Bai Giang Mon Khi Cu Dien

cng bi ging mn Kh c in BI 1 KHI NIM V KH C IN I. KHI NIM V KH C IN 1. Khi nim v kh c in. Kh c in l thit b dng ng ct, bo v, iu khin, iu chnh cc li in, mch in, cc loi my in v cc my trong qu trnh sn xut. Kh c in lm vic lu di trong cc mch dn in, nhit ca kh c in tng ln gy tn tht in nng di dng nhit nng v t nng cc b phn dn in v cch in ca kh c. V vy kh c in lm vic c trong mi ch khi nhit ca cc b phn phi khng qu nhng gi tr cho php lm vic an ton lu di. 2. S pht nng ca kh c in Vt liu lm kh c in Vt liu khng bc cch in hoc xa nht cch in. Dy ni dng tip xc c nh. Vt liu c tip xc dng hnh ngn Tip xc trt ca ng v hp kim ng Tip xc m bc. Vt khng dn in v khng bc cch in. Vt liu cch in Vi si, giy khng tm cch in Vi si, giy c tm cch in. Hp cht tng hp Mica, si thu tinh Mica, si thu tinh c tm cch in Cht tng hp Silic S cch in. Nhit cho php (oC) 110 75 75 110 120 110

Cp cch nhit Nhit cho php (oC) Y 90 A 105 E 120 B 130 F 155 H 180 C >180

Tu theo ch lm vic khc nhau, mi kh c in s c s pht nng khc nhau: A. Ch lm vic lu di ca kh c in Kh c in lm vic lu di, nhit trong kh c in bt u tng v n nhit n nh th khng tng na, lc ny s to nhit ra mi trng xung quanh.

B. Ch lm vic ngn hn ca kh c in Ch lm vic ngn hn ca kh c in l ch khi ng in nhit ca n khng t ti nhit n nh, sau khi pht nng ngn hn, kh c c ngt nhit ca n st xung ti mc khng so snh c vi mi trng xung quanh.

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cng bi ging mn Kh c in

C. Ch lm vic ngn hn lp li ca kh c in Nhit ca kh c in tng ln trong khong thi gian kh c lm vic, nhit gim xung trong khong thi gian kh c ngh, nhit gim cha t n gi tr ban u th kh c in lm vic lp li. Sau khong thi gian, nhit tng ln ln nht gn bng nhit gim nh nht th kh c in t c ch dng.

3. Tip xc in 3.1. Khi nim Tip xc in l ni m dng in i t vt dn ny sang vt dn khc. B mt tip xc ca hai vt dn c gi l tip xc in. Cc yu cu c bn ca tip xc in: - Ni tip xc in phi chc chn, m bo. - Mi ni tip xc phi c bn c kh cao. - Mi ni khng c pht nng qu ga tr cho php. - n nh nhit v n nh ng khi c dng in cc i i qua. - Chu c tc ng ca mi trng (nhit , cht ho hc...) m bo cc yu cu trn, vt liu dng lm tip im c cc yu cu: - in dn v nhit dn cao. - bn chng r trong khng kh v trong cc kh khc. - bn chng to lp mng c in tr sut cao. - cng b gim lc nn. - cng cao gim hao mn cc b phn ng ngt. - bn chu h quang cao (nhit nng chy). - n gin gia cng, gi thnh h. Mt s vt liu dng lm tip im: ng, Bc, Nhm, Vonfram... 3.2. Phn loi tip xc in Da vo kt cu tip im, c cc loi tip xc in sau: a) Tip xc c nh Cc tip i c ni c nh vi cc chi tit dn dng in nh l: thanh ci, cp in, ch ni kh c vo mch. Trong qu trnh s dng, c hai tip im c gn cht vo nhau nh cc bu - lng, hn nng hay ngui. b) Tip xc ng m L tip xc ng ngt mch in. Trong trnghp ny ht sinh h quang in, cn xc nh khong cch gia tip im tnh v ng da vo dng in nh mc, in p nh mc v ch lm vic ca kh c in. c) Tip xc trt L tip xc c gp v vnh trt, tip xc ny cng d sinh ra h quang in. 3.3. Cc yu t nh hng n in tr tip xc - Vt liu lm tip im: vt liu mm tip xc tt. - Kim loi lm tip im khng b xy ha. - Lc p tip im cng ln th s to nn nhiu tip im tip xc. - Nhit tip im cng cao th in tr tip xc cng ln. http://www.ebook.edu.vn 2

cng bi ging mn Kh c in - Din tch tip xc. Thng thng dng hp kim lm tip im. 4. Cc nguyn nhn h hng v cch khc phc * Cc nguyn nhn h hng a. n mn kim loi Trong thc t ch to d gia cng th no th b mt tip xc tip im vn cn nhng l nh li ti. Trong vn hnh hi nc v cc cht c hot tnh ha hc cao thm vo v ng li trong nhng l nh s gy ra cc phn ng ha hc to ra mt lp mng mng rt gin. Khi va chm trong qu trnh ng lp mng ny d b bong ra. Do b mt tip xc s b mn dn, hin tng ny gi l hin tng n mn kim loi. b. Oxy ha Mi trng xung quanh lm b mt tip xc b oxy ha to thnh lp oxit mng trn b mt tip xc, in tr sut ca lp oxit rt ln nn lm tng Rtx dn n gy pht nng tip im. Mc gia tng Rtx do b mt tip xc b oxy ha cn ty nhit . 2030oC c lp oxt dy khong 25.10-6mm c. in th ha kim loi Mi cht c mt in th ha hc nht nh. Ly H lm gc c in th m (-) th ta c bng mt s kim loi c in th ha hc nh bng sau: Hai kim loi c in th ha hc khc nhau khi tip xc s to nn mt cp hiu in th ha hc, gia chng c mt hiu in th. Nu b mt tip xc c nc xm nhp s c dng in chy qua, v kim loi c in th hc m hn s b n mn trc lm nhanh hng tip im. d. H hng do in Thit b in vn hnh lu ngy hoc khng c bo qun tt l xo tip im b hoen r yu i s khng lc p vo tip im. Khi c dng in chy qua, tip im d b pht nng gy nng chy, thm ch hn dnh vo nhau. Nu lc p tip im qu yu c th pht sinh tia la lm chy tip im. Ngoi ra, tip im b bn, r s tng in tr tip xc, gy pht nng dn n hao mn nhanh tip im. * Cch khc phc h hng i vi nhng tip xc c nh : nn bi mt lp m chng r hoc qut sn chng m. Khi thit k ta nn chn nhng vt liu : c in th ha hc ging nhau hoc gn bng nhau cho tng cp. Nn s dng cc vt liu khng b oxy ha lm tip im. M in cc tip im: vi tip im ng, ng thau thng c m thic, m bc, m km cn tip im thp thng c m caini, km,... Thay l xo tip im: nhng l xo r, yu lm gim lc p s lm tng in tr tip xc, cn lau sch tip im bng vi mm v thay th l xo nn khi lc nn cn qu yu. Kim tra sa cha ci tin: ci tin thit b dp h quang rt ngn thi gian dp h quang nu iu kin cho php. 5.H quang v cc phng php dp tt h quang 1. Khi nim http://www.ebook.edu.vn 3

