B CNG THNG

Trng Cao ng Cng Nghip Phc Yn

BI GING

MY IN

(Lu hnh ni b)

Vnh Phc, thng 5 nm 2013

a

LI NI U

Cun bi ging ny ra i nhm mc ch cung cp nhng kin thc c bn v my in v cc phng php tnh ton v gii quyt cc loi my in. Ni dung

bi ging phc v cho sinh vin ngnh Cng ngh k thut iu khin v t ng ho ca cc trng Cao ng, i hc. ng thi cng gip ch cho sinh vin cc chuyn

ngnh khc v cc cn b k thut, ging vin quan tm n lnh vc my in. Khi xy dng cun bi ging ny chng ti cng tham kho kinh nghim t

cc nh gio nhiu nm tham gia ging dy mn hc ny ti cc trng Cao ng,

i hc. ng thi cng cp nht nhng ni dung kin thc mi, va p ng yu

cu nng cao cht lng o to phc v s nghip cng nghip ho hin i ho va m bo tnh thc tin t nhng loi my in cng nh nhng phng php tnh

ton m cc cn b k thut in ang vn hnh trong thc t. Tuy cc tc gi cng c gng trong qu trnh bin son, nhng cun bi

ging ny cng khng trnh khi nhng hn ch, khim khuyt. Chng ti mong nhn

c s ng gp kin ca cc qu ng nghip v cc bn sinh vin cun bi ging ny hon thin hn.

Cui cng chng ti xin by t li cm n chn thnh n s ng h, ng gp kin qu bu ca cc thy c gio ng nghip trong b mn in x nghip, cm

n khoa in T ng ho, cm n trng Cao ng cng nghip Phc Yn gip v to iu kin chng ti hon thnh cun bi ging ny.

Tc gi

b

MC LC

Ni dung Trang Chng 1. MY BIN P 1 1.1. Khi nim chung 1 1.1.1. nh ngha my bin p 1 1.1.2. Cc lng nh mc ca my bin p 1 1.2. Cu to ca my bin p 2 1.2.1. Li thp 2 1.2.2. Dy qun my bin p 2 1.2.3. V my bin p 3 1.3. Nguyn l lm vic ca my bin p 1 pha 4 1.4. Phng trnh cn bng in v t ca my bin p 5 1.4.1. Phng trnh cn bng in s cp 6 1.4.2. Phng trnh cn bng in th cp 6 1.4.3. Phng trnh cn bng t 6 1.5. S thay th ca my bin p 7 1.6. Tn hao v hiu sut ca my bin p 8 1.7. Bin i in p ba pha 8 1.7.1. My bin p 3 pha 9 1.7.2. Ni dy my bin p 3 pha 9 1.7.3. T ni dy my bin p 3 pha 10 1.8. My bin p lm vic song song 10 Chng 2. MY IN KHNG NG B 12 2.1. Nguyn l lm vic c bn ca my in khng ng b 12 2.1.1. Khi rto quay theo chiu t trng vi tc 00) 13 2.1.2. Khi rto quay theo chiu t trng quay vi tc n>n1 (s

c

4.1.1. Cu to ca my in mt chiu 4.1.2. Phn loi my in mt chiu 4.2. Nguyn l thun nghch ca my in mt chiu 4.2.1. Ch my pht in 4.2.2. Ch ng c in 4.3. M my ng c in mt chiu 4.3.1. Yu cu m my ng c in mt chiu 4.3.2. Cc phng php m my ng c in mt chiu 4.4. iu chnh tc ng c in mt chiu 4.4.1. ng c in mt chiu kch thch song song 4.4.2. ng c in mt chiu kch t ni tip PHN THAM KHO

37 39 39 40 41 42 42 42 45 45 48 51

d

TI LIU THAM KHO

[1]. ng Vn o (2002), Gio trnh My in, Nh xut bn gio dc [2]. Nguyn Hng Thanh (1997), My in trong h thng t ng, NXB gio dc [3]. Trn Khnh H (2010), My in I, II, Nh xut bn khoa hc v k thut. [4]. Trn Khnh H (2000), Thit k my in, Nh xut bn khoa hc v k thut. [5]. V Trung Hc chuyn nghip dy ngh (2002), Gio trnh my in, Nh xut bn gio dc. [6]. V Trung Hc chuyn nghip dy ngh (2002), Gio trnh k thut iu khin ng c in, Nh xut bn gio dc [7]. L c Ton (2007), Bi ging my in hng hi, Nh Xut Bn thnh ph H Ch Minh. [8]. ng Vn o (2002), Gio trnh My in, NXBGD [9]. Nguyn Hng Thanh (1997), My in trong h thng t ng, NXB GD [10]. http://Tailieu.vn

1

Chng 1. MY BIN P Trong phn ny ni dung ch yu c cp n nhm gip ngi c hiu c cu to v nguyn l lm vic chung ca my bin p, bit phng trnh cn bng p, s thay th v cc thng s k thut ca my bin p.

Ngoi ra ngi c cn hiu c phng php bin i in p ba pha v t ni dy ca my bin p 3 pha. 1.1 Khi nim chung 1.1.1. nh ngha my bin p

My bin p l loi my in tnh, dng bin i in p ca h thng in xoay chiu nhng vn gia nguyn tn s ca h thng. My bin p c 2 ca: ca ni vi ngun in gi l s cp ca my bin p, ca ni vi ti gi l th cp ca my bin p.

Cc i lng, thng s s cp trong k hiu c ghi ch s 1: s vng dy cun dy s cp W1, in p s cp: U1, dng in s cp: I1, cng sut s cp: S1, P1

Cc i lng, thng s th cp trong k hiu c ghi ch s 2: s vng dy cun dy th cp W2, in p s cp: U2, dng in s cp: I2, cng sut s cp: S2, P2. K hiu ca my bin p v s nguyn l ca my bin p mt pha nh hnh 1.1.

(a) (b)

Hnh 1.1. K hiu (a) v s nguyn l (b) ca my bin p mt pha My bin p c vai tr quan trng trong h thng in. C 2 dng my bin p chnh:

My bin p in lc c dng trong h thng truyn ti v phn phi in nng, lm nhim v: nng in p u ra my pht in (thng t 6,3 n 38,5 kV) ln mc in p ca ng dy truyn ti (thng l 35, 110, 220 v 500 kV) v h in p ng dy xung mc in p cung cp cho cc ti (thng c cc mc 3kV hoc 6kV v 110V n 500V).

My bin p chuyn dng c dng trong cc thit b: xe in, l in, hn in, o lng v.v 1.1.2. Cc lng nh mc ca my bin p

Cc lng nh mc ca my bin p l cc thng s k thut ca my do nh sn xut my qui nh.

- in p nh mc s cp, K hiu U1m l in p qui nh cho cun dy s cp.

- in p nh mc th cp, K hiu U2m l in p gia cc cc ca cun th cp khi th cp h mch v in p s cp l nh mc. Theo qui c, vi my bin p 1 pha, in p nh mc l in p pha; Vi my bin p 3 pha in p nh mc l in p dy. n v ca in p ghi trn my bin p thng l kV.

- Dng in nh mc l dng in qui nh cho mi cun dy ca my bin p ng vi cng sut nh mc v in p nh mc. Theo qui c, vi my bin p 1 pha, dng in nh mc l dng in pha. Vi my bin p 3 pha dng in nh mc l dng in dy. Dng in nh mc s cp, k hiu l I1m, dng in nh mc th cp, k hiu l I2m. n v dng in ghi trn my bin p thng l A.

- Cng sut nh mc, k hiu Sm (n v o kVA), l cng sut biu kin a

2

ra cun dy th cp my bin p khi in p, dng in my bin p nh mc. i vi my bin p 1 pha, cng sut nh mc l: Sm = U2m.I2m U1m.I1m (1.1) i vi my bin p 3 pha, cng sut nh mc l: Sm = 3 U2m.I2m 3 U1m.I1m (1.2)

Ngoi ra trn nhn my bin p cn ghi tn s, s pha, s ni dy v t ni dy, in p ngn mch, ch lm vicca my. 1.2. Cu to ca my bin p

My bin p c cc b phn chnh sau: Li thp, dy qun v v my 1.2.1. Li thp Li thp my bin p dng dn t thng chnh ca my. gim dng in xoy trong li thp, ngi ta ghp li thp bng cc l thp k thut in.

Phn li thp c lng cun dy gi l tr ca li thp. Phn li thp ni cc tr vi nhau thnh mch t khp kn gi l gng ca li thp. Tit din ca gng c dng hnh ch nht.

Hnh 1.2. Tit din tr li thp ca my bin p

Tit din ca tr, i vi my bin p cng sut nh th c dng hnh ch nht. i vi my bin p cng sut ln th c dng hnh bc thang nh hnh 1.2.

Gng v tr c th ghp vi nhau theo phng php ghp ni hay ghp xen k. Ghp ni th tr v gng ghp ring, sau dng x p v bu lng vt cht li nh hnh 1.3a.

(a) (b)

Hnh 1.3. Ghp ni gia tr v gng ring (a) v xen k (b) Ghp xen k th ton b li thp phi ghp ng thi, cc l thp c xp xen

k nhau theo th t nh m t hnh 1.3b. an ton li thp c ni vi v v v phi c ni t.

1.2.2. Dy qun my bin p Dy qun my bin p thng c ch to bng dy ng hoc nhm, c tit

din trn hoc ch nht, mt ngoi dy c bc lp cch in. Mi cun dy ca my bin p gm 1 s vng dy qun thnh 1 s lp chng

ln nhau. Gia cc lp dy ca 1 cun dy. Gia cc cun dy vi nhau v gia cun dy vi li thp u c lp cch in.

Mt pha ca my bin p thng c 2 cun dy, cun dy ni vo in p cao gi l cun cao p, cun dy ni vo in p thp gi l cun h p. Khi cun cao p v cun h p cng qun trn 1 tr trong kiu dy qun ng tm, th cun h p c

3

qun st tr, cn cun cao p qun ngoi cun h p nh hnh 1.4. Lm nh vy s gim c vt liu cch in.

Hnh 1.4. Dy qun ng tm Ngoi kiu qun dy ng tm cn c kiu qun dy xen k, nh biu din trn

hnh 1.5. Trong kiu qun ny, mi cun dy cao v h p gm mt s bnh dy t xen k nhau.

Hnh 1.5. Dy qun xen k

1.2.3. V my bin p V my bin p gm 2 phn: Thng v np thng:

Thng my bin p: Thng dng cha my bin p v cha du. Du my bin p dng tn nhit cho my v tng cng cch in. Thng my lm bng thp. Cc my cng sut nh ( 30KVA) thng c v trn. Cc my cng sut va v ln, tng kh nng to nhit, v thng c lm theo kiu dp sng hoc c gn cc ng tn nhit hay b tn nhit nh hnh 1.6.

Np thng: Np thng dng y kn thng v lp cc chi tit nh: Tr s ca cc u dy cao p v h p (c nhim v cch in gia cc u dy ra vi v my). Bnh gin du: l 1 thng hnh tr bng thp, t trn np v ni thng vi thng my bin p bng 1 ng. 1 u ca bnh c gn 1 ng ch mc du dng theo di mc

4

du bn trong. Bnh gin du to khng gian cho du trong thng my bin p gin n t do, m bo cho p sut du khng tng v thng lun y du.

(a) (b)

Hnh 1.6. V thng dp sng (a), v thng c ng tn nhit (b) ng bo him: thng c dng hnh tr, t nghing, mt u thng vi thng

my bin p, mt u bt kn bng 1 a thu tinh. Khi p sut trong thng my bin p t ngt tng ln qu ln, a thu tinh s v du du thot ra ngoi, my bin p s khng b hng.

B phn tuyn ng ca cu dao i ni cc u iu chnh in p ca dy qun cao p. 1.3. Nguyn l lm vic ca my bin p 1 pha

Hnh 1.7 l s nguyn l ca my bin p 1 pha c 2 cun dy: cun s cp c W1 vng, cun th cp c W2 vng.

Hnh 1.7. S nguyn l ca my bin p mt pha

Khi ta ni cun s cp W1 vo ngun in xoay chiu c in p u1, tn s f, trong cun W1 s c dng in i1. Dng in i1 sinh ra t thng bin thin chy kn trong li thp xuyn qua c 2 cun dy W1, W2 v c gi l t thng chnh.

Theo nh lut cm ng in t, t thng bin thin s lm cm ng trong cun dy s cp sc in ng:

dt

dWe

11 (1.3)

v trong cun dy s cp sc in ng:

dt

dWe

22 (1.4)

Khi my bin p khng ti (cun th cp h mch), dng in th cp i =0, t thng chnh ch do dng in s cp i1 sinh ra. Khi my bin p c ti, cun th cp ca my c ni vi ti c tr khng Zt, sc in ng e2 s to ra dng in th cp i2 chy qua ti v cun W2. Cun W2 cng sinh ra t thng chy trong li thp v t thng chnh lc ny do ng thi 2 dng in i1 v i2 sinh ra.

in p u1 l hnh sin nn t thng cng bin thin hnh sin: = maxsint th

5

vo (1.3), (1.4) ta c:

)2

sin(2)2

sin(2...44,4)sin( 11111

tEtWftdt

dW

dt

dWe mm

)2

sin(2)2

sin(2...44,4)sin( 22222

tEtWftdt

dW

dt

dWe mm

Trong : E1 = 4,44.f.W1.m E1 = 4,44.f.W1.m (1.5) E1, E2 trong biu thc (1.5) l gi tr hiu dng ca sc in ng cun dy s cp v th cp my bin p. Biu thc ca e1, e2 cho thy cc sc in ng ny c cng tn s nhng khc nhau v tr hiu dng.

T s: 2

1

2

1

W

W

E

E = K (1.6)

Trong K c gi l h s my bin p. Nu b qua in tr dy qun v t thng tn ra ngoi li thp, s c cc quan

h gn ng U1 E1, U2 E2, v c:

2

1

2

1

2

1

W

W

E

E

U

U = K (1.7)

Ngha l t s in p gia s cp v th cp bin p gn ng bng t s vng dy ca 2 cun.

