Chuong 2

CHNG 2 TIP XC IN

KHI NIM V TIP XC IN Ch tip gip gia hai vt dn in cho dng in chy t vt dn ny sang vt dn kia gi l tip xc in. B mt ch tip gip ca cc vt dn in gi l b mt tip xc in.

PHN LOI TIP XC INDa vo mi lin kt tip xc, ta chia tip xc in ra cc dng sau : Tip xc c nh : l loi tip xc khng tho lp gia 2 vt dn, c lin kt bng bulng, inh vit, inh riv,... Tip xc ng m : l tip xc m c th lm cho dng in chy hoc ngng chy t vt ny sang vt khc (nh cc tip im trong thit b ng ct). Tip xc trt : l vt dn in ny c th trt trn b mt ca vt dn in kia (v d nh chi than trt trn vnh gp my in).

PHN LOI TIP XC INDa vo hnh dng ch tip xc, ta chia tip xc in ra cc dng sau : Tip xc im: l hai vt tip xc vi nhau ch mt im hoc trn b mt din tch vi ng knh rt nh (nh tip xc hai hnh cu vi nhau, hnh cu vi mt phng, hnh nn vi mt phng,...) Tip xc ng: l hai vt dn tip xc vi nhau theo mt ng thng hoc trn b mt rt hp (nh tip xc hnh tr vi mt phng, hnh tr vi tr,...) Tip xc mt: l hai vt dn in tip xc vi nhau trn b mt rng(v d tip xc mt phng vi mt phng,...).

CC YU V TIP XC IN Cc yu cu i vi tip xc in ty thuc cng dng, iu kin lm vic, tui th yu cu ca thit b v cc yu t khc. Mt yu t ch yu nh hng ti tin cy lm vic v nhit pht nng ca tip xc in l in tr tip xc Rtx.

IN TR TIP XC

I CNG V TIP XC IN Xt khi t hai vt dn tip xc nhau, ta s c din tch b mt tip xc : Sbk= a . l.

I CNG V TIP XC IN Nhng trn thc t din tch b mt tip xc thc nh hn nhiu a.l v gia hai b mt tip xc d gia cng th no th vn c nhp nh, khi cho tip xc hai vt vi nhau th ch c mt s im trn tip gip tip xc. Do din tch tip xc thc nh hn nhiu din tch tip xc biu kin Sbk= a.l.

I CNG V TIP XC IN Din tch tip xc cn ph thuc vo lc p ln trn tip im v vt liu lm tip im, lc p cng ln th din tch tip xc cng ln.Din tch tip xc thc mt im(nh mt cu tip xc vi mt phng) xc nh bi: Trong : F l lc p vo tip im [kg]. d l ng sut chng dp nt ca vt liu lm tip im [kg/cm2].

I CNG V TIP XC IN Nu tip xc n im th din tch s ln ln n ln so vi biu thc (2.1).Dng in chy t vt ny sang vt khc ch qua nhng im tip xc, nh vy dng in cc ch tip xc s b tht hp li, dn ti in tr nhng ch ny tng ln.

I CNG V TIP XC IN in tr tip xc ca tip im kiu bt k tnh theo cng thc:K: h s ph thuc vt liu v tnh trng b mt tip im ( theo bng tra).m: h s ph thuc s im tip xc v kiu tip xc vi :Tip xc mt m = 1 Tip xc ng m = 0,7 Tip xc im m = 0,5

I CNG V TIP XC IN Ngoi cng thc (2.2) l cng thc kinh nghim, ngi ta cn dng phng php gii tch dn gii rt ra cng thc tnh in tr tip xc im:Trong : : in tr sut ca vt dn [.cm].n: s im tip xc.F: lc nn [kg].

I CNG V TIP XC IN Do vy r rng in tr tip xc ca tip im nh hng n cht lng ca thit b in, in tr tip xc ln lm cho tip im pht nng. Nu pht nng qu mc cho php th tip im s b nng chy, thm ch b hn dnh. Trong cc tip im thit b in mong mun in tr tip xc c gi tr cng nh cng tt, nhng do thc t c nhiu yu t nh hng n Rtx nn khng th gim Rtx cc nh c nh mong mun.