cng bi ging mn Kh c in Trong cc kh c in dng ng ngt mch in (cu dao, contactor, rle...) khi chuyn mch s pht sinh hin tng phng in. Nu dng in ngt di 0,1A v in p ti cc tip im khong 250 - 300V th cc tip im s phng in m . Trng hp dng in v in p cao hn tr s trong bng sau s sinh ra h quang in. Vt liu lm tip im Platin Vng Bc Vonfram ng Than U(V) 17 15 12 17 12,3 18 - 22 I(A) 0,9 0,38 0,4 0.9 0,43 0,03

2. Tnh cht c bn ca phng in h quang - Phng in h quang ch xy ra khi cc dng in c tr s ln. - Nhit trung tm h quang rt ln v trong cc kh c c th n 6000 n 800000 K. - Mt dng in ti Catt ln (104 n 105)A/cm2. - St p Catt bng 10 n 20V v thc t khng ph thuc vo dng in. 3. Qu trnh pht sinh v dp h quang a) Qu trnh pht sinh h quang in: i vi tip im c dng in b, ban u khong cch gia chng nh khi in p t c tr s nht nh, v vy trong khong khng gian ny s sinh ra in trng c cng rt ln (3.107V/cm) c th lm bt in t t Catt gi l pht x t ng in t (gi l pht x ngui in t). S in t cng nhiu, chuyn ng di tc dng ca in trng lm ion ho khng kh gy h quang in. i vi tip im c dng in ln, qu trnh pht sinh h quang phc tp hn. Lc u m tip im, lc p gia chng c tr s nh nn s tip im tip xc dng in i qua t. Mt dng in tng ng k n hng chc nghn A/cm2, do ti cc tip im s pht nng s tng n mc lm cho , git kim loi c ko cng ra tr thnh cu cht lng v ni lin hai tip im ny, nhit ca cu cht lng tip tc tng, lc cu cht lng bc hi v trong khng gian gia hai tip im xut hin h quang in. V qu trnh pht nng ca cu thc hin rt nhanh nn s bc hi mang tnh cht n. Khi cu cht lng ct ko theo s mi mn tip im, iu ny rt quan trng khi ngt dng in qu ln hay qu trnh ng m xy ra thng xuyn. b) Qu trnh dp tt h quang in iu kin dp tt h quang l qu trnh ngc li vi qu trnh pht sinh h quang: H nhit h quang. Ko di h quang. Chia h quang thnh nhiu on nh. Dng nng lng bn ngoi hoc chnh n thi tt h quang. Mc in tr Shunt tiu th nng lng h quang Thit b dp tt h quang. H nhit h quang bng cch dng hi kh hoc du lm ngui, http://www.ebook.edu.vn 4

cng bi ging mn Kh c in dng vch ngn h quang c xt. Chia h quang thnh nhiu ct nh v ko di h quang bng cch dng vch ngn chia thnh nhiu phn nh v thi kh dp tt. Dng nng lng bn ngoi hoc chnh n thi tt h quang, nng lng ca n to p sut thi tt h quang. Mc in tr Shunt tiu th nng lng h quang (dng in tr mc song song vi hai tip im sinh h quang). 6. Lc in ng Lc in ng chnh l lc tc dng ca in trng v t trng. Lc in ng trong KC (c to ra t dng in ln, dng in ngn mch). Kha nang chu lc tac ong phat sinh khi co dong ien ngan mach chay qua la mot tieu chuan khong the thieu cua kh cu ien, goi la tnh on nh ien ong Trong 1 mch vng c s tc ng ca lc in ng lm bin dng mch vng 2 ch x lp Im khng ln F = kI b ngn mch Inm >> Im F tng ln lm cho thit b nhanh hng hn. II. CNG DNG V PHN LOI KH C IN 1. Cng dng Kh c in l nhng thit b in, c cu in dng iu khin cc qu trnh sn xut, bin i, truyn ti, phn phi nng lng in v cc dng nng lng khc. 2. Phn loi Khi nim iu khin theo ngha rng bao gm : iu chnh bng tay t ng, kim tra v bo v. Theo lnh vc s dng, cc kh c in c chia thnh 5 nhm, trong mi nhm li c rt nhiu chng loi khc nhau. Cc nhm l : a-Nhm kh c in phn phi nng lng in p cao, gm : Dao cch ly, my ngt du (nhiu du v t du), my ngt khng kh, my ngt t sn kh, my ngt chn khng cu chu (cu ch) , dao ngn mch, in khng , bin dng, bin in p b-Nhm kh c in phn phi nng lng in p thp, gm : My ngt t ng , my ngt bng tay, cc b i ni (cu dao, cng tc), cu ch c-Nhm kh c in iu khin : Cng tc t, khi ng t, cc b khng ch v iu khin, nt n , cng tc hnh trnh , cc b in tr iu chnh v m my, cc b khuch i in t, khuch i t, t p d-Nhm cc rle bo v : Rle dng in rle in p, rle cng sut, rle tng tr, rle thi gian e-Nhm kh c in dng trong sinh hot v chiu sng: cng tc, cm, phch cm, bn l, bp in 2.1. Phn loi theo cng dng + ng ngt mch in ca li in : cu dao, CB, cng tc + M my, iu chnh tc , iu chnh in p, dng in: cng tc t, khi ng t, b khng ch + Bo v li in, my in: cu ch , ptmt + Duy tr tham s in gi tr khng i: n p, thit b t ng iu chnh in p, dng in, tn s, tc , nhit + o lng :VOM , volt k, ampe k http://www.ebook.edu.vn 5