Nu b qua tn hao trong my bin p, s c quan h gn ng: U1I1 U2I2

Hay: 1

2

2

1

I

I

U

U K (1.8)

Nh vy, trong my bin p, gia cun dy s cp v th cp khng c s lin trc tip v in, nng lng c truyn t s cp sang th cp nh t thng chnh trong li thp. 1.4. Phng trnh cn bng in v t ca my bin p

vit h phng trnh, ta chn chiu dng in, in p s cp v th cp bin p nh hnh 7. Theo qui tc vn nt chai, chiu t thng ph hp vi chiu i1, chiu e1, e2 ph hp vi chiu . Chiu i2 c chn ngc chiu e2, do chiu t thng do i2 sinh ra ngc chiu .

Trong my bin p, ngoi t thng chnh chy trong li thp cn c cc t thng tn ca cc cun dy, nh ngha nh sau:

- T thng tn mc vng cun dy s cp, k hiu t1 l t thng do cun s cp W1 sinh ra v ch mc vng ring cun s cp.

- T thng tn mc vng cun dy th cp, k hiu t2 l t thng do cun th cp W2 sinh ra v ch mc vng ring cun th cp.

ng i ca t thng tn c nhng on ngoi li thp, c t tr ln, nn t thng tn rt nh so vi t thng chnh.

T thng tn s cp t1 sinh ra trong cun W1 sc in ng cm ng et1

dt

diL

dt

de tt

11

11

(1.9)

Trong : 1

11 i

L t

gi l in cm tn s cp.

T thng tn s cp t2 sinh ra trong cun W2 sc in ng cm ng et2

6

dt

diL

dt

de tt

22

22

(1.10)

Trong : 2

22 i

L t

gi l in cm tn th cp.

1.4.1. Phng trnh cn bng in s cp Trong mch vng s cp c cc in p v sc in ng: in p u1, in p trn in tr dy qun s cp (r1) l i1r1, sc in ng do t thng chnh sinh ra e1,

sc in ng do t thng tn s cp sinh ra dt

diLet

111 . Phng trnh theo lut

Kirhoff 2 vit cho mch vng s cp l:

dt

diLeeeuir t

11111111

Hay: 11

1111 edt

diLiru

Vit di dng phc: 1.

1

.

11

.

1

.

11

.

11

.

EIZEIjXIrU (1.11) Trong : X1 = Lt1 l in cm tn dy qun s cp. Z1 = r1 + jX1 gi l tng tr dy qun s cp. 1.4.2. Phng trnh cn bng in th cp Tng t nh mch s cp, phng trnh theo lut Kirhoff 2 vit cho mch vng th cp l:

dt

diLeeeuir t

22222222

Hay: 22

2222 edt

diLiru

Vit di dng phc: 2.

2

.

22

.

2

.

22

.

22

.

EIZEIjXIrU (1.12) Trong : X2 = Lt2 l in cm tn dy qun th cp. Z2 = r2 + jX2 gi l tng tr dy qun th cp.

V in p th cp u2 chnh l in p trn ti: 2.

22

.

IZU (1.13) 1.4.3. Phng trnh cn bng t

V in khng tn X1 v in tr dy qun s cp r1 rt nh, nn in p trn

cc phn t ),( 1.

11

.

1 IjXIr cng rt nh so vi 1.

E , do t phng trnh (1.11) c quan h gn ng: U1 E1

V in p t vo s cp bin p U1 khng i, nn sc in ng E1 cng khng i. T (1.5) suy ra bin t thng chnh m khng i.

ch khng ti, t thng chnh do sc t ng ca cun dy s cp W1i1 sinh ra. Khi c ti, t thng chnh do tng i s cc sc t ng ca cun s cp v th cp (W1i1 - W2i2) sinh ra. Sc t ng th cp W2i2 ly du m (-) l do chiu i2 khng ph hp vi chiu theo qui tc vn nt chai.

V m khng i nn sc t ng lc khng ti bng sc t ng lc c ti, tc l: q.W1i0 = W1i1 - W2i2 (1.14) Trong i0 l dng in s cp khi khng ti v c gi l dng in khng ti hoc dng in t ho ca my bin p. (1.14) gi l phng trnh cn bng t ca my bin p. Chia c 2 v ca (1.14) cho W1 v thay:

7

2

1

W

W= K;

K

ii 22' ta c: i1 = i0 + i

'2 (1.15)

Trong K

ii 22' gi l dng in th cp qui i v s cp.

Phng trnh cn bng t di dng phc:

'

2

.

0

.

1

.

III (1.16) 1.5. S thay th ca my bin p thun li cho vic phn tch, nghin cu my bin p, ta tm cch thay th my bin p bng mt s mch c qu trnh nng lng tng ng vi my bin p, tc l h phng trnh mch hon ton ng nht vi h phng trnh my bin p.

Nhn 2 v ca (1.12) vi K v thay '

2

.

2

.

IKI v 2.

1

.

EKE , ta c:

1.

2

.

22

2

.

22

2

.

.... EIXjKIrKUK (1.17)

t: 22'

2 .rKr vi '

2r gi l in tr dy qun th cp quy i v mch s cp.

22'

2 .XKX vi '2X gi l in khng tn dy qun th cp quy i v mch s

cp. r2 + jX

2 = K

2(r2 + jX2) = K2.Z2 = Z

2 vi

'2Z gi l tr khng dy qun th cp

quy i v mch s cp. Zt = K

2.Zt vi 'tZ gi l tr khng ti quy i v mch s cp.

2

.

UK = 2.

'U vi 2.

'U gi l in p th cp quy i v s cp.

2

.'U = 2

.

UK = 2.

.. IZK t =.

2'2 .. IZK t = 2

.'' .IZ t v th vo (1.17) ta c:

1

.

2

.'

2'

2

.'

2'

2

.' .. EIjXIrU (1.18)

Xt s hng ( 1.

E ), trong 1.

E l sc in ng do t thng chnh gy ra trong

cun dy s cp. M t thng chnh li do dng 0.

I sinh ra, do ( 1.

E ) c th coi l in p trn mt nhnh (rm + jXm) c dng chy qua gi l nhnh t ho:

1.

E = (rm + jXm) 0.

I (1.19) Trong : rm gi l in tr t ho c trng cho tn hao st t Pst vi Pst = rm.I

20

Xm gi l in khng t ho c trng cho t thng chnh Thay (1.19) vo (1.11), (1.18) v kt hp vi (1.16) ta c h phng trnh:

1.

1

.

11

.

1

.

11

.

11

.

EIZEIjXIrU

1

.

2

.'

2'

2

.'

2'

2

.' .. EIjXIrU (1.20)

'

2

.

0

.

1

.

III H phng trnh (1.20) l h phng trnh vit theo lut K1 v K2 cho s

hnh 1.8a. y gi l s thay th ca my bin p. Thng thng tng tr nhnh t ho rt ln hn tng tr mch th cp qui i

v s cp: rm + jXm = Zm

8

(a) (b)

Hnh 1.8. S thay th my bin p (a) v s n gin (b) 1.6. Tn hao v hiu sut ca my bin p Khi my bin p lm vic c cc tn hao sau: - Tn hao trn in tr dy qun s cp v th cp gi l tn hao ng P ntnmtn PKrIKrIrrIIrIrP ....)(

22221

'21

21

222

211 (1.21)

- Tn hao trong li thp do dng in xoy v do t tr gy ra gi l tn hao st t Pst. Tn hao st t khng ph thuc vo dng in ti m ph thuc t thng chnh, cng tc l ph thuc in p. Tn hao st t bng cng sut lc khng ti:

Pst = P0 (1.22) Hiu sut my bin p c nh ngha l:

nttmt

tmt

st PKPSK

SK

PPP

P

.cos..

cos..2

02

2

(1.23)

Trong : P2 l cng sut ra ti. Ta c P2 = S2.cost = Kt.Sm.cost T (1.23) cho thy hiu sut thay i theo ti. Nu t = const, hiu sut t cc i

khi: 0

tK (1.24)

Thay (1.23) vo (1.24) tnh c: K2t.Pn = P0 Vy hiu sut t cc i khi tn hao st t bng tn hao ng. Minh ho trn hnh 1.9

Hnh 1.9. S ph thuc ca hiu sut theo ti

H s ti khi hiu sut cc i l:

n

t P

PK 0 (1.25)

My bin p in lc thng c thit k hiu sut t cc i Kt = 0,5 n 0,7.

Hiu sut my bin p thay i theo cng sut my v c gi tr ln. My bin p cng sut ln hiu sut c th t ti 99%. 1.7. Bin i in p ba pha

bin i in p 3 pha, ta c th dng 3 my bin p 1 pha ni vi nhau to thnh t my bin p 3 pha, hoc dng 1 my bin p 3 pha.

9

1.7.1. My bin p 3 pha Trn hnh 1.10 l s nguyn l mt my bin p 3 pha kiu tr. Li thp c 3 tr, trn mi tr qun cun dy s cp v th cp ca 1 pha. Ngi ta quy c k hiu cc u dy cun s cp l cc ch in hoa, cn cc u dy cun th cp l cc ch in thng.

Hnh 1.10. S nguyn l my bin p 3 pha kiu tr

Pha u dy s cp u dy th cp A A, X a, x B B, Y b, y C C, Z c, z

Nguyn l lm vic ca my bin p 3 pha tng t nh my bin p 1 pha. Gi s vng ca cun dy s cp v th cp ca 1 pha th t l W1 v W2 , t s in p pha s cp v th cp s l:

2

1

2

1

W

W

U

U

p

p (1.26)

T s in p dy s cp v th cp khng ch ph thuc vo W1, W2 m cn ph thuc vo cch ni dy s cp v th cp. 1.7.2. Ni dy my bin p 3 pha

Cc cun dy s cp v th cp ca my bin p 3 pha hoc t my bin p 3 pha c th ni vi nhau theo hnh sao hay tam gic. cch ni s cp v th cp khng ph thuc ln nhau. Trn hnh 1.11 l s mt vi cch ni v k hiu tng ng.

Hnh 1.11. Cc cch ni dy my bin p 3 pha

T s in p dy s cp v th cp (H s bin p) ca cc s l:

10

S ni Y/Y: 2

1

2

1

2

1

2

1

3

3

p

p

d

d

p

p

U

U

U

U

W

W

U

U

S ni /: 2

1

2

1

2

1

d

d

p

p

U

U

W

W

U

U

S ni /Y: 2

1

2

1

2

1

33 d

d

p

p

U

U

W

W

U

U

1.7.3. T ni dy my bin p 3 pha - T ni dy my bin p 3 pha l k hiu ch r cch ni dy ca my bin p

v gc lch pha gia in p dy s cp v th cp. - Gc lch pha gia in p dy s cp v th cp lun l bi s ca ca 300 vi

h s nhn l cc s nguyn t 1 n 12. Trong k hiu t ni dy, gn, ngi ta qui c khng ghi gc lch m ch ghi h s nhn ng vi gc lch.

V d: T ni dy Y/-11 ch rng: dy qun s cp ni sao, dy qun th cp ni tam gic, gc lch pha gia in p dy s cp v th cp l 11*300 =3300. S ni dy v th vc t in p ca t ni dy ny nh hnh 1.12.

Hnh 1.12. S ni dy v th vct in p

y cc u dy (A, a), (B, b) l cc u dy cng cc tnh. 1.8. My bin p lm vic song song

Khi cn tng cng sut th ni nhiu my bin p lm vic song song vi nhau nh hnh 1.13.

Hnh 1.13. Ni hai my bin p lm vic song song

iu kin cc my bin p c th lm vic song song l: 1- in p nh mc s cp v th cp ca cc my phi tng ng bng nhau (cng

11

c ngha l h s bin p ca my bng nhau). U1I = U1II, U2I = U2II, kI = kII

(Thc t cho php h s bin p ca cc my sai khc nhau khng qu 0, 5%) 2- Cc my phi c cng t ni dy ( in p th cp ca chng trng pha nhau)

iu kin 1 v 2 m bo cho khng c dng in cn bng ln chy qun trong dy qun cc my do s chnh in p th cp ca chng. 3- in p ngn mch ca cc my phi bng nhau. UnI% = UnII% iu kin ny m bo cho phn b ti cho cc my t l vi cng sut ca chng. Tht vy, gi s c: UnI% < UnII%, xt khi my I nhn ti nh mc, tc c II = IIm in p ri trong my I l: IIm.ZnI. Khi dng in trong my II l III v in p ri trong my l IIIm.ZnII. V hai my lm vic song song nn 2 in p ny phi bng nhau: IIm.ZnI = IIIm.ZnII Vi ZnI v ZnII l tng tr ngn mch ca my I v my II (xem hnh 1.14). V UnI% < UnII% nn IIm.ZnI < IIIm.ZnII, do : IIm < IIim tc dng in trong my II nh hn dng in nh mc ca n. Vy khi my I nhn ti nh mc th my II non ti, nu my II nhn ti nh mc th my I s qu ti. Thc t cho php in p ngn mch cc my sai khc khng qu 10%.

Hnh 1.14. Tng tr ngn mch ca my bin p khi lm vic song song

Cu hi n tp chng 1 Cu 1. My bin p l g ? Vai tr ca my bin p trong h thng in lc ? Tc dng ca tng b phn trong my bin p ? Cu 2. ngha cc i lng nh mc ca my bin p ? Xc nh cc dng in nh mc ca my bin p 3 pha nu bit Sm = 100(kVA), U1m/U2m = 6000/230 (V). Cu 3. Ti sao khi tng dng in th cp th dng in s cp li tng ln? lc t thng trong my bin p c thay i khng ? Cu 4. Cch xc nh tr khng mch s cp v th cp ca my bin p? Tn hao ngn mch l g? Tn hao khng ti l g ? Tr s in p ngn mch c ngha g ? Cu 5. S ph thuc ca in p th cp vo ti nh th no ? Cu 6. Cho 2 my bin p ni Y/Y-12 v Y/Y-6 c cng t s bin i K v in p ngn mch Un. chng lm vic song song th phi lm th no ? Cng cc kin trn nu 2 my bin p c t ni dy Y/-11 v Y/-12 ?. Cu 7. My bin p 1 pha c tit din tr li thp l 135cm2, in p s cp/th cp l 10kV/0,23kV, 50Hz. Bit bin cm ng t trong li thp l 1,1T, Tnh s vng dy cun dy s cp, th cp. Cu 8. Mt my bin p 3 pha Y/Y-12 c: Sm = 180kVA; U1 /U2 =6000/400 V; dng in khng ti I0% = 6,4%; tn hao khng ti P0 = 1000W; in p ngn mch Un% = 5,5; tn hao ngn mch Pn = 4000W. Gi s r1 = R

2, x1 = X

2. Hy v s thay th

ca my bin p v tnh cc thnh phn ca in p ngn mch.

