe dang bien oi sang cac dang nang lng khac.Nh ien nang co the t ong hoa toan bo qua trnh san xuat cung

nh cac dch vu ky thuat khac.

Viet Nam co tiem nang het sc to ln ve cac nguon nang lng,nhng do hau qua chien tranh keo dai, anh hng c che quan lieu baocap lam cho nen san xuat con kha lac hau. San lng ien nam 1975 canc ch co 1,5 ty kWh. Sau giai phong chung ta a cung co va xay dngthem nhieu nha may ien ln, Thuy ien Hoa Bnh vi cong suat 1.920MW, thuy ien Tr an (440MW), Nhiet ien Pha lai I (440MW), Nhietien Pha Lai II (600MW), thuy ien Ialy (720MW) , Hien nay angtrien khai xay dng nha may Thuy ien Sn la , d an nha may ien

nguyen t Bnh Thuan. Nam 2003 san lng ien ca nc a at 41 tykWh bnh quan 500kW/ au ngi nam. Theo lo trnh phat trien ti nam2010 se at 70 tykWh, nam 2020 at 170 ty kWh. e ap ng nhu cauphu tai ien en nam 2015 Viet nam se xay dng 61 nha may ien vitong cong suat 21.658 MW, trong o co 32 nha may thuy ien vi tongcong suat 7.975 MW, 17 nha may ien tuabin kh vi tong cong suat9.783 MW va 12 nha may nhiet ien than vi tong cong suat 3.900 MW.Hien nay ng truyen tai ien sieu cao ap 500 kV Bac Nam c xemla huyet mach chnh cua nang lng ien Quoc gia. Tuyen 500 kV thhai ang c xay dng. Toc o tang trng ien nang giai oan 2003 2010 la 15%. Von au t trung bnh 2,16 ty USD moi nam.

Nganh san xuat thiet b ien ang c au t phat trien. Cac maybien ap 110 kV, 25MVA va 63 MVA a va ang c san xuat hangloat. May bien ap 220 kV, 125 MVA au tien i vao san xuat t nam2004 tai cong ty thiet b ien ong Anh. Cac ong c ien cong suat ti1000 kW a c che tao tai cong ty che tao Viet Hung, cong ty che taoien c Ha Noi, Thu c,


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KY THUAT IEN

TS. Lu The Vinh

2

Giao trnh ky thuat ien c bien soan theo chng trnh khungao tao c nhan Vat ly cua Trng ai hoc a Lat bat au thc hien tnam 2001. Tai lieu c bien soan tren c s ngi hoc a hoc xong mon

ien t hoc, do o khong i sau vao mat ly luan cac hien tng ma chuyeu nghien cu cac phng phap tnh toan va cac ng dung ky thuat cuacac hien tng ien t.

Giao trnh c chia lam 2 phan vi 9 chng, trong o phan 1cung cap cac kien thc ve c s ly thuyet va cac phng phap tnh toanmach ien. Phan th 2 cung cap cac kien thc ve nguyen ly, cau tao, actnh va ng dung cac loai may ien c ban.

Tac gia bay to long biet n chan thanh en tap the bo mon ien t

t ong hoa, can bo Khoa Vat Ly a tao ieu kien e tai lieu choan thanh. V la tai lieu bien soan lan au nen chac chan con nhieuthieu sot, rat mong nhan c y kien ong gop t oc gia va cac banong nghiep.

TAC GIA


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Phan th nhat

C S LY THUYET&

CAC PHNG PHAP NGHIEN CU

MACH IENA

thn

B

R1R

a)

2

b)

20V

c)

B

th

th

R

E

A

J

B

R

A

E


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KY THUAT IEN4

TS. Lu The Vinh

Chng 1.NHNG KHAI NIEM C BAN VE MACH IEN

1.1. MACH IEN VA CAC PHAN T MACH.

1.1.1. Mach ien:

Mach ien la to hp cac thiet b ien bao gom nguon, phu tai c

noi vi nhau bang day dan theo mot cach thc nhat nh thong qua cacthiet b phu tr (hnh 1-1).

NGUONIEN

Thiet bphu tr

PHUTAI

Hnh 1-1

* Nguon ien: Ni san sinh ra nang lng ien e cung cap cho mach.Nguon ien co the la nguon mot chieu hoac xoay chieu.Nguon mot chieu: Pin, acquy, may phat ien mot chieu...Nguon xoay chieu: Lay t li ien, may phat ien xoay chieu.Cac nguon ien cong suat ln thng c truyen tai t cac nha

may ien (nhiet ien, thuy ien, ien nguyen t...).Cac nguon ien mot chieu thng c ac trng bang suat ien

ong E, ien tr noi r. Vi nguon xoay chieu thng bieu dien bang congsuat P (cong suat may phat) va hieu ien the loi ra U.

* Phu tai: La cac thiet b s dung ien nang e chuyen hoa thanh motdang nang lng khac, nh dung e thap sang (quang nang), chay cacong c ien (c nang), dung e chay cac lo ien (nhiet nang)... . Cacthiet b tieu thu ien thng c goi la phu tai (hoac tai) va ky hieubang ien tr R hoac bang tr khang Z.

* Day dan: Co nhiem vu lien ket va truyen dan dong ien t nguonien en ni tieu thu.


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KY THUAT IEN 5

TS. Lu The Vinh

* Cac thiet b phu tr: nh cac thiet b ong cat (cau dao, cong tac...),cac may o (ampeke, von ke, oat ke ), cac thiet b bao ve (cau ch,aptomat ... ).

1.1.2. Ket cau hnh hoc cua mach ien.

Mot mach ien phc tap baogom nhieu nhanh ket noi vi nhautao thanh cac mach vong khep kn(mat) giao ket tai cac nut.

R

Re

I

I

e 4

2

A

I1

1

2e

I

R

5

C

2

3

R

1

1

B

3

4

3

D4

e

R

Hnh 1-2

Nhanh: la mot phan cua machien, trong o cac phan t mach

mac noi tiep vi nhau sao cho cocung mot dong ien chay qua. Nut: la cho giao nhau cua cac

nhanh.Mat: la mot mach vong khep

kn lien ket nh cac nhanh.V du: ach ien tren hnh 1-2 gom 5 nhanh AB, AC, CB, CD va BD

ket noi vi nhau tao thanh 4 nut A, B, C va D. Cac mach vong khep kntao thanh cac mat (ACBA), (BCDB) va (ACDBA).

1.1.3. Cac ai lng ac trng qua trnh nang lng trong mach ien.

Nang lng ien tac dung trong mach c ac trng bang cac ailng la dong ien i va ien ap u hoac bang cong suat tac dung p =ui.

* Dong ien. Dong ien i chay trong mach co tr so bang toc o bienthien cua ien tch qua tiet dien ngang cua vat dan.

dqi

dt= (1-1)

i

S

A B

uAB Hnh 1-3Chieu dong ien c quy c ngc vi chieu chuyen ong cua

cac electron (hnh 1-3).


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KY THUAT IEN6

TS. Lu The Vinh

* ien ap u. Tai moi iem trong mach ien co mot ien the. Hieuien the gia hai iem goi la ien ap u. Chang han hieu ien the giahai iem A va B tren hnh 1.3 c goi la ien ap uAB.

uAB = A - B (1-2)Chieu ien ap c quy c la chieu t iem co ien the cao en

iem co ien the thap.

* Cong suat p. Tren cac oan mach ien, cac phan t co the nhan hoacphat nang lng. Khi chon chieu dong ien va ien ap tren nhanh trungnhau (hnh 1.3) ta co cac qua trnh nang lng sau:

Neup = ui > 0 - nhanh nhan nang lng.

Neup = ui < 0 - nhanh phat nang lng.Khi chon chieu cua dong ien va ien ap ngc nhau ta se co cac

ket luan ngc lai.

Trong he n v SI n v dong ien la ampe (A), n v ien ap lavon (V), n v cua cong suat la oat (W).

1.2.MO HNH MACH IEN

e tien li khi tnh toan thiet ke va khao sat cac qua trnh ien t

xay ra trong mach ien ngi ta s dung phng phap mo hnh. Machien thc te vi cac thiet b ien c thay the bang mo hnh mach vicac phan t ly tng ac trng cho mot qua trnh nao o. Mo hnh machcha cac phan t tch cc (active): nguon ap u(t), nguon dong i(t) va cacphan t thu ong (passive): ien trR, ien camL va ien dung C.

1.2.1. Nguon ap u(t).

Nguon ap u(t) hay may phat ien ap con c goi la nguon sc

ien ong e(t) ac trng cho kha nang tao nen va duy tr mot ien apkhong oi tren hai cc cua nguon.

ac tnh quan trong cua nguon ap la co ien tr noi r= 0, hieuien the tren hai cc cua nguon la khong oi va khong phu thuoc vao giatr cua phu tai. Ky hieu quy c cua nguon ap nh hnh 1-4, a. Ta co giatr cua nguon ap:

u(t) = - e(t) (1-3)

1.2.2. Nguon dong ien i(t).


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KY THUAT IEN 7

TS. Lu The Vinh

Nguon dong ien i(t) hay may phat dong ac trng cho kha nangtao nen va duy tr mot dong ien khong oi trong mach. ac tnh quantrong cua nguon dong la co noi tr r = va gia tr cua dong ien trong

mach khong phu thuoc vao phu tai. Ky hieu quy c cua nguon dong chra tren hnh 1-4, b.

Trong thc te, cac bo nguon eu co mot ien tr noi hu han naoo. Do vay, khi thay the trong mo hnh mach chung c bieu dien dang mot nguon sc ien ong e(t) mac noi tiep vi mot mot ien tr r(hnh 1-4, c), hoac dang mot nguon dong ien i (t) mac song song vimot ien tr r(hnh 1-4, d).

e(t) u(t) i(t)e(t)

r

i(t) r

a) b) c) d)

Hnh 1- 4 . Ky hieu quy c nguon ap va nguon donga, b Nguon ap va nguon dong ly tng.c, d Nguon ap va nguon dong thc te

1.2.3. ien tr R.

ien trR ac trng cho vat dan ve mat can tr dong ien. Ve matnang lng ien trR ac trng cho qua trnh tieu thu ien nang va bienien nang thanh cac dang nang lng khac nh nhiet nang, quang nang,

c nang, Quan he gia dong ien va ien ap tren ien tr la:

uR = R.i (1-4)

Cong suat thoat ra tren ien tr:

p = R.i2 (1-5)

Trong he n v SI n v ien tr la om ().

1.2.4. ien cam L.


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KY THUAT IEN8

TS. Lu The Vinh

Mot cuon day co dong ien i chay qua se sinh ra t trng. Tthong gi qua n vong cua cuon day la = n.. ien cam cua cuon dayc nh ngha la:

. = = nLi i

(1-6)

Khi dong ien bien thien trong cuon day xuat hien mot sc ienong t cam eL.

L

de

dt dt

diL

= = (1-7)

Quan he gia dong ien va ien ap tren cuon cam:1

;L L Ldi

u e L i u

dt L

= = = dt

(1-8)

Cong suat tren cuon day:

L L

dip u i L i

dt= = (1-9)

Nang lng t trng tch luy trong cuon day:

2

0 0

1

2

t I

LW p dt Lidi LI = = = (1-10)

Trong he n v SI n v ien cam la henri (H).

1.2.5. ien dung C.

Khi noi hai au cua mot tu ien co ien dung Cvao nguon ien apu, tu ien se c tch ien. o ln cua ien tch q:

q = C u (1-11)Quan he gia dong ien va ien ap tren tu ien la:

( )dq d du

i Cudt dt dt

= = = C (1-12)

va: 1u i dt C

= Nang lng tch luy tren tu ien:

2

0

1

2

u

u du C u= =W C (1-13)

Trong he n v SI n v ien dung la fara (F).

1.2.6. Mo hnh mach.


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KY THUAT IEN 9

TS. Lu The Vinh

Mo hnh mach la s o thay the tng ng cac phan t machbang cac phan t mo hnh ly tng e, i, R, L, C sao cho ket cau hnh hocva cac qua trnh nang lng xay ra trong mach giong nh mach ien

thc.e thiet lap mo hnh mach ta phan tch cac qua trnh nang lng

xay ra trong tng phan t mach va thay the chung bang cac phan ttng ng. Khi phan tch can chu y rang, tuy thuoc vao ieu kien lamviec cua mach ien, ac biet la dai tan cong tac ma s o thay the sekhac nhau.

