chapter 6 - contactors and motor starters

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CHAPTER

6 Contactors and Motor Starters (Contactor v B khi ng ng c)

Chng ny s gip bn hiu cc vn sau :

1. Gip bn hiu v

B khi ng ng c v contactor c s dng ng/ngt cc mch in ng lc. C hai

u s dng mt dng in nh cp in hoc ngt in cho cc ph ti kt ni vi chng.

Chng ny cho bit contactor v b khi ng ng c c s dng nh th no trong mch

c ph ti ng c v mch khng c ph ti ng c.

PART 1 Magnetic Contactor (Contactor kiu in t)

Hip hi cc nh sn xut in Hoa K (National Electrical Manufacturers Association-NEMA)

nh ngha contactor kiu in t l mt thit b c dn ng bng t tnh dng ng ngt

lp i lp li nhiu ln mt mch in ng lc.Contactor kiu in t hot ng nh mt rle

in c. C 2 u c chung mt chc nng quan trng l : to lin lc hot ng cho mch khi

cun dy c cp in. Ni chung, khng ging nh rle, contactor c thit k ng/ngt

mch in c dng ln hn 15A m

khng b nguy him. Hnh 6-1 cho

thy mt loi contactor kiu in t

theo chun NEMA c s dng

ngt mch c ph ti ng c xoay

chiu khi hin tng qu ti m

khng cn phi c thm cc thit b

khc. Bn cnh 3 tip im ng

lc, cn c 1 tip im ph thng

m trang b cho nt n iu khin

3 pha (3 dy).

C 2 mch lin quan n s hot

ng ca contactor kiu in t :

mch iu khin v mch ng lc.

Mch iu khin c kt ni vi

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cun dy, v mch ng lc c kt ni vi cc tip im ng lc chnh. Nguyn tc hot

ng ca contactor kiu in t loi 3 cc c minh ha Hnh 6-2. Khi in p c cp cho

cun dy, s c dng in qua cun dy, to ra in t trng. Sau , cun dy lm nhim t

khung thp c nh, bin n thnh mt nam chm in (electromagnet). Nam chm in ny ko

cun li v pha n, ko cc tip im c nh li tip xc vi nhau. Khi s dng in i qua

contactor t pha dy cp in sang pha ph ti. Thng thng, mt contactor c th c cu

hnh 1, 2, 3 hoc 4 cc.

Switching Loads (chuyn mch ph ti)

Contactor c s dng chung vi cc thit b dn hng (pilot devices) iu khin cc ph

ti c dng in ln mt cch t ng. Thit b iu khin ny c cng sut dng gii hn, c

s dng iu khin dng vo cun dy ca contactor, cc tip im chnh c dng

ng/ngt cc dng ti nng hn. Hnh 6-3 m t mt contactor c s dng vi cc thit b dn

hng kim sot nhit v mc nc trong bnh. Trong ng dng ny, cun dy ca

contactor c kt ni vi cc sensor bo mc v sensor nhit ng/m t ng cc tip

im ng lc cho van in t v cc ph ti gia nhit.

Contactor c th c s dng ngt cc ph ti ng c khi mch ng lc khi xy ra hin

tng qu ti.Cch s dng contactor ph bin nht l dng km vi mt rle qu ti (overload

relay) trong b khi ng ng c xoay chiu (AC motor starter). Hnh 6-4 m t mt contactor

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chun IEC (y Ban K Thut in Quc T) c dng kt hp vi mt rle qu ti ngt

ph ti ng c. S iu khin bng 2-dy hoc 3-dy c th c s dng ng/ngt ng

c.Mch iu khin bng 2-dy thng c s dng ph bin nhng ni yu cu s vn hnh

mch ch t ng, chng hn nh p dng vi bm, b gia nhit bng in, my nn kh,

nhng ni m i hi cc thit b dn hng khi ng ng c mt cch t ng. Mch 3-dy

cng tng t nh mch 2-dy ngoi tr vic n c thm mt tp hp cc tip im ph gn

n vo mch in.ng dng ph bin nht cho s iu khin 3-dy l ng c c iu khin

bng nt n bt/tt tc thi. Trong trng hp ny, cc nt n phi c n cp in hay ngt

in khi cun dy contactor. Nhn chung, cc nt n bt/tt c s dng bt u v kt thc

cc qu trnh ca h thng no .

in p cao c th c s dng cho contactor nhng phi hon ton trnh xa khi nhn vin

vn hnh, v th cn gia tng s an ton trong khu lp t. Trong trng hp s dng my bin

p lm gim in p xoay chiu xung mc ci t cung cp cho mch iu khin no .

