Chuong2-Bao Ve Qua AP&Qua Dong

Gio trnh in T ng Dng 1

+

-

EpIm

SW m

Ti

L

Es

Ep

Im

SW m

qu in pEs

Chng 2: BO V QU P, QU DNG

Cc phn t trong mch cng sut s dng dng c bn dn nh BJT, FET, Thyristor thng hay b hng do cc nguyn nhn qu p,qu dng. m bo tin cy trong hot ng v bo v h thng khi s c phi c mch bo v qu p, qu dng.

Trong chng ny, ta s kho st cc phng php bo v qu p qu dng cho cc phn t cng sut s dng c bn dn v cc mch bo v qu p qu dng in t s dng trong cng nghip. 2.1: Bo v qu p cc phn t cng sut: 2.1.1: Cc iu kin qu p:

Trong cc mch cng sut, ngoi vic qu p do ngun cp in gy nn cn c cc nguyn nhn qu p khc do bn thn hot ng ca mch nh: giao hon bin p, tt ca phn t cng sut, giao hon ti.

1- Giao hon bin p: l ngun gy qu p thng xy ra c bit khi mch cng sut chn cp ngun bng bin p. Cc qu trnh qu xy ra trn th cp khi dng s cp b ngt hoc ng. Cc qu xy ra khi bin p v ti b ngt ngun cp, do dng t ho bn trong bin p. Khi ngun cp cho bin p,dng t ho s gy nn qu p gp i so vi lc xc lp. in dung ghp gia s cp v th cp lm tng qu p th cp nu bin p c t s vng h p ln. Qu p do giao hon bin p c minh ho hnh 2.1, 2.2 v 2.3

Hnh 2.1: Qu in p do ngt dng t ho bin p

Hnh 2.2: Qu in p do np nng lng s cp bin p.


Hnh 2.3: Qu in p do np nng lng bin p h p

2- Tt phn t cng sut: bt c khi no phn t cng sut tt khi lm vic vi dng in cao trong mch in cm, s c dng ngc xut hin x in tch tch tr, gy nn in p cao trn cun dy. in p ny p ngc ln phn t cng sut ang tt v phn nh ln phn t cng sut khc vi cc tnh ngc li. (hnh 2.4)

Hnh 2.4: Qu p do phc hi ngc ca SCR. 3- Giao hon ti: xy ra trong cc mch in cm c kt ni ti hay ngun ca mch

in. Cu ch n hoc cu ch bo v thch hp cho trng hp ny. Nng lng tch tr trong mch in cm l E= LI2 , vi I l dng ang chy qua , cn phi c tiu tn trong cc phn t bo v, khng gy ra qu p. Ni chung, nhng dng c h mch chm tiu tn nng lng ny chm to ra h quang, v nhng sw tc nhanh (cu ch hay mch bo v) s li phn ln nng lng ny cho cc phn t khc hp th. iu kin giao hon ti ch yu ph thuc vo c tnh h mch ca sw.

Hnh 2.5 v hnh 2.6 minh ho qu p do giao hon ti.


Hnh 2.5: Qu p do mch giao hon ti cm ng vo.

Hnh 2.6: Qu p do giao hon ti Qu in p cng xy ra khi ti c mch lc LC (hnh 2.7) v ti ti sinh (hnh 2.8) nh ng c.

Hnh 2.7: Qu p do mch lc LC vi t l L/C cao


+

-E

L

TR

C

V(t)

+

-

i(t)

i(t)

t

V(t)

I

0

t0

IR

E

Dng thyristor di/dt=E/L

Dng snubber

Hnh 2.8: Qu p do ti ti sinh 2.1.2 : Bo v qu p:

bo v cc phn t cng sut, phi c mch bo v qu p cho tng phn t.Trc tin phi chn linh kin sao cho in p lm vic ca n phi gp 1,5 2 ln in p nh lm vic bnh thng. Cc mch bo v thng s dng nh: mch Snubber, cc linh kin phi tuyn, mch bo v in t. 1- Mch Snubber trit dv/dt:

` Trong cc mch in, mch bo v cn phi chng tc bin thin in p thun dv/dt ngang qua cc phn t cng sut, c th gy hng linh kin. Vic gy ra dv/dt c th t vic ng ngun ngoi hay bn thn hot ng ca mch.