V du. Ta hay xet mot mach ien thc te gom mot may phat cungcap ien cho phu tai la mot bong en mac song song vi mot cuon day

theo s o hnh 1-5, a. Khi chuyen sang s o thay the oi vi dong ienxoay chieu, may phat ien c thay the bang (Ef ,Lf ,Rf). Phu tai la bongen thay the bangRz, con cuon day bang (Ld , Rd) (hnh 1-5, b). Tuy nhienkhi chuyen sang s o thay the oi vi ien mot chieu, do cac phan tkhang bang khong nen s o thay the co dang n gian hn (hnh 1-5, c).

1.3. PHAN LOAI VA CAC CHE O LAM VIEC CUA MACH IEN.


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KY THUAT IEN10

TS. Lu The Vinh

1.3.1. Phan loai theo tnh chat cua dong ien trong mach.

Theo tnh chat cua dong ien trong mach ngi ta chia ra 2 loaimach ien mot chieu va mach ien xoay chieu.

Mach ien mot chieu la mach ien c tac dung bi nguon ienap mot chieu trong mach. Dong ien chay trong mach co tr so vachieu khong oi theo thi gian.

Mach ien xoay chieu la mach ien c tac dung bi nguon ienap xoay chieu trong mach, thng la cac nguon ien ap bien thientheo quy luat hnh sin. Dong ien chay trong mach co tr so vachieu thay oi tuan hoan theo thi gian.

1.3.2. Phan loai theo tnh chat cac thong so R, L, C cua mach.

Theo tnh chat cac thong so R, L, C cua mach ngi ta chia ra 2loai mach ien tuyen tnh va mach ien phi tuyen.

Mach ien tuyen tnh khi tat ca cac phan t mach la tuyen tnh.Ngha la gia tr cua cac phan t R, L, C khong thay oi va khongphu thuoc vao dong ien va ien ap tren chung.

Mach ien phi tuyen khi co cha cac phan t phi tuyen. Ngha lagia tr cua cac phan t R, L, C cua cac phan t phi tuyen thay oi

phu thuoc vao dong ien va ien ap tren chung.Trong giao trnh nay chu yeu chung ta nghien cu va khao satmach ien tuyen tnh.

1.3.3. Phan loai theo qua trnh nang lng trong mach.

Theo qua trnh nang lng trong mach ngi ta chia ra 2 loai cheo xac lap va che o qua o.

Che o xac lap xay ra oi vi cac qua trnh a on nh. che oxac lap dong ien va ien ap tren cac nhanh cua mach ien bienthien theo cung mot quy luat cua nguon cung cap. oi vi machien mot chieu: dong va ien ap tren cac phan t mach a on nh(khong oi). oi vi mach ien xoay chieu: dong va ien ap trencac nhanh bien thien theo quy luat sin vi thi gian.

Che o qua o la qua trnh chuyen tiep t trang thai xac lap naysang trang thai xac lap khac. Che o qua o xay ra trong cac machien co cha cac phan t khang L, C cac thi iem ong mach


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KY THUAT IEN

TS. Lu The Vinh

11

va ngat mach. Thi gian qua o thng rat ngan va phu thuoc vao

gia tr ien khang cua mach.1.3.4. Phan loai cac bai toan ve mach ien.Khi nghien cu ve mach ien co hai dang bai toan c ban la bai

toan phan tch va bai toan tong hp.

Bai toan phan tch. Cho trc ket cau hnh hoc va cac thong so cuamach ien. Can phai tm dong ien, ien ap va cong suat tren cacnhanh va cac phan t mach.

Bai toan tong hp. La bai toan ngc lai can phai tm cach thietke mot s o mach thch hp e thoa man cac yeu cau at ra vedong, ien ap va cong suat.

Trong giao trnh nay chu yeu chung ta khao sat bai toan phan tch cacmach ien tuyen tnh che o xac lap.


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Chng 2.

DONG IEN XOAY CHIEU HNH SIN

Dong ien xoay chieu hnh sin la dong ienxoay chieu bien thien ieu hoa theo quy luat hamsin vi thi gian I = I0 sin (t + ) .

Dong ien xoay chieu hnh sin c ng dungrong rai trong nhieu lnh vc khac nhau cua ien kythuat. Nang lng ien trong hau het cac trnghp eu c san xuat, phan phoi va tieu thu didang ien xoay chieu. ieu o c giai thch binhng u iem cua dong xoay chieu la de dangtruyen tai, de dang bien oi va kinh te.


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KY THUAT IEN 13

TS. Lu The Vinh

2.1. CAC AI LNG AC TRNG CUA DONG IEN HNH SIN.

Dong ien xoay chieu hnh sin la dong ien bien thien ieu hoatheo quy luat ham sin:

i = I0 sin (t+ i) (2-1)u = U0sin (t+ u) (2-2)

Trong o: i, u la gia tr tc thi cua dong ien va ien apI0, U0 la tr cc ai (bien o) cua dong ien va ien ap.(t + i), (t + u) la goc pha (hay goi tat la pha) cua dong ien

va ien ap, cho phep xac nh tr so cua dong ien va ien ap thiiem t.

i va u la pha ban au (t= 0). Co gia tr phu thuoc vao goc thigian ma ta chon.

la tan so goc cua dong ien hnh sin, n v la rad/s.Chu ky Tla khoang thi gian ngan nhat e dong ien lap lai tr so

va chieu bien thien. Ngha la trong khoang thi gian Tgoc pha bien thienmot lng la T = 2.

So chu ky cua dong ien trong mot giay goi la tan sof.1

2f

T

= =

n v tan so la hec (Hz).

Nh vay, co the viet:2

2 fT

= = (2-3)

Trong cong nghiep, dong xoay chieu co tan sof = 50Hz.

2.2. TR HIEU DUNG CUA DONG IEN XOAY CHIEU HNH SIN

oi vi dong ien xoay chieu hnh sin, do gia tr tc thi bien thien

mot cach lien tuc, nen trong thc te ngi ta quan tam en gia tr hieu dungcua no. Gia s xet tac dung cua dong ien xoay chieu i tren mot ien trR.Cong suat tac dung c tnh:

2

0 0

1 1T T

P i R dt R i d T T

= = 2 t (2-4)

Vi dong ien mot chieu cong suat tieu tan tren ien trR la:

P = R I2 (2-5)


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KY THUAT IEN14

TS. Lu The Vinh

Neu trong cung mot thi gian, cong suat tac dung nhiet tren ien trR oi vi ca hai dong ien la nh nhau ta co:

2 2

0

1 TP R i dt R I

T= =

T o: 2

0

1T

I iT

= dt (2-6)

Gia trItnh theo (2.6) c goi la tr hieu dung cua dong ien xoaychieu.

Neu thay i =I0 sin tvao (2.6) ta co:

22 2 0 00

0 0

1sin (1 cos2 )

2 2

T TI I

I I tdt t dtT T

= = = (2.7)

Tng t, ta c tr so hieu dung cua ien ap va sc ien ong:

0

2

UU= (2.7,a)

0

2EE= (2.7,b)

Nh vay, co the viet lai (2.1) va (2.2) nh sau:

i = I 2 sin (t + i) (2.8)

u = U 2 sin (t + u) (2.9)

Tr hieu dung thng c ghi tren cac dung cu va cac thiet b tieu

thu ien, cho ta biet cap ien ap s dung va dong ien cho phep. V du(220V-10A).

2.3. BIEU DIEN DONG IEN XOAY CHIEU HNH SIN

e bieu dien dong ien xoay chieu hnh sin co the s dung cacphng phap toan hoc khac nhau:

Bieu dien bang phng trnh lng giac thong qua cac ai lng actrng: tr tc thi, tr bien o, tr hieu dung, tan so, chu ky, goc pha. (cacphng trnh (2.1), (2.2) va (2.8)(2.9)).


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KY THUAT IEN 15

TS. Lu The Vinh

Phng phap o th dang song: bieu dien o th cua cac phng trnh(2.1) va (2.2).

Phng phap gian o vec t quay: bieu dien thong qua tr hieu dung(hoac bien o) va goc pha .

Bieu dien bang phng phap so phc.2.3.1.Phng phap gian o vec t.

e tien li trong viec tnh toan cac ai lng hnh sin cung tan so,ngi ta thng s dung phng phap gian o vec t Fresnel. Theo phngphap nay cac ai lng hnh sin co cung tan so c bieu dien bang cac vect co o ln (moun) bang tr hieu dung va goc pha ban au xac nh olech gia vec t vi truc nam ngang Ox.

V du. Hnh ve (2-1, a) la gian o vec t bieu dien dong ien va ien aptren mot oan mach co phng trnh sau:

i = I 2 sin (t + i )

u = U 2 sin (t - u )

Sau khi a bieu dien cac ai lng dong ien va ien ap bang vec t,hai nh luat Kirchhoff c viet lai di dang vec t nh sau:

0kI

U E

=

=

uur

uur uur (2.10)

Da vao gian o vec t va cac nh luat Kirchhoff ta co the giai machien xoay chieu mot cach thuan tien.


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KY THUAT IEN16

TS. Lu The Vinh

2.3.2.Phng phap so phc.a) oi vi cac mach ien phc tap, phng phap vec t co nhieu han che.

e giai mach ien hnh sin che o xac lap ngi ta thng dung phngphap so phc.

Bieu dien phc cua ai lng hnh sin co c khi thay truc Ox trengian o vec t (hnh 2-1, a) bang truc thc +1, thay truc Oy bang truc ao +j.Nh vay ta a thc hien viec bieu dien ai lng hnh sin bang so phctrong toa o phc (hnh 2-1, b).

So phc bieu dien cac ai lng hnh sin c ky hieu bang cac chcai in hoa co dau cham tren au. Bieu dien phc co the di dang ham mu,

dang lng giac hay ai so.V du. Viet cac dang bieu dien phc cua dong ien i = 10 2 sin (t300) va

ien ap u = 200 2 sin (t+ 600).

Dang ham mu: (2-11)03010ij jI I e e = =&

060200ujU U e e= =& j

0

Dang ham mu con c viet di dang:

(2-12)10 30iI I = = o&

200 6uU U = = o&Dang lng giac hay ai so:

= Icos i +j Isin i = 10 cos (-300) + j 10 sin (-300)I&

I& = 5 3 -j 5 (2-13)

U= Ucos u+j Usin u = 200 cos 600 + j 200 sin 600&

U& = 100 +j 100 3

trong o:Icos i, Ucos u la phan thc cua so phcj Isin i,j Usin u la phan ao cua so phc.

b) Nhac lai mot so phep tnh oi vi so phc.

1. Cong, tr. Bien oi cac so phc ve dang ai so, sau o cong (tr) phanthc vi phan thc, phan ao vi phan ao.

V du: (2 + j 6) + ( 3 j 2) = (2 + 3) + j (6 2) = 5 + j 4( 4 + j 5) ( 2 +j 3g = (4 2) + j (5 3) = 2 +j 2

2. Nhan, chia. Khi nhan chia hai so phc ta nen a ve dang mu.

V du:0 0 0 060 45 (60 45 ) 1055 .10 5.10 50j j j je e e e+= =

0


19/176

KY THUAT IEN 17

TS. Lu The Vinh

00 0 0

0

45(45 30 ) 15

30

200 20040

55

jj j

j

ee e

e

= =

4. Nhan so phc vi e

j

. ( ).j j jA e e A e =

Ngha la khi nhan so phc vi e j ta quay vec t bieu dien so phc ayi mot goc ngc chieu chieu kim ong ho, khi nhan vi e - j ta quay vect i mot goc cung chieu kim ong ho.

5. Nhan so phc vi j .Theo cong thc le :

2 cos sin

2 2

je j

j= + =

2 cos ( ) sin ( )2 2j

e j

j= + =

Nh vay, khi nhan mot so phc vi j ta quay vec t bieu dien so phco i mot goc /2 ngc chieu kim ong ho, neu nhan vi j ta quay vec tcung chieu kim ong ho mot goc /2.

c) Tong tr phc.

Tong tr phc c nh ngha theo bieu thc nh luat Ohm didang phc:

( )u

u i

i

jj j

j

U U e U Z e

I II eZ e

= = = =&

&(2-14)

ay la dang ham mu cua tong tr phc.Ta co:

(cos sin ) cos sinjZ Ze Z j Z jZ = = + = +

R

X

Z

Hnh 2-2

T tam giac tong tr tren hnh 2-2 ta co:Zcos = R la ien tr hoat ong cua machZsin = X la ien khang cua mach.

Do o:

1(jZ Z e R j X R j L

C

= = + = + ) (2-15)

oi vi nhanh thuan tr:0jZ R R e= = (2-16)

oi vi nhanh thuan cam:X = XL , do o:


20/176


21/176

KY THUAT IEN 19

TS. Lu The Vinh

2 cos( ) 2 sin( )2i i

diI t I t

dt

= + = + +

Bieu dien phc tng ng cua i :( / 2) / 2' i ij jjI I e e I e += =

Biet / 2je = j

I

, do o:

' ijI j I e j = =& & (2-24)

Nh vay, ao ham theo thi gian cua dong ien tng ng vi phepnhan dang phc vi tha soj.