cng bi ging mn Kh c in 2.2. Phn loi theo in p + Kh c in cao th: Um 100KV + Kh c in trung th : 1000V Um Khi nim Nam chm in l mt dng c to t trng hay mt ngun sn sinh t trng hot ng nh t trng sinh ra bi cun dy c dng in ln chy qua. Cm ng t ca nam chm in c dn v to thnh ln nh vic s dng mt li dn t lm bng vt liu t mm c t thm ln v cm ng t bo ha cao. Khc vi nam chm vnh cu c cm ng t c nh, nam chm in c cm ng t c th thay i c nh vic iu khin dng in chy qua cun dy. Nam chm in ln u tin c pht minh bi nh in hc ngi Anh William Sturgeon (1783-1850) vo nm 1825. Nam chm in ca Sturgeon l mt li st non hnh mng nga c mt s vng dy in cun quanh. Khi cho dng in sinh ra bi mt pin nh chy qua, li st b t ha v cm ng t sinh ra mnh ht ln c mt hp st nng 7 ounce. Khi ngt dng in, t trng ca li cng bin mt

S nguyn l ca nam chm in u tin. Dng in cung cp bi ngun pin to ra t trng trong cun dy v c khuch i bi li dn t lm bng st non 2> ng dng ca nam chm Cc nam chm c ng dng x l nguyn liu, nng sn phm nng, tch vt liu trong cc dy chuyn sn xut, ... Khng ch trong ngnh cng nghip nng m cn cc n v quy m nh nh n v sn xut in t, my tnh, a CD, TV, ... l nhng ngnh ng dng nam chm. Ngoi vic b s dng trong ngnh cng nghip, nam chm cng c dng cha bnh. T trng ca nam chm, tng dng chy ca mu v do iu tr cc bnh th cht. Nam chm c s dng iu tr cc bin chng th cht nh huyt p, tng huyt p cc vn thn kinh, ... II. RLE T Rle in t lm vic trn nguyn l in t. Nu t mt vt bng vt liu st t (gi l phn ng hay np t) trong t trng do cun dy c dng in chy qua sinh ra. http://www.ebook.edu.vn 14

cng bi ging mn Kh c in T trng ny tc dng ln np mt lc lm np chuyn ng

Khi cung cp in cho cun dy, s to t trng chy trong mch t chnh. Lc ht in t sinh ra thng c lc ht l xo phn lc 7 np mch t c v pha li. ng vi mch t 1 chiu - xoay chiu c cc rle 1 chiu - xoay chiu. III. RLE NHIT (OVER LOAD OL) 1. Khi nim v cu to: Rle nhit l mt loi kh c bo v ng c v mch in khi c s c qu ti. Rle nhit khng tc ng tc thi theo tr s dng in v n c qun tnh nhit ln, phi c thi gian pht nng, do n lm vic c thi gian t vi giy n vi pht

Phn t pht nng 1 c u ni tip vi mch ng lc bi vt 2 v m phin lng kim 3. Vt 6 trn gi nha cch in 5 dng iu chnh mc un cong u t do ca phin 3. Gi 5 xoay quanh trc 4, tu theo tr s dng in chy qua phn t pht nng m phin lng kim cong nhiu hay t, y vo vt 6 lm xoay gi 5 m ngm n by 9. Nh tc dng l xo 8, y n by 9 xoay quanh trc 7 ngc chiu kim ng h lm m tip im ng 11 khi tip im tnh 12. Nt nhn 10 Reset Rle nhit v v tr ban u sau khi phin lng kim ngui tr v v tr ban u. 2. Nguyn l hot ng: Nguyn l chung ca Rle nhit l da trn c s tc dng nhit lm dn n phin kim loi kp. Phin kim loi kp gm hai l kim loi c h s gin n khc nhau (h s gin n hn km nhau 20 ln) ghp cht vi nhau thnh mt phin bng phng php cn nng hoc hn. Khi c dng in qu ti i qua, phin lng kim c t nng, un cong v pha kim loi c h s gin n b, y cn gt lm l xo co li v chuyn i h thng tip im ph. http://www.ebook.edu.vn 15

cng bi ging mn Kh c in Rle nhit lm vic tr li, phi i phin kim loi ngui v ko cn Reset ca Rle nhit.

3. Phn loi Rle nhit: Theo kt cu Rle nhit chia thnh hai loi: Kiu h v kiu kn. Theo yu cu s dng: Loi mt cc v hai cc. Theo phng thc t nng: - t nng trc tip: Dng in i qua trc tip tm kim loi kp. Loi ny c cu to n gin, nhng khi thay i dng in nh mc phi thay i tm kim loi kp, loi ny khng tin dng. - t nng gin tip: Dng in i qua phn t t nng c lp, nhit lng to ra gin tip lm tm kim loi cong ln. Loi ny c u im l mun thay i dng in nh mc ta ch cn thay i phn t t nng. Nhc dim ca loi ny l khi c qu ti ln, phn t t nng c th t n nhit kh cao nhng v khng kh truyn nhit km, nn tm kim loi cha kp tc c m phn t t nng b chy t. - t nng hn hp: Loi ny tng i tt v va t trc tip va t gin tip. N c tnh n nh nhit tng i cao v c th lm vic bi s qu ti ln. 4. Chn la Rle nhit c tnh c bn ca Rle nhit l quan h gia dng in ph ti chy qua v thi gian tc ng ca n (gi l c tnh thi gian - dng in, A - s). Mt khc, m bo yu cu gi c tui th lu di ca thit b theo ng s liu k thut cho ca nh sn xut, cc i tng bo v cng cn c tnh thi gian dng in. La chn ng Rle l sao cho ng c tnh A - s ca Rle gn st ng c tnh A - s ca i tng cn bo v. Nu chn thp qu s khng tn dng c cng sut ca ng c in, chn cao qu s lm gim tui th ca thit b cn bo v. Trong thc t, cch la chn ph hp l chn dng in nh mc ca Rle nhit bng dng in nh mc ca ng c in cn bo v, Rle s tc ng gi tr (1,2 1,3)Im. Bn cnh, ch lm vic ca ph ti v nhit mi trng xung quanh phi c xem xt.


