Tai Lieu Ky Thuat Dien - Nghe Xay Dung

7/27/2019 Tai Lieu Ky Thuat Dien - Nghe Xay Dung

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TRNG CAO NG NGH QUC T VABIS HNG LAM

KHOA IN IN LNH

MN HC: K THUT IN

M S: MH09

NGH: K THUT XY DNG

Trnh Cao ng ngh

Vng tu 2012

Gio trnh lu hnh ni b


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TRNG CAO NG NGH QUC T VABIS HNG LAM

KHOA IN IN LNH

MN HC: K THUT IN

M S: MH09

NGH: K THUT XY DNG

Trnh Cao ng ngh

Gio vin son Khoa in in lnh

L Vit Thnh Nguyn Vn V

Vng tu 2012Gio trnh lu hnh ni b


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Mn hc: K thut in Ngh: K thut Xy Dng

Chng 1: Mch in mt chiu Trang 1

MC LC

Contents

MC LC .................................................................................................................................. 1

GII THIU MN HC ........................................................................................................... 2

CHNG 1: MCH IN MT CHIU ................................................................................ 4

CHNG 2: IN T ............................................................................................................ 20

CHNG 3: DNG IN XOAY CHIU MT PHA ......................................................... 36

CHNG 4: MCH IN BA PHA ...................................................................................... 60

CHNG 5: MY BIN P .................................................................................................. 71

CHNG 6: NG C KHNG NG B ....................................................................... 76

TI LIU THAM KHO ...................................................................................................... 105


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Gii thiu mn hc Trang 2

GII THIU MN HC

I.V TR, TNH CHT CA MN HC:- V tr mn hoc: Mn K thut in l mt trong cc mn k thut c s, c

b tr hc trc cc mn hc/m un chuyn mn ngh.- Tnh cht mn hc: Mn K thut in l mn c s h tr kin thc cho cc

mn khc, ng thi gip cho hc vin c iu kin t hc, nng cao kin thc nghnghip.

II. MC TIU MN HCTrang b cho ngi hc nhng kin thc c bn v mch in mt chiu, xoay

chiu v cc loi my in.Gip cho ngi hc hiu c cu to, nguyn l lm vic ca cc dng c o

lng in v cc loi my in.V kin thc:

Nu c cc kin thc v phn tch c mt s s mch in c bn;Nu c trnh t cc bc o, kim tra bng dng c o m bo an ton.

V k nngS dng cc loi dng c o, tin hnh o v kim tra cc mch in c bn,

cc my in hoc ng c in.V thi :

Rn luyn tnh cn thn, t m khi phn tch v s dng cc dng c kim tra,o mch in.

III. NI DUNG MN HC

STT

Tn chng, mc Thi gian (gi)Tngs

Lthuyt

Thc hnh,BT

Kimtra*

I Mch in mt chiu 4 2 2

II in t 4 2 2

III Mch in xoay chiu hnhsin 1 pha

6 3 2 1

IV Mch in xoay chiu 3 pha 6 3 2 1

V My bin p 4 2 2 0

VI ng c khng ng b 6 3 2 1

Cng 30 15 12 3* Ghi ch: Thi gian kim tra l thuyt c tnh vo gi l thuyt, kim tra

thc hnh c tnh vo gi thc hnhIV. IU KIN THC HIN CHNG TRNH:* Dng c v trang thit b:

- Cc m hnh m phng mch mt chiu, xoay chiu;- Cc bn v, tranh nh cn thit;- Ngun lc khc:- PC, Phn mm chuyn dng;- Projector, Overhead;


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Gii thiu mn hc Trang 3

- My chiu vt th ba chiu.

V. PHNG PHP V NI DUNG NH GI:C th p dng hnh thc kim tra vit hoc kim tra trc nghim. Cc ni dung

trng tm cn kim tra tp trung chng 2, chng 5 v chng 6 l:Chng 2:

- Cc nh lut, biu thc c bn;- Gii mch xoay chiu R-L-C ni tip.Chng 3:- Cu to, nguyn l lm vic ca my pht in 3 pha;- Cc ni ph ti hnh sao, tam gic.Chng 5:- Cu to, nguyn l lm vic ca my bin p;- Ch lm vic ca my bin p.Chng 6:- Cu to, nguyn l lm vic ca ng c in xoay chiu;- Cc thng s k thut ca ng c in xoay chiu.

VI. HNG DN CHNG TRNH :

1. Phm vi p dng chng trnh:

Chng trnh mn hc ny c s dng ging dy cho trnh Trung cpngh K thut xy dng.

2. Hng dn mt s im chnh v phng php ging dy mn hc:

- Trc khi ging dy, gio vin cn cn c vo ni dung ca tng bi hc

chun b y cc iu kin cn thit nhm m bo cht lng ging dy;- Nn p dng phng php m thoi ngi hc ghi nh k ;

- Nn b tr thi gian gii bi tp hp l mang tnh minh ha ngi hc hiubi su hn;

3. Nhng trng tm cn ch :- Phng php gii mch, tnh ton cc thng s trong mch in xoay chiu R-

L-C;- Cu to, nguyn l lm vic ca my pht in 3 pha;- Cu to, nguyn l lm vic ca my bin p, ng c in xoay chiu;

4. Ti liu cn tham kho:- K thut in i cng - Hong Hu Thn - NXB i hc v Trung hc

chuyn nghip - H Ni 1976;

- Bi tp K thut in i cng - Hong Hu Thn - NXB i hc v Trunghc chuyn nghip - H Ni 1980;

- Gio trnh in k thut, in cng trnh NXB Xy dng 2007.


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Mn hc: K thut in

Chng 1: Mch in mt chiu

CHNG 1(L thuy

Mc tiu:

- Phn tch c nhi

ngun in, dy dn, ph ti, t- Hiu v vn dng

Ni dung:

1.1. NHNG KHI NIMDng in mt chiu l

gian.

.

.

Ngh: K

: MCH IN MT CHIt: 2h; Bi tp: 2h; Kim tra: 0h)

v, vai tr ca cc phn t cu thnh

it b o lng, ng ct...;c cc biu thc tnh ton c bn ca nh

BN V MCH IN MT CHI

dng in c tr s v chiu khng tha

:.

.

thut Xy Dng

Trang 4

U

ch in nh:

lut m

U

i theo thi


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Hnh v bn y cho thy ngun pin to ra sc p in v to ra dng in chytrong mch, dng ny lm nng si nung trong n, khi dy kim loi b t nng n s

pht ra nh sng. l nguyn l vn hnh ca loi n nhit quang.

Mt mch in thng 3 trng thi:

* Trng thi mch ang hot ng, lc ny trn cc ng mch c mc p V,

trn cc nhnh c dng chy I v cc linh kin chu tc ng ca cng sut W.* Trng thi mch ang tt. Lc ny trong mch vn cn c ngun nhng khng

c dng chy. Mch s hot ng li khi ng kha in.

* Trng thi mch cht, mch tho pin.

Trong mt mch in c 2 tham s trng thi quan trng m chng ta lun munbit, l: Mc p V trn cc ng mch v cng dng in I chy qua cc linhkin. o in p chng ta dng Volt k cho mc song song vo hai im o bitp, do khi o p dng cch mc song song nn my o t nh hng vo hot ngca mch Bn phi dng my o Volt c ni tr ln, cng ln cng tt. Khi o dngchng ta dng Ampere k cho mc ni tip vo mch, do khi o dng dng cch mcni tip nn my o t nh hng vo hot ng ca mch Bn phi dng myo Ampere c ni tr nh, cng nh cng tt.


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1.2. NH LUT OHM1.2.1. Lut Ohm cho on mch

Lut Ohm cho thy cc mi quan h gia 4 tham s P, V, R, I. Trong mt mchin khi bit R, I, chng ta s tnh cV. Pht biu c bn ca lut Ohm l: Khi cdng in I chy qua 1 in tr R, th trn 2 u ca in tr s xut hin in p V, v

V = R x I.in p V v dng in I l 2 tham s trng thi, cn cc tham s khc, nh in

tr R, in dn G, cng sut P l cc tham s dn xut, n tnh theo V v I.

Lut Ohm cho thy mi quan h nh lng ca 3 tham s trn cc in tr, l: in p V o trn hai u ca in tr, l cng dng in I chy qua in trv tr sc cng Ohm ca in tr.

nh mi quan h ca 3 tham s ny, Bn c th dng hnh v trn. Nu bittrc 2 tham s th lun tnh c tham s th 3. Bn ly ngn tay che ch V s thyIxR, Bn ly ngn tay che ch I s thy V/R v Bn ly ngn tay che ch R s thyV/I.

Trong mt mch in, cc php ton thng c tnh theo cc cng thc c lit k

theo bng sau:


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Cc tnh ton c bn trong mt mch in ch lng vng vi 4 tham s, l:

* Cng sut P tnh bng Watt.

* in p V tnh bng Volt.

* in tr R tnh bng Ohm.

* Cng dng in I tnh bng Ampere

Cch dng bng:Phn 1: Bn dng bng tnh cng sut P theo cc tr V, R, I.

Phn 2: Bn dng bng tnh V theo cc tr P, R, I.

Phn 3: Bn dng bng tnh R theo cc tr P, V, I.

Phn 4: bn dng bng tnh I theo cc tr P, V, R.

Th d: Bn c mt ngun pin V=9V cp dng cho mt in tr l R=1K, vy dngchy trong mch l bao nhiu Ampere? v cng sut t nng in tr l bao nhiuWatt?

Tra bng chng ta thy, ly p V chia cho tr R s bit cng dng chy trongmch.

Tra bng chng ta thy, ly bnh phng p V v chia cho tr R s bit cng sutt nng tr.


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1.2.2. Lut Ohm cho mch knXt nhnh c E v R (Hnh 1-10)

Biu thc tnh in p U :

U = U1 + U2 + U3 + U4

= R1. I E1 + R2 .I + E2

= (R1 + R2 )I (E1 - E2 )Vy : U = ( R)I - E. (1.9)

Trong biu thc (1.9), quy c du nh sau : Sc in ng E v dng in I cchiu trng vi chiu in p U s ly du dng, ngc li s ly du m.

Biu thc tnh dng in :

R

EUI

+=

(1.10)

Trong biu thc (1.10), quy c du nh sau : Sc in ng E v in p U cchiu trng vi chiu dng in I s ly du dng, ngc li s ly du m.

V d: Cho mch in nh hnh 1 11.

Bit E1 = 100V; R1 = 2; I1 = 5A. Tnh UAB ?

E1 R1I1A B

UABHnh 1 - 11


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Li gii: in p UAB : UAB = E1 R1 x I1 = 100 2 x 5 = 90V

1.2.3. in tr trong mch mt chiuin tr l mt linh kin rt ph dng, c 3 tham s lun gn vi mt in tr,

l: in p V o trn hai u ca mt in tr, cng dng in I chy qua intr v sc cn dng ca chnh in tr. Lut Ohm cho thy mi quan h ca 3 tham

s ny.

Cng c th dng cch mc cc in tr theo kiu ni tip hay theo kiu song

song to ra cc in tr ng hiu c tr s Ohm theo mun:Hnh 1 cho thy khi cho 2 in tr mc ni tip, chng ta s c mt in tr ng

hiu R = R1 + R2. Vy khi cho nhiu in tr mc theo kiu ni tip, tr ca in trng hiu s bng tt c cc in tr ny cng li. Nu mun c in tr ln, c thdng nhiu in tr nh cho mc theo kiu ni tip.

Hnh 2 cho thy, khi cho mc 2 in tr song song, chng ta s c mt in trng hiu R = R1xR2/ (R1+R2). Vy khi cho nhiu in tr mc theo kiu song song,tr nghch o ca in tr ng hiu s bng tng ca cc nghch o ca cc in trsong song. Nu mun c in tr nh, c th dng nhiu in tr ln cho mc theo

kiu song song.


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Mn hc: K thut in

Chng 1: Mch in mt chiu

.

song song:

theo logic AND.

logic OR.

Ngh: K

.

thut Xy Dng

Trang 12


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CU HI N TP CHNG 1

1. Nu khi nim mch in, cc thnh phn ca mch in?

2. Pht biu nh lut m.

3. Cho E = 100V; R = 10; I = 5A. Tnh in p U trong hai s sau:

p s : U = 150 V, U = 50V

4. Cho E = 50V; R = 5; U = 40V. Tnh dng in I trong hai s sau:

p s : I = 18 A, I = 2A

E R I E R I

U U

E R I E R I

U U


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5. Mt ti c in tr R = 5 u vo ngun in mt chiu c E = 100V, intr trong Rtr= 1. Tnh dng in I in p U v cng sut P ca ti

p s : I = 5 A, U = 95V; P = 475W.