Neu ,di

i I th j I dt

& &

(2-25)

g) Bieu dien tch phan .idt

Xet dong ien 2 sin( )ii I t = + co bieu dien phc la ijI I e =&

Lay tch phan:

12 sin( ) 2 cos( )i ii dt I t dt I t

= + = + 1

2 sin( )2

iI t

= +

Bieu dien phc tng ng cua tch phan nay la:

( / 2) / 21 1i ij jj Ii dt I e e I ej

= &

=

Nh vay, Tch phan theo thi gian cua dong ien tng ng vi viphep chia dang phc choj.

Neu ,I

i I th idt

j

&& (2-26)

2.4. PHAN NG CUA CAC PHAN T MACH R,L,C OI VIDONG IEN XOAY CHIEU.

2.4.1. Mach thuan tr R.

Xet mach thuan ien tr R. khi cho dong ien i = I0 sin tchay qua,(hnh 2-3, a) . Hieu ien the gia hai au mach se la:

0 0sin sinR Ru Ri R I t U t = = =


22/176


23/176

KY THUAT IEN 21

TS. Lu The Vinh

V cong suat tc thi khong co y ngha thc tien nen ta dung khainiem cong suat trung bnh P, la gia tr trung bnh cua cong suat tc thi trongmot chu ky:

2

0 0

1 1( ) (1 cos2 )

T T

R R RP p t dt U I t dt U I RT T= = = I= (2-29)

2.4.2. Mach thuan ien cam L .

Khi cho dong ien i = I0sin tchay qua oan mach thuan cam L(hnh 2-4, a) . Hieu ien the gia hai au mach se la:

0 0 0

0

( ) ( sin ) cos sin( )2

( ) sin( )2

L

L L

di du t L L I t LI t LI t

dt dt

u t U t

= = = = +

= +

trong o: 0 0L LU LI X 0I= =

0

2L

L LU

U X= = I

XL = L co th nguyen cua ien tr, o bang goi la cam khangcua cuon day. T o quan he gia tr hieu dung cua ien ap va dongien la:

LL L

L

UU X I hoac I

X= = (2-30)

Dong ien va ien ap co cung tan so song ien ap nhanh pha hndong ien mot goc la /2.

Cong suat tc thi tren ien cam L:

0 0

0 0

sin( / 2)sin1

sin2 sin22

L L L

L L

p u i U I t t

U I t U I t

= = +

= =(2-31)

Cong suat trung bnh:

0 0

1 1( ) sin2 0

T T

L L LP p t dt U I t dt T T

= = = (2-32)


24/176


25/176

KY THUAT IEN 23

TS. Lu The Vinh

0 0

0

1 1 1( ) sin cos

( ) sin( )2

C

C C

u t idt I t dt I t C C C

u t U t

= = =

=

Trong o:

00 0 0

1;

2C

C C CU

U I X I U X C

= = = = CI (2-33)

XC = 1/ C co th nguyen cua ien tr, o bang va goi la dung

khang cua tu ien.

Quan he gia tr so hieu dung cua dong ien va ien ap la:, hoac CC C

C

UU X I I

X= = (2-34)

Cong suat tc thi tren ien dung:

0 0( ) sin sin( / 2) sin2C C C C p t u i U I t t U I t = = = (2-35)

Cong suat trung bnh:

0 0

1 1( ) sin2 0

T T

C C CP p t dt U I tdt T T= = = (2-36)

o th bieu dien cac gia tr tc thi cua dong ien, ien ap va congsuat tren ien dung C bieu dien tren hnh 2-5, c. Ta co nhan xet sau:

Trong mach thuan ien dung C, dong ien va ien ap co cung tanso, song dong ien nhanh pha hn ien ap mot goc la /2.

Trong mach co hien tng trao oi nang lng gia ien dung vicac phan mach con lai, do o cong suat tac dung trung bnh trongmach la bang khong. ien dung khong tieu thu nang lng.

e bieu dien qua trnh trao oi nang lng trong mach ngi ta ara khai niem cong suat phan khang QCcua ien dung:

QC = - UCI = - XCI2 (2-37)

n v cua cong suat phan khang la VAR hoac kVAR = 103 VAR.


26/176

KY THUAT IEN24

TS. Lu The Vinh

I

ur

Ci

uC

CU

uuur

uC

pC

i

0 2 /2 t

a ) b)

c)

Hnh 2 - 5

2.4.4.Mach R, L, C mac noi tiep.Khi cho dong ien i = I0sin tchay qua oan mach R, L, C mac noi

tiep(hnh 2-6, a). Phan ng cua cac phan t mach nh a xet tren, tc lahieu ien the tren cac phan t R, L, C lan lt la:

0 sin 2 sin ;R R R

R

u U t U t

U R I

= =

=

0( ) sin( ) 2 sin( )2 2;

L L L

L L L

u t U t U t

U X I X L

= + =

= =

+

0( ) sin( ) 2 sin( )2 2

1;

C C C

C C C

u t U t U t

U X I X C

= =

= =

Hieu ien the hai au mach se la:

2 sin( )R L Cu u u u U t = + + = + (2-38)

Trong o gia tr cua Uva c xac nh bang phng phap vec t:

R L CU U U U = + +uur uuur uuur uuur


27/176

KY THUAT IEN 25

TS. Lu The Vinh

CA

R

U

BU

u

LUUC

a)

R

U

u

U

u Z

X

u

R

c)

R

L

L

b)

L

C

C

AB

I

T o th vec t ta co:

2 2 2 2( ) ( )R L C L CU U U U I R X X I = + = + = Z (2-39)

Trong o: 2 2 2( )L CZ R X X R X= + = +2 = tong tr cua mach.

L CX X X= = ien khang cua mach. (2-40)

T cong thc (1-40), ta thay: ien trR, ien khangXva tong trZhp thanh ba canh cua mot tam giac vuong (hnh 2-6, c) goi la tam giactong tr. Nh tam giac tong tr ta de dang xac nh c moi lien he

gia cac ai lngR, L, Z, goc lech pha .

L C L C

R

U U X X Xtg

U R

R

= = = (2-41)

Phu thuoc vao o ln va dau cua goc lech pha ta co cac trnghp sau:

Khi > 0, ,L C LX X> CU U > , mach co ac tnh cam khang.Dong ien cham pha hn ien ap mot goc (hnh 2-7, a).

Khi < 0, ,L C LX XCU U< < , mach co ac tnh dung khang.Dong ien nhanh pha hn ien ap mot goc (hnh 2-7, b).

Khi = 0, ,L C LX X= = CU U , tg = 1. Tong tr cua mach lucnay cc tieu: Z =R =Zmin . Dong trong mach at gia tr cc ai,trong mach xay ra hien tng cong hng noi tiep (hnh 2-7, c).

maxU

I IR

= = (2-42)


28/176

KY THUAT IEN26

TS. Lu The Vinh

Hnh 2-7

Khi xay ra hien tng cong hng, dong trong mach trung pha viien ap hai au oan mach. NeuXL = XC >> R th tr hieu dung cua ienap tren cac phan t L va C co the ln hn ien ap Unhieu lan, do o conghng con goi la cong hng ien ap.

ieu kien xay ra cong hng ien ap la:

01 1

, tan soLC LC

= = = (2-43)

0 goi la tan so dao ong rieng cua mach.

2.5. CONG SUAT CUA DONG IEN XOAY CHIEU.Trong trng hp tong quat, khi tai cua mach ien xoay chieu bao

gom cac phan t R, L, C, trong mach se xay ra hai qua trnh nang lngsau:

Qua trnh tieu thu ien nang va bien oi sang cac dang nang lngkhac (chu yeu la nhiet nang) tren cac phan t ien tr hoat ong R cuamach. Qua trnh nay c ac trng bang cong suat tac dung P.

Qua trnh trao oi, tch luy va giai phong nang lng ien t trngtren cac phan t ien camL va ien dung Ccua mach. Qua trnh nay cac trng bang cong suat phan khang Q.

2.5.1. Cong suat tac dung P.Cong suat tac dung P la cong suat tieu thu tren ien tr R cua

mach. Theo (2-29) P co gia tr bang cong suat tieu thu trung bnh trongmot chu ky:

2

0

1( )

R

T

RP p t dt U I R

T

= = I= (2-44)


29/176

KY THUAT IEN 27

TS. Lu The Vinh

Theo gian o vec t (hnh 2-5, b) ta co: UR = Ucos, do o ta co:

P = R I2= UIcos (2-45)

Trng hp mach co nhieu ien tr hoat ong th cong suat tac dungtren toan mach bang tong cac cong suat tieu thu rieng re tren tng ien tr:2

kR k kk k

P P R= = I (2-46)

2.5.2. Cong suat phan khang Q.

Cong suat phan khang Q dung e ac trng cho qua trnh trao oi,tch luy nang lng ien t trng tren cac phan t ien cam L va iendung C cua mach.

Q = X I2 = (XL XC)I2 (2-47)

Theo (2-32) va (2-37) ta co the viet:

Q = XLI2 XCI

2 = QL + QC (2-48)

Trng hp mach gom nhieu phan t L va C th cong suat phankhang cua mach bang tong cong suat phan khang tren cac phan t rieng re:

2m n m

2nL C L m

m n m n

Q Q Q X I X = + = C nI (2-49)

Theo hnh (2-5, c) ta coX=(XL XC) =Zsin , do o ta co the viet:

Q = I2 Zsin = UIsin (2-50)

2.5.3. Cong suat bieu kien S.

e ac trng cho kha nang cua thiet b va nguon thc hien hai quatrnh nang lng noi tren ngi ta a ra khai niem cong suat toan phan haycong suat bieu kien S:

2S U I P Q= = + 2 (2-51)

Nh vay, co the viet lai (2-45) va (2-50) nh sau:

P = UIcos = Scos

Q = UIsin = Ssin

Ta thay cc ai cua cong suat tac dung P (khi cos = 1) co giatr bang cong suat bieu kien S. Cc ai cua cong suat phan khang Q (khisin= 1) bang cong suat bieu kien S. Nh vay Scho biet kha nang cua thiet


30/176

KY THUAT IEN28

TS. Lu The Vinh

b ien. Gia tr nh mc cua cong suat bieu kien S thng c ghi trennhan cua cac may ien.

Moi quan he gia P, Q va Sc thehien bang ba canh cua mot tam giac vuong(hnh 2-8), goi la tam giac cong suat.

P, Q va Sco cung th nguyen, nhnge phan biet ngi ta dung cac n v khacnhau: n v cua P: W, kW, MW. n v cua Q: VAR,kVAR, MVAR.

n v cua S: VA, kVA, MVA.2.5.4. Nang cao he so cong suat cos.

Trong bieu thc cong suat tac dung P = UIcos , cos c goi lahe so cong suat. Gia tr cua cos phu thuoc vao cac thong so cua machien. Trong machRLCmac noi tiep ta co:

2 2cos

( )L C

R R

Z R X X = =

+ (2-52)

hoac : 2 2cosP P

SP Q = =+ (2-53)

He so cong suat la ch tieu rat quan trong, co y ngha rat ln vekinh te nh sau:

Nang cao he so cong suat se tang c kha nang s dung congsuat cua nguon (may phat ien, may bien ap ) cung cap cho phu tai.Chang han, mot may phat ien co cong suat nh mc la Sm =10000kVA, neu he so cong suat cua tai cos = 0,5 th cong suat tac dung

cua may phat cho tai P = Sm cos = 10000.0,5 = 5000 kW. Neu cos=0,9 th cong suat tac dung P = 10000.0,9 = 9000 kW. Nh vay khi coscang cao, cong suat phat ra cang nhieu hn.

Khi can truyen tai mot cong suat P nhat nh tren ng day, dongien chay tren day la:

cos

PI

UI =

Neu cos cang cao th dong I cang nho, lam giam ton that ien

nang tren day.


31/176

KY THUAT IEN

TS. Lu The Vinh

29

Trong cong nghiep va ien dan dung tai thng co ac tnh camkhang (cuon day ong c ien, may bien ap, ) do vay cos thap. enang cao he so cong suat cos ngi thng mac song song vi tai mot

tu ien bu (hnh 2-8, a).

Hnh 2-8

Khi cha mac tu ien bu, dong chay tren ng day la I1, he socong suat cua mach la cos1.

Khi mac them tu ien bu song song vi tai, dong chay tren ngday laI, he so cong suat cua mach la cos.