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IV. CU CH 1. Khi nim v yu cu Cu ch l mt loi kh c in dng bo v thit b v li in trnh s c ngn mch, thng dng bo v cho ng dy dn, my bin p, ng c in, thit b in, mch in iu khin, mch in thp sng. Cu ch c c im l n gin, kch thc b, kh nng ct ln v gi thnh h nn c ng dng rng ri. Cc tnh cht v yu cu ca cu ch: - Cu ch c c tnh lm vic n nh, khng tc ng khi c dng in m my v dng in nh mc lu di i qua. - c tnh A - s ca cu ch phi thp hn c tnh ca i tng bo v. - Khi c s c ngn mch, cu ch tc ng phi c tnh chn lc. - Vic thay th cu ch b chy phi d dng v tn t thi gian. 2. cu to v nguyn l hot ng 1. Cu to Cu ch bao gm cc thnh phn sau: + Phn t ngt mch: y chnh l thnh phn chnh ca cu ch, phn t ny phi c kh nng cm nhn c gi tr hiu dng c dng in qua n. Phn t ny c gi tr in tr sut b (thng bng bc, ng hay cc vt liu dn c gi tr in tr sut nh ln cn vi cc gi tr nu trn...). Hnh dng ca phn t c th dng l mt dy (tit din trn), dng bng mng. + Thn ca cu ch: Thng bng thu tnh, ceramic (s gm) hay cc vt liu khc tng ng. Vt liu to thnh thn ca cu ch phi m bo c hai tnh cht: - C bn c kh. - C bn v iu kin dn nhit v chu ng c cc s thay i nhit t ngt m khng h hng. + Vt liu lp y (bao bc quanh phn t ngt mch trong thn cu ch): Thng bng vt liu Silicat dng ht, n phi c kh ngng hp th c nng lng sinh ra do h quang v phi m bo tnh cch in khi xy ra hin tng ngt mch. + Cc u ni: Cc thnh phn ny dng nh v c nh cu ch trn cc thit b ng ngt mch; ng thi phi m bo tnh tip xc in tt. 2. Nguyn l hot ng c tnh c bn ca cu ch l s ph thuc ca thi gian chy t vi dng in chy qua (c tnh Ampe - giy). c tc dng bo v, ng Ampe - giy ca cu ch ti mi im phi thp hn c tnh ca i tng cn bo v. http://www.ebook.edu.vn 17

cng bi ging mn Kh c in + i vi dng in nh mc ca cu ch: Nng lng sinh ra do hiu ng Joule khi c dng in nh mc chy qua s to ra mi trng v khng gy nn s nng chy, s cn bng nhit s c thit lp mt gi tr m khng gy s gi ho hay ph hng bt c phn t no ca cu ch. + i vi dng in ngn mch ca cu ch: S cn bng trn cu ch b ph hu, nhit nng trn cu ch tng cao v dn n s ph hu cu ch: 3. Phn loi, k hiu, cng dng Cu ch dng trong li in h th c nhiu hnh dng khc nhau, trong s nguyn l ta thng k hiu cho cu ch theo mt trong cc dng sau:

Cu ch c th chia thnh hai dng c bn, ty thuc vo nhim v: + Cu ch loi g: Cu ch dng ny ch c kh nng ngt mch, khi c s c hay qu ti hay ngn mch xy ra trn ph ti. + Cu ch loi a: Cu ch dng ny ch c kh nng bo v duy nht trng thi ngn mch trn ti. Mun phn bit nhim v lm vic ca cu ch, ta cn cn c vo c tuyn Ampe - giy (l ng biu din m t mi quan h gia dng in qua cu ch v thi gian ngt mch ca cu ch). Gi ICC: Gi tr dng in ngn mch. IS: Gi tr dng in qu ti. Vi cu ch loi g: Khi c dng ICC qua mch n phi ngt mch tc th, v khi c dng IS qua mch cu ch khng ngtm ch tc th m duy tr mt khong thi gian mi ngt mch (thi gian ngt mch v gi tr dng IS t l nghch vi nhau). Do nu quan st hai c tnh Ampe - giy ca hai loi cu ch a v g; ta nhn thy c tnh Ampe - giy ca cu ch loi a nm xa trc thi gian (trc tung) v cao hn c tnh Ampe - giy ca cu ch loi g.

c tnh Ampe - giy ca cc loi cu ch 4. Cc c tnh in p ca cu ch - in p nh mc l gi tr in p hiu dng xoay chiu xut hin hai u cu ch (khi cu ch ngt mch), tn s ca ngun in trong phm vi 48Hz n 62Hz.. - Dng in nh mc l gi tr hiu dng ca dng in xoay chiu m cu ch c th ti lin tc thng xuyn m khng lm thay i c tnh ca n. - Dng in ct cc tiu l gi tr nh nht ca dng in s c m dy ch c kh nng ngt mch. Kh nng ct nh mc l gi tr cc i ca dodngf in ngn mch m cu ch c th ct. http://www.ebook.edu.vn 18


V. THIT B CHNG R IN 1. Khi nim C th ngi rt nhy cm vi dng in, v d: dng in nh hn 10mA th ngi c cm gic kim chm; ln hn 10mA th cc c bp co qup; dng in n 30mA a n tnh trng co tht, ngt th v cht ngi. Khi thit b in b h hng r in, chm mt m ngi s dng tip xc vo s nhn dng in i qua ngi xung t in p ngun. Trong trng hp ny, CB v cu ch khng th tc ng ngt ngun in vi thit b, gy nguy him cho ngi s dng. Nu trong mch in c s dng thit b chng dng in r th ngi s dng s trnh c tai nn do thit b ny ngt ngun in ngay khi dng in r xut hin. 2. Cu to Thit b chng dng in r hot ng trn nguyn l bo v so lch, c thc hin trn c s cn bng gia tng dng in vo v tng dng in i ra tit b tiu th in. Khi thit b tiu th in b r in, mt phn ca dng in c r nhnh xung t, l dng in r. Khi c dng in v theo ng dy trung tnh rt nh v rle so lch s d tm s mt cn bng ny v iu khin ct mch in nh thit b bo v so lch. Thit b bo v so lch gm hai phn t chnh: - Mch in t dng hnh xuyn m trn c qun cc cun dy ca phn cng sut (dy c tit din ln), chu dng cung cp cho thit b tiu th in. - Rle m mch cung cp c iu khin bi cun dy o lng (dy c tit din b) cng c t trn hnh xuyn ny, n tc ng ngt cc cc. 3. Nguyn tc hot ng