12

Chng 2. MY IN KHNG NG B Ni dung chnh ca chng my in khng ng b nhm gip ngi c hiu

c cu to v nguyn l lm vic chung v nhng thng s k thut ca my in khng ng b, bit cc phng php m my i vi ng c in khng ng b xoay chiu ba pha. Ngoi ra ngi c cn hiu c nhng yu cu khi m my ng c in khng ng b. 2.1. Nguyn l lm vic c bn ca my in khng ng b

Hnh 2.1. S in t v s hnh thnh mmen in t my in khng ng b khi lm vic ch ng c in (a); s u (b). Trong my in khng ng b (hnh 2.1a) c hai dy qun: mt dy qun t

phn tnh - trong li thp stato 1, cn dy qun th hai t phn quay - trong li thp rto 3. Gia stato v rto c khe h khng kh. tng cng s lin h v t gia cc dy qun, tr s khe h khng kh cn phi ch to nh nht khi c th c. Dy qun stato thng c ba pha (hoc tng qut gm c nhiu pha), c cc pha ca n c t cch u nhau theo chu vi stato. Trng hp trn hnh 1, dy qun stato 2 c ba pha AX, BY, CZ ni theo s hnh sao hoc tam gic v c ni vo li in xoay chiu ba pha (hnh 1b). Dy qun rto c ch to thnh ba pha (hoc nhiu pha), c cc pha ca n cng c t cch u nhau theo chu vi rto. Trng hp trn hnh 1, dy qun rto 4 c ba pha ax, by, cz c ni ngn mch li.

Khi c cung cp in xoay chiu ba pha, dy qun stato to ra t trng quay 1 quay vi tc ng b:

p

f60n 11 (vng/pht) (2.1)

Trong : f1 - tn s li in cung cp (Hz); p - s i cc t stato. T trng quay stato 1 qut qua cc thanh dn dy qun rto, cm ng trong

chng s.. cm ng e2 v nu dy qun rto c ni ngn mch, th trong n xut hin dng in i2, bin i vi tn s f2=f1 (khi rto ng yn). Nu dy qun rto c ba pha (hnh 2.1b), th dng in ba pha sinh ra trong n s to ra t trng quay rto 2, c s cc t 2p, quay cng chiu v cng tc vi t trng quay stato khi n=0: n2=60f2/p=60f1/p=n1, vg/ph. Nh vy, t trng quay 1 v 2 quay ng b vi nhau, to thnh t trng quay tng khe h khng kh. Kt lun ny cng ng cho c my in c rto lng sc.

13

Tc dng tng h ca dng in rto vi t trng quay tng s to ra lc in t F v mmen quay in t M, lm quay rto vi tc n. iu kin cn thit sinh ra mmen in t M my in khng ng b l tc quay ca rto n phi khc tc ca t trng quay n1. Ch iu kin trong dy qun rto mi cm ng s.. e2 v do vy mi xut hin dng in i2. Chnh v rto quay khng ng b vi t trng m tn gi ca my in l my in khng ng b. i khi my in ny cn c gi l my in kiu cm ng, v dng in i2 sinh ra trong dy qun rto l nh con ng cm ng t, ch khng phi c cung cp t ngun ngoi. Hiu s tng i ca tc t trng quay n1 v tc quay rto n c gi l h s trt s:

1

1

n

nns

(2.2)

T cng thc (2.2), c th tnh tc quay rto n qua h s trt s: n = n1(1-s) (2.3)

Theo cc cng thc (2.2) v (2.3), c th biu th h s trt s (v tc quay rto n tng ng) nm trong cc phm vi cho bng 1.1, m mi phm vi ng vi mt ch lm vic c th, c xt di y. Bng 2.1 H s trt s +s>1 s=1 1>s>0 s=0 0>s- Tc quay rto n -n

14

Hnh 2.2. S hnh thnh mmen in t my in

khng ng b, khi lm vic ch my pht in (a) v ch hm in t (b).

2.1.3. Khi rto quay ngc chiu t trng n1). Nu v nguyn nhn no , chng hn do ngoi lc, rto quay ngc chiu t

trng n

15

Hnh 2.3. Cu to ca ng c in khng ng b roto lng sc 1 - li thp stato; 2 - dy qun stato ; 3 - np my; 4 - bi; 5 - trc my; 6 - hp

u cc; 7 - li thp roto; 8 v my; 9 - qut gi lm mt; 10 np bo v qut gi.

Hnh 2.4. Cu to ca ng c in khng ng b roto dy qun

tt c cc ng c khng ng b c ng knh ngoi li thp stato nh hn 1m, nhng l thp k thut in stato c dng hnh vnh khn nguyn tm, mt trong ca n c dp sn rnh t dy qun; cn khi ng knh ngoi li thp stato ln hn 1m, phi dng nhng l thp k thut in c hnh r qut. Khi li thp stato ngn c th ghp thnh mt khi; cn khi li thp stato qu di phi ghp thnh tng khi, mi khi di 68cm, gia cc khi c rnh thng gi rng 1cm.

- Dy qun stato c t vo trong cc rnh stato v c cch in tt so vi

16

li thp. 2.2.2.2. Phn quay (rto).

Rto ca my in khng ng b gm ba phn chnh: trc my, li thp v dy qun.

Hnh 2.5. Cu to rto lng sc: lng sc (a); rto lng sc vi

thanh dn bng ng (b); rto lng sc c nhm (c); cc dng rnh rto (d).

1- li thp rto; 2- cc thanh dn; 3- vnh ngn mch; 4- cnh qut gi. - Trc my c lm bng thp. - Li thp rto. Khi my lm vic tn hao st t trong li thp rto rt b nn c

th khng cn dng thp l k thut in. Tuy vy sau khi dp li thp stato, phn thp l k thut in cn li thng c tn dng dp lun li thp rto. Trong cc my in c nh li thp rto c p trc tip ln trc, cn nhng my in ln li thp rto c p ln gi trc.

- Dy qun rto, gm hai loi chnh: rto dy qun v rto lng sc. + Rto dy qun: c dy qun ging nh ca dy qun stato. Trong my in c

nh thng dng loi dy qun ng tm mt lp. Trong my in c trung tr ln thng dng kiu dy qun sng hai lp v bt c nhng u dy ni, kt cu dy qun trn rto cht ch. Dy qun ba pha ca rto thng c u hnh sao, cn ba u kia c ni vo ba vnh trt thng lm bng ng t c nh mt u trc my v thng qua chi than c th ni vi mch in bn ngoi. ng c in rto dy qun c u im l: thng qua vnh trt v chi than c th ni ni tip bin tr m my vo mch rto ci thin tnh nng m my; a s.. ph hoc ni ni tip in tr ph vo mch rto iu chnh tc . Lc my lm vic, dy qun rto c ni ngn mch.

+ Rto lng sc: dy qun rto lng sc c cu to rt khc so vi dy qun stato. Lng sc c th c ch to t cc thanh dn bng ng (hoc nhm), hai u ca chng c ni ngn mch vi nhau bng hai vnh ngn mch (hnh 2.5a). cc ng c in c cng sut ln, lng sc c ch to t cc thanh dn bng ng t vo trong cc rnh rto, phn u thanh dn nh ra khi li thp c hn li vi nhau thnh hai vnh ngn mch (hnh 2.5b). cc ng c in c cng sut nh v trung bnh, lng sc c ch to bng cch c nhm vo trong rnh rto, ng thi c lun c vnh ngn mch v cnh qut gi (hnh 2.5c). Dy qun lng sc khng cn

17

cch in so vi li thp rto. ci thin tnh nng m my, cc ng c in cng sut tng i ln rnh rto lng sc c ch to c hnh dng c bit. ci thin dng sng s.., trong cc my in c nh rnh rto thng c lm cho i mt gc so vi phng dc trc my. 2.2.2.3. Khe h khng kh

Gia stato v rto ca my in khng ng b c khe h khng kh rt nh. hn ch dng in t ho nhm nng cao h s cng sut ca my, tr s khe h khng kh 0,31mm i vi my in c cng sut trn 0,5kW v 0,020,3mm i vi my in c cng sut rt nh. 2.3. M my ng c in khng ng b 2.3.1. Cc yu cu khi m my ng c in khng ng b

Cc phng php m my ng c in khng ng b phi p ng c cc yu cu sau:

- ng c phi c mmen ln (Mmm > Mc) rto tng tc t tc quay nh mc.

- Dng in m my Imm phi c hn ch, trnh lm hng ng c v ph hy ch lm vic bnh thng ca li in.

- S m my phi n gin, chc chn, gi thnh thit b m my phi r. - Tn hao cng sut trong qu trnh m my cng nh cng tt. 2.3.2. Cc phng php m my ng c in khng ng b

2.3.2.1. M my trc tip ng c in khng ng b roto lng sc.

Hnh 2.6. S cc phng php m my ng c in

khng ng b rto lng sc: a m my trc tip; b- m my bng in khng; c- m my bng bin p t ngu

y l phng php m my n gin nht, ch vic u dy qun stator ng c in r to lng sc trc tip vo li in (hnh 2.6a). Khi in p t trn dy qun stator U1=Udm v dng in m my trc tip Imm bng:

Imm = (47)Im (2.4) Cc ng c in khng ng b r to lng sc c thit k, m bo cho

php chng m my trc tip c. Nh vy, cc ng c in r to lng sc hon ton c th m my trc tip c, nu li in c cng sut v dng in m my khng gy ra st p trong li in qu mc cho php.

18

2.3.2.2. Cc phng php m my di in p thp Trong mt s trng hp, c khi khng m my trc tip cc ng c khng

ng b roto lng sc c, do n gy ra st p qu ln trong li in. Lc phi dng cc phng php m my di in p thp.

u im ca phng php m my di in p thp l gim c dng in m my. Song, phng php m my ny c nhc im l: khi gim in p U1, m men m my Mmm b gim bnh phng ln mc gim in p, do vi nhng ti yu cu mmen m my ln s khng dng c phng php m my ny. a) M my bng cun khng

Lc m my tin hnh ng cu dao CD1, ng c c cp in qua b in khng ba pha DK (hnh 2.6b). Tr s in khng Xdk ca b in khng ni ni tip mch stator c tc dng hn ch tr s dng in m my ca ng c. Sau khi m my xong, ng cu dao CD2 loi in khng DK ra khi s , a in p U1 = Um vo cc ng c.

Khi m my trc tip, dng in m my Imm bng:

2 2

dmmm

n n

UI

r x

(2.5)

trong : rn, xn tng ng l in tr v in khng ngn mch ca mt pha ng c in. Khi m my bng in khng, b qua in tr tc dng ca b in khng,

dng in m my Imm bng:

'

2 2( )

dmmm

n n dk

UI

r x x

(2.6)

Bng cch iu chnh tr s in khng xdk s nhn c dng in m my cn thit. T biu thc (2.5) v (2.6), ta c:

2 2

' 2 2

( )mm n n dk

mm n n

I r x xk

I r x

(2.7)

Hay: ' mmmm

II

k (2.8)

Ngha l khi m my bng in khng dng in m my ca ng c in I

mm b gim i k ln so vi khi m my trc tip Imm. Nu coi rng lc m my bng in khng cc tham s ca my in khng i,

th in p trn cc ng c Umm cng b gim i k ln khi vi m my trc tip:

' dmmm

UU

k (2.9)

V do vy, m men m my Mmm b gim i k2 ln so vi khi m my trc tip.

'2mm

mmM

Mk

(2.10)

b) M my bng bin p t ngu u tin ng cu dao CD1 v CD3 a in p thp vo cc ng c in qua

bin p t ngu TN (hnh 2.6c). Sau khi t tc quay nht nh no c th m cu dao CD3, khi ng c c cp in qua mt s vng dy ca my bin p t

19

ngu TN, ging nh trng hp cp in qua cun khng. Sau cng tin hnh ng cu dao CD2 a in p nh mc Udm vo cc ng c.

Nu bin p t ngu TN gim in p m my ng c i kTN ln, th dng in m my trong ng c hay pha h p HA ca bin p t ngu Imm cng b gim i kTN ln, cn dng in m my pha cao p CA ca my bin p t ngu hay pha li in Imm b gim i k

2 TN ln. M men m my ca ng c t l vi in p m

my, cng b gim i k2TN ln. c) M my bng cch i ni tam gic sao Phng php m my bng cch i ni sao - tam gic (hnh 2.7) ch dng

thch hp cho cc ng c in lc lm vic bnh thng dy qun stator u .

Hnh 2.7. S m my ng c in khng ng b

rto lng sc bng cch i ni sao - tam gic. Khi m my tin hnh ng cu dao CD2 trn hnh 2.7 sang tri, ng vi v tr

m my, lm cho dy qun stator u sao (Y); cn khi tc ng c t n nh mc phi nhanh chng ng cu dao CD2 sang bn phi, ng vi v tr lm vic, dy qun stator u li thnh sao ().

So vi khi m my trc tip vi dy qun stator u , phng php m my ny gim c in p pha dy qun stator ln, m men m my gim 3 ln, dng in m my trong mi pha dy qun stator gim ln, cn dng in m my pha li in gim 3 ln.

2.3.2.3. M my ng c in khng ng b rto dy qun bng bin tr m my

ng c in rto dy qun t c dng hn so vi ng c in rto lng sc. Chng c s dng trong cc trng hp sau:

- Khi cc ng c in rto lng sc khng p ng c cc yu cu v iu chnh tc quay.

- Khi m my vi m men cn tnh Mc trn trc ln, ng c in khng ng b r to lng sc khng th m my c bng phng php in p thp, cn m my trc tip th dng in m my li qu ln, khng m bo.

20

- Khi trng lng phn ng ln, nhit nng sinh ra mch in th cp ca ng c in r to lng sc rt ln, t nng dy qun r to lng sc qu mc cho php.

Di y xt qu trnh m my ng c in khng ng b rto dy qun nh bin tr m my ni ni tip vo mch rto nh hnh 2.8.

Hnh 2.8. S m my ng c in khng ng b

rto dy qun nh bin tr m my Trc lc m my, tt c cc cp bin tr m my c ni vo mch r to qua

vnh tip xc v chi in. Trong qu trnh m my ln lt ng cc tip im K3, K2, K1.

u im ca phng php m my ny l to ra c m men m my ln, trong khi dng in m my li nh.