6. Bn in tr mc ni tip u vo ngun in in p U = 12V (khng cin tr trong). Dng in trong mch I = 25mA, in p trn cc in tr R1;R2; R3 l 2,5V; 3V; 4,5V.

V s cch u dy, cch mc ampe k, vn k o cc i lng trn. Tnhin p U

4. Tnh in tr R

1; R

2; R

3; R

4.

p s : U4 = 2V; R1 = 100; R2 = 120; R3 = 180; R4 = 80.

7. c in tr tng ung 150, ngi ta u song song hai in tr R1 =330 v R2 .Tnh R2

p s : R2 = 275;


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8. Xc nh tng tr ca mch mc ni tip cc in tr 20, 75 v 5.

9. Cc in tr 6 ohm, 7 ohm v 12 ohm c mc ni tip vi nhau vo ngunin 100V, xc nh cng dng in v cng sut tiu th ca in tr 7ohm.

10.Tnh tng tr ca mch t hp ni tip sau y v cng sut tiu th trn tngon mch song song.

11.Xc nh tng tr ca mch in di y v cng dng in qua in tr56 .


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12.Xc nh in p ri trn tng in tr trong mch in di y.

13.Xc nh cng dng in tng ca mch in v cng sut tiu th cain tr 5 .


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14.Xc nh in p ca ngun in v cng dng in qua in tr 24 trong mch in di y.

15.Trong mch din di y, xc nh cng dng in tng v cng suttiu th ca in tr 32 .

16.Xc nh cng dng in tng v in p ri trn in tr 50 trongmch in di y.


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17.Xc nh in p ngun in v cng dng in qua in tr 3 trongmch in di y.

18.Xc nh in p ri trn in tr 24 , 36 v 72 cng dng inqua in tr 320 trong mch in di y.


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19.Xc nh gi tr ca in tr R5 trong mch in di y.

20.Xc nh tng tr v cng dng in tng v cng sut tiu th ca intr 128 trong mch in di y.

21.Xc nh in p ca ngun in v cng sut tiu th ca on mch songsong u tin trong mch in di y.


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Chng 2: in t Trang 20

CHNG 2: IN T(L thuyt: 2h; Bi tp: 2h; Kim tra: 0h)

Mc tiu:

- Phn tch c nhim v, vai tr ca cc phn t cu thnh mch in nh:

ngun in, dy dn, ph ti, thit b o lng, ng ct...;- Hiu v vn dng c cc biu thc tnh ton c bn ca nh lut m.

Ni dung:

2.1. Nhng khi nim c bn v t trng.

Nam chm vnh cu, nam chm in, dy dn dn in to ra xung quanh n mtt trng. T trng c biu din bi nhng ng sc t trng (gi l cc ngcm ng t) i t cc bc (N) n cc nam (S) v tr v cc bc (N) qua li namchm. (Hnh 2 1 a,b)

i vi dy dn thng di mang in, chiu t trng c xc nh theo quy tcvn nt chai: Nu chiu dng in trng vi chiu tin ca ci vn nt chai thchiu quay ca ci vn nt chai xc nh cho ta chiu t trng mi im (Hnh2 2a).

Ta cng c th s dng quy tc bn tay phi (Hnh 2 2b): Ngn tay ci hngtheo chiu dng in, bn ngn tay cn li ch hng ng sc t trng.

I

Hnh 2-2 : Xc nh chiu ng sc t trng ca dy dn thng mang dng in

a)

b)

a) b)Hnh 2 1 : ng sc ca nam chm

a) nam chm vnh cu b) nam chm in

+ -


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2.2.1 Vect cng t cm :

T trng c c trng bi i lng vt l l vect cng t cm (gi l

vect t cm, vect cm ng in t). Tr s ca vect t cm cho ta bit t trng

mnh hay yu. Chiu ca vect t cm l chiu ca t trng (chiu ca ng sct trng).

Trong h n v quc t SI, n v ca cng t trng l Tesla, k hiu l T.Trong cc my in, cng t cm B vo khong t 1T - 6 T.

Ngi ta chng minh c rng cm ng t ca mt dng in chy trong dydn di t trong khng kh c tnh bng cng thc:

(2.1)

Chiu ca c xc nh theo quy tc ci inh c 1.

Ngi ta chng minh c rng cm ng t ca mt dng in chy trong ngdy dn t trong khng kh c tnh bng cng thc:

(2.2)

Vi n l s vng dy trn mi mt chiu di ca ng, I l dng in chy trongdy dn.

Chiu ca c xc nh theo quy tc ci inh c 1.

2.2.2 Vect cng t trng :

Trong chn khng, vect t cm m t trng thi ca t trng. Nhngtrong vt cht, ta phi xt n nh hng ca chng ln t trng.

Khi t vt liu nh giy, thy tinh, nha vo trong t trng ca mt nam chm,

ng sc t khng b bin dng. Ngc li, khi ta t vo mt tm st, ng sct s tp trung i vo tm st, t trng b bin dng. xt n nh hng ny ca

mi trng vt cht, ngi ta dng vect cng t trng .

Trong mi trng ng hng (mi trng c tnh cht vt l ng nht theo mi

hng khc nhau), quan h gia v nh sau:

(2.3)Trong :

B

B

r

IB 710.2 =

nIB 710..4 =

B

H

B

H

=+= HHBm

.).1(0


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m : t thm ca mi trng vt cht, c trng nh hng ca mitrng.

0 : h s () t thm ca chn khng.

: h s t thm ca mi trng vt cht.

n v ca cng ca h s t thm l Henry trn mt, k hiu Hm .

n v ca cng t trng l Ampe trn met, k hiu lAm .

Trong thc t h s t thm ca vt liu in t ln gp hng nghn ln i vichn khng, so snh ngi ta a ra mt khi nim h s t thm tng i r:

(2.4)

Trong k thut in, cc vt liu st t dn t rt tt c r t vi trm n vi vnv th vt liu st t c s dng ch to cc mch t cho cc thit b in.

Biu thc (6.1), khi p dng vo cho cc b phn ca thit b in ta c:

Trong cc khe h khng kh hoc cc b phn khng st t:

B = 0H (2.5)

Trong :

0

= 4.10 -7 (H

m)

Trong phn thp:

B = H = 0rH. (2.6)

2.2.3 T thng .

Khi nghin cu, thit k cc thit b in, ngoi khi nim v , ngi tacn s dng khi nim t thng.

T thng l thng lng ca vect xuyn qua mt b mt S.

Khi vect thng gc vi mt S v c tr s bng nhau trn mt phng y th tthng c tnh nh sau:

= B.S (2.7)

n v ca t thng l Vbe, k hiu l Wb.

Biu thc (3.8) c th vit:

B =

S (2.8)

Vy cng t cm B chnh l mt t thng trn din tch S.

0

=r

B

B

B


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2.3. Vt liu st t:

Cc cht st, cban, niken v nhng hp kim ca chng lm thnh nhm vt liust t. Do cu to vt cht ca n, h s t thm ca vt liu st t rt ln (r c trs t vi trm n vi vn) v h s t thm khng phi l hng s m l mt hm cacng t trng H. Ngoi ra vt liu st t, c hin tng t tr, ngha l khi ta t

ha li st bng dng in xoay chiu trong c chu k, qu trnh din ra khng thunnghch, ngha l khi tng cng t trng H th cng t cm B tng, quan htheo nhnh tng, khi ta gim H th B gim theo nhnh gim khng trng vi nhnhtng. ng cong quan h gia B v H trong c chu k gi l ng khp kn t tr.

Trong vic tnh ton mch t th dng ng cong trung bnh B = f(H). Ph thucvo loi thp s c cc ng cong t ha khc nhau.

Hnh 2-5 : ng khp kn t tr ca st.

2.4. nh lut cm ng in t.2.4.1. Sc in ng cm ng khi t thng xuyn qua vng dy bin thin.

Hin tng cm ng in t do Faraday pht hin nm 1831 sau nm 1893Lentz pht hin ra quy tc v chiu. Ni dung nh lut nh sau:

Khi t thng xuyn qua vng dy bin thin, trong dy s cm ng ra scin ng, sc in ng y c chiu sao cho dng in n sinh ra c xu hngchng li s bin thin ca t thng.

Nu chn chiu dng ca sc in ng cm ng ph hp vi chiu ca t

thng theo quy tc vn nt chai (Hnh 2 - 6). Sc in ng cm ng trong mi vngdy c vit theo cng thc Macxoen nh sau:

e = -ddt (2.9)

Nu cun dy c W vng dy,

sc in ng cm ng ca cun dy s l:

e = - Wddt = -

ddt (2.10)

Trong = W gi l t thng mc vng ca cun dy.

B

H

e

Hnh 2-6


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Trong cc cng thc trn t thng o bng Vbe (Wb), sc in ng o bngVn (V).

2.4.2. Sc in ng cm ng trong thanh dn chuyn ng trong t trng.

Khi mt thanh dn chuyn ng ct ng sc ca t trng, trong thanh dn scm ng mt sc in ng e c tr s l:

e = Blvsin. (2.11)

Trong :

B l t cm o bng T.

l l chiu di hiu dng ca thanh dn (phn thanh dn nm trong ttrng) o bng mt.

v vn tc ca thanh dn o bng m/s.

l gc hp bi chiu vn tc v chiu t trng.

Khi chuyn ng vung gc vi chiu t trng (thng gp trong my in, = 900 ) th sc in ng cm ng l:

e = Blv. (2.12)

Chiu ca sc in ng cm ng c xc nh theo quy tc bn tay phi vc pht biu nh sau:

Cho ng sc t trng i vo lng bn tay phi, chiu chuyn ng cathanh dn theo chiu ngn tay ci xe ra, th chiu 4 ngn tay cn li l chiu casc in ng cm ng. (Hnh 2-7)

Khi thanh dn chuyn ng song song vi t trng, trong thanh dn s khng csc in ng cm ng.

2.5. nh lut in t:

Khi thanh dn mang dng in nm trong t trng, thanh dn chu lc in ttc ng c tr s l:

N

S

B

v

e

Hnh 2-7


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sin... lIBFt = Trong :

B - Cng t cm o bng T.

I - Dng in o bng A.

l - Chiu di hiu dng thanh dn o bng m.

- Gc gia chiu dng in v chiu t trng.

Ft - Lc in t o bng N.

Khi thanh dn t vung gc vi t trng, lc in t l:

Ft = B.I.l

Chiu lc in t c xc nh theo quy tc bn tay tri "Cho chiu ng sct trng xuyn vo lng bn tay tri, chiu dng in trng vi chiu bn ngn

tay, th chiu ngn tay ci xe ra l chiu lc in t".

Tm tt

t thm () l i lng th hin kh nng vt liu c th c t ha. a ra cgi tr ca t thm ca vt liu, ngi ta phi cn c vo t thm ca khng kh c gitr l 4 x 10-7.

= r.o trong : l t thm thc t

r l t thm tng io l t thm ca khng kh.

V d. Mt li t c tit din ngang hnh vung 20mm x 25mm c chiu di 110mm, nu c t thm tng i (r,) l 810, xc nh t tr ca li t.

= ro= 810 x 4 x10-7= 0.001018

hay Rm = ______L_________

r x o x A

____ .110_107_______= 810 x 4 x .02 x .025

= 110000 = 86389.961.2733

Cc i lng ca t tnh

Sut t ng (Fm)

Kh nng ca mt cun dy ta ra t thng c gi l Sut t ng (Fm) v c xc nhqua n v Ampe-vng (A/t).


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Fm = IN trong Fm l sut t ng vi n v l amp-vngI l cng dng in (A)N l s vng dy ca cun dy (vng).

V d. Xc nh sut t ng to ra bi cun dy c 960 vng khi c dng in 0,25A chy

qua. Fm = I.N = 0.25 x 960 = 240 A/t

Lc t ha (H).c Electrical Principles for Electrical Trades - 4th Edition, trang.102, phn. 5.5.2

Lc t ha (H) l i lng th hin kh nng ca sut t ng t ha vt liu khi tnh nchiu di ca vt liu c t ha. Do vic tnh ton c th thc hin bng cch chia sutt ng cho chiu di ca mch t. Nu din tch mt ct ngang ca mch dn t l khngthay i th c th p dng cng thc:

H = IN / ltrong : H l lc t ha vi n v amp-vng/mt

IN vi n v amp-vng1 l chiu di ca mch t (m).