CC YU T NH HNG IN TR TIP XC Vt liu lm tip imLc p tip imHnh dng ca tip imNhit ca tip imTnh trng b mt tip xcMt dng in

TIP IM THIT B IN

VT LIU LM TIP IM tha mn tt cc iu kin lm vic khc nhau ca tip im thit b in th vt liu lm tip im phi c c nhng yu cu c bn sau: C dn in cao (gim Rtx v chnh in tr ca tip im). Dn nhit tt (gim pht nng cc b ca nhng im tip xc). Khng b oxy ha (gim Rtx tng n nh ca tip im).

VT LIU LM TIP IM C kt tinh v nng chy cao (gim mi mn v in v gim s nng chy hn dnh tip im ng thi tng tui th tip im). C bn c cao (gim mi mn c kh gi nguyn dng b mt tip xc v tng tui th ca tip im). C do ( gim in tr tip xc). D gia cng khi ch to v gi thnh r.

VT LIU LM TIP IM Thc t t vt liu no p ng c y cc yu cu trn. Trong thit k s dng ty tng iu kin c th m trng nhiu n yu cu ny hay yu cu khc.

VT LIU LM TIP IM ng ki thut in: ng nguyn cht thu c bng in phn. N p ng hu ht cc yu cu trn. Nhc im chnh ca ng ki thut in l rt d b oxit ha. ng caimi: ng ki thut in pha thm caimi c tnh cht c cao chng mi mn tt, kh nng chu c h quang tt hn ng ki thut in thng thng.

VT LIU LM TIP IM Bc: l vt liu lm tip im rt tt do c dn in cao v c in tr tip xc n nh. Nhc im ch yu l chu h quang km nn s dng b hn ch.ng thau: hp kim ng vi km c s dng lm tip im dp h quang Cc hp kim ng khc: hp kim ng vi nhm, ng vi mangan, ng vi niken, ng vi silic v cc hp kim ng khc c s dng lm tip im, ng thi lm l xo p (v d tip im tnh ca cu ch). Nhng tip im nh vy khi b t nng d b mt tnh n hi.

VT LIU LM TIP IM Thp c in tr sut ln: thp thng b oxy ha cao nhng l vt liu r nn vn c s dng lm tip xc c nh dn dng in ln, trong cc thit b thp thng c m.Nhm: c dn in cao, r nhng rt d b oxy ha lm tng in tr sut. Nhc im na l hn nhm rt phc tp, bn c li km.

VT LIU LM TIP IM Vonfram v hp kim vonfram: c mi mn v in tt v chu c h quang tt nhng c in tr tip xc rt ln. Hp kim vonfram vi vng s dng cho tip im c dng nh. Hp kim vi molipen dng lm tip im cho nhng thit b in thng xuyn ng m, khi dng in ln th vonfram v hp kim vonfram s dng lm tip im dp h quang.

VT LIU LM TIP IM Vng v platin: khng b oxy ha do c in tr tip xc nh v n nh, c s dng lm tip im trong thit b in h p c dng in b v quan trng. Vng nguyn cht v platin nguyn cht c bn c thp nn thng c s dng dng hp kim vi mlipen hoc vi irii tng bn c. Than v graphit: c in tr tip xc v in tr sut ln nhng chu c h quang rt tt. Thng dng lm cc tip im m khi lm vic phi chu tia la in, i khi lm tip im dp h quamg.

VT LIU LM TIP IM Hp kim gm: hn hp v mt c hc ca hai vt liu khng nu chy m thu c bng phng php thiu kt hn hp bt hoc bng cch tm vt liu ny ln vt liu kia.Thng vt liu th nht c tnh cht k thut in tt, in tr sut v in tr tip xc nh, t b oxy ha.Vt liu th hai c tnh cht c cao v chu c h quang. Nh vy, cht lng kim loi gm l do tnh cht ca hn hp quyt nh.

MT S KT CU TIP IM THNG DNG

TIP IM KIU CU

TIP IM KIU NGN

TIP IM KIU DAO

TIP IM KIU NM

TIP IM KIU I

NGUYN NHN H HNG

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.

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 . 20-30oC c lp oxt dy khong 25.10-6mm.

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.

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.

CC BIN PHP KHC PHC 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.

CC BIN PHP KHC PHC 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,...

CC BIN PHP KHC PHC 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.