PHN THAM KHO IU CHNH TC NG C IN KHNG NG B XOAY CHIU 3 PHA

I. iu chnh tc ng c khng ng b bng phng php thay i in p 1. S nguyn l iu chnh

iu chnh in p ta dng b bin i BB c tn hiu in p ra thay i theo tn hiu iu khin nh s nguyn l sau

BB

U1 ,

f1

= c

on

st

Rc

U2 = var

Uk 2. c tnh c trong iu chnh a, Nu b qua tng tr ngun v khng dng in tr ph trong mch r to

in p ngun thay i ta thu c mt h c tnh iu chnh c trt ti hn gi nguyn cn Mth thay i t l vi U

2

21

t.tn

MthMth.u

U1

U2

M

Nh vy nhng ng c tnh iu chnh ny c on lm vic ngn, cng

thp v Mth gim nhanh khi in p gim. ci thin c tnh iu chnh v lm gim mc pht nng ca my in

ngi ta ni thm mt in tr Rc vo mch roto. Khi in p t vo stato l nh mc th ta thu c c tnh mm hn c tnh t nhin, ta gi n l ng c tnh gii hn

ththghcd

ththgh MMR

RRss

2

2.

Mthgh , sthgh: m men v trt ti hn gii hn ca c tnh gii hn Mth , sth: m men v trt ti hn ca c tnh t nhin

Khi in p t vo khc nh mc, m men ti hn Mth.u s thay i t l vi bnh phng in p cn trt ti hn sth.u th khng i

constss

UMU

UMM

thghuth

thghdm

thghuth

.

2*

2

. .

Da vo c tnh gii hn Mgh(s) ta suy ra c tnh iu chnh ng vi gi tr U cho trc nh quan h 2*.UMM ghu

Cc ng c tnh iu chnh s c dng nh sau:

t.tn

U1

U2

M

th.gh

MthMth.u b, Nu tnh n c tng tr ngun

Trng hp n gin ta xt b bin i c in tr Rb, in khng Xb v cc thng s ny khng ph thuc vo in p U t vo ng c, khi ta c

2212112

212

11

221

2110

2

221

21

2

)(

)(

)(2

3

)((

XXRR

XXRRM

XXRR

UM

XXR

RRs

ttt

th

ttt

thgh

tt

cdthgh

22

Trong : R1t = R1+Rb ; X1t = X1 +Xb Phng trnh c tnh c ca ng c tnh gii hn s l:

thghthgh

thgh

thghthghgh

sas

s

s

s

saMM

,

,

2

)1(2

vi:

,,2

1,

cd

t

RR

Ra

II. iu chnh tc ng c khng ng b bng phng php thay i tn s ngun f1 1. Khi nim chung

Xut pht t biu thc )1(2

)1( 10 sp

fs

, ta nhn thy khi thay i tn

s f1 ta cng c th thay i c tc ca ng c khng ng b . Ta c s iu chnh nh sau :

BB

U1 , f

1 =

const

U2 , f2 = var

Uk Do my in c thit k lm vic vi mt tn s nht nh nn vic thay

i tn s s lm nh hng n ch cng tc ca my in.

1

1'

1

111

11111

f

UC

Cf

UUfC

ZIUfCE

Nu in p U1 = const th khi tn s f1 tng th t thng s gim do s dn n hin tng gim m men trong my. gi cho m men khng i th ta phi tng dng in. Nh vy ng c s b qu ti v in

Nu ta gim tn s f1 th t thng s tng ln , iu ny s lm t nng li thp v lm cho hin tng bo ho t trong my tng ln.

Nh vy i vi phng php thay i tn s th khi iu chnh tn s th ta cng phi thay i U1 cho ph hp nhm mc ch gi cho l khng i. 2. Quy lut thay i tn s

Khi tin hnh iu chnh nu ta gi cho h s qu ti v m men l mt hng s th ch lm vic ca my in s lun c duy tr mc ti u nh khi lm vic vi ti nh mc.

Nh vy khi iu chnh ta cn phi lun tho mn iu kin : constM

M

c

th

Nu coi 01 r t biu thc ca Mth, ta c:

21

'21

21

1'2110

21

)(4

3

)(2

3

fCCp

U

fxfx

UM th

23

Trong , ta thay th p

f10

2

H s qu ti v m men ca ng c c xc nh da vo Mth v Mc = f ()

)(.

.)(.)(

43

21

21

21

'21

21

cc

c

th

Mf

UA

MfCCp

U

M

M

Thay th Mc = f () bng phng trnh c tnh c dng gn ng ca my sn xut v

coi xxx

x

dmcx

dmcc fBfpMMM

p

f11..

10

)2(.)(

2

Nh vy ta c )2(

1

21

xc

th

f

U

B

A

M

M

v vit biu thc cho trng hp lm vic cc

thng s nh mc v trong trng hp U1, f1 bt k v tho mn iu kin = const lc ta c

)2(

1

)2(1

21

21

)2(1

21

)2(1

21

x

x

dmxx

dm

f

f

U

U

f

U

f

U

T ta rt ra quy lut bin i ca in p )2(

1

)2(1

1

1x

dm

x

dm f

f

U

U

hoc )2(11xfU

Vy in p stato phi thay i ph thuc tn s v c tnh ph ti. Cho x cc gi tr khc nhau ta s c nhng quy lut bin i khc nhau ca in p. Ta c bng biu din quy lut:

Loi ti X Quy lut iu chnh Kiu my tin -1

1f

Kiu my nng 0 1f

Ma st nht 1 31f

Qut gi 2 21f

3. Cc c tnh iu chnh c tnh c ca ng c khi iu chnh tn s khng nhng ph thuc vo f1

m cn ph thuc vo quy lut thay i in p, ngha l ph thuc vo c tnh ti. Khi s dng quy lut iu chnh in p gn ng th m men ti hn ca c

tnh iu chnh cng c xc nh gn ng. Khi tn s v in p l nh mc th m men ti hn s l:

)(4

3

'21

21

21

.

CCfp

UM

dm

dmdmth

So snh vi Mth ta c 21

21

.

f

UMM dmthth v thay

1U bng quy lut bin thin va xc

nh c ta s c xdmthth fMM 1.

trt ti hn c xc nh theo biu thc gn ng

1

.

1'211

'2

f

s

fCfC

Rs dmthth

Trong sth.m l trt ti hn ca c tnh c t nhin. Nh vy khi bit s liu ca c tnh t nhin v c tnh c ca my sn xut ta c th xc nh c Mth v sth ca ng c ti bt k tn s no. Cui cng s dng phng trnh:

24

s

s

s

sM

Mth

th

th

2

ta s dng c c tnh c iu chnh. Di y trnh by dng cc ng c tnh c ng vi cc ph ti khc nhau.

M M MMc Mth

f11fm

f12

f13

f21

fm

f22

fm

f31

f32

Trn thc t h c tnh ny u tho mn iu kin constM

M

c

th

Trong thc t , do ta b qua gi tr R1 nn nhng min tn s thp m men ti hn c s sai khc ng k so vi gi tr tnh ton. nhng min tn s cao th in khng t ho x >>R1 nn ta c th b qua cn khi tn s iu chnh thp th gi tr R1 khng th b qua c nn kt qu tnh ton s khng chnh xc. H s qu ti thc t b gim ng k trong min ny.

cng ca c tnh c cng ph thuc vo tn s iu chnh v c tnh ca m men cn. n gin trong tnh ton ta coi on lm vic ca c tnh c l ng thng v c phng trnh:

2 th

th

MM s

s

Khi cng ca n s c xc nh theo phng trnh:

0

21 th

th

M

s

Thay cc gi tr ca Mth v sth vo ta c cc c tnh iu chnh tng ng III. iu chnh tc ng c khng ng b bng phng php thay i s i cc 1. Nguyn l iu chnh

Khi thay i s i cc ca my in KB, tc t trng quay thay i do tc ca roto cng thay i theo. Quan h c th hin theo biu thc :

10

2(1 ) (1 )

fs s

p

f1: tn s ca li in p: s i cc c th thay i c s i cc ca ng c th my in phi c ch to c bit. Nhng my in kiu gi l my in a tc. S i cc ca my c th c thay i bng 2 cch: + Dng 2 t dy qun stato ring bit, mi t c s i cc ring + Dng mt t dy qun stato nhng mi pha c chia lm 2 on, thay i cch ni dy gia 2 on ta s thay i c s i cc.

25

Thng thng nhng ng c c t 3 cp tc tr ln u c 2 hoc nhiu t dy qun stato. Mi t li c th phn on thay i s i cc theo cch hn hp. Nhng loi ng c kiu ny thng l loi ng c lng sc. Ta kho st phng php thay i s i cc bng cch thay i cch u dy stato : Gi s ta c mt t u dy stato gm 2 on, mi on l mt phn t dy qun, nu ta u ni tip hai on thun cc nhau th s i cc s l p = 2, cn nu ta u ni tip ngc cc hoc song song ngc th p = 1.

* *

* *~

P = 2 ; 0

* *

* *

P = 1 ; 20

**

* *~

P = 1 ; 20

~

Ns Ns s N Ns

Nh vy bng cch i ni n gin ta iu chnh c tc ng c.

Cu hi n tp chng 2 Cu 1. Nguyn l lm vic chung ca my in khng ng b l g ? Vai tr ca my in khng ng b trong cng nghip v dn dng ?. Cu 2. Mi quan h ca h s trt s vi tc quay ca rto n ? Ti sao tc quay ca rto li khc tc ca t trng quay ?. Cu 3. Cu to ca ng c khng ng b ba pha loi lng sc v dy qun ? Chc nng ca mi b phn ? Cu 4. Cc yu cu khi m my ng c in khng ng b, gii thch vai tr v ngha ca cc yu cu khi m my ng c khng ng b ? Cu 5. Gii thch ti sao cn phi p dng cc phng php khi ng cc ng c khng ng b c nhng phng php khi ng no ? Cu 6. c im ca cc phng php m my ng c khng ng b rto lng sc, phm vi ng dng ca tng phng php ? Cu 7. c im ca cc phng php m my ng c khng ng b rto dy qun, phm vi ng dng ca tng phng php ?

26

Chng 3. MY IN NG B Ni dung chnh ca chng my in ng b nhm gip ngi c hiu c

cu to v nguyn l lm vic chung v nhng thng s k thut ca my in ng b, bit cc phng php m my i vi ng c in ng b. Ngoi ra ngi c cn hiu c vai tr, phm vi ng dng ca my pht v ng c in ng b. 3.1. Nguyn l lm vic c bn ca my in ng b Stato ca my in ng b (hnh 3.1a) c cu to ging nh stato ca my in khng ng b. Dy qun stato 3 ca my in ng b thng c ba pha AX, BY, CZ (tng qut c nhiu pha), c cu to ging nh dy qun stato ca my in khng ng b, c s cc t bng s cc t ca r to, c gi l dy qun phn ng.

Hnh 3.1. S in t (a) v s u dy (b)

ca my in ng b Trn li thp rto ca my in ng b t dy qun kch thch 4, c cung

cp in mt chiu t ngun ngoi qua vnh trt 5 v chi in 6. Nh vy, dng in sinh ra trong dy qun rto l do ngun in mt chiu bn ngoi cung cp, ch khng phi t con ng cm ng nh trong my in khng ng b. Dy qun kch thch, c nhim v to ra t trng kch thch trong my. Rto cng vi dy qun kch thch c gi chung l phn cm. Khi ch to my in ng b, cn phi s dng cc bin php sao cho nhn c s phn b t cm ca t trng kch thch dc theo chu vi stato c dng gn sin nht. Nu quay rto ca my in ng b vi tc n no v cp in cho dy

qun kch thch, th t thng kch thch t s qut qua cc thanh dn dy qun stato, cm ng trong cc pha dy qun stato s.. xoay chiu E (hnh 3.1b) bin i vi tn

s 1f .

1 ,60

pnf Hz (3.1)

trong : n-tc quay r to, vg/ph H thng s.. dy qun stato l h thng s.. ba pha i xng. Khi mc ti i xng cho n, dy qun stato s mang ti ba pha i xng v my lm vic ch my pht in. Khi mang ti, dy qun stato to ra t trng quay, ging nh ca dy qun stato my in khng ng b. T trng stato quay theo chiu quay ca r to, vi tc :

27

1160

, /f

n vg php

(3.2)

T cc cng thc (3.1) v (3.2), ta c: 1n n (3.3) Nh vy, t trng stato lun quay cng chiu v cng tc (ngha l quay ng b) vi rto, chnh v vy m my in c gi l my in ng b. T trng stato v t trng r to lun quay ng b vi nhau, to ra t trng

quay tng ging nh my in khng ng b. C th t dy qun phn ng phn tnh, cn dy qun kch thch phn quay nh trn hnh 3.1a (kiu c bn), hoc c th t ngc li (kiu o ngc) v iu khng lm thay i nguyn l lm vic c bn ca my in. My in ng b c th lm vic song song vi cc my pht in ng b khc trn cng mt li in. Khi lm vic vi li in, my in ng b c th cung cp in nng cho li in (t cch ny l my pht in) hoc tiu th in nng t li in ( t cch l ng c in).

Khi dy qun stato ni vo li in c in p 1U , tn s 1f , ging nh my in khng ng b, dng in stato to ra t trng quay stato. Tc dng tng h

gia t trng quay stato vi dng in kch thch ti ca r to s to ra mmen in t M. Khi my lm vic ch ng c in m men in t M ng vai tr m men ch ng; cn khi lm vic ch my pht in l m men hm. Nh vy khc vi my in khng ng b, s sinh ra m men in t my in ng b khng i hi phi c s.. cm ng dy qun r to, v trong dy qun

r to c dng in ti c cung cp t ngun ngoi. Cng chnh v vy, m r to lun quay ng b vi t trng quay stato c ch ng c in, cng c ch my pht in, khng ph thuc vo ti c trn trc r to hoc ti in. 3.2. Phn loi v cu to ca my in ng b 3.2.1. Phn loi C th phn loi my in ng b theo kt cu, theo cch t cc dy qun v theo chc nng ca my. - Theo kt cu, my in ng b c phn thnh: my cc n v my cc li.

Hnh 3.2. My in ng b

kiu c bn (a) v kiu o ngc (b).

