2.2. C TNH C CA NG C KHNG NG B 2.2.1. Xy dng c tnh phn tch trng thi lm vic xc lp ca ng c khng ng b ta s dng s thay th. Gi thit: - in p li hon ton hnh sin v 3 pha ca ng c l i xng. - in tr dy qun rto v stato coi nh khng i trong qu trnh lm vic lu di ca ng c 75oC. - Mch t cha bo ho nn coi in khng stato v in khng rto quy i v pha stator khng thay i. - in tr mch t ho v in khng mch t ho coi nh khng i, dng in t ho I

I1 I Uf X R

X1

R1

I

' 2

' X2

Hnh2.30 S thay th mt pha ca ng c khng ng b

khng ph thuc vo ph ti m ch ph thuc vo in p t vo stator ca ng c. - B qua tn tht do ma st, tn tht trong li thp. T cc gi thit trn ta a ra s thay th ca ng c khng ng b 3 pha bng s thay th 1 pha (cc pha khc tng t) Uf: Tr s hiu dng ca in p pha stato. I, I1, I2: Dng in t ho, dng stato, dng r to quy i v stato. X, X1, X2: in khng mch t ho, in khng Stato, in khng r to quy i v stato. R, R1, R2: in tr tc dng ca mch t ho, in tr cun dy statov ca r to quy i v stato. s: trt ca ng c:s=

R '2 s

1 1

2f1 1: tc gc ca t trng quay: 1 = p

Vi:

f1: p: :

tn s in p ngun t vo stato. s i cc ca ng c. tc gc ca ng c.

a. c tnh dng in rto ca ng c' I2 =

Uf' R2 2 ' ( R1 + ) + ( X 1 + X 2 ) 2 s

s

1 0 Rf=0 Rf 0 0 1 I2nm I1nm I

' I2 =

Uf ( R1 + R 2 2 ) + X nm s' 2

Khi = 1 s = 0 th I2 = 0 Khi = 0 s = 1U f1' 2 2 th I2 = ( R1 + R2 ) + X nm ' = I 2 nm

Hnh2.31 c tnh dng R to ca C KB

Vi I2nm l dng in ngn mch ca roto b.c tnh c ca ng c tm phng trnh c tnh c ta da vo iu kin cn bng cng sut trong ng c. - Cng sut in t chuyn t stato sang r to: Mt : m men in t ca ng c. - Cng sut a ra trc ng c: Pc = Mc. Nu b qua cc tn tht ph th: Mt = Mc =M P2: cng sut tn tht ng trong rto P2 = Pt Pc P2 = M( 1 - ) = M 1s Mt khc: P2 = 3R2(I2)2 (2.69) (2.70)' ' 3I 22 R 2 (2.71) 1 .s

P12 = Mt 1

T phng trnh (2.69) v (2.70) ta c M =

Thay gi tr I2 tnh c trn vo (2.71) v bin i ta c:

M =

' 3U 21 R 2 f

1 s[( R1 +

' R2 2 2 ) + X nm ] s

s

Biu thc trn c gi l c tnh c ca ng c KB. *V dng c tnh c:sthFth M dM 0 tm cc tr ca ng cong ny ta gii phng trnh 1 = 0. Kt qu l ta s c MthF ds th tr s ca M v s ti im cc tr gi l m men v trt ti hn k hiu l Mth, sth.

M

s

sth = M th =

' R2 2 R12 + X nm

=0 s=1 Hnh2.32 . c tnh c ca my in khng ng b

3U 21 f2 21 ( R1 R12 + X nm )

Trong cc biu thc trn du + ng vi trng thi ng c cn du (-) ng vi trng thi my pht. Ngoi ra khi nghin cu cc h truyn ng s dng ng c KB ngi ta quan tm nhiu n trng thi lm vic ca ng c nn ng c tnh c lc ny thng biu din trong khong tc 0 s sth. Phng trnh c tnh c ca ng c KB c th s din thun tin hn bng cch biu 1 lp t s gia M v Mth. Bin i ta c phng trnh m tnh c dng: c (1)2 M th (1 + .s th ) M = s + s th + 2 .s th s th s(2) TN (Rf=0)

Trong =R1/R2

NT (Rf 0)

1 MX Mth M i vi cc ng c c cng sut ln R1 thng rt0nh so vi m nm. Lc ny c th b Hnh 2.33 2M th c C KB c tnh = = trong ch qua R1 ngha l = 0 v phng trnh c tnh c tr thnh: Mf(M) s sth ng c

sthVi sth = R 2/Xnm;