Ch thch: - I1: Ddng in i vo thit b tiu th in. - I2: Dng in i t thit b tiu th in ra. - Isc: Dng in s c. - In: Dng in i qua c th ngi. 1: Thit b o lng s cn bng 2: C cu nh. http://www.ebook.edu.vn 19

cng bi ging mn Kh c in 3: Li t hnh vnh xuyn Trng hp thit b in khng c s c I1 = I2 Trng hp thit b in c s c I1 - I2 = Isc

I1 - I2 do xut hin mt s cn bng trong hnh xuyn t, dn n cm ng mt dng in trong cun dy d tm, a n tc ng rle v kt qu lm m mch in. Ch thch: - I1: Dng in i quapha 1. - I2: Dng in i quapha 2. - I3: Dng in i qua pha 3. - I0: Dng in i qua dy trung tnh. 1: C cu nh. 2: Li t hnh vnh xuyn. Trng hp thit b in khng c s c: I1=I2=I3=I0=0 T thng tng trong mch t hnh xuyn bng 0, do s khng c dng in cm ng trong cun dy d tm. Trng hp thit b c s c: I1-I2-I3-I0=0 T thng tng trong mch t hnh xuyn khng bng 0, do s c dng in cm ng trong cun dy d tm, vy cun dy d tm s tc ng m cc cc in. 4. S TC NG CA THIT B CHNG DNG IN R 4.1. S tc ng tin cy ca RCD - RCD tc ng nhy v tin cy. - Dng in tc ng r thc t lun thp hn dng tc ng r danh nh (ghi trn nhn hiu ca RCD) khong 20 40% khi dng in r xut hin tng dn hay t ngt. - Thi gian tc ng thc t u nh hn thi gian tc ng c nh sn xut quy nh (ghi trn nhn hiu) khong 20 80%. Thng thng thi gian tc ng ct mch c ghi trn nhn hiu ca RCD l 0,1s v thi gian tc ng ct mch thc t nm trong khong 0,02 0,08s. 4.2. S tc ng c tnh chn lc ca RCD bo v h thng in - s in. - Khi xut hin dng in r ln on ng dy in hoc ph ti, RCD c lp t gn nht s tc ng ct mch, tch on dy hoc ph ti b r http://www.ebook.edu.vn 20

cng bi ging mn Kh c in in ra khi h thng cung cp in. Nh vy m bo tnh chn lc, vic cung cp in khng nh hng n phn cn li. - Nu RCD lp t khng ng yu cu k thut th RCD s khng tc ng ct mch khi xut hin dng in r phn ng dy hay ph ti tng ng vi chng, hoc tc ng khng ng yu cu ra. a) Kh nng chn lc tng hp Kh nng chn lc tng hp l nhm loi tr duy nht thit b c s c. t c kh nng ny phi tho mn hai iu kin: - Dng in so lch d nh mc ca RCD pha trn phi c gi tr ln hn dng in so lch d nh mc ca RCD pha di. - Thi gian ti thiu khng lm vic ca RCD pha trn phi c gi tr ln hn thi gian ti thiu khng lm vic ca RCD pha di. b) Kh nng chn lc tng phn Tnh chn lc c gi l tng phn v n khng tip snhn i vi mt s gi tr dng in s c. Tnh chn lc c tho mm khi cc h qu ca mt s s c c th ko theo ngt in tng phn hay ngt in ton b h thng cung cp in. V d v chn lc tng phn:

H thng cung cp in cng nghip vi kh nng chn lc tng 3 mc chm (tr) mc 1: chm 200ms; mc 2: chm 50ms; mc 3 khng c thi gian tr. VI. BIN P O LNG 1.BU Bin in p o lng dng bin i in p t tr s ln xung tr s thch hp (100V hay 100/ V) cung cp cho cc dng c o lng, rle v t ng ha. Nh vy cc dng c th cp c tch khi mch in cao p nn rt an ton cho ngi. Cng v an ton, mt trong nhng u ra ca cun dy th cp phi c ni t. Cc dng c pha th cp ca BU c in tr rt ln nn c th coi BU lm vic ch khng ti. in p nh mc ca cun dy s cp, chnh l in p nh mc ca BU. H s bin in p nh mc : http://www.ebook.edu.vn 21

cng bi ging mn Kh c in Sai s : Sai s in p tng i Sai s gc ca BU Cng sut nh mc ca BU

2. My bin dng BI My bin dng in (BI) hay bin dng l thit in dng bin i dng in c tr s ln v in p cao xung dng in c tr s tiu chun 5A hoc 1A, in p an ton cung cp cho mch o lng, iu khin v bo v. mch in xoay chiu, nguyn l lm vic ca bin dng tng t nh MBA. Cun dy s cp ca bin dng c s vng rt nh, c khi ch mt vi vng, cn cun th cp c s vng nhiu hn v lun c ni t phng khi cch in gia s v th cp b chc thng th khng nguy him cho dng c pha th cp v ngi phc v. Ph ti th cp ca bin dng in rt nh v vy c th coi bin dng lun lm vic trng thi ngn mch. Trong trng hp khng c ti phi ni t cun th cp trnh qu in p cho n. S u dy ca bin dng trong mch in c trnh by nh hnh .Ti ca bin dng c u vo cun th cp w2 ca n v mt u c ni t, th t u v cui ca cc cun dy my bin dng thng c phn bit , u cun dy nh du sao


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Cc thng s in p nh mc l tr s ca in p dy ca li in m bin dng lm vic,in p ny quyt nh cch in gia pha s cp v th cp ca bin dng. Dng in nh mc pha s cp v th cp l dng lm vic di hn theo pht nng c d tr. H s bin i : Km = I1ddm/I2ddm Sai s ca bin dng in K .I I I % = dm 2 1 100 I1 Chn BI Theo v tr t: Trong nh hoc ngoi tri. Cp chnh xc: Ty theo mc ch s dng (cng t cp chnh xc 0,5). in p: UmBI U HT Dng in: ImBI Icb/1,2, V BI cho php qu ti lu di 20% Ph ti ca BI Theo v tr t: Trong nh hoc ngoi tri. Cp chnh xc: Ty theo mc ch s dng (cng t cp chnh xc 0,5). in p: UmBI U HT Dng in: ImBI Icb/1,2, V BI cho php qu ti lu di 20% Ph ti ca BI