28

- Theo cch t cc dy qun, my in ng b c cng sut ln thng c ch to theo kiu c bn, c dy qun phn ng t phn tnh (hnh 3.2a) tin cho vic truyn dn in nng t phn ng ra li in. Mt khc, khi thc hin cp in cho dy qun kch thch qua cc vnh trt khng phi gp tr ngi ln do cng

sut kch thch nh. Cc my in ng b c cng sut nh 2 5kW , thng c ch to theo kiu o ngc (hnh 3.2b). - Theo chc nng, my in ng b c phn thnh: + My pht in ng b: My pht in ng b c s dng bin i c nng thnh in nng. in nng ba pha dng trong sn xut v trong i sng hin nay ch yu c sn xut ra t cc my pht in quay bng tuabin hi hoc kh (gi l my pht tuabin hi) hoc quay bng tuabin nc (gi l my pht tuabin nc). My pht in ng b c quay bng cc loi ng c khc (ng c diezen, ng c t trong, xylanh hi nc) c ch to c cng sut va v nh, dng cho cc ti a phng. + ng c in ng b: Khc vi cc ng c in khng ng b, ng c in ng b c kh nng pht ra ch khng tiu th cng sut phn khng. Cc ng c in ng b thng c dng ko cc ti khng yu cu phi thay i tc , c cng sut ch yu t 200kW tr ln, nh dng truyn ng cho cc my nn xi lanh, qut gi m, nm thy lc, my xc m l thin Cc ng c in ng b c cng sut nh (c bit lm cc ng c nam chm vnh cu) c s dng rt rng ri trong cc thit b t ng v iu khin. + My b ng b: My b ng b ch yu c dng ci thin h s cng sut osc ca li in. Ngoi ra trong thc t cn gp cc my in ng b c bit, nh: my bin i mt phn ng , my ng b tn s cao, cc my ng b cng sut nh dng trong iu khin t ng: ng c ng b phn khc, ng c ng b t tr, ng c bc 3.2.2. Cu to

3.2.2.1. Kt cu ca my in ng b cc n: Kt cu rto ca my in ng b cng sut ln ph thuc ch yu vo tc quay. Kt cu rto cc n hnh 3.3b c dng c bit thch hp cho cc my in ng b c hai cc 2p=2 (n=3000vg/ph) v bn cc 2p=4 (n=1500vg/ph), nh cc my pht tuabin hi. S d khng dng c kt cu r to cc li cho cc my c t cc (c bit l my hai cc) l v vic c nh dy qun kch thch rt kh khn.

Rto ca my in ng b cc n (hnh 3.3a) c lm bng thp hp kim cht lng cao (hp kim Crom, Niken, Mlpen). N c ch to t mt phi thp nguyn vi c u trc, c rn thnh khi hnh tr, sau gia cng v phay rnh t dy qun kch thch. Phn khng phay rnh ca r to hnh thnh nn mt cc t. Mt ct ngang trc li thp r to cho trn hnh 3.3b. Do tc quay ln, hn ch lc ly tm v m bo bn c. Khi n=3000vg/ph ng knh r to D khng vt qu 1,20 1,30m. tng cng sut my , ch c th tng chiu di l ca rto. Chiu di ti a ca rto vo khong l 7,5 8,5m.

Dy qun kch thch t trong rnh r to c ch to t dy ng trn tit din ch nht qun theo chiu mng thnh cc bi dy ng tm. Cc vng dy ca bi dy ny c cch in vi nhau bng mt lp mika mng.

29

Hnh 3.3. R to ca my in ng b cc li (a)

v cc n (b): 1- li thp r to; 2- dy qun kch thch

Hnh 3.4. Hnh dng b ngoi rto cc n (a) v cc li (b)

c nh v p cht dy qun kch thch vo trong rnh, ming rnh c nm kn bng cc thanh nm thp khng t tnh. Phn u ni (nm ngoi rnh) ca dy qun kch thch c ai cht bng cc ng tr thp khng t tnh. Hai u dy qun kch thch i lun trong trc v ni vi hai vnh trt t u trc, thng qua hai chi in dy qun kch thch c ni vi ngun mt chiu bn ngoi. Thng s dng my pht in mt chiu lm my kch thch t ca my in ng b. My kch t c t trn trc my in ng b hoc c ni vi trc ca n. Stato ca my in ng b cc n bao gm li thp, trong c t dy qun ba pha, than my v np my. Li thp stato c ch to t cc l thp k thut in (tn silic) dy 0,5mm, hai mt c ph sn cch in, ri c p cht li. Dc chiu di li thp stato c cch khong 3 6cm li c mt rnh thng gi ngang trc, rng 10mm. Li thp stato c t c nh trong than my. cc my in ng b cng

30

sut trung bnh v ln, thn my c ch to sao cho trong n hnh thnh h thng ng thng gi lm ngui my in. Np my cng c ch to t thp tm hoc t gang c. i vi cc my in ng b cng sut trung bnh v ln, trc c t trn gi trc, t c nh trn b my.

3.2.2.2. Kt cu ca my in ng b cc li Kt cu rto cc li hnh 3.5 c dng trong cc my in ng b c s cc

t 2 4p ( 1500 /n vg ph ), nh my pht tuabin nc, my pht diezen, my b ng b, ng c tc chm

Hnh 3.5. Cu to ca my in rto cc li

My in ng b cc li thng c tc quay thp (c vi chc hoc vi trm vg/ph), v my c s cc t ln ( 2 16 96p ). V vy, khc vi my in ng b cc n, ng knh r to D ca my in ng b cc li c th n 15m trong khi chiu di 1 ca n li nh (t l l/D= 0,15 0,2 ). Kt cu ca my in ng b cc li cng sut nh cho trn hnh 3.5. Li thp r to my in ng b cc li cng sut trung bnh v nh c lm bng thp ng v gia cng thnh khi lng tr hoc khi hnh tr (bnh xe) trn mt c t cc cc t. cc my c cng sut ln, li thp r to c ch to t cc tm thp dy 1 6mm , c dp hoc c nh hnh sn ghp thnh cc khi lng tr v li thp ny khng c lng trc tip vo trc my. Cc t t trn li thp roto. Gi r to mi c lng 1 1,5mm. Cc t c c nh trn li thp nh ui hnh T hoc nh cc bulng xuyn qua mt cc v vt cht vo li thp rto. Dy qun kch thch c ch to t dy ng trn tit din ch nht qun theo chiu mng thnh tng cun dy. Cch in gia cc vng dy l cc lp mica hoc amiang. Cc cun dy sau khi gia cng c lng vo than cc cc t.

Dy qun cn ( my pht in ng b) hoc dy qun m my ( ng c in ng b) c t trn cc u cc. Cc dy qun ny (ging nh dy qun lng sc ca my in khng ng b) c ch to t cc thanh dn bng ng t vo rnh cc u cc v hai u ca chng c ni bng hai vng ngn mch (hnh 3.6b). Dy qun m my ch khc dy qun cn ch cc thanh ln hn, do c lm t nhng vt liu c in tr sut cao hn (ng thau).

Stato ca my in ng b cc li c cu to tng t nh stato ca my in ng b cc n.

31

(a)

(b)

Hnh 3.6. Mt ct ngang (a) v dy qun (b) ca rto cc li Trc ca cc my ng b cc li (ng c in ng b, my b dng b, my pht in diezen) thng c t nm ngang. Trc ca cc my pht tuabin nc c t thng ng.

3.2.2.3. H thng kch thch ca my in ng b Theo phng php cp in cho dy qun kch thch, h thng kch thch ca my in ng b c phn thnh: kch thch c lp v t kch. h thng kch thch c lp, ngun in cp cho dy qun kch thch ly t my pht in mt chiu (my kch t) t trn trc ca my in ng b (hnh 3.7a) hoc ly t my pht in ring, c quay bi ng c in ng b. h thng t kch, dy qun kch thch c cp in t dy qun phn ng cu my in ng b qua b chnh lu c iu khin hoc khng c iu khin dng van bn dn (hnh 3.7b). Cng sut cn thit kch thch ch bng 0,3 3,0% cng sut ca my in ng b.

Hnh 3.7. S kch thch ca my pht in ng b

Trong cc my pht in cng sut ln, ngoi my kch t chnh, i khi cn c thm my kch t ph (l my pht in mt chiu cng sut nh), dng cp in

32

cho cun kch thch ca my kch t chnh. Trong trng hp ny c th s dng my pht in ng b kt hp vi b chnh lu bn dn lm my kch t chnh. Vic cp in cho dy qun kch thch qua chnh lu bn dn dng it hoc thyrist c p dng rng ri cc ng c in ng b v my pht in ng b cng sut nh v trung bnh, cng nh c cc my pht tuabin nc v tuabin hi cng sut ln (h

thng kch thch dng thyristor). Vic iu chnh dng in kch thch ti c thc hin t ng nh b iu chnh kch thch c bit. cc my pht in cng sut nh i khi ngi ta iu chnh dng in kch thch bng tay, nh thay i tr s bin tr mc trong mch dy qun kch thch. nng cao tin cy cho h thng kch thch, ngi ta dng h thng kch thch khng c chi in (hnh 3.7c), ngha l trong mch cp in cho dy qun kch thch ca my pht in ng b khng c vnh trt v chi in. Trn hnh 3.7c, my kch t chnh l my pht in ng b c dy qun phn ng t trn r to v c b chnh lu c gn trc tip ln trc. Cun dy kch thch ca my kch t chnh c cp in t my kch t ph qua b iu chnh in p. 3.3. ng c in ng b 3.3.1. u, nhc im v phm vi s dng ca ng c in ng b Trong thc t ch yu dng ng c in khng ng b, do chng c nhng u th: cu to n gin, lm vic chc chn, bo qun d dng v gi thnh h. Tuy vy, so vi loi ng c in khng ng b, cc ng c in ng b c nhng u im: - Nh c kch thch bng dng in mt chiu, chng c th lm vic vi

os =1c , v khi khng tiu th mt cng sut phn khng no ca li in. Cn khi lm vic ch qu kch thch, thm ch cc ng c in ng b s pht cng sut phn khng vo li in, ci thin c h s cng sut ca li in, lm gim hao tn in p v tn hao cng sut trong li in, cng nh nng cao h s cng sut ca my pht in, lm vic trn trm pht in. - Mmen cc i ca ng c in ng b t l vi U, cn ca ng c in

khng ng b t l vi 2U . V vy khi in p gim, ng c in ng b vn c

kh nng duy tr s lm vic vi ti ln. Ngoi ra, s dng kh nng tng dng in kch thch ca cc ng c in ng b s cho php tng tin cy lm vic ca chng khi in p trong li in b gim xung do s c v ci thin c iu kin lm vic ca c li in. - Do tr s khe h khng kh ln, cc tn hao ph trong thp v trong lng sc rto ca cc ng c in ng b nh hn so vi ng c in khng ng b, v vy m cc ng c in ng b thng c hiu sut cao hn. Cu to ca cc ng c in ng b phc tp hn so vi ng c in khng ng b r to lng sc, v thm vo , cc ng c in ng b cn phi c my kch t hoc mt thit b no khc cung cp dng in mt chiu cho dy qun kch thch. V vy, ni chung ng c in ng b t hn ng c in khng ng b r to lng sc. Vic m my v iu chnh tc quay ca ng c in ng b cng phc tp hn.

Cc ng c in ng b c u th ln ch khi 200 300dmP kW v chng c s dng thch hp nhng ni khng i hi phi m my, dng v iu khin tc quay thng xuyn (t hp ng c in-my pht in, bm cng sut ln, my nn kh, qut gi m, ng c dn ng chnh trong cc my xc EKG-8U trn m khai thc l thin). Theo nghin cu ca cc tc gi L.V.litvak,

33

L.A.Stramianhikv, cc ng c in ng b c os =1dmc xt v mt gi thnh v tn hao nng lng lun u vit hn so vi cc ng c in khng ng b c dng

kt hp vi t in b h s cng sut n os =1c . Khi 300dmP kW dng cc ng

c in ng b c os =0,9dmc lm vic ch qu kch thch l c li; cn khi 1000dmP kW - dng vi os =0,8c . Cc ng c in ng b c ch to c cng

sut n 50000dmP kW . Ngy nay, cc ng c in ng b c s dng ngy cng rng ri. 3.3.2. Cc phng php m my ng c in ng b

3.3.2.1. M my theo phng php khng ng b a s cc ng c in ng b s dng phng php m my ny.

Qu trnh m my khng ng b ng c in ng b c chia thnh hai giai on. Giai on I: khi m my, dy qun phn ng ca ng c in ng b (hnh 3.8b) c ni vi li in. T trng quay phn ng c sinh ra tc dng tng h vi dng in trong dy qun m my (hnh 3.8a), to nn lc in t F v mmen in t khng ng b, lm quay rto.

Dy qun m my c cu to kiu lng sc t trong cc rnh mt cc, hai u ni vi hai vnh ngn mch (hnh 8a) v c tnh ton m my trc tip. mt s ng c in, cc mt cc bng thp nguyn khi c ni vi nhau hai u r to bng hai vng ngn mch, thay th cho rto lng sc m my. Cn i vi cc ng c in ng b cc n, vic m my theo phng php khng ng b c kh khn hn, v dng in cm ng lp mng mt ngoi r to nguyn khi s gy nng cc b ng k. Trong trng hp , m my c d dng, cn phi h thp in p bng cch dng bin p t ngu hoc cun khng. Giai on II: cui qu trnh m my khng ng b, khi 0,05s r le tc tc ng, tip im 7 ca cng tc t ng li, cn tip im 8 b m ra. Kt qu l dy qun 2 c cp dng in kch thch v rto ca ng c in c li vo ng b sau mt vi qu trnh dao ng.

Hnh 3.8. Cu to dy qun m my

ca ng c in ng b (a) v s m my khng ng b (b,c).

34

My kch t ca ng c in ng b t ngay trn trc ca n, thng l my pht in mt chiu kch thch song song (hnh 3.9).

Khi m my theo s hnh 3.9a tip im 7 m, cn tip im 8 ng. Khi dy qun kch thch 2 ca ng c in ng b c ni tt qua in tr 6, nh vy qu trnh m my c din ra trong nhng iu kin thun li nht. S d phi ni tt dy qun kch thch 2 vi in tr 6, l v nu h mch dy qun kch thch th lc bt u m my t trng quay phn ng qut qua vi tc ng b s sinh ra s.. cm ng rt ln c th ph hng cch in dy qun. ng c in ng b c li vo ng b mt cch chc chn, nu h s trt cui giai on I cha kch thch tha mn iu kin:

2 20,04m dm tdb

odtdmdm

k P iS

iGD n (3.4)

Trong . odS mk -bi s mmen cc i ch ng b, khi dng in kch thch nh mc tdmi ; dmP l cng sut nh mc ca ng c in, kW; tdbi l dng in

kch thch khi ha ng b; 2GD l mmen ng lng ca ng c in v c cu ni trc vi n, 2.kG m ; 2dmn - tc quay nh mc ca ng c in, vg/ph.