V d. Lc t ha ta ra trn li c chiu di 150mm v d trn c th tnh ton c nhsau:H = IN/l

= 0.25 x 960 / 0.15=1600 At/m (i khi c vit l A/m)

T thng ()T thng trong mt mch t c nh lng thng qua n v Vbe. Mt Vbe bng100.000.000 (108) ng sc t. Tuy nhin tng lng t thng li khng thc s quan trng

bng mt t thng trn b mt cc t ca nam chm.

Mt t thng (B)

Mt t thng (B) c xc nh thng quan lng t thng tng v din tch m t thngi qua:

B = /A trong : B l mt t thng vi n v Tesla (wb/m2) l tng lng t thng vi n v Vbe (Wb)A l din tch vi n v m2.

V d. Mt mch t vi din tch mt ct ngang l 120mm2 dn dng t thng 0.24 Wb. Xcnh mt t thng trong li dn. B = /A

= 0.24/ 120 x 10-6

= 2x10-9T


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T tr (Rm)

T tr (Rm) l i lng c trng cho kh nng cn tr s i qua ca t thng trong vt liu.T tr c nh hng tng t nh in tr trong mch in v ph thuc vo chiu di camch t (l), din tch mt ct ngang ca mch dn t (A) v t thm ca vt liu

__ l__

Rm = Atrong : Rm l t tr vi n v amp-vng/Vbe1 l chiu di ca mch t vi n v mt l t thm thc ca li vt liu dn tA l din tch mt ct ngang ca li dn t vi n v m2.

Mch t

Mch tc th so snh vi mch in nh th hin trong bng di y:

Trut ng Mch in Mch t

S cn tr in tr (R) T tr (Fm)

p sut/Lc Sut in ng (emf) Sut t ng (mmf)

Dng Cng dng in (I) - T thng ()

Sut t ng (mmf) l c hnh thnh bi mt cun dy solenoid vi (N) vng ccng dng in (I) i qua. n v ca sut t ng l ampe vng (At).

mmf = NI (ampe vng)

T thng l chm cc ng sc t i vo v i ra khi bt k mi nam chm vnhcu hay nam chm in. n v ca t thng l Vbe (Wb) v c k hiu bng kt Hy lp (phi).

T tr l i lng ca mch t th hin s ngn cn dng t thng v c th xcnh c thng qua n v ampe-vng / Vbe (At/Wb).

Lc t ha (H) to ra trong mch t c xc nh thng qua sut t ng (mmf) cacun dy sonlenoid v chiu di (l) ca phn m t thng i qua.

H = mmf/1 At/m


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t thm tuyt i () l c trng cho php cc ng sc t i qua ca vt chtv c so snh vi t thm ca khng kh a ra t thm tng i (pr).

= o r (Ch : t thm ca khng kh (o) = 4 x 107) Mt t thng(B) l s phn b ca t thng ()trn mt din tch (A).

B = /A (Tesla)

Mt t thng ca mt t trng ph thuc vo lc t ha (H) v t thm tuyti ca mi trng t tnh.

B = H (Tesla)

T thng tht thot l mt phn ca lng t thng tng khng thc hin theo mcch mong mun. T thng tht thot c nh gi thng qua h s tht thot v

thng c gi tr tuqf 1,1 n 1,3 i vi cc trang thit b in.

h s tht thot = tng t thng to ra / t thng hu ch

Phn tn txy ra khi t thng i qua khe h khng kh v c khuynh hng phntch. Hiu ng ny dn n lm gim mt t thng qua khe h khng kh. (c nghal khe h cng ln th s phn tn cng xy ra mnh hn).

Khe h khng kh: khng xy ra s gin on ca mch t vi vt liu l st t ngayc khi c s phn tch bng khng kh, ng, g hay mi loi vt liu khng c t tnh

khc.


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BNG CNG THC

mmf ___l ___Fm = IN H = l Rm = ro A

IN IN

H = l mmf = NI = Rm

I V = L. t F = B l I L. = l

ro A N mmfB = A L = l Rm =

N NL = I E = t

Trong :mmf = sut t ng theo ampe vng (At)Fm = sut t ng theo ampe vng (At)I = cng dng in (A)N = s vng qun ca cun dyH = lc t trng (At/m)l = chiu di ca mch t - mt (m)B = t thng : tt-la (T=Wb/m) = t thng tng - webers (Wb)A = din tch - (m)Rm = t tr ca mch t (At/Wb)

V = in th (V) = bin thinF = Lc - Newtons (N)r = t thm tng i ca vt liu lio = t thm ca khng gian t do (H/m) = t trL. = t cm HenryE = sut in ng phn khng


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CU HI N TP CHNG 2

1. Xc nh lc tc ng ln dy dn c chiu di 0.2032m t trong t trng1.395T vi cng dng in i qua l 50 A.

2. Xc nh lc tc ng ln on dy dn c chiu di 0,15m t trong t trng2,5T v c dng in chy qua l 20A.

3. Xc nh chiu di cn thit ca dy dn ca cun dy t trong t trng ccng 8 Tesla, mang dng in 5A c lc tc ng l 10N.

4. Xc nh cng dng in chy trong dy dn c chiu di 0,2m t trong ttrng 0,3T v to ra lc tc ng 10N.

Hiu in th cm ng

nh lut Faraday th hin rng hiu in th cm ng tng nu:

tng s vng dy ca cun dy tng cng t trng tng tc chuyn ng tng i gia dy dn v t trng

IV = N t V = L t


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5. Mt nam chm c ko nhan ra khi mt cun dy c 800 vng. T thngtng thay i trong cun dy l 0.2 Wb v vi thi gian ko nam chm ra l 20ms. Xc nh hiu in th cm ng.

Kt qu _________________

6. Nu nam chm c ko ra khi cun dy tng t trong khong thi gian 2msv tng tc ln 10 ln th hiu in th cm ng s l:

Kt qu _________________________________________

7. Mt nam chm di chuyn vo trong mt cun dy c 200 vng trong khongthi gian 20ms v hnh thnh mt hiu in th cm ng 25V. Vy hnhthnh hiu in th cm ng 50V vi cng tc di chuyn ca nam chm thcun dy phi c bao nhiu vng qun?

Kt qu ______________________________________


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8. Mt nam chm di chuyn vo trong mt cun dy c 200 vng qun trongkhong thi gian 20ms v hnh thnh hiu in th cm ng 25V. Vy numun hnh thnh hiu in th cm ng l 50V trong khi khng thay i svng dy th tc di chuyn ca nam chm phi tay i nh th no?

Kt qu __________________________________

9. Mt nam chm di chuyn vo trong mt cun dy c 200 vng qun trongkhong thi gian 20ms v hnh thnh hiu in th cm ng 25V. Nu khngthay i cun dy hoc tc di chuyn ca nam chm th cng t trngca nam chm phi thay i nh th no hnh thnh hiu in th cm ng l100V?

Kt qu .

10. Mt b phn cm in c t cm l 5H c cng dng in bn trong l15A. carries a current of 15 amps. Cng dng in gim xung 0 vikhong thi gian l 2ms. Xc nh hiu in th cm ng.

Kt qu


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11. T trng ca mt b phn cm ng in b mt i trong khong thi gian 10giy v sinh ra hiu in th cm ng 100V. Nu t trng mt i trong thigian 1 giy th hiu in th cm ng s c gi tr nh th no?

Kt qu _______________________________________

12. Mt cun cm c t cm l 5H mang dng in 3A. nu cng dng ingim v 0 vi vi khong thi gian l 1ms th s hnh thnh hiu in th cmng bng bao nhiu.

Kt qu _____________________________________

13. Mt cun cm 400 mH mang cng dng in 300 mA. Nu gim cng dng in v 0 trong khong thi gian 3ms, xc nh hiu in th cm ng.

Kt qu _______________________________________

14. Mt cun cm c hiu in th cm ng l 12,5VA khi c dng in 100mA hxung 50mA trong khong thi gian 20ms. Xc nh gi tr ca t cm.

Kt qu


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15.Nu cng dng in i qua mt cun cm 2mH hnh thnh hiu in thcm ng l 20V khi tng dng in t 1A ln 5A th thi gian din ra s thayi cng dng in nh vy s l bao lu.

Kt qu

16. Mt cun dy thng c 100 vng qune, Xc nh hiu in th cm ng tngng vi cc trng hp sau:

a. t thng khng i 106 Wb

b. t thng thay i vi tc 2 Wb/s

c. t thng thay i vi tc 0.01 Wb/s

Kt qu a...........................................................

b...........................................................

c............................................................

17. 10. Xc nh sut t ng cn thit to ra t thng 0.2 Vbe trong mt cundy nam chm c t cm l 2000 At/Wb.

Kt qu


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41/109


Chng 3: Dng in xoay chiu mt pha Trang 37

3.2. Cc nh ngha v dng in hnh sin.

Dng in xoay chiu hnh sin l dng in bin i mt cch chu k theo quylut hnh sin vi thi gian.

(3.1)

Trong :

i, u : tr s tc thi ca dng in, in p.

Imax, Umax : tr s cc i (bin ) ca dng in, in p.

i, u : pha ban u ca dng in, in p.

Hnh 3 3: Dng in xoay chiu.

3.2.1. Chu k, tn s, tn s gc :

Chu k T l khong thi gian ngn nht dng in lp li tr s v chiu binthin.

Tn s fl chu k ca dng in trong thi gian 1s.

f =1T (3.2)

n v tn s l hc k hiu l Hz.Tn s gc l tc bin thin ca dng in hnh sin, n v l rad/s.

= 2f =2T (3.3)

3.2.2 Tr s tc thi ca dng in, in p:

Tr s tc thi l tr s ng vi mi thi im t.

Trong biu thc (3.1) tr s tc thi ph thuc vo bin Imax (hoc Umax) vgc pha (t + i) hoc (t + u).

Bin Imax, Umax l tr s cc i, ni ln dng in (hay in p) ln hay nh.

( )

( )u

i

tUu

tIi

+=

+=

sin

sin

max

max


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Gc pha (t + i) hoc (t + u) ni ln trng thi ca dng in hay in p thi im t. thi im t = 0 gc pha ca dng in (in p) l i (u). i (u) cgi l gc pha ban u (hoc gi ngn gn l pha u) ca dng in (in p).

Gc pha u ph thuc vo thi im chn lm gc thi gian (thi im t = 0).Gc pha u l on ON trong N l im dng in i qua tr s 0 t m ndng, gn im O nht. Trn hnh 3 4 ch ra gc pha u i khi chn gc to khc nhau.

3.2.3. Gc lch pha gia cc i lng (gia in p v dng in).

L hiu s pha u ca chng. Gc lch pha gia in p v dng in thng k

hiu l:

= u - i (3.4)

> 0 in p vt trc (sm pha) dng in (hoc dng in chm sau in p)

< 0 in p chm pha so vi dng in (hoc dng in vt trc in p)

= 0 in p trng pha vi dng in.

= 180O in p nghch pha vi dng in.

3.3. Tr hiu dng ca dng in hnh sin.

Ta bit rng tc dng nhit v lc in t t l vi bnh phng dng in. i

vi dng in bin thin c chu k T th tc dng ny t l vi tr s trung bnh bnhphng ca dng in trong mt chu k T.

Tr s trung bnh bnh phng trong mt chu k T c gi l tr s hiu dng I.Tr s hiu dng ca dng in c tnh l :

Thay th i = Imax sint vo biu thc trn.

=T

dtiT

I0

21

==TT

tdtTItdtITI 022max0

22max sin1sin1

i i

O O O

i

i i

i > 0 i = 0 i > 0

a) b) c)

Hnh 3 - 4


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Vy tr hiu dng ca hnh sin l:

(3.5)

Tng t, ta c tr s hiu dng ca in p v sc in ng xoay chiu hnh sinl :

(3.6)

(3.7)

Tr hiu dng l i lng quan trng ca mch in xoay chiu. Ta ni dngin xoay chiu bng bao nhiu Ampe hoc in p xoay chiu bng bao nhiu Vn ln tr hiu dng ca chng.

Trong thc t, gi tr c trn cc c cu o dng in I, o in p U, o cngsut P ca dng in hnh sin l tr s hiu dng ca chng.

Cc gi tr U, I, P ghi nhn mc ca dng c v thit b in l tr s hiu dng.

3.4. Biu din dng in hnh sin bng vect.

Mt phaso l mt ng thng, c v theo t l, do vy chiu di ca n thhin ln (kch thc) ca gi tr, v hng ca n th hin mi quan h pha.