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BI 4

KH C IU KHIN

I. CNG TC T 1. khi nim Contactor l mt kh c in dng ng ngt cc tip im, to lin lc trong mch in bng nt nhn. Nh vy khi s dng Contactor ta c th iu khin mch in t xa c ph ti vi in p n 500V v dng l 600A (v tr iu khin, trng thi hot ng ca Contactor rt xa v tr cc tip im ng ngt mch in). Phn loi Contactor tu theo cc c im sau: - Theo nguyn l truyn ng: ta c Contactor kiu in t (truyn in bng lc ht in t), kiu hi p, kiu thu lc. Thng thng s dng Contactor kiu in t. - Theo dng dng in: Contactor mt chiu v Contactor xoay chiu (Contactor 1 pha v 3 pha). 2. Cu to Contactor c cu to gm cc thnh phn: C cu in t (nam chm in), h thng dp h quang, h thng tip im (tip im chnh v ph). a) Nam chm in: Nam chm in gm c 4 thnh phn: - Cun dy dng to ra lc ht nam chm. - Li st (hay mch t) ca nam chm gm hai phn: Phn c nh v phn np di ng. Li thp nam chm c th c dng EE, EI hay dng CI. - L xo phn lc c tc dng y phn np di ng tr v v tr ban y khi ngng cung cp in vo cun dy.

Trng thi nam chm cha ht Trng thi nam chm to lc ht b) H thng dp h quang in: Khi Contactor chuyn mch, h quang in s xut hin lm cc tip im b chy, mn dn. V vy cn c h thng dp h quang gm nhiu vch ngn lm bng kim loi t cnh bn hai tip im tip xc nhau, nht l cc tip im chnh ca Contactor. c) H thng tip im ca Contactor H thng tip im lin h vi phn li t di ng qua b phn lin ng v c. Tu theo kh nng ti dn qua cc tip im, ta c th chia cc tip im cu Contactor thnh hai loi: http://www.ebook.edu.vn 24

cng bi ging mn Kh c in - Tip im chnh: C kh nng cho dng in ln i qua (t 10A n vi nghn A, th d khong 1600A hay 2250A). Tip im chnh l tip i thng h ng li khi cp ngun vo mch t ca Contactor lm mch t Contactor ht li. - Tip im ph: C kh nng cho dng in i qua cc tip im nh hn 5A. Tip im ph c hai trng thi: Thng ng v thng h. Tip im thng ng l loi tip im trng thi ng (c lin lc vi nhau gia hai tip im) khi cun dy nam chm trong Contactor trng thi ngh (khng c cung cp in). Tip im ny h ra khi Contactor trng thi hot ng. Ngc li l tip im thng h. Nh vy, h thng tip im chnh thng c lp trong mch in ng lc, cn cc tip im ph s lp trong h thng mch iu khin (dng iu khin vic cung cp in n cc cun dy nam chm ca cc Contactor theo quy trnh nh trc). Theo mt s kt cu thng thng ca Contactor, cc tip e ph c th c lin kt c nh v s lng trong mi b Contactor, tuy nhin cng c mt vi nh sn xut ch b tr c nh s tip im chnh trn mi Contactor, cn cc tip im ph c ch to thnh nhng khi ri n l. Khi cn s dng ta ch ghp thm vo trn Contactor, s lng tip im ph trong trng hp ny c th b tr tu . 3. Nguyn l hot ng ca Contactor

Khi cp ngun in bng gi tr in p nh mc ca Contactor vo hai u ca cun dy qun trn phn li t c nh th lc t to ra ht phn li t di ng hnh thnh mch t kn (lc t ln hn phn lc ca l xo), Contactor trng thi hot ng. Lc ny nh vo b phn lin ng v c gia li t di ng v h thng tip im lm cho tip im chnh ng li, tip im ph chuyn i trng thi (thng ng s m ra, thng h s ng li) v duy tr trng thi ny. Khi ngng cp ngun cho cun dy th Contactor trng thi ngh, cc tip im tr v trng thi ban u. Cc k hiu dng biu din cho cun dy (nam chm in) trong Contactor v cc loi tip im. C nhiu tiu chun ca cc quc gia khc nhau, dng biu din cho cun dy v tip dim ca Contactor Cun dy Tip im thng ng Tip im thng h


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cng bi ging mn Kh c in4. cc thng s c bsrn ca contactor 4.1. in p nh mc in p nh mc ca Contactor Um l in p ca mch in tng ng m tip im chnh phi ng ngt, chnh l in p t vo hai u cun dy ca nam chm in sao cho mch t ht li. Cun dy ht c th lm vic bnh thng in p trong gii hn (85 105)% in p nh mc ca cun dy. Thng s ny c ghi trn nhn t hai u cun dy Contactor, c cc cp in p nh mc: 110V, 220V, 440V mt chiu v 127V, 220V, 380V, 500V xoay chiu. 4.2. Dng in nh mc Dng in nh mc ca Contactor Im l dng in nh mci qua tip im chnh trong ch lm vic lu di. Dng in nh mc ca Contactor h p thng dng c cc cp l: 10A, 20A, 25A, 40A, 60A, 75A, 100A, 150A, 250A, 300A, 600A. Nu t trong t in th dng in nh mc phi ly thp hn 10% v lm km mt, dng in cho php qua Contactor cn phi ly thp hn na trong ch lm vic di hn. 4.3. Kh nng ct v kh nng ng Kh nng ct ca Contactor in xoay chiu t bi s n 10 ln dng in nh mc vi ph ti in cm. Kh nng ng: Contactor in xoay chiu dng khi ng ng c in cn phi c kh nng ng t 4 n 7 ln Im. 4.4. Tui th ca Contactor Tui th ca Contactor c tnh bng s ln ng m , sau s ln ng m y th Contactor s b hng v khng dng c. 4.5. Tn s thao tc L s ln ng ct Contactor trong mt gi: C cc cp: 30, 100, 120, 150, 300, 600, 1200, 1500 ln/gi. 4.6. Tnh n nh lc in ng Tip im chnh ca Contactor cho php mt dng in ln i qua (khong 10 ln dng in nh mc) m lc in ng khng lm tc ri tip im th Contactor c tnh n nh lc in ng. 4.7. Tnh n nh nhit `Contactor c tnh n nh nhit ngha l khi c dng in ngn mch chy qua trong mt khong thi gian cho php, cac tip im khng b nng chy v hn dnh li. II. KHI NG T 1. Khi qut v cng dng Khi ng t l mt loi kh c in dng iu khin t xa vic ng ngt, o chiu v bo v qu ti (nu c lp thm rle nhit) cc ng c khng ng b ba pha rto lng sc. Khi ng t c mt Contactor gi l khi ng t n thng ng ngt ng c in. Khi ng t c hai Contactor l khi ng t kp dng thay i chiu quay ca ng c gi l khi ng t o chiu. Mun bo v ngn mch phi lp thm cu ch. 2. Cc yu cu k thut ng c in khng ng b ba pha c th lm vic lin tc c hay khn gtu thuc vo mc tin cy ca khi ng t. Do kh ng t cn phi tho mn cc yu cu k thut sau:


