Dong Dien 3 Pha

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Mn L

BI 17 - DNG IN 3 PHAA. Tr li cu hi k trc :1.Ti sao trong my pht in xoay chiu : stato c hai cun dy ? My pht in xoay chiu s dng nguyn tc cm ng in t : mi khi t thng qua mt khung dy hay cun dy bin thin th trong khung dy hoc cun dy xut hin sut in ng cm ng V th nu stato ca my pht ch c mt cun dy th khi roto quay trong cun dy cng xut hin sut in ng cm ng. T ta cng thy ngay nu t trn stato 2 hay 3, hay nhiu cun dy th trong cc cun dy cng u xut hin sut in ng cm ng. V vy, nu t nhiu cun dy lp kn khong khng gian c trong stato ta s tn dng c s bin thin ca t thng c sn do s quay ca roto. Tuy nhin, dng in cm ng sinh ra trong cc cun dy l cc dao ng in. Do chng c t lch nhau nn pha ca cc sut in ng sinh ra trong cc cun dy s khc nhau. Khi u cc cun dy ni tip hoc song song th cc dao ng lch nhau c th li lm yu nhau i. Nu t hai cun dy i nhau th hai sut in ng sinh ra s ngc pha nhau, do khi mc ni tip hoc song song 2 cun dy vi cc u dy u thch hp th hai sut in ng s cng vo nhau lm cho sut in ng pht ra mnh hn. Trong qu trnh t thm cc cun dy tn dng s bin thin ca t thng th ngi ta thy pht ra dng ba pha s c nhiu li ch nht. 2.Ti sao roto ca my pht thng dng nam chm in. + Nh ta bit, bin ca sut in ng do my pht ra l E0 = NBS th cho t trng B mnh hn khi s dng nam chm vnh cu. + Dng in cung cp cho roto s l mt phn dng in do chnh my pht ra. C 2 cch gii quyt : Cch 1 : Mc 1 my pht mt chiu ng trc vi my pht xoay chiu nhng c cng sut nh hn. Dng in do my pht mt chiu pht ra s c a vo nui nam chm in ca roto.

E0 t

l thun vi cm ng t B ca nam chm, do nu s dng nam chm in th c th lm

1


Mn L

Cch 2 : Tch mt phn dng in xoay chiu do my pht ra, ri dng ir chnh lu nn n thnh dng 1 chiu, ri a vo nui nam chm in ca roto.

B. Bi ging :

Dng in 3 pha

1/ nh ngha : Dng 3 pha l mt h thng ba dng mt pha, c gy ra bi ba sut in ng c cng tn s, c cng bin nhng lch nhau v pha l 2/ My pht in 3 pha. a) Cu to : (Xem SGK) b) Hot ng : Cc em cn ch : my pht in ba pha c cu to sao cho pht ra c dng in ba pha ng theo nh nh ngha nu trn. + V ba cun dy ging ht nhau nn khi roto quay, trong 3 cun dy xut hin 3 sut in ng c

2 . 3

E0 ging nhau v f nh nhau.

+ Khi cc N ca nam chm trc cun 1 th quay thm

e1 = 0 nhng e2, e3 0 , roto phi

2 th 3

e2 = 0, ri 23

na s n

e3 = 0 . Nh vy ba sut in ng sinh ra lch

pha nhau ng

2 . 3

c/ Mc mch 3 pha. Mc sao : Trong cch mc ny : dng in chy qua 3 pha l nhau thnh dng in i chy trong dy trung ho.

i1,i2,i3 s tng hp vi

!! !!" !!" !!" " i = i1 + i2 + i3 I 0 = I 01 + I 02 + I 03Nu ba pha hon ton ging nhau th i = 0. Tuy nhin, trong thc t khng bao gi ba pha ging nhau hon ton, do , trn dy trung ho lun c 1 dng i 0 . Trong cch mc sao Udy = Idy = Ipha * Mc tam gic ; Ch s dng khi 3 pha hon ton u c c im : Upha = Udy ;Idy =

3U

pha

3 Ipha.2


Mn L

V d 1 : Cho mch in 3 pha mc theo hnh sao, c hiu in th hiu dng trong 1 pha l

U p = 220V , tn s f = 50 Hz .a) Chng minh rng hiu in th gia hai dy pha khc nhau c gi tr hiu dng Udy =

3U

pha

.

b) mch tiu th : Pha 1 : Gm 1 cun dy

1 L = (H )

mc ni tip vi mt in

tr R1 = 100 3 . Pha 2 : Gm 1 in tr R2 = 100 3 , ni tip 1 t C = Ch c mt in tr R3 = 400 .

104

( F ) . Pha 3 :

+ Tnh dng in chy trong cc pha v dng in chy trn dy trung ho. + Tnh cng sut ca mch ba pha ny. GII a) Chng minh : U d = 3U p Ti 1 thi im bt k, theo nh lut m, ta lun c : A

U phaL

R1

U AB = U AO + U OBV mi hiu in th l mt dao ng iu ho nn c th biu din bng vecto bin . Nu chia c hai v cho

U day

Trung hoa

R2 R3 B C

2 , ta chuyn

C

v cc gi tr hiu dng :

!!!! !!!! !!!! !!!! " " " " !!!! " UAB = UAO +UOB = UAO + (UBO )UABl hiu in th ng dy;

U AO l hiu in th ca pha 1 v U BO l hiu in!" !

th ca pha 2. Vy :