Hnh 3.9. S mch kch thch ca ng c in ng b vi my kch t lc m my c dy qun

kch thch c ni tt qua in tr trit t (a) v c ni thng vo my kch t (b)

M my theo s hnh 3.9b din ra trong nhng iu kin km thun li hn: - Th nht, v ng c in ng b c kch thch qu sm s to nn dng in ngn mch:

2 2 2

(1 )

(1 )n

u d

s EI

r s x

(3.5)

Trong E-s.. cm ng bi dng in kch thch ti ; dx - in khng ng b dc trc,

khi s=0. Do , ng c in phi ti thm cng sut:

2

un nP mI r (3.6) V trn trc ng c in c thm momen cn:

35

1

nn

pPM

(3.7)

Khin cho qu trnh li rto ng c in vo ng b gp kh khn hn. - Th hai, so vi s hnh 8, ng cong momen khng ng b c dng km thun li hn. S hnh 3.9b m bo li ng c in vo ng b chc chn, nu momen

cn trn trc khi : (0,4 0,5)dm c dmn n M M Khi dy qun m my c thit k hon

ho, ng c in c th c li vo ng b tin cy khi c dmM M . M my theo s hnh 3.9b rt n gin, ging nh m my ng c in khng ng b rto lng sc, v vy cch m my ny ngy cng c p dng rng ri. Trn thc t cc ng c in ng b thng c m my trc tip theo phng php khng ng b, ngha l tin hnh ni trc tip dy qun phn ng ca n vo li in c in p . Khi iu kin m my kh khn (gim p trong li in ln, qu nhit dy qun m my hoc rto nguyn khi n mc nguy him) th phi m my di in p thp bng in khng hoc bng bin p t ngu, ging nh i vi ng c in khng ng b rto lng sc. Trn hnh 3.10 cho cc ng cong thay i ca dng in phn ng I, dng in kch thch ti , in p U, tc my quay n, khi m my khng ng b trc tip

ng c in ng b cng sut ln ( 1500 , 6 , 1000 / )dm dm dmP kW U kV n vg ph lc khng ti, bng cch ni thng dy qun kch thch. ng c c li vo ng b sau 11 giy di tc dng ca mmen phn khng. 3.3.2.2. Cc phng php m my khc Ngoi phng php m my khng ng b, trong nhng trng hp c bit, cn c th s dng mt vi phng php m my khc. Chng hn, c th m my theo phng php ha ng b, bng cch quay ng c in ng b lc khng ti nh mt my ni vi n (th d, trong t hp my ng c in ng b-my pht in mt chiu). Khi c th dng phng php ha ng b vi li in, nh i vi my pht in ng b. Trong mt s trng hp c th m my bng ngun c tn s thay i. Theo phng php ny, ng c in ng b c cp in t mt my pht in ng b ring, v tn s c tng ln dn dn t khng. Khi , ng c in ng b c quay ng b thm ch ngay t khi tc n cn rt thp. Dy qun kch thch ca my pht in v ca ng c in trong trng hp ny cn phi c cung cp t ngun ngoi. M my bng ngun c tn s thay i s din ra thun li hn khi dng in kch thch ca my pht in lc bt u m my c bng nh mc, cn dng in kch thch ca ng c in bng dng in kch thch ly theo c tnh khng ti ng vi khi dm n=ndmU U v . 3.3.3. Cc c tnh lm vic ca ng c in ng b Cc c tnh lm vic ca ng c in ng b l quan h ca

1 2 t, , , os =f(P ) khi U=const, f=const v iP I c const c dng nh trn hnh 3.10. Cng ging nh my pht in ng b, ng c in ng b thng lm vic vi

gc ti 020 30dm .

36

Hnh 3.10. Cc ng cong c trng cho qu trnh m my

trc tip ng c ng b bng cch ni thng dy qun kch thch

Cu hi n tp chng 3 Cu 1. Nguyn l lm vic chung ca my in ng b l g ? Vai tr ca my in ng b trong cng nghip v h thng phn phi v truyn ti in nng ?. Cu 2. Mi quan h ca tc t trng quay stator n1 vi tc quay ca rto n ? Ti sao tc quay ca rto li bng tc ca t trng quay ?. Cu 3. Kt cu ca my in ng b loi cc n v cc li ? Chc nng ca mi b phn ? Cu 4. Cc s kch thch ca my pht in ng b, phm vi ng dng ca mi s ? Cu 5. u nhc im v phm vi s dng ca ng c in khng ng b ?. Cu 6. c im ca cc phng php khi ng cc ng c ng b, c tnh lm vic ca ng c in ng b ?

37

Chng 4. MY IN MT CHIU Trong chng ny ni dung c gii thiu v my in mt chiu nhm gip

ngi c hiu c cu to, nguyn l thun nghch v nhng thng s k thut ca my in mt chiu, bit cc phng php m my i vi ng c in mt chiu. Ngoi ra ngi c cn bit nhng yu cu khi m my ng c in mt chiu v cc phng php iu chnh tc ng c in mt chiu. 4.1. i cng v my in mt chiu 4.1.1. Cu to ca my in mt chiu

Hnh 4.1. Cu to ca my in mt chiu

1-lp bo v c gp chi in, 2-chi in, 3-li thp rto, 4-li thp Stato 5-cc t chnh, 6-v my, 7-lp bo v cnh qut, 8-qut gi

4.1.1.1. Phn tnh (Stator), gm cc b phn sau: - Cc t chnh (hnh 4.2a) l b phn sinh ra t trng, gm li thp cc t 4 v dy qun kch thch 2 lng bn ngoi li thp. Li thp cc t lm t nhng l thp k thut in dy 0,5-1mm, c p v tn cht vi nhau. i vi nhng my in nh, cc t c th c ch to t thp khi. Cc t c gn cht vo v my nh cc bulng 5. Dy qun kch thch bng ng bc cch in, qun thnh tng cun ri bc cch in thnh mt khi v c tm sn cch in trc khi t trn cc cc t. Cc cun dy kch thch trn cc cc t, c ni ni tip vi nhau.

Hnh 4.2. Cc t chnh (a) v cc t ph (b)

ca my in mt chiu

38

- Cc t ph (hnh 4.2b) t gia cc cc t chnh, dng ci thin i chiu. Li thp 1 ca cc t ph thng c lm bng thp khi, trn c t dy qun 2 c cu to nh dy qun cc t chnh. Cc t ph cng c gn vo v my bng bulng.

- Gng t (hnh 4.1 - s 6) dng ni lin mch t gia cc cc t, ng thi lm thnh v my. i vi nhng my in nh v va, gng t c ch to t cc tm thp dy sau khi un li c hn, i khi cng c c bng gang. i vi nhng my in ln gng t thng c ch to t thp c.

- Cc b phn khc, gm: + Np my (hnh 4.1 - s 1 v s 7), dng bo v khi nhng vt l ri vo trong my v m bo an ton cho ngi s dng, trnh nguy him do in git. nhng my in nh v va, np my cn c tc dng lm gi bi chng thng c c bng gang. + C cu chi than (hnh 4.3) dng a dng in t phn quay ra ngoi hoc ngc li. C cu chi than, gm: Chi than 3 t trong hp chi than 4 v nh l xo 2 n c t cht xung c gp.

Hnh 4.3. C cu chi than

Hp chi than t c nh trn gi chi than v c cch in so vi gi. Gi chi than c th xoay c, mc ch iu chnh v t chi than ng v tr. Sau khi iu chnh xong, dng vt c nh cht li.

4.1.1.2. Phn quay (rotor) gm cc b phn sau:

Hnh 4.4. Li thp phn ng my in mt chiu khi khng

c dy qun (a); L thp ca li thp phn ng (b); phn ng c vi phn t dy qun (c)

- Li thp phn ng (hnh 4.4a) dng dn t, c ghp t cc l thp k thut in dy 0,5mm, sn cch in mng hai mt ri p cht li vi nhau nhm

39

gim tn hao dng in xoy. Trn tng l thp c dp hnh dng rnh (hnh 4.4b), sau khi p chng li dy qun phn ng s c t vo cc rnh (hnh 4.4c). i vi nhng my in c trung bnh tr ln trn cc tm thp cn dp thm cc l thng gi, khi p chng li thnh li thp, l thng gi dc trc c to ra. i vi nhng my in va v ln, li thp thng c chia thnh tng on nh, gia cc on c mt khe h. i vi nhng my in c nh li thp phn ng c p trc tip vo trc. - Dy qun phn ng l b phn sinh ra s.. v c dng in chy qua, thng c ch to t dy ng bc cch in c tit din trn hoc ch nht. Dy qun phi c cch in so vi rnh v so vi np my.

Hnh 4.5. C gp my in mt chiu

- C gp hay cn gi l vnh gp (hnh 4.5), dng kt hp vi chi than i chiu dng in xoay chiu trong phn ng thnh dng in mt chiu mch ngoi (hoc ngc li). - Cc b phn khc, gm c: Cnh qut dng lm ngui my (hnh 4.2 s 8); trc my t li thp phn ng v bi. 4.1.2. Phn loi my in mt chiu - Phn loi my in mt chiu theo cng sut: n 0,5kW l my c cng sut rt nh; t 0,5 n 20kW l my cng sut nh; t 20 n 250kW l my cng sut trung bnh v trn 250kW l my in cng sut ln. - Phn loi theo in p: n 24V l my in p rt thp; t 60 n 80V l my in p thp; t 110 n 220V l my in p thng; t 400 n 600V l my in p tng i cao v trn 750V l my in p cao. - Phn loi theo v tr t cun kch t so vi phn ng ng c in mt chiu gm c: ng c in mt chiu kch t c lp, ng c in mt chiu kch t ni tip, ng c in mt chiu kch t song song v ng c in mt chiu kch t hn hp. 4.2. Nguyn l thun nghch ca my in mt chiu

T thng chnh do phn cm to ra i t cc t bc N qua phn ng n cc nam S v qua gng t tr v khp kn mch cc N.

My in mt chiu c tnh cht thun nghch, ngha l my c th lm vic ch my pht in, cng nh c ch ng c in. Di y s ln lt xt tng ch lm vic ca n.

40

Hnh 4.6. S nguyn l my in mt chiu n gin

4.2.1. Ch my pht in Gi s quay phn ng ca my in mt chiu (hnh 4.6 v hnh 4.7a) theo chiu kim ng h. Khi , theo nh lut cm ng in t trong cc thanh dn dy qun phn ng cm ng ra s.., c chiu c xc nh theo quy tc bn tay phi, cn gi tr tc thi ca n bng: etd = Blv (4.1) trong : B - t cm khe h khng kh ni thanh dn qut qua; l - chiu di tc dng thanh dn; v - tc chuyn ng thanh dn trong t trng. Do cc thanh dn b tr i xng, nn s... cm ng trong cc thanh dn etd ging nhau, theo mt vng kn cc s... ny c cng li. Nh vy s... dy qun phn ng c mt vng dy: E = 2etd =2Blv (4.2) V cc thanh dn quay lun chuyn trong t trng t N n S v ngc li, nn s... cm ng trong thanh dn etd v trong dy qun phn ng E l nhng i lng xoay chiu. Nu tc quay phn ng n c th nguyn l vg/s, th tn s f bin i ca s... my in 2 cc: f= n cn my in c p s i cc t: f=pn. Nu my pht in mang ti, dy qun phn ng khp kn mch vi ti bn ngoi qua vnh gp v chi than (hnh 4.3, hnh 4.5), th trong dy qun phn ng c c mt dng in xoay chiu I bin i tng t nh dng E. mch ngoi, do tc dng ca vnh gp v chi than m cc tnh ca chi than lun khng i, v vy s.. E v dng in I mch ngoi l nhng i lng mt chiu bin i p mch. Tht vy, khi phn ng cng c gp quay i 900 so vi v tr trn hnh 4.7a, chiu s.. trong thanh dn b i li ng thi vi vic thay th phin gp di chi than. Nhn thy chi than bn trn lun tip xc vi phin gp ni vi thanh dn nm di cc N, cn chi than bn di cc S, chnh v vy m cc tnh ca chi than lun khng i, v E,I mch ngoi l nhng i lng mt chiu. Nh vy ch my pht, vnh gp v chi than l b chnh lu c kh bin i dng in xoay chiu trong dy qun phn ng thnh dng in mt chiu mch ngoi. nhn c s.. E mch ngoi c tr s ln v bt b p mch, trong thc t dy qun phn ng gm nhiu bi dy, cc bi dy t cnh nhau lch nhau

41

trong khng gian mt gc no . Tt nhin khi c gp cng bao gm nhiu phin gp t cch in so vi nhau v cch in so vi trc my. Khi my pht in mang ti, in p trn cc my pht U nh hn s.. E mt lng bng in p ri trn in tr dy qun phn ng: U=E-IR (4.3)

trong : R in tr ton phn ca mch in phn ng. Khi c dng in I chy trong thanh dn nm trong t trng, s xut hin lc

in t Ftd tc dng ln thanh dn (hnh 4.6). Chiu ca lc in t Ftd c xc nh theo quy tc bn tay tri, cn tr s ca n c tnh theo cng thc:

Ftd = BlI (4.4) Lc in t Ftd to ra m men in t M M=FtdD = BlID (4.5) trong : D ng knh phn ng.

Hnh 4.7. Nguyn l ca my in mt chiu n gin ch

my pht in (a) v ng c in (b). ch my pht in, m men in t M tc dng ngc chiu quay phn

ng nn n ng vai tr m men hm. 4.2.2. Ch ng c in

Mun my in mt chiu ang xt lm vic ch ng c in, phi dng ngun in mt chiu t bn ngoi t vo cc chi than khc cc tnh ca my. Nh c dng in I chy trong dy qun phn ng m lc in t Ftd xut hin tc dng ln thanh dn. Chiu ca lc in t Ftd c xc nh theo quy tc bn tay tri, cn tr s ca n cng c xc nh theo cng thc (4.4). Kt qu l m men in t M, c tr s xc nh theo cng thc (4.5) c to ra. Khi mmen in t M ln thng c mmen cn phn ng s quay, to ra cng sut c trn trc ng c in. ch ng c in, chiu quay ca my do mmen in t quyt nh nn M ng vai tr l mmen ch ng.