Mch Snubber bao gm in tr ni tip t in v mc song song vi phn t cng sut nh hnh 2.9.Vic mc song song C vi phn t cng sut s to mt dng in C(dv/dt) qua t in.in cm L hn dng qua t C nn cng hn ch dv/dt, Ngoi ra L cn hn ch tc bin thin dng di/dt khi phn t c kch dn.Mt mt do mch Snubber rt b c th b qua nht l f cao.

a) b) Hnh 2.9: a) Mch Snubber vi SCR b) dng sng phc hi ngc

Trong hnh 2.9a khi SCR T tt xut hin xung dng phc hi ngc tng n gi tr nh trong thi gian tt. Nu khng c mch RC, dng phc hi ngc b ngt trong mch in cm s to ra mt qu in p L(di/dt) c th lm hng phn t cng sut.

Nu ni mch Snubber RC song song vi phn t cng sut, dng phc hi ngc c th i qua mch RC (hnh 2.9b) Trn nhnh Snubber s xut hin mt dao ng tt dn to p ngc p trn phn t cng sut. Gi tr RC thch hp s hn ch in p ngc v tc bin thin ca n.

Khi SCR T dn, t snubber C x dng qua T v R chng tng dng di/ti ti thi im dn. Tuy nhin khi T off, xut hin mt in p thun IR p vo t. iu ny lm hn ch vic gii hn dv/dt ca mch snubber.


Hnh 2.10 minh ho mch snubber c cc tnh. R1 c gi tr b ci thin kh nng bo v dv/dt thun, R2 c gi tr ln hn gii hn dng x khi SCR dn.

Trong cc mch cp ngun cho mch cng sut, mch snubber thng c ni vo ng cp ngun AC hay th cp bin p trit cc qu gy ra do bn thn mch in. Tuy nhin mch snubber ch c hiu qu phn no, t hiu qu cao hn ngi ta s dng nhng phn t phi tuyn trit p, cc phn t ny c mt mt nh hn trong iu kin lm vic Hnh 2.10: Mch snubber bnh thng. c cc tnh

Thit k mch snubber cho mch dc: Mch snubber s dng trong mch dc nh hnh 2.11. Trong thc t ngun dc c ng ngt cp cho SCR T, v tc bin thin dv/dt phi di gi tr in p t kch T dn.

Khi S ng, C xem nh c ngn mch, SCR tt xem nh h mch. RL l in tr ti v L l in cm ngun, ta c:

dtdiLiRRE Ldc /.).(

Li gii ca phng trnh trn:

)1( /teIi (2.1)

Vi : )/( Ldc RREI v )/( LRRL

t l thi gian o t lc S ng. Vi phn (2.1) theo t:

1. / teIdt

di Hnh 2.11: Mch snubber

/tL

L

dc eL

RR

RR

E

/tdc eL

E

Gi tr di/dt max khi t=0:

L

E

dt

di dcmax (2.2)

hay max)/( dtdi

EL dc (2.3)

in p trn SCR T: iRv .

dt

diR

dt

dv

maxmax

dt

diR

dt

dv

Thay (2.2) vo ,ta c:

max

dt

dv

E

LR

dc

(2.4)

Cc thng s L,RL,C c chn sao cho mch c m ti hn. C phi tch in y trong thi gian ngn nht. T , theo phn tch trn ta c :

04

)( 2 C

LRR L

C

LRR L 2 (2.5)


S dng gi tr cho php dv/dt v L, RL cho trc, ta c th xc nh c R v C t (2.4) v (2.5) . V d 2.1: Mch in nh hnh 2.11.in p cp ngun 400V,cc thng s cho SCR (di/dt)max=50A/s v (dv/dt)max=200V/s.Tnh L m bo di/dt v R,C mch snubber. Gii: Ti thi im SW ng,SCR cha dn,C xem nh ngn mch,p trn SCR VAK p ton b

trn R.

Theo (2.3): max)/( dtdi

EL dc H8

50

400

Theo (2.4):

4

400

2008

maxdt

dv

E

LR

dc

Nu gi tr R b,nng lng tiu tan cao v dng x t C vo SCR khi SCR dn s cao! Gi tr R c chn thng ln hn gi tr yu cu gii hn dv/dt. Gi tr C c chn nh trnh gy qu dng cho SCR khi x dng vo SCR. Thng thng ngi ta chn gi tr tiu biu l R=10,C=0,1F. Nu chn R=10,gi tr L s tng m bo dv/dt cho php:

T (2.4): Hdtdv

REL dc 20

200

10400

)/( max

Gi tr L tnh trn m bo di/dt cho php! Thit k mch snubber cho mch ac.