!!!! !!! " " !!! " U day = U P1 + (U P2 )3nn gc gia

!!! M " U P2

UH

d

" 2 !!! " !!! V gc gia U P1 v U P2 l

O

!!! " U P1

N

3

!!! " U P2


Mn L

!!! " !!! " # U P1 v U P2 l M O N = OM N u :3 OH = ON Ud = U P1 2 3 2 3 U d = U P1 2

3 = 220. 3 (V )

b) Tnh dng in I : * pha 1 :

Z L = L = Z1 =

1

!!" UL.2 .50 = 100 (100 3 ) 2 + 100 2 = 200 1

!!! " U P1

2 R12 + Z L =

220 I1 = = = 1,1 A 200 Z1

Up

! " I1

!!! " U R1

i1 tr pha hn U* Ti pha 2 :

p1

gc 1 c

tg1 =

U L ZL 100 1 = tg1 = = 1 = U R1 R1 6 100 3 3!!!" ! UR2

ZC = ZL = I2 =

1 1 = 4 = 100 C 10 .2 .50

2 C

2

!! " I2

R +Z2 L

=

(100 3 ) + 100 = 200 2

2

Up Z2

=

220 = 1,1 A 200

i2

sm pha hn

U P2 gc 2

tg 2 =

UC Z 100 1 = C = = 2 = U R2 R2 100 3 6 3

!!! " UC

!!! " UP2

* Ti pha 3 :

I3 =

U

p

R3

=

220 = 0, 5 5 A 400

i 3 ng pha vi U 3* tm dng in trn dy trung ho ta phi tng hp cc dng in chy trong cc pha :

4


Mn L

+ Ta c 3 vct

!!! !!! !!! " " " U P1 , U P2 , U P3 to vi nhau mt gc 23:

.

V

i1 tr hn U 1

6

!!! " U P3

!" !! " Suy ra, gc gia I1 , I 2 l

!" I3

# 2 I10I2 = ( + ) = 3 6 6 3V ln

!" 1 I 1H

!!! " UP1

I2 = I 1 0I1I2 u3 I 3 12 = I1 . 2 2 2

2

H = I1Vy :

!!! " UP2

!" ! I2

!!" I 12

D thy

!! " I12 hng theo ng phn gic ca gc !! " # UP UP , do I12 hng ngc vi I 3 ,1 2

I12 = I1 3 = 1,1. 3 $ 1,905 A

suy ra dng in trn dy trung ho :

I = I12 I3 = 1,905 0,55 = 1,355 A* Cng sut ca dng ba pha.2 P = Ppha1 + Ppha2 + Ppha3 = I12 R1 + I2 R2 + I32 R3

= 1,12.100 3 +1,12.100 3 + 0,552.400 $ 540WV d 2 : Cho mt mch in ba pha mc theo hnh sao c U p = 120V , tn s mi pha u c mt cun dy L =

f = 50Hz .

2

H mc ni tip vi mt in tr R = 200 .

Tnh I chy trong mt pha. Chng minh dng in trong dy trung ho Nu ba pha b t th dng in trn dy trung ho bng bao nhiu. Ba pha vn b t v pha 2 : thay cun L bng t C =

I = 0.

10 4 F th dng in trn dy 2

trung ho bng bao nhiu ? GII a)

5


Mn L

Z L = L = Z = I1 =2

2

.100 = 200 200 2 + 200 2 = 200 2

!!" UL

!!! " U P1

2 R + ZL =

U P1 Z

=

120 = 0, 3 2 ( A ) $ 0, 424 A 200 2

1!" I1

i 1 tr pha hn UP1 gc 1 c tg1 =

UL ZL = =1 UR1 R1

!!! " UR

* Vi pha 2 v 3, ta cng thu c kt qu tng t : i u tr pha hn U P tng ng gc

4

.

Nh vy, c 3 dng in c ln bng nhau v cng tr pha hn UP :

!" !! !" " 2 gc gia cc vct I1, I2 , I3 cng l . 4 3

!! " I2

Tng hp

!" I3ln

! " !! " !!" I1 vi I2 ta c vct I12

hng ngc

. D thy

OI1I12 l tam gic u, suy ra v !! " !" I12 = I2 = I3 tng hp I12 vi I3 s bng 0.

!! " I12

!" I3

Dy trung ho khng c in. I = 0 . b) Nu pha 3 b t th dng in chy trn

!" I3

!" !! " dy trung ho l tng hp ca I1 , I 2 nh chng minh trn l I12 = I = 0,424 A . !! " !!! " c/ Thay L pha hai bng C. I2 UR 1 1 ZC = = 4 = 200 . 2 C 10 .100 2suy ra, v ln tng tr ca pha 2 khng i

I 2 = 0, 3 2 ( A) nh c nhng

i2 sm pha hn

!!! " UC

!!! " UP2

U

P2

gc

4

.

Dng in trn dy trung ho :

" ! !" " ! I = I1 + I2 .

!!! " UP1

# 2 ( + ) = = = 5 D thy I1 0I 2 = 3 4 4 6 6 6Theo nh l hm cos : 6

2

H

1 !"I1

!!! " U P2

!! " I2

" I


Mn L

2 I = I12 + I2 2I1I2cos = I1 1 +1 2.cos

5 = 6

I = 0,3 2. 2 + 2.

3 = 0,3 4 + 2 3 $ 0,82 A 2

CU HI V BI TP V NH 1/ Lm th no chng minh c cng thc : I day = 3I pha trong cch mc tam gic ? 2/ Trong mch in mt chiu v mch xoay chiu mt pha : dy no l dy mt ? 3/ Lm cc bi tp trong B thi i hc : 61 (2), 77 (2), 80 (2).

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Dong Dien 3 Pha