Khi thanh dn quay trong t trng, trong thanh dn cm ng s.. etd c chiu c xc nh theo quy tc bn tay phi, cn tr s cng c xc nh theo cng thc (4.1). Kt qu l trong dy qun phn ng c mt s.. E.

ch ng c in, in p t trn cc U cn bng vi s.. E v in p ri trn in tr dy qun phn ng:

U=E+IR (4.6)

42

ch ng c in, vnh gp v chi than ng vai tr b nghch lu dng in, bin i dng in mt chiu mch ngoi thnh dng in xoay chiu trong dy qun phn ng.

Nu cc tnh cc t chnh v chiu quay phn ng ch my pht in v ng c in ging nhau (hnh 4.7), th chiu tc dng ca mmen in t M v chiu dng in I hai ch ny ngc nhau. 4.3. M my ng c in mt chiu 4.3.1. Yu cu m my ng c in mt chiu

- Phi to c mmen m my Mmm c gi tr ln nht c th c, ng c in hon thnh qu trnh m my trong khong thi gian ngn nht.

- Dng in m my Imm phi c hn ch n mc nh nht, nhm trnh cho dy qun phn ng khi b chy hoc nh hng xu n i chiu.

m my khng ti ng c in, c th s dng cc phng php nh m my trc tip, m my bng bin tr v m my bng in p thp. 4.3.2. Cc phng php m my ng c in mt chiu

4.3.2.1. M my trc tip: y l phng php m my n gin nht, ch vic ng in trc tip cho

ng c in vo ngun c in p U=Um. ta c:

u

uu R

EUI

(4.7)

Lc rto cha quay n=0, s.. E=Cen=0, v vy dng in phn ng:

u

u R

UI (4.8)

Trong h n v tng i, vi U=Um, ta c:

uuu RR

UI

1 (4.9)

cc my in kiu thng thng, c R=0,020,10, nn khi m my trc tip vi in p nh mc (U=1):

I=(5010)Im (4.10) Dng in phn ng I ln nh vy l khng cho php.

V vy phng php m my trc tip ch dng ph hp m my cc ng c in mt chiu cng sut c vi trm W, v chng c R tng i ln, lc m my c I(46)Im, mt khc thi gian qu trnh m my li ngn 12s.

4.3.2.2. M my dng in tr, bin tr: y l phng php hay c dng nht m my cc ng c in mt

chiu kch t song song. S m my ng c in kch thch song song nh bin tr m my c

trnh by trn hnh 4.8. trnh nguy him cho ng c in v dng in m my Imm qu ln, lc m my phi ni ni tip vo mch phn ng ca ng c in bin tr m my (hnh 4.8a) hay in tr m my (hnh 4.8b). Khi , theo biu thc (4.8) ta c:

mmu

uu RR

EUI

(4.11)

43

Hnh 4.8. S m my ng c in mt chiu kch t

song song dng bin tr (a) v in tr (b). thi im bt u m my, n=0, E=0:

mmu

u RR

UI

(4.12)

Rmm l ca bin tr m my (hnh 4.8a) hay in tr m my (hnh 4.8b). Tr s ca Rmm phi c tnh chn sao cho i vi cc ng c in cng sut

ln lc bt u m my c I=(1,41,7)Im, cn i vi cc ng c in cng sut nh - I=(22,5)Im.

Trc lc m my cng tc ng CT ca bin tr m my t v tr 0 trn hnh 8a, mch in ca ng c in h.

Lc bt u m my (t=0), dng tay xoay cng tc ng CT v v tr 1. Trong qu trnh m my cng tc ng CT ln lt c xoay v cc v tr 2, 3, 4, 5, nh vy tng cp in tr ca bin tr m my ln lt c loi ra khi s . Khi kt thc qu trnh m my v tr 5 (loi ht cc cp bin tr m my ra khi s ), ng c in lm vic di in p U=Um.

Khi thnh lp s hnh 4.8a phi lu : trong sut qu trnh m my mch kch thch ca ng c in lun t di in p , dy qun kch thch KT phi c ni c nh vi cung ng , cng tc ng CT lun tip xc trt trn n, v trc lc m my phi t con trt ca bin tr iu chnh v tr sao cho rc=0. iu ny l cn thit cho khi m my lun c It v t gi tr cc i v khng i, v nh vy ng vi mi tr s I cho s nhn c mmen M cc i.

S thay i ca dng in phn ng I, mmen M v tc quay n theo thi gian trong qu trnh m my, c trnh by trn hnh 4.9.

Hnh 4.9. Quan h ca I, M, n vi thi gian

khi m my ng c in.

44

Vng gch cho trn hnh 4.9 l mmen ng: M = M MC (4.13)

Di tc dng ca mmen ng ny, din ra qu trnh tng tc n. S cp bin tr m my v tr s in tr tng cp phi c tnh chn sao cho trong khong thi gian chuyn tip gia cc cp, tr s cc i v cc tiu ca dng in I tt c cc cp phi bng nhau.

Cc cp in tr c tnh chn theo ch lm vic ngn hn di ti. Trong cc thit b t ng, thay cho bin tr m my ngi ta s dng cc in

tr m my (hnh 4.8b). Khi m my, chng ln lt c ni sun mt cch t ng nh cc tip im ca cng tc t K1, K2, K3. n gin s v gim bt s lng thit b, s cp in tr m my cn ly nh nht (vi ng c in cng sut nh thng ly bng 12 cp).

Khi dng ng c in, khng c h mch kch thch, m phi khp kn mch ca n qua phn ng (hnh 4.8a,b). Lm nh vy c tc dng cho dng in trong dy qun kch thch khng b gim xung 0 mt cch tc thi sau khi ct in ng c, m phi sau thi gian ln, nhm ngn nga s.. t cm c tr s rt ln cm ng ra trong dy qun kch thch, c th ph hng cch in ca dy qun ny.

Cn c bit lu l: trong bt c trng hp no cng khng cho php lm t mch dy qun kch thch ca ng c in.

i chiu quay ng c in cn phi i chiu dng in phn ng I, hoc i chiu dng in kch thch It ngay trc lc m my. Mun i chiu quay lc ng c in ang quay, ch c php i chiu dng in phn ng I, v nu i chiu dng in kch thch th s xut hin s.. t cm rt ln (do dy qun kch thch c in cm ln) c th gy ra qu in p nh thng cch in ca n.

4.3.2.3. M my bng in p thp: Mc ch ca phng php m my ny l hn ch dng in m my.

m my bng in p thp i hi phi c ngun in mt chiu c in p iu chnh c (my pht in mt chiu, chnh lu c iu khin...) cung cp in cho phn ng ca ng c in. Trong khi , mch kch thch ca ng c phi c t di in p U=Um, ly t ngun in mt chiu c lp khc. Phng php m my ny thng s dng cho cc ng c in cng sut ln, c kt hp vi vic iu chnh tc quay ca ng c.

4.3.2.4. M my bng cch thay i in p phn ng:

Hnh 4.10. S iu chnh tc quay ng c in kch thch c

lp bng cch thay i in p phn ng, khi cp in cho ng c t my pht in (a) v t b chnh lu c iu khin (b).

45

Phng php iu chnh tc ny c th p dng c i vi ng c in kch t song song lm vic ch kch t c lp. Mun iu chnh tc , phi c ngun in mt chiu cho php iu chnh m in p cung cp cho phn ng ca ng c in, trong khi mch kch thch ca n phi c cung cp t ngun in mt chiu c lp khc ( =const).

Trn thc t, c th s dng my pht in mt chiu kch thch c lp trong t hp "my pht in & ng c in mt chiu " (hnh 4.10a), hoc b chnh lu bn dn thyrist c iu khin - iu chnh c in p ra (hnh 4.10b), lm ngun in mt chiu cung cp cho phn ng ng c.

Khi thay i in p U cung cp cho dy qun phn ng, s nhn c mt h c tnh c cng dc nh trn hnh 4.11.

V khng cho php ng c in lm vic in p U>Um, nn phng php iu chnh ny ch cho php iu chnh tc v bn di tc nh mc. Khi in p U cng gim, tc ng c in cng thp.

phng php iu chnh tc ny c phm vi iu chnh tc kh rng, n 1:10 v ln hn, v hiu sut ng c in khi iu chnh tc vn c duy tr cao.

Hnh 4.11. c tnh tc v c tnh c

4.4. iu chnh tc ng c in mt chiu 4.4.1. ng c in mt chiu kch thch song song

4.4.1.1. c tnh tc v c tnh c t nhin c tnh tc t nhin v c tnh c t nhin ca ng c in kch thch

song song l ng thng (ng 1 hnh 4.12). Khi lm vic trn c tnh t nhin, s

Hnh 4.12. c tnh tc t nhin v c tnh c t nhin ca ng c in mt chiu kch thch song song.

46

thay i tc quay n ng c in kch thch song song khi ti thay i t khng ti (I=I0) n ti nh mc (I=I.m) l rt nh, bng (28)%nm. c tnh c s suy gim tc nh nh vy c gi l c tnh cng. ng c in kch thch song song c c tnh cng c s dng trong cc thit b yu cu tc quay duy tr gn nh khng i khi ti thay i (my ct gt kim loi...).

4.4.1.2. iu chnh tc quay bng cch gim t thng: Phng php iu chnh tc bng cch gim t thng l phng php

tin li, ph bin v kinh t nht. Tnh kinh t th hin ch: khi tin hnh iu chnh mch kch thch (dng in kch thch ch bng (110)% dng in nh mc phn ng) tn hao khi iu chnh rt nh, v vy hiu sut ng c khi iu chnh vn c duy tr cao.

Cc ng c in c tnh ton lm vic ch nh mc c tr s l ln nht, ngha l c tc quay n l nh nht. V vy, trn thc t ch c th iu chnh tc bng cch gim t thng nh tng in tr iu chnh rc trong mch kch thch.

Hnh 4.13. c tnh c v c tnh tc ca ng c in

mt chiu kch thch song song khi iu chnh tc bng cch gim t thng .

Khi khng c in tr ph trong mch phn ng Rf=0 v U=const, cc tr s t thng

47

ch lm vic khng ti ni trn, nh c mmen ngoi ng c in c th chuyn sang lm vic ch my pht in trn gc phn t th hai.

- iu chnh tc bng cch thay i in tr ph Rf trong mch phn ng. Khi mc ni tip in tr ph Rf vo mch phn ng (hnh 4.14a), ta c:

e

ufu

e C

IRR

C

Un

)( (4.14)

2)(

Me

fu

e CC

MRR

C

Un (4.15)

in tr ph Rf c tnh chn lm vic ch di hn, v c th iu chnh c.

Khi U=const, It=const, vi cc tr s Rf=const khc nhau s nhn c cc c tnh M=f(n) v I=f(n) nh trn hnh 4.14b. c tnh trn cng hnh 4.14b c Rf=0, l c tnh t nhin. Khi tr s Rf cng ln, c tnh cng mm hn, tc thay i nhiu hn khi ti thay i, nhng tc khng ti l tng n0 vn gi nguyn. Mi c tnh ct trc honh (n=0) ti mt im c:

fu

u RR

UI

(4.16)

fu

M

RR

UCM

(4.17)

Hnh 4.14. S iu chnh tc quay ca ng c in kch

thch song song bng cch thay i in tr ph Rf trong mch phn ng (a); v cc c tnh tc , c tnh c tng ng (b).

Giao im ca cc ng c tnh ni trn vi c tnh mmen cn MC=f(n) (ng nt t m nt trn hnh 4.14b) s xc nh tc quay n ch lm vic xc lp ca ng c in ng vi mi tr s ca Rf. Phng php iu chnh tc ny ch cho php iu chnh tc v bn di tc nh mc v c phm vi iu chnh tc ph thuc vo mmen ti MC bng qung 1:5.

Phn tip tc ca cc c tnh ni trn v bn di trc honh trn hnh 4.14b l ch hm u ngc ca ng c in, n

48

cn mmen M ca ng c in tc dng ngc chiu quay, l mmen hm. Nu ch khng ti (I=I0) nh c mmen quay bn ngoi lm tng tc

ca ng c, th ban u t n ch I=0, cn sau I b i chiu, my chuyn sang lm vic ch my pht in (gc phn t th hai trn hnh 4.14b).

Khi tr s Rf cng ln, tc quay n cng nh, v hiu sut ng c in cng thp. 4.4.2. ng c in mt chiu kch t ni tip

4.4.2.1. c tnh tc v c tnh c t nhin ng c in mt chiu kch thch ni tip c It=I=I, nn c th vit

= k I (4.19) trong

k - h s ph thuc vo ti, c gi tr khng i khi I(0,80,9)Im do mch t bo ho.

i vi ng c in mt chiu kch thch ni tip ta c:

IkC

IR

IkC

Un

e

u

e

(4.20)

k

CIkCICM MMM2

2 (4.21)

kC

R

MkC

UCn

e

u

e

M (4.22)

c tnh tc ca ng c in kch thch ni tip I=f(n) rt mm, c dng hypebon nh trn hnh 4.15. Khi k=const, c tnh I=f(n) c dng ng nt t. Khi dng in I nh, tc ng c in tng ln rt ln, khng cho php. V vy, khng cho php ng c in kch thch ni tip lm vic khng ti hoc c th ri vo tnh trng lm vic khng ti (chng hn khng c dng b truyn ai hoc ly hp ma st... truyn ng cho my cng tc t loi ng c in ny). Thng thng ch cho php ng c in kch thch ni tip lm vic vi ti ti thiu P2=(0,20,25)Pm.

Hnh 4.15. c tnh tc t nhin ca ng c in mt chiu

kch thch ni tip. Ch nhng ng c in kch thch ni tip c cng sut rt nh (c vi chc

W) mi cho php chy khng ti. c tnh c t nhin ca ng c in kch thch ni tip M=f(n) xc nh theo

biu thc (4.22) c biu din bng ng 1 trn hnh 4.15.

49

So vi ng c in kch thch song song, ng c in kch thch ni tip to ra mmen m my ln hn ng k v khi b qu ti v mmen n t b nguy him hn do cng sut P2 thay i t hn

(*) khi mmen ti MC=M thay i trong phm vi rng. Cc ng c in kch thch ni tip c dng thch hp nhng ni c iu

kin m my nng n (yu cu phi c mmen m my Mmm ln) v c mmen ti thay i trong phm vi rng. Chng thng c s dng trong cc u tu in, vn ti m hm l,...