Phaso in p c biu din bng mt ng thng vi u mi tn h:

Phaso dng in c biu din bng mt n thng vi mi tn u c:

biu din mt gi tr ny dn trc (vt pha so vi) gi tr khc, v d Idn trc V mt gc 45o, ta cn bit ng no hng v pha trc.Theo qui c,

pha c chiu quay ngc chiu kim ng h, do ta c:

Mt cch hiu biu phaso l coi mi pha nh ang chy ua vi nhng phakhc. ng i ca cuc ua l theo chiu ngc chiu quay ca kim ng h, vchxut pht nm ngang, v hng sang bn phi, ging nh phaso in p trong biu trn.

Vch xut pht ny c gi l "phaso chun". im xut pht ca tt c cc

pha ny c gi l "im gc".Xt mt dng in hnh sin c biu thc ca tr s tc thi l :

maxmax 707,02

II

I =

maxmax 707,02

UU

U =

maxmax 707,02

EE

E =

I

V


44/109



Trong : Im l bin , I l tr hiu dng, l tn s gc v l gc pha u.

Ta hy ly mt h trc to c cc c trc Ox nm ngang (hnh 3-5a). Ta v

mt vc t vi di biu din cho tr s hiu dng I ca dng

in theo mt t l t l xch mI cho sao cho di c tr s thch hp.

V d: Vi I = 10A, nu ta chn t l xch l th :

Ta t vc t v tr sao cho gc hp bi v trc Ox tnh theo chiu

t trc Ox n bng gc pha u , tc l . Theoquy c, ngi ta chn chiu dng ca gc l chiu ngc vi chiu quay ca kimng h.

Vc t xc nh nh trn gi l vc t dng in hiu dng , hoc mt

cch vn tt hn, l vc t dng in I.

V d hai dng in hnh sin c cng tn s gc nhng c cc tr s hiu dngv gc pha u khc nhau.

s tng ng c biu din bng hai vc t dng in hiu dng v

nh hnh 3-5b.

( ) ( ) +=+= tItIi m sin2sin

OM OMOM=OM

cm

AmI 2=

cm

cmA

A

m

IOM

I

52

10===

OM

OM

OM =),( OMOx

OM

( ) ( )( ) ( )AttIti

AttIti0

222

0111

45sin220sin2)(

30sin210sin2)(

+=+=

+=+=

1I

2I

x

M

Oa)

I1

I2

O300

450

x

y

b)

Hnh 3 - 5


45/109



Tnh tng hai dng in hnh sin c cng tn s bng phng php th vc t:

Gi s ta cn tm tng ca hai dng in hnh sin ni trn :

Ta t

tm tr hiu dng I3 v gc 3 ta ch cn thc hin php cng vc t i vicc dng in (hiu dng)

Hay bng phng php hnh hc nhhnh v hnh 3-6.

bit ln ca I3 ta ly di nhn vi t l xch mI:

xc nh s ca 3 ta ch cn o gc 3 trn hnh v.

Cn ch rng, vic cng vc t cc dng in nh trn ch p dng cho ccdng in hnh sin c cng mt tn s gc (hoc cng tn s f).

T th vc t hnh 3-6 ta thy r rng ni chung I 3 I1 + I2 v I3 = I1+I2 ch

khi 1 = 2.

Hnh 3-6 c gi l th vc t cho cc dng in.

( ) ( ) ( ) ( ) ( )2211213 sin2sin2 +++=+= tItItititi( ) ( )333 sin2 += tIti

213 III +=

213 OMOMOM +=

3OM

33 .OMmI I=

I1

I2

O1

x

y

Hnh 3 - 6

M2I3

M3

M1 32


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3.5. R L C TRONG MCH IN XOAY CHIU

Mch in xoay chiu, ch cha in tr (R)

Khi mch in c in tr c u vo ngun xoay chiu, n s hot ng nh khi n nhn

c dng in mt chiu. Tt nhin l dng xoay chiu, nhng tr s ca n, cho d l tr

s tc thi hay tr s hiu dng, cng u c tnh da trn nh lut m, v cng sut tiu

th c tnh theo cng thc P = V x I = I2R = V2/R.

Cu hi n tp

1. Mt l si in tiu th dng in 12A t ngun 240V v 50Hz. Hy tnh tng cng sut

m mch in ny tiu th.

2. Mt phn t nung c ni vo ngun in li 480V v 50Hz v c tr s dng in tiu

th. Hy tnh tng cng sut ca mch in ny.

3. Nu mt phn t tiu th in c cng sut 5kW th n s tiu th dng in c tr s bao

nhiu nu ni n vo ngun in 240V?

4. Hy tnh cng dng in m mt phn t in tr 4,8kW tiu th khi ni phn t ny

vi ngun in 240V v 50Hz.


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5. Nu cc in tr c tr s 10 v 15 c mc ni tip vi nhau v u vo ngun 250V

th dng in m chng tiu th l bao nhiu?

6. C 3 in tr c mc ni tip vi nhau v c in p 10V trn in tr th nht v in

p 15V trn in tr th ba.

a. Hy tnh in p trn in tr th 2 nu in p ngun l 50V.

b. Nu tr s dng in o c qua in tr th 3 l 2A th tr s dng in chy qua in tr

th nht R1 v in tr th hai R2 l bao nhiu?

c. Tng tr s dng in m mch in ny tiu th t ngun l bao nhiu?

d. Hy tnh tng cng sut ca mch in ny.

7. Mt mch in xoay chiu cha 3 in tr c tr s ln lt l 5, 10 v 15. in tr 10

c in p l 25V.

Hy v s mch in ny v tnh:

(i) in p trn in tr 15.(ii) in p ngun(iii) Cng sut m in tr 5 tiu th.

Gi cch gii:

Tnh cng dng in chy qua ti 10.

y l mch ni tip, dng in ny ng thi l IT.

Tnh in p trn ti 15, s dng IT v 15.

Tnh tng in tr RT, sau tnh in p ngun VS.

Tnh cng sut ti ti 5, s dng IT v 5.


48/109



IN CM (L) TRONG MCH IN XOAY CHIU

in cm l tnh cht ca mch in cho php gy ra mt sut in ng cm ng bn trong

mch.

Mt cun cm v c bn gm mt s nht nh cc vng dy cch in c qun thnh mt

cun. Cc vng dy ny c th c qun quanh nhiu loi li khc nhau.

Cc cun cm c th c li khng kh, li st, v.v.

i khi cun cm cn c gi l cun cm khng.

n v o ca in cm l Henry (H) K hiu ca in cm l: (L) K hiu ca cun cm l:

t cm

Khi dng in xoay chiu bin i c ni vo mt cun cm th s hnh thnh mt t

trng thay i.

T trng ny ct ngang cc vng dy ca cun cm v s gy nn sut in ng cm ng

bn trong cun cm.

iu ny c ngha l t trng thay i ca n t to ra mt in p trong cc vng dy.

S h cm (cm ng h tng)

Khi nim s h cm c dng m t hiu ng khi dng in bin i chy qua mt dydn gy nn sut in ng trong mt dy dn gn k n. Khng nht thit phi c bt k s

kt ni no v mt in gia hai dy dn .

CM KHNG (XL)

Cm khng (XL) l s cng li dng in thay i m in cm ca mch to nn.

n v o: m ().

V dng in xoay chiu dng sng hnh sin bao gi cng bin i, cm khng tr thnh yu

t quan trng trong cc mch in xoay chiu. V chnh dng in bin i to nn cm khng

nn suy ra, tc thay i (tn s) ca dng xoay chiu s phn no quyt nh tr s ca cm

khng, nh sau:

XL = 2fL, trong XL = cm khng tnh bng (m)

f = tn s tnh bng Hz (Hertz)

L = in cm tnh bng H (Henry).

Th d:

Cun dy vi tr s 0,15H c ni vo ngun in 240V v 50Hz. Hy tnh cm khng (XL)

ca n.


49/109



XL = 2fL

= 2 x 3,14 x 50 x 0,15

= 47,12.

Khi mt s cun cm c ni thnh mt mch in, c th tnh tng in cm v tng cm

khng bng cch dng nhng cng thc tng t nh nhng cng thc dng tnh intr trong cng loi mch in.

LT = L1 + L2 + L3 v.v., v

XL tng = XL1 + XL2 + XL3 v.v.

Bi t kim tra:

1. Cm khng 25 c mc ni tip vi cm khng 30. Hy tnh tng cm khng ca

mch in ny.

2. Bn cun cm mc ni tip vi nhau c tr s cm khng ln lt l 40, 50, 65 v

35. Hy tnh tng cm khng ca chng.

3. Mt cun cm c tr s in cm 0,25H c ni vi mt ngun in 250V v 50Hz. Hy

tnh cm khng ca mch in ny.

4. Hy tnh cm khng ca mch in c cun dy 0,65H ni vi ngun 400V v 50Hz.

5. Hai cun cm c mc ni tip vi nhau. Hy tnh tng cm khng ca mch in ny nu

hai cun cm c tr s in cm ln lt l 1,5H v 2H v chng c u ni vo ngun

in 150V v 50Hz.


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6. Mt cun cm c in cm 0,7H v mt cun cm khc c cm khng 65 c mc nu

tip vi nhau v ni vo ngun in 240V v 50Hz. Hy tnh tng cm khng ca mch in

ny.

Mch in song song:

1.321

111

LLLLT ++=

2.321

,

111

LLLTongL XXX

X ++=

Bi t kim tra:

1. Hy tnh tng cm khng ca mch in sau y nu cm khng 25 c mc song song

vi cm khng 35.

2. Cc cm khng 8, 6 v 4 c ni song song vi nhau. Hy tnh tng cm khng ca

mch in ny.

3. Nu mt cun cm c cm khng 40 c ni song song vi mt cun cm khc c cmkhng 10 th tng cm khng ca mch in do chng to nn l bao nhiu?

4. Hy tnh tng cm khng ca mch in song song cha cc cun cm c cc tr s sau

y: 50, 60, 40 v 30.


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Mt cun cm ni chung l mt cun dy dn, c ngha l n phi c c tr s in tr v in

cm no . Trong mch in, cun cm hot ng nh l in cm thun ty mc ni tip

vi in tr thun ty. Tuy nhin, i khi thun tin, chng ta coi chng ring bit.

V vy, i khi trong ti liu c ghi nhng cu nh: trong mt mch in cm ng thun

ty Bng cch ny, chng ta c th xem xt mt cch ring bit cc thnh phn ca mchin, ri sau kt hp kt qu ca chng li vi nhau.

Nu xem xt mt mch in cm ng thun ty, khi m cm khng (XL) l thnh phn duy

nht cn li dng in, c th tnh dng in chy qua cun cm bng cch chia in p trn

cun cm cho cm khng.

I = V / XL

I = V / XL, trong : I = dng in chy qua cm khng, tnh bng A

V = in p trn cun cm, tnh bng V

XL = cm khng, tnh bng .

V, cng nh i vi nh lut m, nu bit trc bt k hai tr s no trong nhng i lng

, ta c th tnh c tr s ca i lng th ba.

Th d:

Mt in cm 0,2H ng vi dng in 1,25A chy qua trong mch in 240V v 50Hz. Hy

tnh mc st in p.

(i) XL = 2fL

= 2 x 3,14 x 50 x 0,2

= 62,83.

(ii) I = V / XL do , ta dng cng thc bin i tnh V

(iii) V = I x XL = 1,25 x 62,83 = 78,54V.

Ngoi vic cn dng in trong mch in xoay chiu, in cm cng gy nn hiu ng lm

cho dng in b tr so vi in p. Ni cch khc, dng in tr nn lch pha so vi in p.

Mc tr ny ph thuc vo lng cm khng v vo lng ca cc thnh phn cn dng

khc trong mch in. Nu in cm l thun ty, dng in s b tr 90o.

Sau y l hai phng php minh ha s tr ny.

Bt k cun dy c li st no cng u c tnh cht ny mc no . Mt s trong cc

th d l: ng c in, my pht in, bin th, cun cm khng, rle v cngtct.


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Trong mt s mch in, cn phi hn ch ln dng in chy qua thnh phn c th ca

mch. Mt trong nhng cch thc lm iu l mc ni tip thm mt in tr vo thnh

phn . iu ny s khin cho in p st ti in tr, v lm cho in p trn thnh phn

b gim bt. in p gim c ngha l dng in chy qua thnh phn cng gim.

Tuy nhin, in tr cng tiu th cng sut theo cng thc I2

R v cng sut ny coi nh blng ph. i khi tt hn ht l dng mt cun cm mc ni tip vi thnh phn . N s hn

ch dng in bng cch to ra sut in ng ngc vi in p, do dng in chy qua

ch l kt qu ca s chnh lch gia in p tc ng v in p cm ng.