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cng bi ging mn Kh c in - Tip im c bn chu mi mn cao. - Kh nng ng - ct cao. - Thao tc ng - ct dt khot. - Tiu th cng sut t nht. - Bo v ng c khng b qu ti lu di (c Rle nhit). - Tho mn iu khi ng (dng in khi ng t 5 n 7 ln dng in nh mc). 3. Kt cu v nguyn l lm vic 3.1. Khi ng t thng c phn chia theo: - in p nh mc ca cun hy ht: 36V, 127V, 220V, 380V, 500V. - Kt cu bo v chng cc tc ng bi mi trng xung quanh: h, bo v, chng bi, nc n - Kh nng lm bin i chiu quay ng c in: Khng o chiu quay v o chiu quay. - S lng v loi tip im: Thng h, thng ng. 3.2. Nguyn l lm vic ca khi ng t a) Khi ng t v hai nt nhn: Khi cung cp in p cho cun dy bng nhn nt khi ng M, cun hy Contactor c in ht li thp di ng v mch t khp kn li: Lm ng cc tip i chnh khi ng ng c v ng tip im ph thng h duy tr mch iu khin khi bung tay khi nt nhn khi ng. Khi nhn nt dng D, khi ng t b ngt in, di tc dng ca l xo nn lm phn li di ng tr v v tr ban u; cc tip im tr v trng thi thng h. ng c dng hot ng. Khi c s c qu ti ng c, Rle nhit s thao tc lm ngt mch in cun dy, do cng ngt khi ng t v dng ng c in. b) Khi ng t o chiu v ba nt nhn Khi nhn nt nhn MT cun dy Contactor T c in ht li thp di ng v mch t khp kin li; lm ng cc tip im chnh T khi ng ng c quay theo chiu thun v ng tip im ph thng h T duy tr mch iu khin khi bung tay khi nt nhn khi ng MT. o chiu quay ng c, ta nhn nt nhn MN cun dy Contactor T mt in, cun dy Contactor N c in ht li thp di ng v mch t khp kn li; lm ng cc tip im chnh N, lc ny trn mch ng lc o hai dy trong ba pha in lm cho ng http://www.ebook.edu.vn 27

cng bi ging mn Kh c in c o chiu quay ngc li v tip im ph thng h N duy tr mch iu khin khi bung tay khi nt nhn khi ng MN. Qu trnh o chiu quay c lp li nh trn. Khi nhn nt dng D, khi ng t N (hoc T) b ngt in, ng c dng hot ng. Khi c s c qu ti ng c, Rle nhit s thao tc lm ngt mch in cun dy, do cng ngt khi ngt v dng ng c in. 4. La chn v lp rp khi ng t Hin nay nc ta, ng c khng ng b ba pha rto lng sc c cng sut t 0,6 n 100KW c s dng rng ri. iu khin vn hnh chng, ta thng dng khi ng t. V vy thun li cho vic la chn khi ng t, nh sn xut thng khng nhng ch cho cng dng in sut nh mc m cn cho c cng sut ca ng c in m khi ng t phc v ng vi cc in p khc nhau. khi ng t lm vic tin cy, khi lp t cn phi bt cht cng khi ng t trn mt mt phng ng ( nghing cho php so vi trc thng ng 50), khng cho php bi m vo cc tip im v cc b phn ng. Sauk hi lp t khi ng t v trc khi vn hnh, phi kim tra: - Cho cc b phn chuyn ng bng tay khng b kt, vng. - in p iu khin phi ph hp in p nh mc ca cun dy. - Cc tip im phi tip xc u v tt. - Cc dy u in phi theo ng s iu khin. - Rle nhit phi t khi ng t cn t km theo cu ch bo v.III. Rle trung gian 1. Khi nim v cu to Rle trung gian l mt kh c in dng trong lnh vc iu khin t ng, c cu kiu in t. Rle trung gian ng vai tr iu khin trung gian gi cc thit b iu khin (Contactor, Rle thi gian...). Rle trung gian gm: Mch t ca nam chm in, h thng tip im chu dng in nh (5A), v bo v v cc chn ra tip im. 2. Nguyn l hot ng Nguyn l hot ng ca Rle trung gian tng t nh nguyn l hot ng ca Contactor. Khi cp in p bng gi tr in p nh mc vo hai u cun dy ca Rle trung gian (ghi trn nhn), lc in t ht mch t kn li, h thng tip im chuyn i trng thi v duy tr trng thi ny (tip im thng ng h ra, tip im thng h ng li). Khi ngng cp ngun, mch t h, h thng tip im tr v trng thi ban u. im khc bit gia Contactor v Rle c th tm lc nh sau: - Trong Rle ch c duy nht mt loi tip im c kh nng ti dng in nh, s dng cho mch iu khin (tip im ph). - Trong Rle cng c cc loi tip im thng ng v tip im thng h, tuy nhin cc tip im khng c bung dp h quang (khc vi h thng tip im chnh trong Contactor hay CB). Cc k hiu dng cho Rle trung gian: Trong qu trnh lp rp cc mch iu khin dng Rle hay trong mt s mch in t cng nghip, ta thng gp cc k hiu sau y: - K hiu SPDT:


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cng bi ging mn Kh c in K hiu ny c vit tt t thut ng SING POLE DOUBLE THROW, Rle mang k hiu ny c mt cp tip im, gm tip im thng ng v thng h, cp tip i ny c mt u chung.