1 CI I I= +ur uuur uuur

T o th hnh (2-7, b) ta thay dong ienIgiam cos tang len:

I < I1 ; < 1 va cos > cos1Gia tr ien dung cua tu bu c tnh nh sau:V cong suat tac dung cua tai la khong oi nen cong suat phan

khang cua mach la:

Khi cha bu: Q1 = P tg1

Khi mac tu bu: Q = Q1 + QC = P tg 1 + QC= P tgT o: QC= - P (tg1 tg ) . (2-54)

Mat khac, cong suat QC cua tu ien c tnh:

QC= - UCIC= -U.U.C = - U2C (2-55)

So sanh (2-54) va (2-55) ta co:

12(tg tg )

PC

U

= (2-56)


32/176

Chng 3.

CAC PHNG PHAP PHAN TCH

VA GIAI MACH IEN

e phan tch va giai mach ien co nhieu phng phap

khac nhau. Viec la chon phng phap nao la tuy thuoc vaotng s o mach cu the.

oi vi cac oan mach n gian co the ap dung nhluat Ohm e tnh toan. Vi cac mach ien phc tap th hai nhluat Kirchhoff la c s e giai quyet bai toan.

Trong phan ln cac trng hp, e phan tch machchung ta phai tien hanh cac phep bien oi tng ng can

thiet ea s o mach phc tap ve cac s o n gian hn.Trong chng nay chung ta se khao sat cac phep bien oitng ng nh: bien oi tr khang song song, noi tiep, bienoi sao tam giac. Ngoai ra, trong mot so trng hp chungta se dung cac nh ly Thevenin, nh ly Norton e thc hiencac phep bien oi mach.

Khi nghien cu mach ien xoay chieu hnh sin ta sdung phng phap vec t va bieu dien phc e viet cac phng

trnh mach. oi vi cac mach ien mot chieu che o xac lap,co the xem nh trng hp rieng cua dong ien hnh sin (vitan so = 0). Khi o cac phan t ien khang cua mach sekhong co tac dung lam cho s o mach se n gian hn nhieu.


33/176

KY THUAT IEN 31

TS. Lu The Vinh

3.1. CAC PHEP BIEN OI TNG NG.

3.1.1. Tong tr mac noi tiep.

Gia s co cac tr khang mac noi tiep vi nhau, tongtr tng ng cua mach se la:1 2, ,..., nZ Z Z

(3-1)=

= + + + + + = t 1 21

... ...n

i ni

Z Z Z Z Z iZ

Trong o cac tr khang co the la ien tr hoat ong R, cam

khangXL hoac dung khangXC. Nh vay co the viet di dang:(3-2)

iZ

= +t t t Z R jX

Trong o: . (3-3)= = ;t t L C R R X X XV du: Tnh tong tr tng ng cua oan mach sau (hnh 3-1).

C1

Z

LR3 2R 21

RL

t d

Hnh 3-1

Ap dung cac cong thc (2.2) va (2.3)cho mach hnh 3.1 ta co:

t ( ) (4 3 1) (2 5 6) (8L CZ R j X X j j= + = + + + + = + 3) 3.1.2. Tong tr mac song song.

Tong tr tng ng cua cac nhanh mac song song co gia tr:

1 2 1

1 1 1 1...

n

t n iiZ Z Z Z == + + + =

1

Z (3-4)

Hoac tong dan tng ng:(3-5)= + + + = 1 2 ...t n i

i

Y Y Y Y Y

Vi trng hp co 2 nhanh:

(3-6)=+1 2

1 2t

Z ZZ

Z Z

V du. Tnh tong tr tng ng cua mach ien hnh 3-2, trong o:


34/176

KY THUAT IEN32

TS. Lu The Vinh

Z1 = 1+j1;Z2 = 1+j1.

Z

Z

Z1

2

t d

Hnh 3-2

Ta co:+

= = =+ + + 1 2

1 2

(1 1)(1 1)1

(1 1 1 1)tZ Z j j

ZZ Z j j

3.1.3. Bien oi tng ng sao tam giac. (hnh 3-3)

Goi: 1 2 3, ,Z Z Z la tong tr cua cac nhanh noi hnh sao.

12 23 31, ,Z Z Z la tong tr cua cac nhanh tam giac tng

ng, ta co cac cong thc bien oi sau:a) T sao sang tam giac:

= + +

= + +

= + +

1 212 1 2

3

2 323 2 3

1

3 131 3 1

2

Z ZZ Z Z

Z

Z ZZ Z Z

Z

Z ZZ Z Z

Z

(3-7)

I1

.

3

2

.

Z

.

12

Z

1

1

3

1

.

23

3

2

I.

3

Z

.

3

Z

I

I

2

31

1

Z

Z

2

I 2

I

a) b)

Hnh 3-3. Bien oi sao tam giac


35/176

KY THUAT IEN 33

TS. Lu The Vinh

b) T tam giac sang sao.=

+ +

=+ +

=

12 311

12 23 31

23 122

12 23 31

31 233

Z ZZ

Z Z Z

Z ZZ

Z Z Z

Z ZZ

+ +12 23 31Z Z Z

(3-8)

Ta co the chng minh lai cac cong thc (3-7) va (3-8) cua phepbien oi tng ng. That vay, phep bien oi phai thoa man ieu kienla khong c lam thay oi dong ien va ien ap tai cac iem noi vi

mach hnh sao hoac tam giac. Ngha la cac dong ien 1 2 3, ,I I I

va cac

ien ap trong ca hai s o phai bang nhau .12 23 31, ,U U U

Gia s 1 0I =

, theo s o hnh sao (3-3, a) ta co:

2 323 2 (U I Z Z = +

)

Theo s o hnh tam giac (2-3, b) ta co:

12 31 2323 2[( ) // ]U I Z Z Z = +

Suy ra:12 31 23

2 3

12 23 31

( )Z Z ZZ Z

Z Z Z

++ =

+ +(a)

Tng t, neu cho 2 0I =

, theo hnh sao ta co:

3 131 3 ( )U I Z Z = +

Theo s o tam giac: 12 23 3131 3[( ) // ]U I Z Z Z = +

Suy ra: 12 23 313 112 23 31

( )Z Z ZZ ZZ Z Z

++ =+ +

(b)

Neu cho 3 0I =

, theo hnh sao ta co:

1 212 1( )U I Z Z = +

Theo s o tam giac: 31 23 1212 1[( ) // ]U I Z Z Z = +

Suy ra:23 31 12

1 2

12 23 31

( )Z Z ZZ Z

Z Z Z

++ =

+ +(c)


36/176

KY THUAT IEN34

TS. Lu The Vinh

Cong (a)+(b)+(c) ve vi ve, roi chia oi ta co:

12 23 23 31 31 121 2 3

+ ++ + =

12 23 31+ +

Z Z Z Z Z ZZ Z Z

Z Z Z(d)

Thay (d) lan lt tr ve (a),(b),(c) ta se tm lai c cac cong thcbien oi (3-8) va (3-7) tng ng.

V du: Tnh dong ien I chay qua nguon cua mach cau hnh (3-4, a), bietrang R1 =12, R2 = R3 = 6, R4 = 21, R0 = 8, E = 240V, Rn = 2.

Hnh 3-4

a) b)

Bai giai:

Bien oi tam giac ABC (R1,R2,R0) t hnh (3-4, a) thanh hnh sao

RA,RB,RC (hnh 3-5)Ta co: 1 2

1 2 0

12 62

12 6 18A

R RR

R R R

= =

+ + + +=

1 0

1 2 0

12 186

12 6 18B

R RR

R R R

= =

+ + + +=

0 2

1 2 0

18 63

12 6 18C

R RR

R R R

= =

+ + + +=

ien tr tng ng cua 2 nhanh song song la:

3 4

3 4

( ) ( ) (6 6) (3 21)8

6 6 3 21B C

OD

B C

R R R RR

R R R R

+ + + += =

+ + + + + +=

ien tr tng ng toan mach la:

= + + = + + = 2 2 8 12t n A ODR R R R

Dong ien chay qua nguon la:

= = =240

20

12t

EI A

R


37/176

KY THUAT IEN 35

TS. Lu The Vinh

3.1.4. Mang hai cc Phep bien oi nguon ien tng ng.

a)Mang hai cc.

Khi can khao sat mot nhanh nao o cua mot mach ien phc tap,ta tach nhanh o ra khoi mach. Phan con lai cua mach ien tao thanhmot mang hai cc, v co hai cc e noi ti nhanh va tach ra.

May phat ien, accu, mot ien tr eu co the coi nh mot manghai cc. Mang hai cc co cha nguon ien, tc kha nang cung cap motdong ien cho phu tai mac vao hai cc cua mang c goi la mang haicc co nguon. Ngc lai, neu mang hai cc khong co kha nang cung capdong ien cho phu tai mac vao hai cc cua no goi la mang hai cc khongco nguon.

b)nh ly Thevenin.

oi vi mang hai cc khong co nguon, ta ap dung cac phngphap bien oi tr khang tng ng (noi tiep, song song, sao tam giac)e a mang ve dang mot ien tr tng ngRt goi la ien tr tngng thay the cua mang hai cc.

oi vi mang hai cc co nguon ta co the ap dung phep thay thetng ng Thevenin (hay nh ly Thevenin) sau:

Mot mang hai cc phc tap co nguon gia hai iem A va B co the cthay the bang mot mach ien hai cc n gian gom mot nguon sc ienong Eth noi tiep vi mot ien tr Rth , trong o gia tr cua sc ien ong

Eth bang ien ap gia hai cc UAB khi h mach ngoai, Rth la ien trtng ng gia hai cc cua mang khi sc ien ong cua mang bangkhong (hnh 3-5).

AA

B

R

B

E

th

thMANG 2C C CONGUON

Hnh 3-5. Phep bien oi Thevenin


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KY THUAT IEN36

TS. Lu The Vinh

c) nh ly Norton.

Mang 2 cc co nguon co the c thay the bang nguon dong tngng theo nh ly Norton: Mot mang hai cc phc tap co nguon giahai iem A va B co the c thay the bang mot mach ien hai cc ngian gom mot nguon dong ien JN mac song song vi mot ien tr Rth,trong o JN bang dong ien ngan mach gia hai cc A va B, con Rth laien tr tng ng gia hai cc khi cac sc ien ong cua mang bangkhong (giong ien tr Thevenin).(hnh 3-6)

J

A

B

n

A

B

Rth

MANG 2

C C CONGUON

Hnh 3-6. Phep bien oi Norton.

V du: Cho mang hai cc nh s o hnh (3-7, a), trong o E = 20V,R1 = 2, R2 = 8. Hay thay the mang 2 cc tren bang s o Thevenin vabang s o Norton.

A

R

n

b)

B

E

A

th

A

R

c)a)

J R

th

th

B

1

20V E

B

2R

Hnh 3-7.

S o thay the Thevenin nh hnh (3-7, b), trong o:

21 2

208 16

2 8th ABE

E U RR R

= = = =+ +

V

= = = =

+ +1 2

1 2

2 81,6

2 8th AB

R RR R

R R

S o thay the Norton nh hnh (3-7, c), trong o:


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KY THUAT IEN 37

TS. Lu The Vinh

1

2010

2N ABngan machE

J J AR

= = = =

1 2

1 2

2 81,62 8th AB

R RR R R R

= = = =+ +

3.2. PHNG PHAP DONG IEN NHANH

Phng phap dong ien nhanh s dung hai nh luat Kirchhoff ethiet lap he phng trnh giai mach ien. So nhanh cua mach ien bangso an so phai tm.

Neu mach co n nhanh tc la ng vi n an so ta phai thiet lap tnhat n phng trnh oc lap. Gia s mach co m nut ta se thiet lap c m-1 phng trnh oc lap cho nut, con lai n-(m-1) phng trnh viet cho mat.Cac bc e giai bai toan nh sau:

Bc 1. at cho dong ien chay tren cac nhanh mot mot chieu tuy y.

Bc 2. Dung nh luat Kirchhoff 1 viet m-1 phng trnh cho nut.Bc 3. Chon n - (m-1) mat vi chieu dng tuy y va dung nh luat

Kirchhoff 2 e viet cac phng trnh cho mat.

Bc 4. Giai he n phng trnh e tm ra ap so la cac dong ien nhanh.Neu dong ien tm c co gia tr am ngha la chieu thc te cuadong ien ngc vi chieu a chon ban au.

ac iem cua phng phap nay la co the giai c cac mach ienphc tap, nhieu nguon, nhieu nhanh. Do so phng trnh can thiet lapbang so nhanh cua mach, nen khi mach phc tap so lng phng trnhse tang len. Tuy nhien viec giai he vi mot so lng ln cac phng trnhtuyen tnh co the s dung phng phap matran va cac chng trnh maytnh tr giup.