Dng in ra ca my bin p c th l 12V, 24V hoc 120V; dng in cp vo my bin p c

th l 208, 230, 240, 460, 480, hoc 600V. Trong mi trng hp, in p cun dy contactor

phi bng vi in p ca mch iu khin.

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Cc tip im ph ca contactor c dng nh hn nhiu so vi cc tip im chnh v c dng

trong cc mch iu khin kha lin ng (interlocking), duy tr (holding), v bo hiu trng

thi. Hnh 6-5 trnh by s mch cho mt mch gia nhit 3 pha c iu khin bi mt

contactor kiu in t loi 3 cc v c vn hnh bi mch iu khin 3-dy. C ch hot ng

ca mch c th c tm tt nh sau :

Mt my bin p iu khin c s dng h thp in p t 480V xung 120V cho

cc mc ch iu khin.

Mch iu khin 3-dy c s dng ng/ngt in cho cc b gia nhit.

Cng tc on/off b ng li, nt n heat on c nh ra cp in cho cun dy CR

ca contactor.

Cc tip im ng lc chnh CR-1, CR-2 v CR-3 ng cp in cho cc b gia nhit

theo in p dy.

Tip im ph CR-4 ng li gi cun dy contactor bng cch to thnh mt mch

kn quanh nt nhn heat on.

ng thi, tip im ph CR-5 m ngt n bo mu xanh l cy v lin lc CR-6

ng li bt n bo mu .

n nt nhn heat off hoc m cng tc on/off s ngt in khi cun dy, lm mch

quay tr li trng thi off.

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Cc contactor c th (definite-purpose contactors) c thit k c bit cho cc ng dng

ring nh iu ha khng kh, k thut lm lnh, thit b gia nhit bng in tr, x l d liu, v

chiu sng. Cc contactor chiu sng a ra cch iu khin hiu qu cho cc ta nh vn phng,

cc khu cng nghip, bnh vin, sn vn ng, sn bay. Chng c th c dng iu khin

cc ph ti n si t bng kh tr (dy tc nng sng) hoc chn lu (n hunh quang) cho ti

cc ph ti phi ng c khc. Cc contactor c th c duy tr bng in hoc bng c. Vi

contactor duy tr bng in (electrically held contactor) th cun dy cn c cp in lin tc

trong mi thi gian cc tip im chnh c ng li. Contactor c duy tr bng c

(mechanically held contactor) yu cu duy nht mt xung in ca cun dy thay i trng

thi. Khi thay i, mt kha lin ng c kh duy tr cc tip im chnh ng v tr iu

khin ng lc

c th c loi

b. kt qu l s

vn hnh

contactor m,

mt v hiu qu

hn. Hnh 6-6

m t mt v d

v cc contactor

chiu sng loi

duy tr bng

in v bng c

nm trong hp

bo v.

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Hnh 6-7 m t mt contactor chiu sng loi 2 cun dy duy tr bng c kh v mch in tng

ng. Cc mch chiu sng l loi mt pha c in p 120V hoc 277V. Nguyn tc hot ng

ca mch ny c th c tm tt nh sau :

Khi nt ON c bt,

cun cam (cun dy mch

cht latch coil) c cp

in qua tip im lin h

thng ng (NC clearing

contact).

Kt qu l, contactor ng

li v kha c kh ng

cc tip im chnh (main

contacts - k hiu l M)

thp sng cc bng n,

vi iu kin l CB

c ng.

Cc cun dy lin h tip

xc (coil clearing

contacts) thay i trng

thi (thng ng sang

thng m v ngc li)

gy ra s thay i ca cc v tr cht hm trong contactor (a change in contactor latching

position).

m cht contactor, bng cch tt ln lt cc bng n, n nt OFF m cht hm

contactor m cc tip im M.

V cc cht hm ca cc cun dy khng c thit k lm vic lin tc nn chng t

ng ngt kt ni khi cun dy ngn nga cun dy b chy khi cc nt n vn ng.

Hnh 6-8 m t mt s dy cho mt contactor chiu sng loi duy tr c kh vi mt cun dy

n c cp in c th ng hoc m contactor. Vi ng dng ny, contactor chiu sng

c iu khin t 2 v tr t xa (3 v tr, tc thi, im gia l tt) t ti t iu khin. Mi v

tr iu khin c trang b mt n tn hiu, bo trng thi ng/m ca cc tip im chnh ca

contactor chiu sng. C rt nhiu thit b iu khin nh thit b iu khin theo chng trnh

logic (PLC) v h thng qun l nng lng c th c ghp ni vi contactor.