+

s

3U 21 f Mth = 21X nm

Nhiu trng hp cho php ta s dng nhng c tnh gn ng bng cch tuyn tnh ho cc c tnh trong on lm vic. V d vng c trt s >sth b qua sth/s, phng trnh c tnh c tr thnh: M=

2M thsth ; s

=

2M thsth 1s2

Trong on ny dng, c gi tr bin i. 2.2.2. nh hng ca cc thng s ti c tnh c T phng trnh c tnh c KBM =' 3U 21 R 2 f ' R2 2 2 ) + X nm ] s

1 s[( R1 +

Ta thy c cc thng s sau nh hng n c tnh c: - in tr, in khng mch stato R1, X1 - in tr mch r to (ni thm in tr ph R2f sr to vi ng c KB r to dy vo Mc2 TN qun). 1 (U ) - in p li cp cho ng c. - Tn s ca li in. - S i cc P Khi nghin cu nh hng ca thng s no n c tnh c ta coi cc tham s cn li l khng i. a. nh hng ca in p li ti c tnh cM 0 1 Mnm3 M Mnm2 nm1 MnmTN MthTNm

m

Mc1

U3

U2

U1

Hnh2.34 c tnh c ca ng c KB khi gim in p

sth = M th =

' R2 2 R12 + X nm

3U 21 f2 21 ( R1 R12 + X nm )

1 =

2f1 p

Khi Uf thay i di Um th m men ti hn s gim bnh phng ln suy gim ca in p cn tc ng b 1 gi nguyn v trt ti hn sth khng i. Ta c dng c tnh c khi gim in p li trn hnh v. c tnh ny ph hp vi ti l bm v qut gi, khng thch hp cho ti l hng s. Khi thay i in p t vo stato th ta c th iu chnh tc ca ng c khng ng b v c th hn ch dng in khi ng. b. nh hng ca in tr, in khng ph mch statoA B C A B C

R1f

R1f

R1f

X1f

X1f

X1f

C

C

sth = sth = M th =

' R2

(b) (a) Hnh2.35. S nguyn l vi R1f (a), vi X1f (b)

2 ( R1 + R1 f ) 2 + X nm ' R2

;

1 m sth

s

TN

X1f

R12 + ( X nm + X 1 f ) 2 3U 21 f2 21 ( R1 + R1 f ) + R1 + R1 f ) 2 + X nm )

R1f 0 1 Mnm MnmTN Mth

(Mth khi a X1fvo tng t )

M

2f1 1 = p

Hnh2.36 c tnh c ca ng c KB khi a thm R1f v X1f

Ta thy rng khi ni thm in tr ph R1f hoc in khng ph X1f vo mch stato th 1 = const, sth gim , Mth v Mk gim nn c tnh c c dng nh hnh v ng dng a in tr ph R1f hoc in khng ph X1f vo mch stato c th gim dng khi khi ng v dng iu chnh tc . Nu X1f = R1f ta thy c tnh i vi X1f tt hn v cng c tnh c khi a in khng ph ln hn cng c tnh khi a in tr ph vo mch stator v tn tht trn in tr ph nhiu hn nn trn thc t ngi ta thng a thm in khng ph vo mch stator. c. nh hng ca s i cc p Vi ng c khng ng b rto lng sc nhiu cp tc iu chnh tc ca n ngi ta thay i s i cc mch stator bng cch thay i cch u dy stato.

1 =sth =

2f1 ; p' R2

s=

1 1M th =

= 1(1 - s)3U 21 f

12

s P2

2 R12 + X nm

;

Khi thay i s i cc p th tc ng b 1 thay i v do cng thay i theo. Cn sth khng ph thuc p nn khng i ngha l cng c tnh c khng i ta c h c tnh nh sau d. nh hng ca tn s li in f1Hnh 2.37 M Mnm2 tc Mnm1 Mth1 Nu cung cp cho ng c bi ngun in c tn s thay i th Mth2 ca ng c s 0 1 thay i v dng ca c tnh c cng thay i.' 2f1 R2 1 = ; sth = 2 p R12 + X nm

2 21 ( R1 R12 + X nm ) 11

P1

s

11 1m 12 f11 f1m f12 M 0 1 Mth Hnh2.38 c tnh ca ng c KB khi thay i tn s f12