I3

Z

B

1

E1

I

1

ba

3

Z

E

Z

I

3

A

2

2

Hnh 3-7

V du. Cho mach ien nh hnh 3-7, trongo e1 = e3 = 120 2 sin t (V);

1 2 3 2 2Z Z Z j= = = + .

Hay tm dong ien chay trong cacnhanh.


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KY THUAT IEN 39

TS. Lu The Vinh

Bai giai:

Mach ien co 2 mat a va b.Goi dong ien vong chay trong cac

mat tng ng laIa vaIb. (hnh 3-8).E

1

1

B

1

I A

I

3

Zb

E

Z3

I

2

2

Z

I

I

3

a

Hnh 3-8

Viet phng trnh nh luatKirchhoff 2 cho 2 mach vong ta co :

( )

( )

1 2 2 1

2 3 2

a b

b a

Z Z I Z I E

Z Z I Z I E

+ =

+ =

& & &

& &3

&(a)

Thay cac gia tr

1 2 3 2 2Z Z Z j= = = + ,0

1 3 120jE E e= =& & ta co:

( ) ( ) 04 4 2 2 120 ja bj I j I e+ + =& &

( ) ( )4 4 2 2 120b aj I j I+ + = 0je

10 10

10 10

=

= +

&

&

a

b

I j

(b)& &

Giai he (b) ta co :

I j

1

2

3

10 10

20 20

10 10

= =

= =

= = +

& &

& & &

& &

a

a b

b

I I j A

T o, dong ien cac nhanh se la:

I I I j A

I j A

I

Hay ve o ln:

2 21 10 10 10 2= + =I A

2 22 20 20 20 2= + =I A

2 23 10 10 10 2= + =I A

3.4. PHNG PHAP IEN AP NUT3.4.1.nh luat Ohm cho oan mach cha nguon.

Cho oan mach nh hnh 3-9,a. Gia s ien ap at vao hai aumach la AB&U , dong chay qua mach c xac nh theo nh luat Ohm:


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KY THUAT IEN 41

TS. Lu The Vinh

1 31 3

1 2 3AB

E Y E YU

Y Y Y

+=

+ +

& &&

Trong trng hp tong quat,neu gia 2 nut A va B co k nhanhmac song song th bieu thc ienap 2 nut co dang:

kk

kAB

k

k

E Y

UY

=

&

& (3-11)

Z ZZ

3

I

E1

13

B

3

2

I IA

2

1

E

UAB

.

.

..

..

Hnh 3-9

Ap dung cho bai toan tren hnh 3-9 ta co:

1 31 3

1 2 3

EAB

Y E YU

Y Y Y

+=

+ +

& && =

0

0

1 1120

2 2 2 280 80

1 1 12 2 2 2 2 2

j

j

ej j

e V

j j j

+ + + = =

+ ++ + +

11 1 1120 80

( ) (10 10) ; 10 22 2AB

I E U Y j A Ij

A

= = = =+

& & &

22 280

) ( 20 20) ; 20 22 2AB

I U Y j A Ij

= = = =

+& & A

33 3 3120 80

( ) (10 10) ; 102 2AB

I E U Y j A Ij

2A

= = = =+

& & &

Ket qua ta thay so vi phng phap dong nhanh, dong mach vongth phng phap ien ap 2 nut cho ket qua nhanh hn.

3.5. PHNG PHAP XEP CHONGPhng phap xep chong c ap dung e xac nh dong ien trong

mach co nhieu nguon ien tac ong. Phng phap nay rut ra t tnh chatc ban cua he phng trnh tuyen tnh. Neu trong mot nhanh co nhieudong ien do cac nguon khac nhau cung cap, th dong ien tong bangtong ai so cac dong ien qua nhanh do tac dung rieng re cua tng scien ong (luc o cac sc ien ong khac c coi nh bang khong).ien ap tren moi nhanh cung bang tong ai so cac ien ap gay nen do

tac dung rieng re cua tng sc ien ong.


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KY THUAT IEN42

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V du. Xet mach ien tren hnh 3-10, a: R=2,2

100L

= H. Tnh dong

ien chay trong cac nhanh oi vi 2 trng hp sau:

a) Cac nguon ien xoay chieu : e1= e2 = 120 2 sin 314 t(V)b) Cac nguon ien mot chieu :E1 = E2 = 60 V.

I2

R

R

R

I'

L

2

e

L

1

31

2

L

2

3

R

e

R

R

2I"

R

A

L

L

1

A

L

E

1

b)

R

L

I

E

I

a)

I'

I'

L

B B

c)

R

I"3

L

I"1 A

B

Hnh 3-10

Bai giai.Ap dung phng phap xep chong, ta co 2 s o tng ng vi s

tac ong rieng re cua moi sc ien ong (3-10, b,c).

a) Khi e1= e2 = 120 2 sin 314 t (V).Ta co: Cam khang

2314. 2

314LX L= = =

Tong tr: 1 2 3 2 2Z Z Z j= = = +

Giai mach hnh 3-10,b . Dung phep bien oi tng ng ta co tongtr tng ng cua hai nhanh 2 va 3 la:

2 21 1

2t

jZ j

+= = +

Dong ien: 111

120' 2(2 2) (1 1)t 0 20

EI jj jZ Z= = = + + ++

&

A

12 3

'' ' 10 10

2

II I j= = = A

Mot cach tng t, oi vi mach hnh 3-10,c ta co:

2" 20 20I j= A

1 3" " 10 10I I j= = A

Dung nguyen ly chong chat ta co:


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KY THUAT IEN

TS. Lu The Vinh

43

1 1 1

2 21

' " (20 20) (10 10) 10 10

10 10 10 2

I I I j j j

I A

= = =

= + =

A

2 2 22 2

2

' " (10 10) (20 20) 10 1010 10 10 2

I I I j j jI A

= = = += + =

A

3 3 3

2 21

' " (10 10) (10 10) 20 20

20 20 20 2

I I I j j j

I A

= + = + =

= + =

A

b) Trng hp nguon mot chieu E1 = E2 = 60 VKhi giai mach oi vi dong mot chieu che o xac lap ta ch can chu

y rang cac thanh phan khang bang khong, tong tr cua moi nhanh ch co

thanh phan ien tr hoat ong R. Cac ai lng khac c bieu dienbang so thc. ien tr tng ng cua 2 nhanh 2 va 3 (Hnh 3-10,c) :

12t

RR = =

Cac dong ien: 111

60' 2

2 1t0

EI A

R R= = =

+ +

12 3

'' ' 10

2

II I= = = A

Tng t, vi s o hnh 3-10, c ta co:I2 = 20 A

21 3

'" " 10

2

II I= = = A

Xep chong ket qua ta co:I1 = I1 I1 = 20 10 = 10 AI2 = I2 I2 = 10 20 = -10 AI3 = I3 + I3 = 10 + 10 = 20 A

Ket qua tnh toan cho gia tr dong I2 < 0, nh vay dong thc tengc vi chieu a chon.


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KY THUAT IEN

Chng 4.

MACH IEN BA PHA

He thong ien 3 pha la tap hp ba he thong

ien mot pha c noi vi nhau tao thanh mot hethong nang lng ien t chung, trong o sc ienong moi mach eu co dang hnh sin, cung tan so,lech pha nhau mot phan ba chu ky.


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KY THUAT IEN 45

TS. Lu The Vinh

4.1. HE THONG IEN 3 PHA

4.1.1. nh ngha. He thong ien 3 pha la tap hp ba he thong ien motpha c noi vi nhau tao thanh mot he thong nang lng ien t chung,trong o sc ien ong moi mach eu co dang hnh sin, cung tan so,lech pha nhau mot phan ba chu ky.

He thong ien 3 pha co nhieu u iem hn han he thong ien motpha. e truyen tai ien mot pha ta can dung 2 day dan, nhng e truyentai he thong 3 pha ch can dung 3 hoac 4 day dan do vay tiet kiem vakinh te hn. He 3 pha de dang tao ra t trng quay, lam cho viec che

tao ong c ien n gian. Cac ong c cong suat ln eu phai s dungnguon ien 3 pha.

4.1.2.Nguyen ly may phat ien ba pha.

He thong ien ba pha uc tao ra t may phat ien ong bo bapha, hoat ong da tren nguyen ly cam ng ien t.

Cau tao nguyen ly cua may phat ien 3 pha gom hai phan:a) Stator (phan tnh). Gom ba cuon day giong nhau (goi la cac

cuon day pha) at lech nhau 1200 trong cac ranh cua loi thep stator. Caccuon day ba pha thng ky hieu tng ng la AX, BY, CZ.

b)Rotor (phan quay). La mot namcham ien N-S (hnh 4-1)

Hnh 4-1

Khi rotor quay, t trng cua no lanlt quet qua cac cuon day pha, gay ra cacsc ien ong hnh sin co cung bien o, cungtan so, nhng lech pha nhau 1200. Neu chon

pha ban au cua sc ien ong eA trong cuonday AX bang khong ta co bieu thc cac scien ong trong cac pha la:

2sin

22 sin( )

34 2

2 sin( ) 2 sin( )3 3

A

B

c

e E t

e E t

e E t E t

=

=

= = +

(4-1)


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KY THUAT IEN46

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Hoac bieu dien di dang phc:0

2

3

2

3

jA

jB

j

E E e

E E e

CE E e

=

=

&

&

=&

(4-2)

o th hnh sin va gian o vec t sc ien ong 3 pha c thehien tren hnh 4-2.

Hnh 4-2a) b)

oi vi nguon 3 pha oi xng ta co:eA + eB + eC= 0 (4-3,a)

Hoac di dang phc:0A B CE E E+ + =

& & & (4-3,b)

Neu noi rieng re tng pha vi tai tac 3 he thong mot pha oc lap, hay he

thong 3 pha khong lien he vi nhau (hnh4-3). He thong nay t s dung trong thc tedo khong kinh te v can ti 6 day dan.

Hnh 4-3

Thong thng 3 pha nguon c noivi nhau, 3 pha tai cung c noi vi nhauva co ng day 3 pha noi gia nguon vatai. Co 2 phng phap noi mach 3 phathng s dung trong cong nghiep la noi

hnh sao (Y) va noi hnh tam giac ().


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KY THUAT IEN 47

TS. Lu The Vinh

4.2. PHNG PHAP NOI HNH SAO

4.2.1. Nguyen tac noi.

Moi pha cua nguon va tai eu co iem au va iem cuoi. Tathng ky hieu cac iem au pha la A,B,C, cac iem cuoi pha la X,Y,Z.e noi hnh sao ngi ta noi 3 iem cuoi cua cac pha lai vi nhau taothanh iem trung tnh.

oi vi nguon, 3 iem cuoi X, Y, Z cua cac cuon day may phatien c noi lai vi nhau tao thanh iem trung tnh O.

oi vi tai, 3 iem cuoi X, Y, Z c noi lai vi nhau tao thanhiem trung tnh O (hnh 4-4, a).

Ba day noi cac iem au cua nguon va tai AA, BB, CC goi lacac day pha.

Day dan noi cac iem trung tnh OO goi la day trung tnh.

a) b)

Hnh 4-4

4.2.2. Quan he gia cac ai lng day - pha.

a) Quan he gia dong ien day Idva dong ien pha Ip.

Dong ien phaIp la dong ien chay trong moi pha cua nguon (hoactai). Dong iendayId la dong chay trong cac day pha noi gia nguon vatai. T hnh (4-4, a) ta thay dong ien day Id co gia tr bang dong ienchay trong cac phaIp.

Id = Ip (4-4)


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KY THUAT IEN48

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b) Quan he gia ien ap day va ien ap pha.

ien ap pha Up la ien ap gia iem au va iem cuoi cua moipha (hoac gia day pha va day trung tnh).

ien ap day Ud la ien ap gia 2 day pha:

AB A

BC B C

CA C A

U U U

U

U U U

=

=

=

& &

& &

& & &

B&

&U U (4-5)

Hnh 4-5

e ve o th vec t ien ap day,trc het ta ve o th vec t ien ap pha

UA, UB, UC, sau o da vao cong thc (4-5)ta dng o th vec t ien ap day nh trenhnh (4-4, b), hoac hnh (4-5). Ta co:

Ve tr so, ien ap day Ud ln hnien ap pha Up la 3 lan. That vay, xettam giac OAB t o th hnh (4-4, b) ta co:

AB = 2AH= 2 OAcos 300 =3

2 32

OA OA=

3d pU=

0 0A B CI I I I= + + =& & & &

U (4-6)

De thay rang, khi ien ap pha oi xng, th ien ap day oi xng.

Ve pha, cac ien ap day UAB, UBC, UCA lech pha nhau mot goc1200 va vt trc ien ap pha tng ng mot goc 300.