Cc phng trnh sau thng c s dng tnh ton:

fV

VAC

s

6010

2 (2.6)

C: gi tr C min cn thit (F) VA: cng sut biu kin bin p Vs: in p hiu dng th cp f: tn s lm vic

C

LR 2 (2.7)

: h s m, thng ly khong 0,65 R: in tr m mch snubber L: im cm hiu dng C: in dung min cn thit mch snubber

Nu cho gi tr max dv/dt, phng trnh (2.6) c th vit thnh: 2

/

564,0

2

1

dtdv

E

LC m (2.8)

Em l in p nh dy/dy. V d 2.2: bo v dv/dt cho nhnh SCR trong mch chnh lu cu 1 pha,tnh mch snubber.SCR c (dv/dt)max=50V/s.in p nh AC ng va 380V v in cm ngun 0,1mH. Gii:

T (2.8): Fdtdv

E

LC m 092,0

50

10380564,0

102

1

/

564,0

2

126

4

2

T (2.7):

86,4210092,0

1065,022

6

4

C

LR

2- Dng c phi tuyn trit qu p:

Cc dng c phi tuyn trit qu p c dng c tuyn nh hnh 2.12. iu kin bnh thng, ch c mt dng in b qua n.Nu c t bin in p, n s dn vi dng in ln


hn rt nhiu ln v gii hn hay kp in p mt mc Vz no . Cc loi dng c ny gm: Varistor, diode selenium, diode zener (Avalanche).

Hnh 2.12: c tnh dng c phi tuyn trit qu p Hnh 2.13: K hiu MOV

Varistor hay MOV (Metal Oxide Varistor) Nguyn l lm vic l in tr thay i theo in p. Khi in p di gi tr ngng, gi

tr in tr rt ln. Khi in p vt qua gi tr ngng, in tr s gim nhanh v to dng qua MOV tng vt ln dn n hn ch in p ngang qua n. c tnh ca varistor nh c tnh vng phn cc nghch ca diode Zener nhng lm vic c hai chiu dng in.

Diode Selenium: Diode selenium(selen) l loi diode chnh lu cao p thng c dng trc kia. Tip xc kim loi selen c in p nghch rt cao .Diode selen lm vic nh diode Zener nh c tuyn hnh 2.12, n c kh nng trit p qu gp 2 ln in p lm vic bnh thng.

Diode selenium c 2 loi: phn cc v khng phn cc. Loi phn cc s dng trong mch dc v loi khng phn

cc s dng trong mch ac. Diode selen gm tng cell ghp li vi nhau, mi cell chu in p RMS bnh thng 25V v in p ghim khong 72V.

Diode Zener: Diode Zener cng c s dng trit qu p trong Hnh 2.14 a) Diode selen phn cc nhng mch dc. N c kh nng hp th nng lng ln b) Diode selen khng phn cc qu p,tuy nhin cng sut tiu tn tng.

Cc dng c phi tuyn c u im hn mch snubber ch khng ph thuc vo gi tr in cm/dung trong mch v trit qu p theo nguyn tc ngn mch, khc vi mch snubber cho p dao ng tt dn nn hiu qu bo v cao hn.

3. Mch bo v qu p: Nh vo c tnh giao hon nhanh ca SCR, c th s dng n vo cc mch bo v qu p do ngun cp in hoc hot ng giao hon gy ra .SCR c ni song song vi ti.Khi c in p vt qu gi tr n nh, SCR c kch dn to mt dng in ln t ngun qua lm gim in p ra ti. Hnh 2.15 minh ho mt dng mch bo v qu p s dng SCR. V ngun cp in l xoay chiu nn c 2 SCR, 1 cho chiu dng v 1 cho chiu m.Cc mch bo v qu p in t s c phn tch phn sau.

R1 hn dng ngn mch khi SCR dn. Dng dn qua SCR ln gy st p trn tng tr ngun Z lm gim in p ra ti . Diode Zener DZ ni tip R1, R2 to mc ngng nhn dng qu p. Khi in p AC in cao gy ra qu p, Dz dn.


Hnh 2.15: Mch bo v qu p

Trong k bn dn dng ,T1 phn cc thun. Dng kch cc G T1 t L ZR1D1DZ R2D3. Trong bn k m ,T2 phn cc thun. Dng kch G T2 t ND2DZR2D4.Khi in p vo gim tr v gi tr an ton, DZ off ct dng kch cc G cc SCR, cc SCR tt, mch tr v trng thi bnh thng. 2.1.3: B tr mch bo v qu p:

Trong thc t, ngi ta thng b tr cc mch bo v qu p cc v tr ngun vo, phn t cng sut, ng ra ti. Chng hn:

Mch cc dng c phi tuyn trit p ng cp ngun vo chng qu p gy ra bi bin p , cc in cm ng vo.