V cun cm c lm t dy dn, n s c in tr no , c ngha l s sinh nhit (tn tht

nhit), nhng mc nh hn so vi s dng in tr thun ty. Th d v cun cm c

chc nng ny l cun cm (chn lu) trong b n hunh quang.

Trong n cng sut 36W, chn lu s gim in p ngun xung cn 110V, nhng tn tht

ch l 7W.

Cu hi t kim tra:

1. Hy tnh dng in chy qua mch in 240V v 50Hz cha mt in cm thun ty 24.

2. Nu mt cun cm thun ty c ni vo ngun 400V v 50Hz v tiu th dng in 16A

th cm khng ca mch in ny l bao nhiu?

3. Mt cun cm 0,85H c mc ni tip vi mt cun cm 0,4H. Hy tnh dng in chy

qua mi cun cm nu chng c ni vo ngun 100V v 50Hz.

4. Mt cun cm thun ty 0,95H tiu th dng in 3A trong mch 240V v 50Hz. Hy tnh

in p trn cun cm .


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DUNG KHNG

Mt loi in khng na l dung khng (XC).

Dung khng (XC), l s cn li dng in bin i m in dung ca mch in to ra. Mt

ln na ta thy, v y l thnh phn in khng, dung khng c o bng , v cng chu

nh hng ca tn s.

fCXC

2106

= trong : XC = dung khng, tnh bng

f = tn s, tnh bng Hz

C = in dung, tnh bng F.

Th d:

Hy tnh dung khng ca mt t in 20F khi t ny c ni vo mch in 240V v 50Hz.

fCXC

2

106=

205014,32

000.000.1

=CX

XC = 159,2.

Nh thy trong phn in ng dng 2, vic mc ni tip cc t in s cho ta tng in

dung nh hn tng tr s ring bit, v khi mc song song th tng in dung li tng ln. Khi

bit trc tng in dung, cng thc ni trn c th c dng tnh dung khng.

Cng thc tnh mt trong nhng i lng dng in, in p v dung khng tng t nh

nh lut m:

IC = V / XC trong IC = dng in chy qua t in, tnh bng A

V = in p trn t in, tnh bng V

XC = dung khng, tnh bng

Khi cc t in c ni vo mch in xoay chiu, chng cng khin cho dng in b lch

pha so vi in p, nhng trong trng hp ny dng in khng tr m li dn trc so vi

in p. in dung thun ty s lm cho dng in dn trc in p mt gc 90o.


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Mt cp cun cm rt di t cnh nhau c th c in dung o c, nhng ni chung, in

dung khng phi l vn i vi th in. Thc ra, v hiu ng lch pha ca n ngc li so

vi ca in cm, chng ta s dng t in dung b li hiu ng lch pha ca cun cm.

Khi dung khng v cm khng tn ti trong mt mch in, chng c xu hng trit tiu ln

nhau, v vy tng in khng (X) c tnh nh sau:X = XL XC

in khng

in khng (X) l s cn li dng in chy qua do c s tn ti ca in cm v/hoc in

dung trong mch in.

n v o:

Nu X < 0 th I dn trc V. Nu X > 0 th I b tr so vi V.

TR KHNGTr khng (Z) l ton b s cn tr dng in chy trong mch in xoay chiu.

n v o: .

Cu hi t kim tra:

1. Cn c in cm (L) bao nhiu to ra cm khng (X L) 60 trong mch in xoay chiu

50Hz.

2. Hy tnh dung khng (XC) ca t in 19F khi ni t ny vo mch in 50Hz.

3. Nu c hai thnh phn ni trn c mc ni tip vi nhau trong mch in 50Hz th tr s

in khng (X) ca mch l bao nhiu?


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MCH IN XOAY CHIU NI TIP

i vi mch ni tip, tr khng c tnh theo cng thc:

Z2 = R2 + X2

Ta thy cng thc ny gn ging vi nh l Pytago cho tam gic vung: H2 = B2 + A2, v vy

c th minh ha cng thc tnh tr khng trn hnh tam gic vung m chng ta gi l tam

gic tr khng.

Tam gic tr khng ca mch ni tip

Ta hy xt mch in ni tip cha in tr thun ty, in cm thun ty v in dung thun

ty.

Th d:

in tr, in cm v in dung trong mch in xoay chiu mc ni tip.

S n gin mch RLC ni tip

Vi s ni trn, gi thit rng in tr (R) l 20, cm khng (XL) l 60 v dung khng

(XC) l 45. Hy tnh tr khng (Z) ca mch in ny.

X = XL XC= 60 45

= 15

Z2 = R2 + X2

22 1520 +=Z

625=Z

Z = 25.

Nu in cm v in dung bit trc thay v cc gi tr ca in khng th trc ht phi

chuyn i chng thng qua tn s.


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CU HI T KIM TRA:

1. Mt mch in xoay chiu ni tip gm mt in tr 50, mt in cm 0,1H v mt t

in 15F c ni vo ngun in 250V v 50Hz.

Hy tnh: (i) Cm khng (XL)

(ii) Dung khng (XC)(iii) Tr khng (Z)(iv) Tng dng in (IT)(v) Tng cng sut tiu th (P = I2R)(vi) Tng cng sut tiu th ca mch (P = I2Z)

Cng hng in l mt iu kin c bit ca mch in xoay chiu, tn ti khi cm khng

(XL) bng dung khng (XC).

Hy nh rng, cm khng (XL) = 2fL v dung khng (XC) = 106 / 2fC. Trong c hai trng

hp, tn s l mt trong nhng yu t. iu ny c ngha l i vi mt cun cm hoc mt tin, cm khng hoc dung khng thay i theo tn s.


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Trong mch xoay chiu mc ni tip R, L v C, nu XL = XC (v d mi thnh phn l

40), ta s tnh c:

X = XL XC

= 40 40

= 0V vy, cn tr duy nht i vi s chuyn ng ca dng in l in tr cn li trong mch

in ny. Do , iu quan trng l phi c in tr trong mch hn ch cng

dng in xung ti mc an ton nht nh.

Trong mt mch in xoay chiu song song cha R, L v C, khi nhnh c cun cm c

cng tr s cm khng nh tr s dung khng ca nhnh cha t in, chng s nui nhau.

Nng lng tch t trong mt nhnh s c chuyn sang nhnh kia, v ngc li.

iu ny c ngha l cc nhnh c tr khng ly dng in t li in, do tr khng (Z)

ca mch in l bng vi in tr, v dng in s c gim thiu.

C th tnh c tn s m ti mt mch in cng hng nh sau:

LCf

2

10 =

trong : f0 = tn s cng hng [Hz]

L = in cm

C= in dung ca mch [F]

CU HI T KIM TRA:

1. Cng hng l mt iu kin c bit ca mt mch in xoay chiu khi:

2. Hy tnh tn s m ti mch in cng hng, nu n cha mt t in c in dung

45F v mt cun cm c in cm 0,0563H. Hy nh chuyn i t F sang F.

3. Hy tnh in khng ca mch in nu cm khng l 65 v dung khng l 65.

4. Mt mch in ni tip RLC gm mt t in, in tr v cun cm. Hy tnh tng in

khng ca mch nu:

XL = 150, R = 10 v XC = 150.


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3.6. Cng sut dng in hnh sin.

3.6.1. Cng sut tc dng P.

Cng sut tc dng P l cng sut in tr R tiu th, c trng cho qu trnhbin i in nng sang dng nng lng khc nh nhit nng, quang nng,....

P = R.I2 = UR.I = U.I.cos.

Cng sut tc dng l cng sut trung bnh trong mt chu k.

3.6.2. Cng sut phn khng Q.

c trng cho cng qu trnh trao i tch ly nng lng in t trng,ngi ta a ra khi nim cng sut phn khng Q.

Q = X.I2 = UX.I = U.I.sin.

Cng sut phn khng ca in cm QL:

QL = XL.I2.

Cng sut phn khng ca in dung QC:

QC = -XC.I2.

3.6.3. Cng sut biu kin S.

c trng cho kh nng ca thit b v ngun thc hin cc qu trnh nnglng, ngi ta a ra khi nim cng sut biu kin S c nh ngha nh sau:

22. QPIUS +==

Biu thc P, Q c th vit theo S nh sau:

P = U.I.cos = S.cos

Q = U.I.sin = S.sin

n v ca P: W, kW, MW.

n v ca Q: VAr, kVAr, MVAr.

n v ca S: VA, kVA, MVA.


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CU HI N TP CHNG 3.

1. Cho hai dng in hnh sin c cng tn s gc nhng c cc tr s hiu dngv gc pha u khc nhau.

)45sin(25)sin(2)(0

111 +=+= ttIti

)60sin(215)sin(2)( 0222 +=+= ttIti

Hy biu din cc gi tr dng in trn bng vec t v tnh dng in tng.

2. Cho hai dng in hnh sin c cng tn s gc nhng c cc tr s hiu dngv gc pha u khc nhau.

)60sin(225)sin(2)( 0111 +=+= ttIti

)30sin(230)sin(2)( 0222 +=+= ttIti

Hy biu din cc gi tr dng in trn bng vec t v tnh dng in tng.

3. Cho cc dng in hnh sin:)45sin(230)( 01 += tti

)60sin(215)( 02 += tti

Hy biu din cc gi tr dng in trn bng s phc v tnh dng in tng.


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8/ V mt biu phaso n gin vi dng in l ng chun v in p tr pha mt gc60 so vi n.

9/ V mt biu phaso n gin vi ng chun l in p v dng in dn trc mtgc 300 hoc tr mt gc 60 .

1. in dung c o bng n v _________________

2. iu g xy ra i vi dung khng ca mt mch in nu tng tn s ca ngun in u

ni vi mch ny?____________________________________________________________________

____________________________________________________________________

3. Dung khng c o bng n v _________________

4. Hy tnh dung khng ca mt t in 8,4F khi ni n vo ngun in 50Hz.

5. Hy cho bit tr s ca t in to ra mt cm khng 70,7 khi n c ni vo ngun in

c tn s 50Hz.


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6. Hy cho bit tr s ca dng in chy qua mt t in 100F khi ni t ny vo ngun

in 230V v 50Hz.

7. Hy tnh tng dung khng ca mt b gm hai t in 4,5F v 50F mc ni tip vi

nhau.

8. Cm khng c o bng n v ____________________________

9. in cm c o bng n v ____________________________

10. Nu tng tn s ca mch in cha mt cun cm, cm khng ca n s _________

11. Hai cun cm thun ty 40mH v 120mH c u ni tip vo mt ngun in 230V v

50Hz. Hy tnh dng in i qua chng.

13. Hai cun cm thun ty 40mH v 120mH c u song song vo mt ngun in 230V

v 50Hz. Hy tnh dng in i qua chng.

14. Tr khng l g?


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15. Hy vit ra cng thc tnh tr khng nu bit trc tr s in p v dng in ca ngun

in.

16. Mt mch ni tip cha mt in tr 40, mt cm khng 20 v mt dung khng 50

c mc vo ngun in c in p 230V. Hy tnh tr s dng in ca ngun.

17. Mt mch ni tip cha mt in tr 40, mt cun cm 220mH v mt t in 50F

c mc vo ngun in c in p 230V. Hy tnh tr s dng in ca ngun.

19. Mt mch ni tip cha mt in tr 40, mt cun cm 0,75H v mt t in 50F c

mc vo ngun in c in p 230V. Hy tnh tr s dng in ca ngun.

20. Mt mch ni tip cha mt in tr c in p 80V, mt cun cm c in p 90V v

mt t in c in p 150V. Hy tnh tr s in p ngun.

22. H s pha (cosphi) ca mch in tr l g?

23. Cng sut biu kin l g?


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24. n v o ca cng sut biu kin l g?

25. Cng sut phn khng do nhng thnh phn no to thnh?

a)

b)

26. Hy cho bit cng thc tnh cng sut phn khng.

27. Mt mch ni tip cha mt in tr c in p 160V, mt cun cm c in p 180V v

mt t in c in p 300V. Hy s dng biu phaso tnh tnh:

a) tr s in p ngun

b) gc h s pha (gc phi).


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Chng 4: Mch in ba pha Trang 60

CHNG 4: MCH IN BA PHA(L thuyt: 3h; Bi tp: 2h; Kim tra: 1h)

Mc tiu:

- Phn tch c khi nim v cc ngha, c im v mch xoay chiu ba

pha;- Phn tch v vn dng c cc dng s u dy trong mng ba pha;

- Gii c cc dng bi ton v mng ba pha cn bng.

- Phn tch c ngha ca h s cng sut v cc phng php nng cao hs cng sut.