Khi tai oi xng, dong ien qua day trung tnh bang khong:

(4-7)

Trong trng hp nay co the khong can day trung tnh, ta co machba pha ba day. V du, ong c ien ba pha la tai oi xng, ch can a baday pha noi en ong c.

Thong thng trong thc te, tai ba pha la khong can bang, khi odong ien qua day trung tnh la khac khong, do o bat buoc phai co daytrung tnh.


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KY THUAT IEN 49

TS. Lu The Vinh

4.3. PHNG PHAP NOI HNH TAM GIAC.

4.3.1. Nguyen tac noi.

e noi hnh tam giac ngi ta noi au pha nay vi cuoi pha kia, vduA noi viZ, B noi viX, Cnoi vi Y(hnh 4-6).

a) b)

Hnh 4-6

4.3.2. Quan he gia cac ai lng day pha.

Ky hieu cac ai lng day, pha nh tren hnh 4-6, a.

a) Quan he gia ien ap day va ien ap pha.T hnh ve ta thay khi noi tam giac th ien ap gia hai day chnh

la ien ap pha:

Ud = UP (4-8)

b) Quan he gia dong ien day Idva dong ien pha Ip.

Ap dung nh luat Kirchhoff 1 cho cac nut, ta co:

Tai nutA: A AB CAI I I= & & &

Tai nutB: B BC ABI I I= & & & (4-9)

Tai nut C: C CAI I I= & & &BC

o th vec t cac dong ien dayIA, IB, ICva dong ien phaIAB, IBC,ICAve tren hnh (4-6, b). Ta co:

Ve tr so, dong ien day ln gap 3 lan dong ien pha. Thatvay, xet tam giac OEF t o th hnh (4-6, b) ta co:


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53/176

KY THUAT IEN 51

TS. Lu The Vinh

Trong o la goc lech pha gia ien ap pha va dong ien phatng ng:

2 2cosP

p

R

Rp X = +

4.4.2. Cong suat phan khang Q.

Cong suat phan khang cua mach ba pha la:

sin sin sinA B C A A A B B B C CQ Q Q Q U I U I U I C = + + = + +

Khi mach oi xng ta co:

3 sinp pQ U I = (4-14)

Hoac: 23 p pQ X I= (4-15)

Trong o Xp la ien khang cua pha. Neu bieu dien theo cac ailng day ta cung co:

3 sind dQ U I = (4-16)

4.4.2. Cong suat bieu kien.

Khi oi xng, cong suat bieu kien ba pha la:2 2 3 3p p dS P Q U I U = + = = dI (4-17)

4.5. GIAI MACH IEN BA PHA OI XNG.

oi vi mach ien ba pha oi xng, dong ien va ien ap cac phaco tr so bang nhau va lech pha nhau mot goc 1200. Khi giai mach ba phata ch can tnh cho mot pha, sau o suy ra cac pha con lai.

Khi noi vao nguon co ien ap Ud, bo qua tong tr cua ng day,neu biet tong tr tai, cac bc tnh toan se thc hien nh sau:

Bc 1. Xac nh cach noi phu tai : hnh sao hay tam giac?

Bc 2. Xac nh ien ap pha cua tai:

+ Neu noi hnh sao:

3d

pU

U =

+ Neu tai noi tam giac: Ud = Up


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KY THUAT IEN52

TS. Lu The Vinh

Bc 3. Xac nh tong tr pha va he so cong suat cua tai.

2 2p p pZ R X= +

He so cong suat :2 2

cos p p

p p p

R RZ R X

= =+

Bc 4. Tnh dong ienIp cua phu tai.

+ Neu tai noi sao: Id= Ip

+ Neu tai noi tam giac: 3d pI I=

Bc 5. Tnh cong suat tieu thu tren phu tai.23 3 os 3 cosp p p p d dP R I U I c U I = = =

23 3 sin 3 sinp p p p d dQ X I U I U I = = =

23 3 3p p p p dS Z I U I U I = = = d

V du 1. Mot tai 3 pha co ien tr pha Rp = 20, ien khang pha Xp =15, noi hnh tam giac va au vao li ien 3 pha co ien ap day Ud =220V. (hnh 4-7, a). Tnh dong ien pha Ip, dong ien day Id, cong suat taitieu thu va ve o th vec t ien ap day va dong ien pha phu tai.

a) b)

Hnh 4-7

Bai giai. Theo s o au day (hnh 4-7) tai noi tam giac, do o ienap pha cua tai la:

Ud= Up = 220 V


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KY THUAT IEN 53

TS. Lu The Vinh

Tong tr pha cua tai:2 2 2 220 15 25Z R Xp p p= + = + =

Dong ien pha cua tai:220

8,825

pp

p

UI A

Z= = =

Dong ien day cua tai:

3 3 8,8 15,24d pI I= = = A

W

R

Cong suat tai tieu thu:2 23 3.20.8,8 4646,4p pP R I= = =

2 23 3.15.8,8 3484,8p pQ X I VA= = =

3 3 .220.15,24 5870,21d dS U I V = = = A

He so cong suat:20

cos 0,825

p

p

R

Z = = = , 36,87 = o

Dong ien cham pha hn ien ap mot goc la . o th vec

t dong ien va ien ap pha ve tren hnh (4-7, b).

36,87 = o

V du 2, Mot tai 3 pha la 3 cuon day c au vao li ien 3 pha coien ap day la 380 V. Cuon day co ien tr R = 2 , ien khang X = 8va c thiet ke lam viec ien ap nh mc la 220V.

2) Xac nh cach noi cac cuon day thanh tai 3 pha.2) Tnh cong suat P, Q va cos cua tai.

Hnh 4-8


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KY THUAT IEN54

TS. Lu The Vinh

Bai giai.

a) Cac cuon day phai noi hnh sao (hnh 4-8. a), v khi au vao liien 3 pha th ien ap pha at len moi cuon day bang ien ap nh mc:

380 2203 3d

pUU V= = = .

Neu tai noi tam giac, ien ap pha at len cuon day (hnh 4-8, b):

Up = Ud= 380 VGia tr ien ap 380 V ln hn ien ap nh mc cua cuon day, nen

cuon day se b hong.b) Tong tr pha cua tai la:

2 2 2 2

2 8 8,24p p pZ R X= + = + = He so cong suat:

2cos 0,242

8,24p

p

R

Z = = =

8sin 0,97

8,24p

p

X

Z = = =

Dong ien phaIpcua tai:

220 26,78,24ppp

UI AZ

= = =

Dong ien day:Id= Ip = 26,7 A

Cong suat tac dung cua tai:3 cos 3.380.26,7.0,242 4252,6d dP U I = = = W

Cong suat phan khang cua tai:

3 sin 3.380.26,7.0,97 17045,7d dQ U I VA= = = RCong suat bieu kien:

3 3.380.26,7 17572,8d dS U I V = = = A

4.6. GIAI MACH IEN BA PHA KHONG OI XNG

Mang ien ba pha cung cap ien cho cac khu dan c la mang ien3 pha 4 day. Nang lng ien cung cap cho sinh hoat c lay t mot phava day trung tnh, do o cap ien ap nh mc cua cac thiet b tieu thuien la ien ap pha Up. Do tai cua cac pha la khong ong nhat va thay


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oi nhieu nen mang ien cua he thong la mang 3 pha khong oi xng(hnh 4-9).

a) b) c)

Hnh 4-9

Neu bo qua tr khang cua day trung tnh tc la bo qua sut ap trenng day th ien the iem trung tnh cua tai O xem nh trung vi ienthe iem trung tnh cua nguon O. Khi tai cac pha thay oi ien ap pha Uptren tai van gi gia tr bnh thng (hnh 4-9, b) khong vt qua ien ap 3pha.

ay la tac dung cua day trung tnh. Neu khong co day trung tnh,hoac day trung tnh b t th trong trng hp tai cac pha thay oi iemtrung tnh O cua tai se lech khoi iem trung tnh O cua nguon lam choien ap cac pha tai UA, UB, UCthay oi rat nhieu (hnh 4-9, c) so viien ap pha nh mc, dan ti qua ap hoac thieu ap cho cac dung cu tieuthu ien.

oi vi mach 3 pha noi hnh sao khong co day trung tnh, khi taikhong oi xng ta tien hanh giai bai toan oi vi tng pha rieng re.

V du. Mang ien 3 pha 380 / 220V co day trung tnh cung cap iencho tai 3 pha gom 90 bong en si ot loai 220V-60W, moi pha 30 bong.

a)

Ve s o mach ien.b) Tnh IA, IB, IC, I0, P khi tat ca cac bong eu bat sang.c) Tnh IA, IB, I0, P khi pha A co 10 bong en bat sang, pha B co 20

bong bat sang, pha C cat ien.d) Tnh ien ap at len cac en pha A va pha B trong cau c, khi

pha C cat ien va day trung tnh b t.

Bai giai.


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a) Mach ien 3 pha 380/220V la mang 3 pha 4 day: 3 day pha vaday trung tnh. ien ap day Ud= 380 V. ien ap pha Up= 220 V.

Cac bong en co ien ap nh mc la 220V nen phai s dung ien

ap pha. S o mac mach nh hnh 4-10.

Hnh 4-10

b)V ien ap at vao cac bong en bang ien ap nh mc, nen congsuat tieu thu tren moi bong en se bang cong suat nh mc 60W.

Tat ca cac bong eu bat sang, mach tai 3 pha la oi xng, congsuat cac pha la bang nhau:

PA = PB = PC= Pp = 30.60 = 1800 W.

Cong suat 3 pha:

P= 3 Pp = 3. 1800 = 5400 W.

Tai la cac bong en, xem nh thuan tr R nen goc lech pha = 0,cos = 1. T o ta co dong ien chay trong cac pha tai la:

18008,18

cos 220.1

pA B C p

p

PI I I I

U = = = = = A

V nguon va tai oi xng nen:

0 0A B CI I I I= + + =uur uur uur uur

o th vec t ve tren hnh 4-11, a, trong o dong ien trung pha viien ap

c) Khi pha C cat ien, IC = 0, cac pha khac van bnh thng, ien aptren cac bong en van gia tr nh mc:


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10.602,73

cos 220.1A

AA

PI A

U = = =

20.605,45cos 220.1

BB

B

PI AU = = =

P = PA + PB = 10.60 + 20.60 = 1800 W

o th vec t cho tren hnh 4-11, b.

0 A CI I I= +uur uur uur

2 2

02 cos120 4,72

A B A BI I I I I= + + =o A

d) Khi pha Ccat ien, ong thi neu day trung tnh b t mach iencon lai nh hnh 4-12, a. Cac bong en pha A va pha B mac noi tiep vanoi vao ien ap day UAB.

Ta co ien tr moi bong en:2 2220

806,660

m

m

UR

P= = =

ien tr tng ng cua 10 bong en mac song song phaA:806,6

80,6610 10

A

RR = = =

ien tr tng ng cua 20 bong en phaB:

806,640,33

20 20

BR

R = = =

Mach ien tng ng ve tren hnh 4-12, b.


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58

Dong ienIchay trong mach la:

3803,14AB

UI A= = =

80,66 30,33A BR R+ +

ien ap at len cac bong en phaA la:

UA = RA.I = 80,66. 3,14 = 253,27 V

a) b)Hnh 4-12

Ta thay UA = 253,27 V > 220 V = Um , en phaA qua ap nen cothe b chay

ien ap at len cac bong en phaB la:

UB = RB.I = 40,33. 3,14 = 126,63 V

V UB = 126,63 < 220V = Um , nen en phaB kem ap nen khongsang bnh thng.

Nh vay ta thay, neu day trung tnh b t se rat nguy hiem chophu tai v co the b qua ap hoac kem ien ap.


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KY THUAT IEN

Phn th2. MAY IEN

Chng 5

KHAI NIEM CHUNG VE MAY IEN

May ien la thiet b ien t, nguyen ly lam viecda vao hien tng cam ng ien t va tng tac giat trng va dong ien, dung e bien oi cac dangnang lng khac nhau thanh ien nang (may phatien), hoac ngc lai bien oi ien nang thanh cnang (ong c ien), hoac dung e bien oi cac thongso ien nang nh ien ap, dong ien , tan so, v.v.

May ien co rat nhieu loai khac nhau, s dungrong rai trong cong nghiep, giao thong van tai, trongsan xuat va i song.


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5.1. NH NGHA VA PHAN LOAI

5.1.1. nh ngha.May ien la thiet b ien t, nguyen ly lam viec da vao hien

tng cam ng ien t va tng tac gia t trng va dong ien, dung ebien oi cac dang nang lng khac nhau thanh ien nang (may phatien), hoac ngc lai bien oi ien nang thanh c nang (ong c ien),hoac dung e bien oi cac thong so ien nang nh ien ap, dong ien ,tan so, v.v.