Cc phn t cng sut u phi c mch snubber chng dv/dt . Mch cc dng c phi tuyn trit p ng ra ti chng qu p gy ra do ti in cm.

Hnh 2.16 minh ho s b tr chng p qu ti tiu biu.

Hnh 2.16: Mch bo v qu p tiu biu. 2.2: Bo v qu dng: 2.2.1: Cc iu kin qu dng:

Cc trng hp gy ra qu dng: - Ngn mch ng ra - S c trong mch cng sut - S c trong chuyn i nhnh dn ca phn t cng sut - Ngn mch gia mt pha v 1 nhnh cu diode chnh lu. 1- Ngn mch ng ra:

Trong bt k h thng no, vic ngn mch ng ra s lm dng ti hay dng qua cc phn t cng sut tng rt nhiu ln so vi iu kin bnh thng. Dng ngn mch lc xc lp ph thuc vo tng tr ca mch:

in p h mch Dng ngn mch xc lp =

Tng tr mch


Trong cc mch giao hon cu hoc y ko, vic ngn mch xut hin khi 1 phn t chuyn t dn sang tt v 1 phn t chuyn t tt sang dn.

Hnh 2.17: Ngn mch ng ra do chuyn trang thi dn gia cc phn t cng sut. Trng hp ti l ng c cng xy ra ngn mch khi khi ng ng c trong trng

hp ng c c ng yn (v nht l khi ng c c gn ti). Lc sc in ng cm ng v tng tr tng ng ca ng c xem nh bng 0. 2- S c trong mch cng sut:

S c xy ra khi phn t cng sut dn sai, c th do xung kch sai hoc xut hin dv/dt vt qu p thun cho php, hoc in p ngc cao qu mc cho php. 3. S c trong chuyn i nhnh dn ca phn t cng sut:

S c ny c cp hnh 2.17, khi mt nhnh chuyn t dn sang tt v mt nhnh chuyn t tt sang dn m khng c thi gian tr chuyn trng thi.

Xem mch cu BJT nh hnh 2.18.Bnh thng Q1Q4 dn, Q2Q3 tt, dng ghp ra ti theo chiu VccQ1RLQ4GND. Khi o trng thi Q1Q4 tt, Q2Q3 dn, dng ghp ra ti theo chiu Vcc Q3RLQ2GND.

Trng hp xung kch sai hoc ti mang tnh cm, thi gian chuyn trng thi di d dn n tnh trng ngn mch do dn trng Q1Q2 dn hoc Q3Q4 dn lm qu dng v hng cc phn t cng sut.

Hnh 2.18:Mch cu BJT b dn trng Q1Q2 hoc Q3Q4 do khng c thi gian tr chuyn trng thi.

2.1.2: Bo v qu dng: 1- Cu ch:

bo v qu dng cho cc phn t cng sut, c th s dng cu ch t v tr thch hp.Hnh 2.19a t cu ch ng vo AC l phng php n gin nht. Trong hnh 2.19b, t cu ch ng hng vi phn t cng sut bo v cho tng phn t mt, cch ny thng hay s dng. Trong mt s trng hp, gim tc bin thin di/dt, ngi ta t thm cc cun dy trn tng nhnh cc phn t cng sut, tuy nhin cun dy s nh hng n hot ng bnh thng ca mch.


Hnh 2.19: a) Cu ch bo v t ng vo

b) Cu ch bo v t ng hng vi phn t cng sut c) t cc cun dy chng di/dt

Hnh 2.20 minh ho cu to cu ch v hnh 2.21 l c tnh dng in ca cu ch.

Hnh 2.20: Cu to cu ch

Hnh 2.21: c tnh dng in cu ch: Khi c dng s c tng, nhit cu ch tng cho n khi t=tm, l thi gian chy cu ch

v h quang bt u xut hin xuyn qua cu ch. Do h quang, tng tr ca cu ch tng, lm gim dng in.

Tuy nhin, n s to ra mt in p h quang ngang qua cu ch . Nhit pht ra lm bc hi cc phn t cu ch, lm tng chiu di h quang v gim dng in. Qu trnh ny lm tt h quang trong thi gian ngn. Khi h quang xut hin v tt trong thi gian ta ,s c b ct. Thi gian ct cu ch cng nhanh, in p h quang cng cao.


+

-

Vc

Vi

Thi gian ct tc l tng hai thi gian chy tm v thi gian h quang ta,tm ph thuc vo dng ti v ta ph thuc vo h s cng sut ca mch gy s c. Thng thng s c b ct trc khi dng s c t n gi tr nh u tin. Dng s c t n gi tr cc i nu khng c cu ch gi l qu dng t ti.