Contactor Assemblies (lp rp contactor)

Hnh 6-9 minh ha 3 loi c cu hot ng c trng ca contactor kiu in t : c cu n

khuu (bell-crank), c cu tc ng ngang (horizontal-action) v c cu qu lc (clapper). C

cu hot ng ca contactor nn c kim tra nh k m bo chc nng hot ng v trnh

b kt hay bm dnh. Mch t ca c cu hot ng bao gm cc thanh thp mm c dn t

cao v hin tng t tnh d thp. Lc ht t tnh bi cun dy phi ln ng li thp,

thng li c trng lc v lc ca l xo tip im.

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Cun dy contactor c ph mt lp nha epoxy chng m v tng tui th cun dy. Hnh

dng ca n thay i ty theo chc nng ca contactor (hnh 6-10). Khe h thng trc gia

mch t trng thi ng ngn

khng cho li thp b gi bi hin

tng t d.

Nu cun dy c biu hin ca s

qu nhit (b rn nt, b chy, lp

cch in b chy) th n phi

c thay th. o in tr ca

cun dy (coil resistance), ngt

kt ni mt trong cc dy dn cp

in ca cun dy v o in tr

bng cch thit lp ng h o

m k (ohmmeter) v mc in

tr nh nht ca n. Cun dy

hng s cho ch s bng 0 hoc

v cng, chng t cun dy b

ngn mch hay b h (a short or

opened coil).

Cc cun dy ca contactor

c rt nhiu vng dy c

cch in vi nhau to ra

cc vng dy ampe cn thit

(necessary ampere-turns )

to ra mt dng in nh. V

cc contactor thng c

s dng iu khin cc

in p dy khc nhau, nn

in p s dng iu

khin cun dy cng khc

nhau. Do , khi chn la

cc cun dy phi chn cun

dy c in p iu khin

sn c tng ng. Gii hn

lm vic ca contactor l t

85% n 110% in p nh mc ca cun dy. S thay i in p cun dy trong khong 5%

s lm gim ti thiu s hao mn cun dy. L do ca vic ny l in p cao hn s lm tng

vn tc ht ca nam chm in khi ng. in p thp hn s lm gim vn tc ht ca nam

chm khi ng. C 2 nhn t ny c th dn n mc rung ny cao hn ca cc tip im khi

ng, iu ny l nguyn nhn chnh gy ra s bo mn.

Cc loi in p ca cun dy bao gm : in p nh mc (rated voltage), in p tc ng

(pick-up voltage), in p duy tr (hold-in voltage), v in p st (dropout voltage).

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in p nh mc lin quan n in p cp cho cun dy v phi ph hp vi

ngun in ca mch iu khin.

in p tc ng l lng in p yu cu thng c cc lc c hc nh trng

lc v lc nn ca l xo, khi c gng duy tr cc tip im trng thi ng.

in p duy tr l lng in p cn thit duy tr cc tip im v tr ng ca

chng sau khi in p tc ng t c (in p duy tr thng nh hn in p

tc ng). Tt c cc contactor duy tr bng in rt nhy cm vi dao ng

in p ca ngun cp in.

in p st l lng in p thp m in t trng tr nn qu yu khng th

duy tr cc tip im v tr ng.

Cc cun dy ca contactor DC (mt chiu) v AC (xoay chiu) c cng in p nh mc

thng khng th thay th hay i ln cho nhau c, l do l v cun dy DC ch c in tr

thun (ohmic resistance) gii hn dng in i qua, trong khi cun dy AC th c in tr v

in khng (reactance or impetance) gii hn dng in i qua. Cc cun dy ca contactor DC

c rt nhiu vng dy v in tr thun cao hn so vi cun dy AC.

Vi cun dy hot ng mt chiu, v dng in i qua ch b gii hn bi in tr nn dng i

qua cun dy lc khp kn mch bng vi dng in bnh thng c cung cp. Tuy nhin, iu

ny khng ng cho trng hp cun dy hot ng xoay chiu. Vi mt cun dy xoay chiu

th ng i ca t trng c mt khe h bi v li thep khng b ko vo trong (Hnh 6-11). Khi

contactor ng, li thp khp kn ng i ca t trng lm tng in khng cm ng ca cun

dy v gim dng in qua n. Kt qu l, xy ra hin tng dng ln khi ng contactor v

dng nh duy tr n. Dng in kch t hay dng in khi ng (inrush current) cho cun

dy xoay chiu c th t 5 n 20 ln so vi dng lm vic bnh thng. Khi c dng in trong

mt ph ti in cm (current in an inductive load), chng hn nh mt cun dy contactor b ct

in s c mt xung in p cao c sinh ra. Nu khng b chn li, nhng xung in p ny c

th ln ti hng ngn vn v to ra dng tng vt t bin gy nguy him. iu ny thc s ng

cho cc ng dng yu cu ghp ni vi cc thnh phn bn dn nh module PLC. Hnh 6-12

minh ha mt module dp h quang c ni song song vi cun dy contactor. in tr v t

in u ni tip lm tr hon mc gia tng ca in p tc thi.