May ien co rat nhieu loai khac nhau, s dung rong rai trong cong

nghiep, giao thong van tai, trong san xuat va i song.5.1.2. Phan loai.

Co the phan loai theo cac tieu ch khac nhau: theo cong suat, theocau tao, theo loai dong ien, theo nguyen ly lam viec, .

Theo nguyen ly bien oi nang lng ta co 2 loai may ien: mayien tnh va may ien quay.

1) May ien tnh.

May ien tnh lam viec theo nguyen ly cam ng ien t gianhng cuon day khong co chuyen ong tng oi vi nhau. Loai mayien tnh thong dung la may bien ap.

Cac may ien tnh c dung e bien oi cac thong so ien nang,qua trnh co tnh chat thuan nghch. V du, may bien ap bien oi iennang au vao cac thong so la U1, I1, f(s cap) thanh ien nang loi ra cocac thong so tng ng la U2, I2, f(th cap). Ngc lai, neu au vao la

U2, I2, fth au ra se laU1, I1, f (hnh 5-1).

a) b)

Hnh 5-1


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2) May ien quay.

La loai may ien lam viec theo nguyen ly cam ng ien t vatng tac ien t gia t trng va dong ien trong cac cuon day co

chuyen ong tng oi vi nhau.

Loai may ien quay thng dung e bien oi c nang thanh iennang (may phat ien) hoac bien oi ien nang thanh c nang (ong c).Qua trnh bien oi cung co tnh thuan nghch (hnh 5-1, b). Tren hnh 5-2la s o phan loai cac may ien thong dung.

May ien tnh

May ien mot chieu

MAY IEN

May ien quay

May ien xoay chieu

May ienong bo

May ienKhong ong bo

ongc

ongbo

Mayphatong

bo

. Ckhongong

bo

ongc

motchieu

Mayphatmot

chieu

M.Pkhong

ongbo

May

Bienap

Hnh 5-2


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5.2. CAC NH LUAT IEN T C BANDUNG TRONG MAY IEN

Nguyen ly c ban cua moi may ien eu da tren c s hai nhluat cam ng ien t la lc ien t. Khi tnh toan mach t thng s dungnh luat dong toan phan.

5.2.1. nh luat cam ng ien t.

Moi khi t thong gi qua mot khung day dan kn bien thien thtrong khung day se xuat hien mot sc ien ong cam ng. Theo nh luatFaraday va quy tac Lenx th sc ien ong cam ng xuat hien trongmach co gia tr bang va ngc dau vi toc o bien thien cua t thong:

ded t

= (5-1)

Trng hp mach ien h, chang han mot thanh day dan chuyenong trong t trng, trong thanh day dan se xuat hien mot sc ien ongcam ng. V du tren hnh 5-3, a, khi thanh day dan chuyen ong thanggoc vi cac ng sc t, sc ien ong cam ng xuat hien trong thanhla:

e Blv= (5-2)Trong o: B : cam ng t (T)

l : chieu dai hieu dung cua thanh dan (m) v : toc o chuyen ong cua thanh dan (m/s)

Chieu cua sc ien ong cam ng xac nh theo quy tac ban tayphai (hnh 5-3, a).

5.2.2. nh luat lc t

Khi cho mot thanh dan co dong ien Ichay qua at trong mot ttrng eu

uu, thanh dan se chu tac dung cua mot lc ien t xac nh

theo bieu thc:

Br

[ . ]F I l B=uur r uru

l

(5-3)

Neu thanh dan chuyen ong thang goc vi t trng, lc ien t seco gia tr:

F I B= (5-4)


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Chieu cua lc ien t c xac nh theo quy tac ban tay trai (hnh5-3,b)

a)b)

Hnh 5-3

5.3. NGUYEN LY VA TNH THUAN NGHCH CUA MAY IEN

Cac may ien eu lam viec theo nguyen ly cam ng ien t neneu co tnh thuan nghch, ngha la co the lam viec hai che o: che omay phat va che o ong c.

5.3.1. Che o may phat ien.

Che o may phat ien la che o c nang c bien oi thanh iennang. Ta hay xet nguyen tac nay nh sau: dung c nang t mot ong cs cap tac dung vao mot thanh dan mot lc c hoc Fc lam cho no chuyenong vi mot van toc v trong t trng cua mot nam cham N-S. Trongthanh dan se xuat hien mot sc ien ong cam ng co o ln xac nhtheo (5-2).

Neu noi hai au thanh dan vao ien tr tai R, qua R se co dongien i. Neu bo qua sut ap tren thanhdan ta co the viet:

u = eCong suat may phat ien cung

cap cho tai la:P= ui = ei

Dong ien i nam trong ttrng lai chu tac dung cua mot t lcFt= iBl co chieu nh hnh ve (hnh 5-4).


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Khi may quay vi mot toc o khong oi th lc ien t se can bangvi lc c cua ong c s cap: Fc= Ft.

Nhan hai ve vi van toc v ta co: Fc .v = Ft..v = iBlv = ei.

Nh vay, cong suat c cua ong c s cap Pc = Fc.v a c bien oithanh cong suat ien Pt = ei. Noi cach khac, c nang a c chuyenhoa thanh ien nang.

5.3.2. Che o ong c ien.

Dong ien at trong t trng se chu tac dung cua t lc. Ta haycung cap mot ien ap u cho thanh dan at trong t trng eu cua motnam cham N-S, qua thanh se co dong ien i. Theo nh luat lc t thanh

dan se chu tac dung cua mot t lc Ft = ilB va lam thanh dan chuyenong vi van toc v (hnh 5-5).

Nh vay cong suat ien a vaoong c la Pt = ui a c bien oithanh cong suat c Pc = Ft v . iennang c bien oi thanh c nang.

Ta thay, cung mot thiet b ien t(thanh dan at trong t trng) tuythuoc vao nang lng a vao ma mayien co the lam viec che o ong choac che o may phat. ay la ac tnh thuan nghch cua may ien, motac tnh het sc quan trong va quy bau lam cho may ien co u the hnhan so vi cac bo bien oi nang lng khac (tua bin hi nc, ong cdiezel, ong c phan lc )

5.4. TNH TOAN MACH T.

5.4.1. nh ly mach t.nh ly mach t chnh la nh ly dong toan phan ap dung cho mach

t. Trong cac may ien, loi thep la mach t cua may dung e tap trung ttrng va dan t thong. Ta hay xet mot mach t n gian nh tren hnh(5-6, a). ay la mach t khong phan nhanh ong nhat bang thep ky thuatva co mot cuon day co dong i chay qua.

Ap dung nh ly dong toan phan H dl i= cho mach t tren

ta co:


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H l wi= (5-5)

Trong o:

H t trng trong mach t (A/m)l chieu dai trung bnh cua mach t (m)w so vong day cua cuon day.

Tch so wi goi la sc t ong, Hl c goi la t ap ri trongmach t.

Trong trng hp mach t khong ong nhat va co nhieu cuon day,v du mach t hnh (5-6, b) th cong thc (5-5) co dang:

H1l1 + H2l2 = w1i1 w2i2

Trong o:H1, H2 cng o t trng trong cac oan mach t 1, 2.L1, l2 chieu dai cac oan t 1, 2.H1l1, H2l2 t ap tren cac oan 1, 2.w1i1, w2i2 sc t ong cua cac cuon day w1va w2.

Dau (-) trc w2i2 v chieu dong i2 khong thuan vi chieu cua t

thong a chon theo quy tac van nut chai.Mot cach tong quat, oi vi mach t co n oan va co m cuon day,

phng trnh (5-5) c viet :

1 1

n m

k k j jk j

H l w i= =

= (5-6)

Dau cua dong ij >0 neu thuan chieu vi chieu cua t thong achon theo quy tac van nut chai, ngc lai lay dau am.

5.4.2. Tnh toan mach t.


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66

Khi tnh toan mach t th au:Bai tnh dong t hoa

t thong ng vi mot mach t

ng gap 2 bai toan s toan thuan. Cho biet trc t thong, can phai(hoac so vong day) e sinh ra t thong o.

Bai toan ngc. Cho dong ien, can tnhxac nh.V du. Cho mach t khong phan nhanh nh hnh 5-7, b. Cho t thong

1, 2 la:

trong mach t la . Hay tnh dong trong cac cuon day.

Ta co, cam ng t tng ng trong cac oan mach

1 21 2

,B BS S

= =

S1, S2 tng ng la tiet dien cac oan mach 1, 2.o t trng trong

khe laoi vi oan mach 2 la khe khong kh, cng

:

22

0

BH =

vi 0 = 4.107 H/m la hang so t.

o t, ta

Tiep theo, ap dung nh luat macht e

Sau o ap dung (5-5) e tnh dong

. Cho biet dong ien, can tnh t thong. ayla bai

i vi oan mach 1 la vat lieu sat

Hnh 5-7

phai tra bang ng cong t hoa

cua vat lieu sat t tng ng B = f(H)(hnh 5-7). BietB1 ta tm cH1 bang oth.

tnh tong:

H l = + 1 1 2 2k kk

H l H l

t hoa (hoac so vong day ).

oi vi bai toan ngctoan phc tap hn phai ap dung phng phap do, hoac cac phng

phap o th, phng phap so dung cho mach phi tuyen. Trong pham vigiao trnh ta ch gii han xet bai toan thuan.


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Chng 6. MAY BIEN AP

May bien ap c s dung rong rai trong cac hethong truyen tai va phan phoi ien nang, trong cac cac loien, trong may han, cac bo chnh lu, trong cac thiet bvo tuyen ien, trong ky thuat truyen ong, ieu khien vat ong hoa.

May bien ap la mot thiet b ien t tnh dung ebien oi dong ien xoay chieu t ien ap nay sang ien

ap khac vi cung mot tan so.


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6.1. NGUYEN LY CAU TAO CUA MAY BIEN AP.

6.1.1. nh ngha. May bien ap (MBA) la mot thiet b ien t tnh dunge bien oi dong ien xoay chieu t ien ap nay sang ien ap khac vicung mot tan so.6.1.2. Nguyen ly cau tao:

Cau tao cua MBA gom 2 bo phan chnh la: loi sat t va cac cuonday quan (Hnh 6-1).

- Cuon day W1 noi vi nguon ien goi la cuon s cap- Cuon day W2 noi vi tai c goi la cuon th cap

I1 I2

U1U2

Hnh 6-1. Nguyen ly cau tao may bien the

a) Loi thep :Loi thep la phan mach t cua may bien ap dung e dan t thong

chnh trong may, no c ghep t cac la thep ky thuat ien. Loi thepc chia lam hai phan :

+ Tru : la ni e at day quan.+ Gong : la phan khep kn mach t gia cac tru.

b) Day quan:Day quan may bien ap thng c che tao t day ong hoac

nhom, co tiet dien tron hoac ch nhat, ben ngoai day co boc lp cachien. Day quan c quan thanh nhieu vong va long vao tru loi thep.Gia cac vong day quan co cach ien vi nhau va cach ien vi loi thep.

May bien ap mot pha co hai cuon day: mot cuon day noi vinguon xoay chieu goi la cuon s cap , con cuon day noi vi tai goi lacuonth cap.


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Hnh 6-4. MBA kieu boc

Cac MBA cong suat ln thng phai lam mat bang dau, toan bo loithep va cac cuon day c ngam trong thung dau, cac au day c a

ra ngoai qua s xuyen bat chat tren nap thung (hnh 6-5).

Hnh 6-5. May bien ap ien lc1- Cac cuon day, 2- Gong mach t3- au s cao ap, 4- au s ha ap5- Thung dau, 6- Ong dau oi lu toa nhiet

6.2. NGUYEN LY HOAT ONG

Nguyen ly hoat ong cua MBA da tren nh luat cam ng ient. Gia s at vao 2 au cuon s cap mot the hieu xoay chieu u1, dongxoay chieu i1 tren cuon s cap se tao ra mot t thong bien thien . Domach t khep kn nen t thong nay se moc vong qua cac cuon day s cap

va th cap (hnh 6-6).


73/176


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TS. Lu The Vinh

k goi la he so bien ap .oi vi may tang ap ta co: U1 < U2 , W1 < W2 , k < 1oi vi may ha ap ta co: U1 > U2 , W1 > W2 , k > 1.

Neu bo qua ton hao trong MBA co the coi gan ung quan he giacac lng s cap va th cap nh sau:U I1 1 2 2U I=

Hoac: 1 22 1

U Ik

U I= = (6-4)

6.3. MO HNH TOAN HOC CUA MAY BIEN AP

e bieu dien cac qua trnh ien t xay ra trong MBA ngi ta s

dung cac phng trnh can bang ien t gia 2 mach s cap va th cap.Xet MBA mot pha co 2 cuon day quan nh hnh 6-7, trong o

mach s cap noi vi nguon ien ap U1. Mach th cap noi vi tai co tongtrZt.