Ni dung:

4.1. Khi nim chung.

Nhng h thng in ln thng gm ba dng in ring bit c hiu in thbng nhau, lch pha nhau 120o v vi cc cun dy qun ca my in lch nhau 120o.H thng in dng ny gi l h thng in ba pha vi nhiu u im m h thngin mt pha khng c c.

U IM CA MY PHT IN BA PHA

C th truyn in n mt a im cch xa ngun v lng kim loi dng lmdy dn cho mch in ba pha t tn km hn.

C hai loi hiu in th khc nhau c th cng hot ng. My pht in ba pha c mt s u im so vi my pht in mt pha. l:

(i) Vn hnh m hn nn gim c rung ca c h thng my.(ii) Mmen khi ng cao hn.(iii) i vi cng cng sut u ra, my c kch thc vt l b hn my pht

mt pha.

(iv) Gi thnh r hn do c kt cu n gin hn.(v) Chi ph chy v bo dng my thp hn.(vi) Cng sut ca my ba pha t ti 96% so vi cng sut ca my mt pha ch

t 64%.4.1.1. Ngun in ba pha.

Vi my pht in ba pha,ta c th s dng ba pha 230V ring r hoc c th kthp ba pha ny theo cc cch khc to ra mt hiu in th khc, v d 400V.

Cng c th to ra mt cng sut ba pha bng cch s dng ba my pht inxoay chiu mt pha ring r, cc my c cng hiu in th v tn s, v cn c nighp mt cch hp l.

Tuy nhin, thng thng, ngi ta s dng mt nam chm quay trong mt Statocha ba cun dy mt pha ging nhau b tr lch gc quay 120o hiu in th cachng lch pha nhau 120oE (lch sng in) trn dao ng hnh sin.


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C ba cun dy c cng mt Roto, nn chng hot ng mt cch ng b v ccng hiu in th ra cho cc ti ging nhau.

Hiu in th u ra c th c minh ha bng nhng sng hnh sin hoc vngpha nh sau:

Th t m ba hiu in th ln lt t c mt gi tr tng t nhau trn snghnh sin c gi l th t pha hoc chiu quay pha. hnh trn, th t pha l L1, L2,L3 hay ng , ng Trng, ng Xanh lam.

Mi dy mang in c th c nhn bit theo mu (, trng, xanh lam) hoc

theo nhn R.W.B hoc L1, L2, L3 hoc A.B.C.Ch rng Roto quay theo chiu ngc chiu kim ng h, tc l (R), Trng

(W), Xanh lam (B).

Mc quy chiu L1 l ng nm ngang, L2 v L3 c t lch 120o nh hnhv.

Thng thng, vng pha c chiu quay ngc chiu kim ng h, khi L2 schuyn vo v tr ca L1, v L3 chuyn vo v tr ca L2. Phng php minh ha nhtrn gip ta hnh dung c nhng gi tr sau:

a) Gi tr Cng dng in (Ampe)b) Gc lch pha (sm hoc tr pha)c) Chiu quay.

Nhng gi tr trn cng c th c minh ha m khng cn s dng n thsng hnh sin.

Mt mch xoay chiu c mc v vn hnh sai trnh t th cn phi tin hn omch.

Trnh t pha cn phi c kim tra, nu thy cn thit, bng mt ng h o

trnh t pha. Trnh t pha s cho bit nhng thng tin v:1.Ni dy vo mt thit b quay nh th no.


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2.Th t t cc i ca mi pha.3.ng c c b o mch hay khng.

4.1.2. Cch ni mch in ba pha.

Nu mi pha ngun in ba pha ni ring l vi mi pha ca ti, th ta c hthng ba pha khng lin h vi nhau. Mi mch in nh vy gi l mt pha ca mchin ba pha.

Thng mch ba pha ca ngun in ni vi nhau, ba pha ca ti ni vi nhau vc ng dy ba pha ngun vi ti, dn in nng ti ti.

Sc in ng, in p, dng in mi pha ca ngun (hoc ti) gi l sc inng pha k hiu l Ep, in p pha k hiu Up, dng in pha k hiu Ip.

Dng in chy trn dy t ngun ti ti gi l dng in dy k hiu I d, in pgia cc dy pha gi l in p dy k hiu Ud.

i lng pha: Phase (P); i lng dy: Line (L) hoc dy (d).

Quan h cc i lng ph thuc vo cch ni.

4.1.3. Mch in ba pha i xng.

Ngun in gm ba sc in ng hnh sin cng bin , cng tn s, lch phanhau 2/3 gi l ngun ba pha i xng.

Hnh 4 - 1: Ngun ba pha i xng

i vi ngun i xng ta c: eA + eB + eC = 0.

Nu tng tr phc ca cc pha ti bng nhau ZA = ZB = ZC th ta c ti i xng.Mch in ba pha gm ngun, ti v ng dy i xng gi l mch in ba phai xng.

Nu khng tho mn mt trong cc iu kin nu gi l mch ba pha khngi xng.

4.2. Cch ni hnh sao (Y).

4.2.1. Cch ni.

Mi pha ca ngun hoc ti c u v cui. K hiu u pha l A, B, C cui pha

l X, Y, Z. Mun ni hnh sao ta ni ba im cui ca pha vi nhau to thnh imtrung tnh.


67/109



i vi ngun, ba im cui X, Y, Z ni vi nhau thnh im trung tnh O cangun.

i vi ti, ba im cui X', Y', Z' ni vi nhau thnh im trung tnh O' ca ti.

Cc dy ni ngun vi ti gi l dy pha.

Hnh 4-2: Cch ni hnh sao.

4.2.2. Cc quan h gia i lng dy v pha trong cch ni hnh sao i xng.

4.2.2.1. Quan h gia dng in dy v pha.

Id = Ip

4.2.2.2. Quan h gia in p dy v in p pha.

(4.1)

(4.2)

(4.3)

V tr hiu dng:UAB = UBC = UCA = Ud = 3 Up (4.4)

V pha, in p dy UAB, UBC, UCA lch pha nhau mt gc 1200. in p dy

vt trc in p pha tng ng mt gc 300.

4.3. Cch ni hnh tam gic ().

4.3.1. Cch ni:

Mun ni hnh tam gic ta ly u pha ny ni vi cui pha kia.

Theo tiu chunl: A ni vi Z, B ni vi X, C ni vi Y.

Ngoi ra ta cng c th kt ni: A ni vi Y, B ni vi Z, C ni vi X.

= BAAB UUU

= CBBC UUU

= CAAC UUU

IpA

B

A

C CB

Up

UA

Id IA

I0

IB

IC

UpOO


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Hnh 4-3: Cch ni ti hnh tam gic.

4.3.2. Cc quan h gia i lng dy v pha khi i xng.

4.3.2.1. Quan h gia in p dy v in p pha.

Ud = Up (4.5)

4.3.2.2 Quan h gia dng in dy v dng in pha.

p dng nh lut Kichp 1 ti cc nt

(4.6)

(4.7)

(4.8)

V tr s dng in dy ta c : Id = 3 Ip

V pha, dng in dy chm sau dng in pha tng ng mt gc 300.

4.4. Cng sut ca mch in ba pha.

4.4.1. Cng sut tc dng.

Cng sut tc dng P ca mch ba pha bng tng cng sut tc dng ca cc phacng li. Gi PA, PB, PC tng ng l cng sut tc dng ca cc pha A, B, C ta c:

P= PA + PB + PC = UA IA cosA + UB IB cosB + UC IC cosC

Khi mch ba pha i xng: UA = UB = UC = UP ; IA = IB = IC = IP

v cosA= cosB = cosB = cos

Ta c: P = 3Up Ip cos = 3 Rp I2p ; trong Rp l in tr pha.

i vi ni sao i xng: Ip = Id ; Up =Ud

3

= BAAB III

= CBBC III

= CAAC III

EC EA

EB

Ud

UpIAB

ICA IBC

IpIp

Id IA

IBIC

A

C

B

A

CB


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i vi ni tam gic i xng: Up = Ud ; Ip =Id

3

Cng sut tc dng ba pha vit theo i lng dy, p dng cho c trng hp nisao v ni tam gic i xng: P = 3 Ud.Id.cos

4.4.2. Cng sut phn khng Q:

Q = QA + QB +QC = UA IA sinA + UB IB sinB + UC IC sinC

Khi mch ba pha i xng : Q= 3Up Ip sin = 3Xp I2p; trong Xp l in khng

pha

Hoc vit theo i lng dy: Q = 3 Ud.Id.sin

4.4.3. Cng sut biu kin:

Khi mch ba pha i xng, cng sut biu kin ba pha:

pp IUS .3=

dd IUS .3= 2.3 pp IZS =

TM TT

1) H s cng sut l cosin ca gc pha gia V v I.2) H s cng sut l h s m khi nhn n vi cng sut biu kin ta c cng sut

thc hay l t s ca cng sut thc v cng sut biu kin.

H s cng sut:

3)

Gi tr ca h s cng sut ph thuc vo lch pha (gc pha) gia hiu inth cung cp v dng in.

4) Khi U v I cng pha, h s cng sut = 1.5) Khi U v I lch pha 900, h s cng sut = 0.6) H s cng sut cng b th dng in cng phi ln cung cp cng sut thc

theo yu cu cho trc.

Cng sut thcPf =

Cng sut biu kin


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NGUYN NHN LM GIM H S CNG SUT

Phn ln cc mch in l cm ng nn dng in b tr pha so vi hiu in th. Dngin sm pha trong mch in l rt him.

Nguyn nhn ch yu lm gim h s cng sut l do n hunh quang, bin p hoc

m-t in c ti thp.NH HNG DO H S CNG SUT THP

Trong trng hp h s cng sut thp, ngun s phi cung cp mt lng in thmvo, nh hng n mt s yu t nh:

1) Tit din dy dn phi ln hn.2) Bin p phi ln hn.3) C cu ng ngt mch phi tt hn.4) Hiu in th ca dy dn phi cao hn.5) Lng ng tiu hao nhiu hn.6) Gim hiu sut ca my (v vy tng nhiu liu v gi thnh pht in).HIU CHNH H S CNG SUT

hiu chnh h s cng sut, cn gim lch pha gia hiu in th v dng in theo

cc phng php sau:

1) Mc t in song song vi ph ti gy chm pha.2) Mc nhiu t in vo bng in chnh.3) Trong cc x nghip ln, ngi ta thng mc ng c ng b khng ti vo mch.


71/109



CU HI N TP CHNG 4

1. Nu nhng u im ca mch in ba pha.

2. Mt iu ha nhit 3kW c ni vi mt ngun in xoay chiu 230V 50Hz.Cho h s cng sut l 0,6.

Tm :a. Dng in chy t ngun ti h s cng sut 0,6 ?

b. Dng in chy t ngun ti h s cng sut 0,9 ?

3. Cho mt ti vi h s cng sut 0,7, cng 8A ni vi mt ngun in xoay chiu240V 50Hz. Tnh cng sut thc ca mch.

4. Mt ti 8KW nhn 50A t mt ngun 240V, 50Hz. Tnh cng sut biu kin, cng sutphn khng, h s cng sut v gc lch pha.

5. Mt ng c c ni vi mt ngun in 240V, 50Hz nhn c dng in c cng 10A nu hiu in th v dng in lch pha 10o. Tnh cng sut thc.

6. Mt ti 800W c cng 5A c ni vi mt ngun 240V, 50Hz. Tnh cng sut biukin, cng sut phn khng v h s cng sut.


72/109



7. Xc nh dng in dy trong ba mch ba pha mc hnh TAM GIC, vi:a) IP = 15A

b) IP = 30A

c) IP = 70A8. Xc nh dng in pha trong ba mch ba pha mc hnh TAM GIC, vi:a) IL = 100A

b) IL = 60A

c) IL = 14A

9. Mt Vn k c mc song song vi mt mch ba pha mc hnh TAM GIC ch230V. Cho dng in trong mi pha l 15A, hy xc nh:

a) Hiu in th dy

b) Dng in dy.

10. Cho mt my pht in xoay chiu ni hnh tam gic. Mi cun dy ca my cungcp hiu in th 450V v nhiu nht l 25A.

Cho mch xoay chiu chy ht cng sut v cung cp in cho ti thun tr.Hy xc nh:(a) Hiu in th dy (VL)

(b) Dng in dy (IL)

11.Mt ngun in ba pha ni sao, Upn = 120V cung cp in cho ti ni sao c dytrung tnh. Tic in tr pha Rp = 180.