Cn bit rng, khi tng tc quay, ng c in kch thch ni tip khng th chuyn sang lm vic ch my pht in c. iu ny th hin ch, c tnh I=f(n) trn hnh 4.15 khng ct trc tung.

4.4.2.2. iu chnh tc quay bng cch thay i t thng c th thc hin bng hai cch:

+ Gim t thng bng cch ni sun dy qun kch thch vi in tr Rs.t (hnh 4.16a) hoc gim s vng dy kch thch, thng qua cc u dy a ra t dy qun ny.

Hnh 4.16. S iu chnh tc quay ng c

in mt chiu kch thch ni tip bng cch ni sun dy qun kch thch (a), ni sun phn ng (b) v mc ni tip in tr ph Rf vo mch phn ng (c).

V in tr dy qun kch thch Rt nh v ging p trn n nh, cho nn Rs.t cng cn phi nh. V vy, tn hao trn in tr Rs.t cng nh, v tng tn hao trn mch kch thch khi ni sun thm ch b gim i, do hiu sut ng c in vn duy tr cao. Phng php iu chnh tc ny c s dng rng ri trn thc t.

Khi iu chnh tc bng cch gim t thng, phi thay k trong cc cng thc (4.20) (4.21) bng k.kg.t, vi kg.t l h s suy gim t thng kch thch. Trng hp ni sun kch thch:

tst

tstg RR

Rk

.

..

(4.23)

cn trng hp gim s vng dy kch thch:

.

..

t

lvttg W

Wk (4.24)

50

trong : Wt.lv - s vng dy kch thch lm vic; Wt. - tng s vng dy dy qun kch thch. Trn hnh 4.17 cho cc c tnh c M=f(n) ng vi: kg.t=1 - ng 1, l c tnh

c t nhin; kg.t=0,6 - ng 2; kg.t=0,3 - ng 3. Cc c tnh trn hnh 4.17 cho trong h n v tng i, ng vi khi k=const

v R*=0,1. + Tng t thng bng cch ni sun dy qun phn ng vi in tr Rs.

(hnh 4.16b). V ging p RtI nh, nn c th ly Rt0, v thc t in tr Rs. t di in p ca li in, v vy tr s ca Rs.t cn phi ln, lm cho tn hao trn n ln v hiu sut ng c in b gim mnh.

Ngoi ra, ni sun phn ng ch mang li hiu qu khi mch t khng bo ho. V nhng l trn, m trn thc t rt t khi s dng phng php iu chnh tc ny. Trn hnh 4.17 ng 4 l c tnh c M=f(n) khi:

dmus

us IR

UI 5,0

..

4.4.2.3. iu chnh tc quay bng cch ni ni tip in tr Rf vo mch phn ng: Phng php ny cho php iu chnh tc n v bn di tc nh mc trn hnh 4.16c. Do c thm tn hao trn in tr ph Rf lc iu chnh tc , lm cho hiu sut ng c in b gim i ng k, v vy phng php iu chnh tc ny t c s dng.

Khi iu chnh tc vi Rf*=0,5, c tnh c M=f(n) c dng ng 5 trn hnh 4.17.

Hnh 4.17. Cc c tnh c ca ng c in mt chiu

kch thch ni tip cc phng php iu chnh tc quay khc nhau. 4.4.2.4. iu chnh tc quay bng cch thay i in p: Phng php ny cho php iu chnh tc n v bn di tc nh mc, v

ng c in vn lm vic vi hiu sut cao. y l phng php iu chnh tc c dng ph bin trong cc thit b vn ti, trong c vn ti tu in m hm l, thc hin bng cch chuyn t cch ni song song cc ng c vi li in, sang ni

51

ni tip cc ng c (hnh 4.18). Trn hnh 4.17 ng 6 l c tnh c M=f(n) khi iu chnh tc vi U=0,5Um.

Hnh 4.18. iu chnh tc quay ng c in mt chiu kch t ni tip bng cch chuyn t cch ni song song cc ng c vi li in (a) sang cch

ni ni tip cc ng c (b).

Cu hi n tp chng 4 Cu 1. Cu to v nguyn l thun nghch ca my in mt chiu ? Vai tr ca my in mt chiu trong cng nghip v dn dng ?. Cu 2. S nguyn l v nguyn l lm vic ca my in mt chiu ch my pht in v ng c in ?. Cu 3. Cc yu cu khi m my ng c in mt chiu, gii thch ngha ca mi yu cu ? Cu 4. S v c im ca mi phng php m my ng c in mt chiu ? Cu 5. c im ca cc phng php iu chnh tc ng c mt chiu kch thch song song ?. Cu 6. c im ca cc phng php iu chnh tc ng c mt chiu kch t ni tip ?

PHN THAM KHO CC KIU DY QUN CA MY IN A. DY QUN MY IN XOAY CHIU I. Dy qun 1 pha 1. Dy qun ng khun

52

Dy qun ca my in xoay chiu c nhim v to ra sc in ng v ng thi cng tham gia vo vic to nn t trng cn thit cho s bin i nng lng c in trong my. - Kt cu ca dy qun phi m bo - Tit kim c dy ng (ch yu l phn u ni). - Bn v c, in, nhit. - Ch to n gin, lp rp, sa cha d dng.

tit kim kim loi v ci thin dng sng sc in ng, dy qun 1 pha thng qun ri.

Cc kiu dy qun 1 pha ca my in c Z=12. Dy qun ng tm v dy qun bc ngn

II. Dy qun ba pha

B dy 3 pha l t hp ca 3 b dy 1 pha t lch nhau mt gc 1200 trong khng gian. 1. Dy qun 3 pha mt lp

Dy qun 3 pha mt lp thng c dng trong cc ng c in c cng sut di 75kW v trong cc my pht in tuabin nc. loi dy qun ny trong mi rnh ch t mt cnh tc dng ca mt bi dy do s bi dy S=Z/2.

Xt dy qun mt lp vi s pha m=3; Z=24; 2p=4. V gc lch pha gia 2 rnh

lin tip l =Z

p 0360.=300 nn sc in ng ca cc cnh tc dng t 1-12 di i

cc th nht lm thnh hnh sao sc in ng c 12 tia nh hnh v bn di: Do v tr ca cc cnh t 13-24 di i cc th 2 hon ton ging v tr cc

cnh 1-12 di i cc th nht nn sc in ng ca chng c th biu th bng hnh sao sc in ng trng vi hnh sao sc in ng th nht.

53

S rnh ca mt pha di mt cc l: 22.3.2

24

2

mp

Zq

Ta c vng pha =q.=2.300=600. V 2 cnh tc dng ca mi phn t cch nhau y==m.q=2.3=6 rnh, nn pha A

gm 2 phn t to thnh bi cc cnh tc dng (1-7) v (2-8) di i cc th nht v 2 phn t (13-19); (14-20) di i cc th 2. Do cc pha lch nhau 1200 nn pha B gm cc phn t (5-11); (6-12); (17-23); (18-24). Pha C gm:(9-15); (10-16); (21-3); (22-4).

Hnh sao sc in ng rnh hay hnh sao sc in ng phn t ca dy qun c Z=24; m=3; 2p=4; q=2.

Cng tt c cc vc t sc in ng ca cc phn t thuc cng 1 pha ta s c

cc sc in ng EAEBEC. em ni cc phn t thuc cng 1 pha vi nhau ta s c dy qun 3 pha.

S khai trin dy qun 3 pha ng khun c Z=24;2p=4;q=2 nh hnh di: V mi pha c 2 nhm phn t c v tr di 2 i cc hon ton ging nhau

nn c th to thnh mt mch nhnh (nu ni cui ca nhm phn t trc vi u ca nhm phn t sau) hay thnh 2 mch nhnh ghp song song (nu ni u ca 2 nhm phn t vi nhau v ni cui ca chng vi nhau). Tng qut, nu my c p i cc th th s mch nhnh song song ca mi pha l k vi iu kin k chia ht cho p.

54

T hnh v trn ta thy sc in ng ca mi pha khng ph thuc vo th t ni cc cnh tc dng, th d vi pha A chng hn ta c th ni cc cnh tc dng 1-8-2-7 di i cc th nht v 13-20-14-19 di i cc th hai v ta c 2 nhm c 2 phn t kch thc khng ging nhau v gi l dy qun ng tm. dy qun ng tm, kh thc hin cc nhnh song song hon ton ging nhau v chiu di ca cc nhm bi dy trong tng pha khng bng nhau. 2. Dy qun 3 pha hai lp

Dy qun 2 lp l dy qun m trong mi rnh c t 2 cnh tc dng ca phn t. Nh vy s phn t S bng s rnh Z. Dy qun 2 lp c u im l thc hin c bc ngn, lm yu c sc in ng bc cao, do ci thin c sc in ng. Nhc im ca n l vic vo dy qun hay sa cha dy qun kh khn hn.

Dy qun 2 lp ca my in xoay chiu c ch to theo 2 kiu: dy qun xp v dy qun sng.

Dy qun xp thng c dng cn dy qun sng ch dng qun rotor dy qun ca ng c khng ng b v my pht tuabin hi nc cng sut ln.

S khai trin ca dy qun 3 pha ng tm 2 mt vi Z=24, 2p=4, q=2

Dy qun 2 lp thng c thc hin vi vng pha =q.=600

55

Hnh trn trnh by cch trin khai ca dy qun xp c Z=24, 2p=4, m=3, vng pha

=q.=600 v bc ngn y=65

. Do gc lch pha gia 2 rnh lin tip l

=Z

p 0360.=300 nn di mi cc t mi pha c q=/=2 bi dy.Th t ni cc bi

dy nh sau: - Pha A: A-1-2-8-7-13-14-20-19-X - Pha B: B-5-6-12-11-17-18-24-23-Y - Pha C: C-9-10-16-15-21-22-4-3-Z

l dy qun xp 3 pha 2 lp vi (z=24,2p=4,q=2;y=5, )65

Cn dy qun sng 3 pha 2 lp vi (Z=24;2p=4;q=2;y=5; )65

th nh sau:

V cc nhm phn t ca mt pha lin tip c t di cc cc t khc nhau

nn sc in ng cm ng ca chng c chiu ngc nhau (u cc nhm phn t, v d pha A c ghi k kiu *). mi pha hnh thnh mt mch nhnh, ta phi ni cui ca nhm phn t trc vi u ca nhm phn t tip theo nh hnh. Nu mun mi pha c nhiu mch nhnh song song th phi ni u ca cc nhm bi dy ca pha vi nhau v cui ca cc nhm bi dy vi nhau. Ni chung s nhnh song song ca mt pha c th l k vi iu kin l k chia ht cho 2p. B. DY QUN MY IN MT CHIU I. Dy qun phn ng my in mt chiu

Dy qun phn ng l b phn quan trng nht ca my in v n tham gia trc tip vo qu trnh bin i nng lng t in nng thnh c nng v ngc li. Ngoi ra dy qun phn ng cn chim mt t l kh cao trong c cu gi thnh ca my pht in mt chiu. Cc yu cu t ra cho dy qun phn ng l: - Sinh ra mt sc in ng cn thit, c th cho mt dng in nht nh chy qua sinh ra mt m men cn thit m khng b nng qu nhit quy nh, ng thi m bo i chiu tt. - Kt cu n gin, tit kim vt liu, lm vic chc chn an ton. - Dy qun phn ng chia lm dy qun xp n v xp phc tp v dy qun sng n v sng phc tp. Mt s my in mt chiu c cng sut ln cn dng loi dy qun hn hp, kt hp gia dy qun sng v xp. 1. Cu to:

56

Dy qun phn ng gm nhiu phn t dy qun ni vi nhau theo mt quy lut nht nh. Phn t l mt bi dy gm mt hay nhiu vng dy m 2 u ca n ni vo 2 phin gp. Cc phn t ni vi nhau thng qua cc phin gp v lm thnh mch vng kn.

Mi phn t c 2 cnh tc dng, l phn t vo rnh ca li st. Phn ni 2 cnh tc dng nm ngoi li st gi l phn u ni.

d ch to, mt cnh tc dng ca phn t t lp di ca mt rnh cn cnh tc dng kia t lp trn ca mt rnh khc. Cc phn t khc cng xp th t nh vy vo cc rnh k bn cho n khi y cc rnh. Nu trong mt rnh phn ng (gi l rnh thc) ch t 2 cnh tc dng (mt cnh nm lp trn v mt cnh nm lp di rnh) th ta gi rnh l rnh nguyn t. Nu trong mt rnh thc c t 2u cnh tc dng (u=1,2,3,4n) th ta c th chia rnh thc thnh u rnh nguyn t. V vy quan h gia s rnh thc Z ca phn ng vi s rnh nguyn t l: Znt=uZ Gia s phn t ca dy qun S, s rnh nguyn t Znt v s phin gp G cng c mi quan h nht nh. V mi phn t c 2 u ni v 2 phin gp, ng thi mi phin gp li ni 2 u ca 2 phn t li vi nhau, nn s phn t S bng s phin gp G. Tc l S=G.

V mi rnh nguyn t t 2 cnh tc dng m mi phn t cng c 2 cnh tc dng nn Znt=S=G. Rnh thc c 1,2,3 rnh nguyn t:

Tu theo kch thc ca cc phn t m ta chia dy qun thnh dy qun c

phn t ng u v dy qun theo cp. - Dy qun c phn t ng u l dy qun m kch thc cc phn t hon ton ging nhau. - Dy qun theo cp l dy qun m khi cnh tc dng th nht ca cc phn t cng nm trong mt rnh thc th cnh tc dng th 2 ca chng li nm trong rnh thc khc nhau. V vy kch thc ca cc phn t khng bng nhau.

Dy qun c phn t ng u v theo cp

2. Cc bc dy qun Cc bc dy qun xp v sng: Quy lut ni cc phn t dy qun c th c xc nh theo bc dy qun sau:

57

- Bc dy qun th nht y1: l khong cch gia 2 cnh tc dng ca mt phn t o bng s rnh nguyn t.

- Bc dy qun th hai y2: l khong cch gia cnh tc dng th 2 ca phn t th nht vi cnh tc dng th nht ca phn t th hai ni tip ngay sau v o bng s rnh nguyn t.

- Bc dy qun tng hp y: l kh