Hnh 6-7

e thiet lap cac phng trnh, trc het ta chon chieu dong i1 nhhnh 6-7. Chieu cua t thong chnh phai thuan chieu i1 theo quy tac vannut chai. Tiep theo chieu cua e1 va e2 phai phu hp vi , ngha la e1 vai1 trung chieu, con dong i2 ngc vi chieu e2 (theo ung ng luat cam

ng ien t). Trong mach ngoai t thong chnh moc vong qua ca haicuon day ong vai tro truyen nang lng ien t con co cac t thong tan.Cac t thong tan ch moc vong rieng re vi moi cuon day quan va tan rangoai khong kh. T thong tan do dong s cap i1 gay ra ky hieu la t1, tthong tan do dong th cap gay ra ky hieu la t2. Gia tr cua t thong tanc ac trng bi ien cam tan L1 va L2 tren cac day quan s cap vath cap tng ng:

1 21 2

1 2

,t tL Li i

= = (6-5)


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6.3.1. Phng trnh can bang ien ap s cap.

Trong mach s cap gom co cacai lng: nguon ien ap u1, sc ienong e1, ien tr day quan s cap R1,ien cam tan L1. S o mach tngng co the bieu dien nh hnh (6-8)

R1

e

L1

U1

i 1

1

Theo nh luat Kirchhoff 2 ta co:Hnh 6-8

U2

i

R 2 L2

eZ t

2

2

11 1 1 1 1di

R i L u edt

+ = +

hay: 11 1 1 1 1di

u R i L edt

= + (6-6)

Neu chuyen sang dang phc ta co:1 1 1 1 1 1 1 1 1 1 1 1 1( )I j X I E R j X I E Z I E= + = + = & & & & & & & &U R (6-7)

trong o 1 1 1Z R j X= + 1 1la tong tr phc vi X L= la ien khang

tan cua day quan s cap.

6.3.2. Phng trnh can bang ien ap th cap.

Mach th cap gom sc ien ong e2, ien tr day quan R2, iencam tan th cap L2, ien ap a ra tai u2, tong tr tai Zt. S o mach

tng ng nh hnh 6-9.Theo nh luat Kircchoff 2 ta co:

22 2 2 2 2

diR i L u e

dt+ + =

hay: 22 2 2 2 2di

u e R i Ldt

= (6-8)

Khi chuyen sang dang phc ta co: Hnh 6-9

2 2 2 2 2

2 2 2 2

2 2 2

( )U E R I j X 2I

E R jX I

E Z I

= = (6-9) +

&

& &

& &=

& &

trong o: 2 2 2Z R j X= + la tong tr phc vi 2X L 2= la ien

khang tan cua day quan s cap.ien ap th cap U2 chnh la ien ap at len tai:

2 2tU (6-10)Z I=& &


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6.3.3. Phng trnh can bang sc t ong.

Trong phng trnh can bang ien s cap (6-7): 1 1 1U Z I E = & & 1&

ien ap ri 1 1Z I&

thng rat nho, nen gan ung co the coi ,hay U1E1.1 1U E & &

Do ien ap li at vao MBA la U1 khong oi nen gia tr E1 sekhong oi va do o t thong m se cung khong oi.

che o khong tai (I2= 0) t thong chnh do sc t ong cua dayquan s cap W1i0 sinh ra, con khi co tai (I2 0) t thong chnh do sc tong cua ca 2 cuon day quan s cap va th cap sinh ra. Sc t ong khico tai la: i1W1 i2W2. (dau trc i2 do chieu i2 khong phu hp vi chieu

t thong

theo quy tac van nut chai).V gia tr m khong oi, nen sc t ong khi khong tai bang sc tong luc co tai. Phng trnh can bang sc t ong tc thi co dang:

i0W1 = i1W1 i2W2Chia hai ve cho W1 ta co:

= = = = 2

'2 2 20 1 2 1 1 1

11

2

W i ii i i i i i i

WW kW

hay

'

1 0 2i i i= +

(6-11)trong o: k = W1/W2 la he so bien ap,i2 = i2/k goi la dong th cap a quy oi ve pha s cap.

Khi viet di dang phc, phng trnh (6-11) se co dang:'

1 0 2I I I= +& & & (6-12)He cac phng trnh (6-6, 6-8, 6-11) hoac (6-7, 6-9, 6-12) c goi

la mo hnh toan hoc cua MBA.

6.4. S O THAY THE CUA MAY BIEN AP

6.4.1. Quy oi cac ai lng th cap ve s cap.

e tien li khi nghien cu cac qua trnh ien t trong MBA, cacai lng th cap thng c quy oi ve s cap. Phep quy oi phai thoaman ieu kien bao toan nang lng. Khi quy oi ngi ta gia thiet sovong W2 cua cuon th bang so vong W1 cua cuon s. Trong trng hpnay, s.., dong ien va tong tr cuon th eu thay oi, nhng cong suat,goc pha trong MBA van gi nguyen tr so cu. Cac ai lng quy oi c

ky hieu bang ch cai co dau phay, v du

'' ' '

22 2 2 2; ; ; ;E I R X Z& &


77/176


78/176


79/176

KY THUAT IEN 77

TS. Lu The Vinh

6.5. CHE O KHONG TAI CUA MAY BIEN AP

6.5.1. Phng trnh va s o thay the MBA khong tai.

Che o khong tai cua MBA la che o h mach th cap (I2=0), concuon s cap noi vao nguon xoay chieu u1.

Phng trnh can bang ien ap:

1 1 0 1 0 1 0 0( )thI E I Z Z I Z= = + =& & & & &U Z (6-28)

trong o: 0 1( thZ Z Z= + ) la tong tr MBA khong tai.

S o thay the MBA khong tai ve tren hnh 6-11

6.5.2. Cac ac iem cua MBA khong tai.

U1

i

X1

X

R

R1

0

th

th

a) Dong khong tai.T phng trnh (6-28) ta co dong

khong tai I0:

1 10 20 1( )

21 th th

U UI =

Z (R + R ) X X =

+ +

Tong tr Z0 thng rat ln, nendong khong tai I0 ch nho bang 2% - 10% dong ien nh mc.

Hnh 6-11

b) Cong suat khong tai.

che o khong tai, cong suat a ra pha th cap bang khong,nhng MBA van tieu thu mot cong suat P0 . Cong suat nay bao gom tonhao nhiet jun-lenx tren ien tr day quan s cap PR1 va cong suat tonhao sat t Pst tren loi sat. V dong khong tai nho nen ton hao nhiet jun-lenx khong ang ke, va co xem gan ung:

P0Pst (6-29)

c) He so cong suat khong tai. che o khong tai cong suat phan khang Q0 rat ln so vi cong

suat tac dung P0, do o he so cong suat luc nay rat thap:

0 02 2 2 2

0 0 0 0

cos 0,1 0,3R P

R X P Q = = =

+ + (6-30)

T o ta thay rang, khi s dung khong nen e MBA che okhong tai hoac non tai.


80/176

KY THUAT IEN78

TS. Lu The Vinh

6.5.3. Th nghiem khong tai MBA .

Th nghiem khong tai nham xac nh cac tham so cua MBA nhhe so bien ap k, ton hao sat t va cac thong so cua may che o khongtai. S o th nghiem nh tren hnh 6-12.

WA

**I 0

P0

V1 VU2 20U1

Nguon

ap

ieu

chnh

u ~

Hnh 6-12

at ien ap nh mc vao cuon s cap, th cap h mach, cacdung cu o cho ta cac so lieu sau:

Oat ke ch cong suat khong tai P0Pst .Ampeke ch dong khong taiI0.Cac von ke ch gia tr U1, U20..T o ta tnh c:

a) He so bien ap k1 1

2 2

W E Uk

W E U= = 1

20

b) Dong ien khong tai0

01

% 100% 3% 10%m

II

I= =

c) ien tr khong tai0

020

0 1 th

PR

IR R R

=

= +

V 1thR R nen co the lay gan ung:

0thR R

d) Tong tr khong tai.1

00

mUZI

=

Cung nh tren tong tr khong tai lay gan ung la:


81/176

KY THUAT IEN 79

TS. Lu The Vinh

0thZ Z

e) ien khang khong tai.

= 2 20 0X Z R0 ien khang t hoa lay gan ung la:

0thX X

f) He so cong suat khong tai.

= = 0

01 0

cos 0,1 0,3.m

P

U I

6.6. CHE O NGAN MACH CUA MAY BIEN AP

Che o ngan mach cua MBA la che o ma cuon th cap b noitat, trong khi cuon s cap van noi vi nguon cung cap. Trong qua trnhvan hanh, do nhieu nguyen nhan co the xay ra s co ngan mach cuaMBA nh 2 au day cuon th cap co the b chap, ri xuong at hoac bnoi tat bang mot day dan co tong tr thap.

6.6.1. Phng trnh va s o thay the cua MBA ngan mach.

Xn

U1

Rn

i1n

S o thay the cua MBA ngan mach ve tren hnh 6-13. Do tong trZ2 rat nho so vi nhanh t hoa Zth nenmot cach gan ung co the bo qua nhanht hoa. Dong ien s cap luc nay la dongien ngan machI1n.

Phng trnh can bang ien ap:

1 1( )'

n 2 nU I Z Z I Z = + =& & & n (6-31)

trong o: Hnh 6-13

1 1 1( ) ( )' ' '

n 2 2 2 n n nZ Z Z R R j X X R jX Z ej n= + = + + + = + =

1'

n 2R R R= + la ien tr ngan mach cua MBA

1'

n 2X X X= + la ien khang ngan mach cua MBA2

n nZ R X= +2n la tong tr ngan mach cua MBA

nZ la tong tr phc ngan mach cua MBA


82/176

KY THUAT IEN80

TS. Lu The Vinh

6.6.2. Cac ac iem cua che o ngan mach.a) Dong ien ngan mach.

T (6-31) ta co dong ngan mach la: 1 1'1 2n n

U UI =

Z Z + Z=&

& &

, hay ve gia

tr ta co:

2 21 1( ) ( )

1m 1mn ' '

n2 2

UI =

ZR R X X=

+ + +

U(6-32)

V tong tr ngan mach Zn rat nho nen dong ien ngan mach Inthng ln gap 1025 lan dong ien nh mc, gay nguy hiem cho MBAang van hanh va cac phu tai.

b) ien ap ngan mach. Khi ngan mach ien ap th cap U2 = 0, do oien ap ngan mach Un chnh la ien ap ri tren tong tr day quan thcap. V dong ngan mach rat ln nen de dan en chay no cuon day quanth cap cua MBA.

e tranh nguy hiem cho MBA phai co bien phap bao ve bang cacthiet b t ong ngat mach ca 2 pha cua MBA khi xay ra s co nganmach.

6.6.3.Th nghiem ngan mach MBA.

e xac nh cac thong so ngan mach va ton hao tren cac ien trday quancua MBA ngi ta thc hien cac th nghiem ngan mach MBAtheo s o thc nghiem sau ay (hnh 6-14).

W*

*

A

I2

1P

Un

Nguon

ap

ieu

chnh

u ~

mm

m

A

1V

1

2

Hnh 6-14

Day quan th cap c noi tat qua ampeke A2. Day quan s capc noi vi mot bo nguon co the ieu chnh mc ien ap sao cho dongngan mach nam trong gii han nh mc. Gia tr ien ap cho phep atvao cuon s cap goi la ien ap ngan mach Un c xac nh vao khoang

35% ien ap nh mc U1m.


83/176

KY THUAT IEN 81

TS. Lu The Vinh

1(%) 100% (3 10)%nn

m

UU

U= = (6-33)

V ien ap ngan mach nho nen t thong nho, do o ton hao sat

t khong ang ke. Cong suat o c trong th nghiem ngan mach Pnchnh la cong suat ton hao tren ien tr day quan s cap va th cap. To ta tnh c cac thong so trong s o thay the:

a) Tong tr ngan mach Zn.

1

nn

m

UZ

I= (6-34)

b) ien tr ngan mach.

21

nn

m

PR I= (6-35)

c) ien khang ngan mach2 2

n nX Z R= n (6-36)e tnh cac thong so day quan cua MBA ta co the s dung cac

cong thc gan ung sau ay:'

1 2 2nRR R (6-37)

'1 2 2

nXX X (6-38)

T o, khi biet he so bien ap k ta tnh c cac thong so cua MBAcha quy oi.

'2

2 2 ;R

Rk

va'2

2 2 ;X

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