Tnh Ud, Id, Ip, I0, P ca mch 3 pha.p s: Ud = 207,84V; Id = Ip = 667mA; I0 = 0; P = 240W


73/109



12.Mt ngun in ba pha i xng u sao cung cp cho ti ba pha i xng utam gic. Bitdngin pha ca ngun Ipn = 17,32A, in tr mi pha cati Rp = 38. Tnh in p pha cangun v cng sut P ca ngun cung cpcho ti 3 pha.

p s: Upn = 220V; Pn = Pt = 11400W

13.Mt ti ba pha i xng u hnh tam gic, bit Rp = 15; Xp = 6, u vomng in 3 phaUd = 380V. Tnh Ip, Id, P, Q ca ti.p s: Ip = 23,5A; Id = 40,7A; P = 24893,5W; Q = 9957,4A.

14.Mt ng c in 3 pha u vo mng 3 pha Ud = 380V, bit dngin dy Id =26,81A; h scng sut cos = 0,85. Tnh dng in pha ca ng c, cng sutin ng c tiu th.p s: Ip = Id = 26,81A; Pin = 15kW


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15.Mt ng c khng ng b c s liu nh mc sau: cng sut c nhmc Pm = 14kW.Hiu sutm = 0,88; h s cng sut cosm = 0,89; Y/-380V/220V. Ngi ta u ng c vomng 220V/127V.a. Xc nh cch u dy ng c

b. Tnh cng sut in ng c tiu th khi nh mc.c. Tnh dngin dy Id v dng in pha Ip ca ng c.

16.Mt ti ba pha c in tr pha Rp = 20, in khng pha Xp = 15, ni hnh tamgic, u vo mch in c in p Ud = 220V. Tnh dng in pha Ip, dng indy Id, cng sut tiu th v v th vec t in p dy v dng in pha.

17.Mt ti ba pha gm ba cun dy u vo mng in ba pha c in p dy l Ud =380V. Cun dy c thit k lm vic vi in p nh mc 220V, cun dy cc in tr R = 2, in khng X = 8.

a) Xc nh cch ni cc cun dy thnh ti ba pha.

b) Tnh cng sut P,Q, cos ca ti.


75/109

Mn hc: K thut in

Chng 5: My bin p

C(L

Mc tiu:

- Phn tch c c

- Phn tch c cNi dung

5.1. Khi nim

- L nhng thit b c khchiu) v khng lm thay

5.2. Cu to v nguyn

a. Cu to:

- Gm c hai cun dy :cun th cp c N2 vng.mng ghp cch in vimch.- S vng dy hai cunc th N1 > N2 hoc ng- Cun s cp ni vi mtiu th in.- Trong thc th th mymy bin p th c dng

b. Nguyn tc hot ng:

- t in p xoay chiuthng trong hai cun. G- T thng qua cun s c- Trong cun th cp xu

T ta thy nguyn tct.

5.3. Kho st my bin

Gi N1. N2 l s vng cGi U1, U2 l hiu in tGi I1, I2 l cng hiTrong khong thi gian

ca c hai cun sut in

Sut in ng trn mt

Sut in ng trn cun=> T s in p 2 u cu

g

NG 5: MY BIN Pthuyt: 3h; Bi tp: 2h; Kim tra: 1h)

u to, nguyn l lm vic ca my bin

lm vic ca my bin p.

nng bin i in p (xoayi tn s ca n.

c hot ng

un s cp c N1 vng vLi bin p gm nhiu l stnhau trnh dng Fu-c v tng cng

phi khc nhau, tu thuc nhim v cac li.ch in xoay chiu cn cun th cp ni

in p c dng nh hnh 1, cn trong vih hnh 2

n s f hai u cun s cp. N gy rat thng ny l: = 0cost

p v th cp ln lt l : 1 = N10costt hin sut in ng cm ng e2 c biu

hot ng ca my bin p da vo hin

p

cun s cp v th cp. 2 u cun s cp v th cp.dng ca dng in 2 u cun s cp

t v cng nh t thng bin thin gy ra

ng bng:

un s cp l:

th cp:n th cp bng t s vng dy ca 2 cu

: K thut Xy Dng

Trang 71

p.

Hnh 1

t thng quaHnh 2

y m

i ti

c biu din s

s bin thin t

v 2 = N20costhc

ng cm ng in

th cp.rong mi vng dy

tng ng


76/109

Mn hc: K thut in

Chng 5: My bin p

T s khng i theo thi g

, (1)

in tr thun ca cun s c(2)

T (1) v (2) ta c Nu N2 > N1 => U2 > U1 : g Nu N2 < N1 => U2 < U1 : gV hao ph my bin p rt nnh nhau.

T (*) v (**) ta cKt lun: Dng my bin pgim by nhiu ln v ngcCh : Cng thc (*) lun c p dng khi hao ph kh

Ngh: K

ian nn ta c th thay bng gi tr hiu d

rt nh nn , khi mch th cp h

, (*)i l my tng p.i l my h p., coi nh cng sut 2 u cun th c

, (**)

ng in p bao nhiu ln th cng li.

c p dng cho my bin p cn cng thg ng k hoc hai u cun th cp

thut Xy Dng

Trang 72

ng ta c

nn ,

p v s cp

dng in

c (**) ch.


77/109


Chng 5: My bin p Trang 73

CU HI T KIM TRA:

1. Hy gii thch ngha ca thut ng b gim th.

2. Hy cho bit tn gi cun dy ca my bin th m ngun in c ni vo.

3. Hy cho bit tn gi cun dy ca my bin th m ti c ni vo.

4. Mt my bin th mt pha c 250 vng dy trn cun dy ni vi ngun in v c 25

vng dy trn cun dy ni vi ti.a) Hy tnh t s ca bin th ny.

b) Nu 250 vng dy c ni vo ngun in xoay chiu 240V, hy tnh in p ti cun

dy th cp.

c) Nu dng s cp o c l 2A, hy tnh dng th cp.

5. Mt b gim th c 24000V ni vo cun dy s cp ca n. Hy tnh in p th cp nu

t s ca bin th ny l 50 : 1.

6. Hy gii thch hin tng h cm ca my bin th.

7. Dng in 8A chy qua ti t mt my bin th. Cun s cp ca bin th ny c ni vi

ngun in 240V v 50Hz. Cun th cp ca bin th c in p 12V.

a) Hy tnh dng in t li in chy vo bin th.


78/109



b) Cun dy s cp ca bin th c 960 vng. Hy tnh s vng dy ca cun th cp.

21. Mt bin th mt pha c cun dy s cp cha 1000 vng dy v cun dy th cp cha

50 vng dy. Hy tnh in p u ra khi u bin th ny vo ngun in 240V v 50Hz.

22. Cn dng mt bin th cp dng in 12V v 12W cho n chiu sng trong vn. Hy

tnh dng in ngun cp cho bin th ny khi u ni n vo in li 240V v 50Hz.

23. Hy m t ngn gn hot ng ca bin th (cch thc hot ng ca n).

30. Nu mt bin th c 960 vng dy s cp, v 960 vng dy th cp, n c gi l loi

..

31. Trong b gim th, dng trong cun dy s cp s nh hn dng trong

33. (khng c cu 32) Nu mt bin th c t s vng dy 4 : 1 th c ngha l cun dy th

cp c

34. Nu cun s cp ca bin th c ni vo ngun 230V v in p th cp l 1150V,

dng th cp l 10A th dng s cp chy t ngun vo bin th l bao nhiu?

35. Hy v s mt my bin th v ch thch y .

56. Mt bin th c 480V ni vo cun s cp v 48V ti pha cun th cp. Hy tnh dng

in s cp khi bit dng in th cp l 20A.

57. Mt bin th c 300 vng dy trn cun s cp v 60 vng dy trn cun th cp. Hy

tnh in p m ngun cung cp cho n nu in p th cp l 12V.


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58. Hy tnh in p s cp ca bin th khi bit t s ca bin th l 12 : 1 v ti pha th cp

o c tr s in p 20V.

Hy tnh dng in s cp nu bit dng in th cp l 20A.

59. Mt bin th c tr s 240V trn cun s cp v 6000V trn cun th cp.

Hy tnh t s ca bin th.

Hy tnh dng in s cp nu dng in th cp l 0,025A.

Cun dy th cp phi c bao nhiu vng dy nu cun s cp c 960 vng dy?


80/109


Chng 6: ng c khng ng b Trang 76

CHNG 6: NG C KHNG NG B(L thuyt: 3h; Bi tp: 2h; Kim tra: 1h)

Mc tiu:

- Phn tch c cu to, nguyn l lm vic ca ng c xoay chiu 3 pha,

ng c khng ng b xoay chiu 1 pha.- Gii thch c cc nguyn tc iu chnh tc ca ng c xoay chiu.

Ni dung

6.1. CU TO V NGUYN L HOT NG CA NG C IN XOAYCHIU BA PHA

6.1.1. Cu to ng c khng ng b ba phaGii thiu

ng c khng ng b l loi ng c c ng dng ng dng rng ri nht

trong cng nghip. L do l n c cu to n gin, cng sut ln v gi thnh thp.ng c khng ng b c mt rotor khng kt ni vi in p ngun cp bn ngoi.ng nh tn gi, ng c khng ng b ly nng lng t hin tng cm ng int.

C hai loi ng c khng ng b l ng c lng sc (SCIM) v ng c rotordy qun (WRIM).

Hnh 1 (a)

C hai loi ng c trn u c mt stator v mt rotor l cc phn cu to chnhca chng

Mt iu rt quan trng l phi ch cc b phn cu to ca tng loi my

Stator

y l mt li c ghp bng cc l thp m cc l thp ny c lm t ccthp tm mng c dp to thnh, c cc rnh b mt bn trong. Cc bi dy lnlt ca cc pha ngun cp c qun u n trong cc rnh ny. Sau khi cc cundy c t vo trong cc rnh tng ng, ton b stator c lp ph bn ngoi mt

ng c khngng b

ng c lng scRotor thng thng

Rotor i

ng c rotor dyqun

Vn chia


81/109



lp sn hoc nha epoxy. iu ny gi cho lp cch in ca stator v rotor khng bm.

Cu to chi tit v cc phn chnh ca stator

Rotor lng sc

Rotor lng sc c mt li tr c ghp t cc l thp vi cc rnh c to ratrn b mt. Thanh dn rotor c lm t vt liu c th l ng, nhm hoc mt vtliu hp kim ph hp, c t trong cc rnh v ni ngn mch cc u bng ccvng dn.

Mt rotor hon chnh bao gm mt trc, bc, cc li thp ghp l v cc thanhdn rotor

T s lp rp cc thanh dn rotor c n gin ha, rt d c th ch ra tisao rotor ny li c gi l lng sc

Hnh 2

Khi kim tra k bn s thy rng cc thanh rotor c t nghing mt gc (thanhxin) dc theo thn ca rotor.iu ny m bo vic tng tc trn v u trong sutqu trnh khi ng.

Lng c lm t hpkim nhm c

Cnh qut

Thanh rotor(thanh xin)

Trcthp


82/109



Mt ch gi tr khc na l cc thanh dn rtor ny c th c in tr ln hocin tr nh

C rt nhiu cch kt hp cc thanh rotor trong vic thit k rotor thay i mmen quay ca my

Theo tiu chun c - Australian Standards 1359.41 cho php ch c hai loi rotor l

1) Thanh dn rotor tiu chun hoc thng thng (Hnh 3)2) Rotor lng i hoc mmen ln (Hnh 4)

Xem xt c th hai loi rotor ny v cch cc thanh dn rotor c t vo trong lithp

Hnh 3

Hnh 4

. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . .ng c rotor dy qun

Trong mt ng c rotor dy qun, cc khe ca rotor c qun trn bi cccun dy cch in, ging nh cc cun dy qun trn stator, v thng c kt ni

theo kiu SAO v kt thc hai u c ni vi ba vng bc trt (c gp) c gnvo trc rotor.

Hnh 5

Khe h khng kh


83/109



Cc cun dy rotor khng c ni trc tip vi ngun cp, cc c gp ch nthun c ngha kt ni vi hp iu khin bin tr bn ngoi vi mch rotor.

Loi rotor ny rt t tin v yu cu bo dng mc cao.

Bin tr rotor s tng tr khng mch rotor trong qu trnh bt u khi ng.Khi ng c tng tc, bin tr dn dn b ct b, do tr khng ca rotor gim. V

vy, m men quay ca ng c c iu khin ti gi tr m men ln nht phc vcho sut qu trnh tng tc ca ng c.

Cc c gp c ngn mch khi ng c t ti tc nh mc, hot ng can khi cng ging nh ng c lng sc

Bi v mch rotor c th thay i trong qu trnh hot ng, iu ny c ngha lcung cp kh nng iu khin tc ca ng c.