Cc cun dy ca contactor hot ng

nh ngun in xoay chiu to ra in t

trng xung quanh chng. Lc ht ca

nam chm in thc hin c l nh

dng xoay chiu l tun hon v bng 0

hai ln sau mi chu k. Khi dng in

tin n 0 th lc t gim xung v c xu

hng tch ri n by ra. Khi t tnh v

lc tc dng c ti to li, n by li

c ko tr li. S chuyn ng ca

n by ra/vo lm cho contactor pht ra

ting ku vo ve v lm bo mn cc

phn di ng ca contactor.

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Ting ku v s bo mn ca cc contactor xoay chiu c th c ngn chn bng vic s dng

cc vnh chn (shading coils or rings) nh m t hnh 6-13. Khng ging nh cun dy ca

contactor cc vnh chn ny khng c kt ni vi ngun cp in nhng c nh v thnh

cp cho cun dy contactor. Vnh chn ny gm mt vng n c cht liu dn in (thng l

ng hoc nhm) c nh v pha i din khi sinh t trng. N to ra mt lc ht t

trng ph ngc pha vi t trng chnh v ln gi n by gn cht vi li thp ngay c

khi t trng chnh bng 0 trn sng hnh sin. Vi cc vnh chn c thit k tt, cc contactor

xoay chiu c th hot ng rt m, yn tnh. Khi vnh chn b v hay b h s lm cho contactor

b ku ngay lp tc.

Li thp v n by ca contactor xoay chiu c lm t nhiu lp thp, trong khi contactor

mt chiu l thp nguyn khi. iu ny l do khng c dng in xoy (dng Phuco) sinh ra

cng vi dng mt chiu lin tc. Dng in xoy l dng in nh sinh ra trong li thp v n

by bi s bin thin ca t trng do dng xoay chiu qua cun dy contactor. Vic s dng li

thp c s sinh ra dng in tun hon ln v v l do ny li thp ca cun dy xoay chiu

c lm t nhiu l thp cch in ghp vi nhau.

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Ngy nay, hu ht cc tip im ca contactor u c lm t hp kim bc c in tr thp

(hnh 6-15). Ngi ta s dng cc tip im bng bc v chng c in tr thp hn cc loi hp

kim t tin khc. Ph thuc vo kch c contactor, cc tip im ng lc chnh c th chu

c dng nh mc vi trm ampe. Hu ht cc chi tit m bng bc thng thng c hn

vi cc tip im bng ng ( phn nghing (on the heel)), v vy bc s dn dng in, ng s

dn h quang trong bung ngt h quang. Hu ht cc nh sn xut u ngh rng cc tip

im bng bc khng bao gi b mi mn. Cc tip im bng bc khng cn phi lm sch bi

v s bc mu thnh mu en xut hin l oxit bc, n l mt vt liu dn in tng i tt.

Cc tip im phi chu c s

mi mn do dng in v do c

hc khi chng hnh thnh v

ng ngt dng in. Trong hu

ht mi trng hp, s mi mn

c hc l nh hn so vi s mi

mn ca dng in. Xut hin h

quang khi cc tip im tip xc

vi nhau hay ngt dng vi nhau

l nguyn nhn gy ra s n mn

do dng in. Bn cnh , cc

tip im s chu s qu nhit nu

chng chuyn ti qu nhiu dng

in qua, nu chng khng m

nhanh chng, hoc nu chng

ng qu thng xuyn s gy ra

s h hng nghim trng b mt

tip im v s lm vic khng n

nh ca contactor.

Arc Suppression (loi tr h quang)

Mt trong nhng nguyn nhn chnh gy mn cc tip im l do h quang in xut hin khi

cc tip im c m ra khi c ph ti. Khi cc tip im m ra s vn c dng in chy qua

gia b mt cc tip im nu in p 2 im tip xc l ln (hnh 6-16). ng dn cho

dng in lin tc ny chnh l do mi trng xung quanh b ion ha to nn h quang in. Khi

khong cch gia cc tip im ny tng dn, s cn tr h quang gia tng dng in gim dn

v in p cn thit duy tr h quang gia cc tip im s tng dn.