SPDT

SPST

DPST

- K hiu SPST: K hiu ny c vit tt t thut ng SING POLE SINGE THROW, Rle mang k hiu ny gm c mt tip im thng h. - K hiu DPST c vit tt t thut ng DOUBLE POLE SINGE THROW, Rle mang k hiu ny gm c hai tip im thng h. Ngoi ra, cc Rle khi c lp ghp trong t iu khin thng c lp trn cc chn ra. Tu theo s lng chn ra c cc ku khc nhau: 8 chn, 11 chn, 14 chn...IV. RLE THI GIAN 1. Khi nim Rle thi gian l mt kh c in dng trong lnh vc iu khin t ng, vi vai tr iu khin trung gian gia cc thit b iu khin theo thi gian nh trc. Rle thi gian gm: Mch t ca nam chm in, b nh thi gian lm bng linh kin in t, h thng tip im chu dng in nh ( 5A), v bo v cc chn ra tip im. Tu theo yu cu s dng khi lp rp h thng mch iu khin truyn ng, ta c hai loi Rle thi gian: Rle thi gian ON DELAY, Rle thi gian OFF DELAY. 2.Nguyn tc hot ng a) Rle thi gian ON DELAY

K hiu: Cun dy in p t vo hai u cun dy Rle thi gian c ghi trn nhn, thng thng 110V, 220V... - H thng tip im: Tip im tc ng khng tnh thi gian: Tip im ny hot ng tng t cc tip im ca Rle trung gian. + Thng ng: + Thng m: * Tip im tc ng c tnh thi gian: + Tip im thng Tip im thng ng, m chm, ng nhanh + Tip im thng ng, m chm, ng nhanh: http://www.ebook.edu.vn 29


* Nguyn l hot ng: Khi cp ngun vo cun dy ca Rle thi gian ON DELAY, cc tip i tc ng khng tnh thi gian chuyn i trng thi tc thi (thng g h ra, thng h ng li), cc tip i tc ng c tnh thi gian khng i. Sau khong thi gain nh trc, cc tip im tc ng c tnh thi gian s chuyn trng thi v duy tr trng thi ny. Khi ngng cp ngun vo cun dy, tt c cc tip im tc thi tr v trng thi ban u. Sau y l s chn ca Rle thi gian ON DELAY:

b) Rle thi gian OFF DELAY

K hiu: Cun dy in p t vo hai u cun dy Rle thi gian c ghi trn nhn, thng thng 110V, 220V... - H thng tip im: Tip im tc ng khng tnh thi gian: Tip im ny hot ng tng t cc tip im ca Rle trung gian. + Thng ng: + Thng m: * Tip im tc ng c tnh thi gian: + Tip im thng m, ng chm, m nhanh:

+ Tip im thng ng, m chm, ng nhanh:

* Nguyn l hot ng: Khi cp ngun vo cun dy ca Rle thi gian OFF DELAY, cc tip im tc ng tc thi v duy tr trng thi ny. Khi ngng cp ngun vo cun dy, tt c cc tip im tc ng khng tnh thi gian tr v trng thi ban u. Tip sau mt khong thi gian nh trc, cc tip im tc ng c tnh thi gian s chuyn v trng thi ban u. http://www.ebook.edu.vn 30


V. MT S S CN BN V NGUYN L IU KHIN, VN HNH NG C. 1. Mch in khi ng dng mt ng c kb 3 pha a) Nguyn l: Dng mch khi ng mt ng c KB 3 pha, c tip im duy tr ng c lm vic, sau dng ng c. b) S mch c) Th t thc hin: - Nhn nt S2, Contactor K1 c in, cc tip im chnh ng li, ng c hot ng, cc tip im ph thay i trng thi, tip im ph thng ng h ra lm cho n H1 tt, tip i ph thng h ng li duy tr ngun cho Contactor K1 v n H2.

Hnh 1: S mch in khi ng - dng mt ng c KB 3 pha2. Mch in khi ng th t hai ng c kb 3 pha a) Nguyn l Dng mch khi ng th t hai ng c KB 3 pha. ng c 1 (iu khin bi Contactor K1) chy trc, sau ng c 2 (iu khin bi Contactor K2) chy theo. Nu c s tc ng nhm lm, mch in khng hot ng. Cui cng dng c hai ng c. b) S mch: c) Th t thc hin: Nhn S3, ng c M1 hot ng, n H1 sng. Nhn S4, ng c M2 hot ng, n H2 sng. Nhn S2, dng ng c M2, n H2 tt. - Nhn S1, dng ng c M1, dng ton b mch iu khin, n H1 tt


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cng bi ging mn Kh c in Hnh 2: S mch khi ng th t hai ng c KB ba pha 3. Mch in o chiu ng c kb ba pha a) Nguyn l: o chiu quay ng c KB ba pha bng cch o hai trong ba dy ngun trc khi a ngun vo ng c. Mch in ny dng iu khin ng c KB ba pha lm vic hai chiu quay, sau dng ng c. b) S mch: c) Th t thc hin: - Nhn S2, ng c hot ng theo chiu thun, n H1 sng. - Nhn S3, ng c hot ng theo chiu nghch, n H1 tt, n H2 sng. - Nhn S1, dng ton b mch iu khin, ng c ngng hot ng

Hnh 3: S mch o chiu ng c KB ba pha 4. Mch in khi ng mt ng c kb 3 pha t ng dng a) Nguyn l: Dng mch khi ng mt ng c KB 3 pha, c tip im duy tr ng c lm vic, sau thi gian lm vic nh trn Timer, tip im thng ng m chm ca Timer h ra, ng c dng. b) S mch: c) Th t thc hin: - Nhn S2, ng c hot ng, n H1 tt, n H2 sng. - Rle thi gian KTON c in v bt u tnh thi gian ng c lm vic. Khi ht khong thi gain nh, tip im thng ng KTON h ra lm ngng cp in cho Contactor K1, ng c ngng hot ng n H1 sng, n H2 tt. - Nhn S1 dng ng c khn cp.


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Hinh 4: S mch in khi ng - dng mt ng c KB 3 pha 5. Mch in t ng khi ng theo th t ca hai ng c kb 3 pha a) Nguyn l Mch in s dng T ON. Dng mch khi ng th t hai ng c KB 3 pha. ng c 1 (iu khin bi Contactor K1) khi ng trc, sau thi gian khi ng ca ng c th tip im thng h ng chm li ca Rle thi gian TON ng li ng c (iu khin bi Contactor 2) khi ng. Cui cng dng c hai ng c, ta nhn S1. b) S mch: c) Th t thc hin: - Nhn S1 ng c M1 hot ng n H1 sng. - Rle thi gian KTON chuyn trng thi, ng c M2 hot ng, n H2 sng. - Nhn S1 dng c hai ng c.

Hnh 5: S mch khi ng th t hai ng c KB ba pha


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