Khi tr khng ca rotor c tng ln, m men quay cng s ln hn tng ngvi mt h s trt ln hn. Kh nng thay i tr khng ca rotor lm cho n c khnng thay i m men khi ng vi cc gi tr khc nhau

Cun dy rotor khng c ni vi ngun cp, cc c gp ch n thun c mt ngha l kt ni b iu khin bin i tr khng bn ngoi vo trong mch rotor

Loi rotor ny rt t tin v yu cu bo dng mc cao.

Quan st k s rotor di v ch v tr ca ba c gp

Hnh 6

V ng c

Kch thc v hnh dng ca v my rt a dng, ph thuc vo iu kin ning c c lp t v hot ng.

Tt c cc khung v ca ng c u c thit k c gi vng bi t rotor,bo v cc cun dy v h tr vic g lp ng c vo cc phn khc ca h thng.

Hu ht cc khung v ng c hin i u c lm t vt liu hp kim nhm,iu ny gip cho trng lng ca my gim i v gip cho vic tn nhit d dng hnthng qua cc gn hoc cc cnh trong cc thit k khung v.

C gp

Cun dy rotor


84/109



Cc nh sn xut ng c thng cung cp mt di cc loi v my khc nhau ph hp vi cc ng dng c th

Sau y l ba loi v my c bn, mi loi bao gm mt s c im khc nhau,nhng xu hng hin i ngy nay ch tp trung vo cc loi v kn hon ton .

V kn - thng gi lm mt t nhin

y l cc loi m vic thng gi lm mt khng b hn ch do khng nh hngti cu trc c kh.

y l cc loi m cc u dn khng kh vo v ra c ph mt lp li l,cho mt khong m nh nht

y l cc loi m phn h lm thng gi c bo v trnh nc v biri xung theo chiu thng ng.

Hnh 8

V kn hon ton (TEFC v kn hon ton c qut lm mt)

Hnh 9


85/109



My c qut lm mt c cu to khng kh c bao kn v khng thng vikhng kh bn ngoi nhng khng cn thit phi airtight - b kn hon ton. S lmmt c thc hin bng vic lu thng khng kh trong khung v my

Hu ht cc ng c u thuc loi kn hon ton, vic lm mt c thc hinbi qut, qut c hot ng nh chnh ng c v thi cc lung kh bn ngoi

vo cc mt lm mt ca ng c. Vi ng c c cp lin tc kh lm mt, thkhung v c b tr cc ng dn kh vo ra lm mt.

V my chng la

My c thit k theo tiu chun ca c Australian Standard (AS1359) cho ccng c chu la

Hnh 10

By gi, s dng cc kin thc m bn thu c trong rt nhiu cc dngng c khng ng b ba pha, ta s kim tra cc c tnh hot ng v cc ng dngin hnh cho tng loi ng c.

Cc ng c tnh hot ng ca ng c rto lng sc

ng c khng ng b rto lng sc c th c v nh l mt my bin th viphn th cp quay. u tin khi ngun cp c cp cho cc cun dy stator (s cp),kt qu l t trng to ra quay tc ng b. Vi hot ng ca rotor nh l mtcun dy th cp ngn mch , mt dng in tun hon gi tr cao c hnh thnhtrong cc thanh rotor. iu ny cng to ra dng in c gi tr cao trong cc cun dystator v dng ny s gim khi rotor t c tc mong mun. Khi ng c chy ch khng ti th tc s gn vi tc ng b v dng trong rotor rt nh. Khi

ng c c ti, th rotor s quay chm i. Tuy nhin t trng quay s tip tc quayvi tc quay ng b

M men quay ca ng c tng ln khi tc gim i v nng lng u ra tngln

c tnh k thut Dng in/Tc


86/109



Hnh 11Hnh v 11 biu th mi quan h gia tc ca rotor v dng ca stator. Khi tc

rotor tip cn tc nh mc, dng stator s gim mnh bt u t dng khing cao lc u (cao gp 6-7 ln dng ti) n khi n t c ch nh mchon ton

Rotor tiu chun ng c tnh M men quay/ Tc

Khi ti ca ng c tng ln (Hnh 12) th m men quay tng ln v tc gimi cho ti khi m men quay t n tc ln nht v c gi l m men ti hn.

Nu ng c c t ti nm ngoi im ny, th c m men quay v tc s gim

v ng c s rt nhanh chng chuyn v ch dng.

Hnh 12

Rotor lng sc kp - c tnh M men quay/ Tc

M men quayM men tihn

Tc nhmc

Tc trt

M men nh mc

M men khing

Dng inTc nh mc

Dng in nh mc


87/109



Loi rotor ny c hai nhm thanh dn rotor. Cc thanh dn rto bn trong (Trkhng nh) c gi tr t nht l mt na so vi cc thanh dn rotor ngoi (c tr khngcao)

Kt qu l m men khi ng (hnh 13) ca loi rotor ny xp x 225% m menquay nh mc, kt qu l cho dng khi ng cao gp khong 5 ln dng chy nh

mc

Hnh 13

ng c tnh lm vic ca ng c khng ng b

Hnh 14

S ny (hnh 14) ch ra mt cch ton din tt c cc ng c tnh hotng chnh ca mt ng c khng ng b ba pha in hnh

Ch rng tc trt khi ti ch xp x 4%. Trong iu kin dng quay,in khng ca rotor ca loi ng c ny c th gp 5 ln in tr ca rotor

Tuy nhin ch ti, in khng ca rotor ca loi ng c ny nh hn

in tr ca rotorC hai m men quay v tc trt u tng ti im bin th

Khng ti ti

Hiu sut (%) vh s cng sut

M menquay Nm

ng c khng

ng b ba pha 4cc12 kW, 50 Hz,415V, 4 cc

Nng lngu ra kW

Rpm vng/pht

Ampe

Tc trt

M men quay % M men nh mc

Tc nhmc

Tc

Speed : tc

Power factor : h scng sut

Efficiency : hiusut

Torque : M menquay

Stator current :Dng stator

Percentage Slip : tc trt


88/109



iu chnh tc trong ng c lng sc rt hon ho, c th thay i khngnhiu ch rt t phn trm trong ln trn dy bt u t ch ti sang ch khngti (Xp x 250 rpm)

Trong qu trnh m my khi ng, h s trt ca ng c l ln nht, mtdng in ln c cm ng vo trong rotor, chnh dng in ny to ra m men quay

cn thit khi ng ng c.Tng kt

ng c khng ng b l loi ng c ph bin nht trong cng nghip C hai loi ng c khng ng b l ng c khng ng b rto lng sc v

ng c khng ng b rto dy qun.

Tt c cc ng c khng ng b u gm c STATOR v ROTOR Stator ca ng c khng ng b c cu to chnh gm cc cun dy ba pha c

nh

Rotor lng sc c cu to bao gm cc thanh dn ng hoc nhm c t votrong mt li ghp t cc thp l v c ni ngn mch hai u

Rotor dy qun ca ng c khng ng b c mt rotor gm t hp ba cundy kt ni vi cc c gp

C rt nhiu loi rotor lng sc ph hp vi ng dng cn m men m my caohay thp

Rotor dy qun ca ng c khng ng b c thit k cho php tc vm men khi ng d thay i ph hp vi cc iu kin ti trng ln

ng c c th dng ba loi v my gm: v kn- thng gi t nhin, kn honton v v my chng chy v c rt nhiu dng khc nhau trong tng loi

c tnh khi ng v vn hnh ca mt ng c khng ng b lng sc cxc nh bng vic thit k cc bi dy dn ca rotor, v c rt nhiu cc thit k phhp vi cc iu kin ti trng khc nhau.

6.1.2. Nguyn l hot ng ca ng c khng ng b ba phaGii thiu chung

ng c xoay chiu l ng c hot ng vi ngun cp l in xoay chiu mt

pha hoc nhiu pha. Tuy vy, cc ng c ny u c mt nguyn l hot ng chung.Nguyn l ny l: dng in xoay chiu cp cho ng c (phn Stator) s sinh ra

mt t trng quay, t trng quay ny s to lc quay phn rtor ca ng c.

T trng quay (RMF)

Trc khi nghin cu lm cch no mt t trng quay c to ra trong statorca ng c th cn hiu r cch kt ni in trong phn stator ca ng c.

S sau (Hnh 1) minh ho mt stator 3 pha vi ngun cp l ngun xoaychiu3 pha. Cc cun dy ni theo kiu tam gic. Hai cun dy trn mi pha c

qun theo cng mt hng


89/109



Ti bt c mt thi im no, t trng c to ra bi mi pha ring bit uph thuc vo cng dng in i qua pha . Nu dng in bng khng thcng t trng cng bng khng. Nu dng in ln nht th cng t trngcng ln nht.Do dng in trong ba pha ln lt lch nhau mt gc 120 th ttrng c to ra t ba dng cng ln lt lch nhau mt gc 120 .

By gi, ti mt thi im bt k th ba t trng tn ti s kt hp vi nhauto ra mt t trng chung duy nht tc ng ln rtor. Hng dch chuyn ca ccng sc t trong t trng t hp ny lun lun thay i theo mt vng trn. Saukhi kt thc hon ton mt chu k ca dng xoay chiu th t trng s chuyn ngquay mt gc 360 hay mt vng quay.

S sau (hnh 2) minh ho hnh dng sng ca ba dng in hnh thnh trongcc cun stator. Cc sng pha ny lch pha nhau 120 ln lt gia cc pha. Ccsng ny c th l t trng xoay chiu c to ra bi ba pha hoc l dng trong mi

pha. Cc sng c nh k hiu ph hp vi cc cc pha tng ng ca n.

Hnh 2: Qu trnh to ra mt t trng quay

u vo

xoay chiu

Stator 3 pha

u voxoay chiu

Hnh 1: Stator 3 pha ni kiu tam gic


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S dng cc dng sng, ta c th kt hp cc t trng ti tng thi im saumt phn su chu k (60oE) xc nh chiu ca t trng tng. Ti im 1, sngC dng v sng B m .

iu ny c ngha l dng i qua theo hai hng i nghch nhau qua cc pha

B v C, v v vy thnh lp s phn cc t trng ca hai pha B v C.S phn cc t trng c ch ra trn s rt gn trn vi v tr l im 1

Bn s quan st thy rng im B1 l cc bc v im B l cc nam, cng tngt im C l cc bc v im C1 l cc nam. Ti thi im ca im 1 khng c dngi qua pha A, do t trng bng khng. T trng di chuyn t cc B1 v Cs tin v trc ti cc cc gn nht l C1 v B nh hnh v. Trong khi t trngca B v C c bin bng nhau nn t trng tng hp ca hai t trng trn s nm gia hai t trng v n s c hng nh hnh v.

Ti im 2, sau mt gc 60oE, dng sng ca dng in u vo ti cc pha Av B l nh nhau v c ln tri ngc v sng ti im C l khng. T trng tnghp s quay i mt gc 60oE.

Ti im 3, sng ti B l khng v t trng tng hp s quay i mt gc 60oEv s quay nh vy bt u t im 1 ti im 7 (theo ng mt chu k ca dng xoaychiu), bn c th nhn thy t trng quay tng hp quay mt vng vi mi chu kca ngun xoay chiu cp cho stator

By gi c th thy r rng rng vic cp in 3 pha xoay chiu vo cc cun dyt i xng quanh stator th s to nn mt t trng quay trong .

T trng quay ng dng trong c ng c khng ng b v ng c ng b.Tc ca t trng quay (Rmf)

Tt c hot ng v tc quay ca ng c khng ng b u da vo s tn ti cat trng quay (a rotating magnetic field - rmf).

Trong kiu qun dy gm hai cc (cho mt pha) c ch ra trn hnh 1, t trngtrong s quay c mt vng hon chnh trong mt chu k ca dng in.

Trong kiu qun dy gm bn cc trong mi pha c hai nhm cun dy ring bitmc ni tip vi nhau, t trng quay (rmf) s to ra mt vng quay trong hai chu kca dng in.

Trong kiu qun day gm su cc, t trng quay (rmf) s to ra mt vng quay trongba chu k ca dng in.

Tc quay trn ca t trng quay c gi l tc ng b ca ng c. i vitt c ng c in khng ng b u c mi lin h ging nhau gia s cc v tn sngun cp trong vic xc nh tc ca t trng quay

Tc ca t tng quay tnh theo s vng trn mt pht (rpm) c th c nhngha t biu thc sau:


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Mn hc: K thut in Ngh: K thut Xy

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Tai Lieu Ky Thuat Dien - Nghe Xay Dung