Cui cng, mt khong cch ln c to ra ton b in p dy khng duy tr c h

quang. Dng h quang c th to ra s gia tng nhit v cng ln trn b mt cc tip im.

S tng nhit ny c th ln lm cho b mt cc tip im b nng chy v bc hi kim

loi trong khong trng gia cc tip im. Do , cng dp c h quang cng sm cng tt,

nu dng h quang vn tip tc, n s lm tan chy b mt cc tip im. Hu ht cc contactor

u c bung dp h quang.

Cc yu t quan trng lin quan n vic to ra h quang gm :

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Mc in p v dng c ng ct : khi in p v dng in tng, khong

trng gia cc tip im ang m b ion ha nhanh chng to thnh mi trng

dn in.

D in p c ngt l dng mt chiu hay dng xoay chiu : dng h quang

mt chiu th kh dp tt hn cc dng h quang xoay chiu. Dng h quang xoay

chiu th t tt, dng h quang thng tt khi chu k dng xoay chiu i qua mc

0. Trong trng hp ca dng mt chiu, khng c dng bng 0, v dng ny lun

i theo mt hng v th tnh t dp tt khng tn ti.

Loi ph ti (in tr thun hay in tr cm ng) : Vi loi ph ti in tr

thun, thi gian h quang xy ra bng vi tc m cc tip im tch ra. Vi

ph ti cm ng, s nh nng lng c tch tr trong trng in t gip duy

tr dng v gy ra s tng vt in p (voltage spikes). Cc ph ti cm ng trong

mch xoay chiu t gp phi vn hn trong mch mt chiu.

Contactor lm vic nhanh chng nh th no ? Tc tch cc tip im cng

nhanh th vic dp tt h quang cng nhanh.

H quang cng c th xut hin trong contactor khi chng ng li, v d, nu cc tip im n

gn m xut hin s nh thng in p (voltage breakdown) v h quang c th to thnh

cu ni khng gian m gia cc tip im. Ni mt cch khc, iu ny ch c th xy ra nu

mp tip xc ca mt tip im chm vo tip im khc ln u tin v b chy ra, to nn mi

trng ion ha cho php dng in chy qua, dng h quang s c duy tr cho n khi cc b

mt tip im c ng kn hon ton.

Mt s khc bit ch o gia

contactor mt chiu v contactor

xoay chiu l cc yu cu thit

yu v c hc v in hc cho

vic dp tt h quang to ra trong

khi ng m cc tip im trong

trng hp c ti. chin u li

dng h quang ko di trong mch

in mt chiu th vic ngt

contactor bng c hc phi c

thit lp cc tip im s tch

ra nhanh chng v to ra khong

trng khng kh dp tt h

quang sm nht c th ngay khi

m. Cc contactor mt chiu

thng c kch thc ln hn so

vi contactor xoay chiu b

sung thm khong trng (hnh 6-

17). iu ny cng rt cn cho

vic ng cc tip im trong

mch mt chiu di chuyn cc

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tip im li vi nhau nhanh nht c th

trnh gp phi mt vi vn ging nh

khi m chng. V l do , tc lm vic

ca cc contactor mt chiu c thit k

lm vic nhanh hn so vi nhng contactor

xoay chiu.

Mt bung dp h quang (arc chute) hay

tm chn (shield) l mt thit b c thit

k gip giam hm, chia tch v lm mt

tia h quang, cc tia h quang ny

khng t duy tr c n. C mt bung

dp h quang cho mi cp tip im c

lp trn phn tip im di ng v tip

im c nh (hnh 6-18). Cc bung dp

h quang chia tch cc tia h quang c

hnh thnh ngay ti cc tip im ca

contactor cng vi vic ph v dng

dp tt h quang. Ngoi ra, chng cng to nn cc hng ro chn gia cc in p dy.

Bung dp h quang c s dng trong cc

contactor xoay chiu cng tng t nh trong

contactor mt chiu. Tuy nhin, ngoi vic c thm

cc bung dp h quang, hu ht cc contactor mt

chiu cn c trang b thm cun dy dp tt h

quang t trng (magnetic blowout coils) h

tr cho vic dp tt h quang. Cun dy dp tt h

quang bao gm cun dy ng nng c gn

trn cc tip im v c mc ni tip vi nhau

(hnh 6-19). Dng in i qua cun dy to nn mt

trng in t gia..

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