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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