Hnh 2.22 minh ho c tnh dng in/ thi gian ca phn t cng sut v cu ch. Hnh 2.22 c tnh dng in /thi gian ca dng c v cu ch

a) Bo v hon ton b) Ch bo v ngn mch

Trong hnh 2.22a, c tnh cu ch lun nm pha di c tnh dng c nn cu ch s bo v dng c trong ton tm lm vic. Trong hnh 2.22b, cu ch ch bo v khi bt u xy ra ngn mch, cn qu dng bnh thng s dng mch ct CB hoc hn dng.

Nu gi R l in tr mch khi c s c v I l dng s c tc thi trong khong thi gian xy ra s c n lc dp tt h quang, nng lng nui mch:

dtRiWe 2 (2.9)

Nu R = const, gi tr I2t t l vi nng lng nui mch. Gi tr I2t gi l nng lng cho qua v p ng vi vic nng chy cu ch. Gi s dng sng dng ct cu ch hnh 2.21 c dng tam gic u v tm=ta=tc/2.Dng nh xuyn qua Ip.T (2.9),ta c:

cp

t

c

p tI

dttt

IdtitI

c

3

22

2

2

2

0

22

Suy ra: c

pt

tII

22 3 (2.9a)

chn cu ch thch hp, trc tin phi d on dng s c v tho cc iu kin: 1- Cu ch phi m bo dn lin tc dng nh mc 2- Gi tr I2t ca cu ch trc khi dng s c b ct phi nh hn gi tr nh mc I2t ca

dng c. 3- Cu ch phi chu c in p sau khi dp tt h quang 4- in p h quang nh phi nh hn gi tr nh mc nh ca dng c.

2.3: Bo v qu p&qu dng s dng mch in t: 2.3.1: Cm bin qu p&qu dng: 1. Cm bin p:

in tr: phng php thng dng nht l dng in tr phn p nh hnh 2.23.

ic VRR

RV

21

2

(2.10)

Chn t l R1,R2 t h s phn p nh yu cu. Lu : tng tr vo nhn t Vc l:

Hnh 2.23: Cm bin p dng in tr phn p.


+

-

Vp

+

-

Vs

Ic

Rs+

-

Vc

21 // RRRc (2.11)

Ty thuc vo cng sut v in p ng vo Vi,ta phi chn cng sut in tr ph hp p ng cng sut tiu tn v in p chu ng trn in tr. V d: Vi=220V,Pi=10W,chn cu phn p sao cho Vc=10V.

Ta c: mAV

PI

i

ii 45

220

10

Chn dng qua cu phn p c 1mA khng nh hng cng sut ngun.

H s phn p: 045,0220

10

21

2 RR

R

KI

VRR

c

i 22010

220321

Suy ra: KRKR 1,210,9,9 12

Cng sut tiu tn v in p trn in tr:

VVWP

VVWP

RR

RR

10,01,010.10

210,21,010.1,210

23

2

13

1

Tng tr vo nhn t Vc: KRRRc 45,9// 21

Cm bin p bng in tr c th s dng lm cm bin p dc hoc ac. Bin p: s dng bin p ch cm bin p ac.

Ta c in p trn th cp:

p

p

ss nV

N

NV (2.12)

Ns: s vng th cp Np: s vng s cp Lu p ng tn s ca bin p v trnh Hnh 2.24: Bin p cm bin p ac.

bin p bo ha t. 2. Cm bin dng:

in tr shunt: phng php ny s dng cho dng dc hoc ac,chnh xc vi dng Ic b v khng cch ly vi mch iu khin.

scc RIV (2.13)

C th to in tr shunt t cng thc tnh in tr dy qun:

S

lR (2.14)

Hnh 2.25: Cm bin dng bng in tr shunt

: in tr sut ca vt liu lm in tr(.m)

l: chiu di dy/thanh in tr(m) S: tit din ngang ca dy/thanh in tr(m2)

Bin dng: ch p dng cho cm bin dng ac. Lu : vi bin dng k hiu Ip/Is=200/5 chng hn,c ngha l nu t dng s cp Ip=200A,ngn mch th cp s c dng th cp Is=5A. Th cp bin dng c ni tr Ri t vi n vi chc . cm dng th cp,ta mc in tr Rs gia hai u th cp bin dng vi iu kin Rs


IsTh cp IpS cp

Dy dn Ic

Phn t Hall

K

Li st vt liu t mm

Vc

Hnh 2.26: Cm bin dng bin dng

Cm bin dng hiu ng Hall: Hin nay cm bin dng hiu ng Hall c s dng rt rng ri,vi nhng u im nh sau:

- Cm bin c dng dc v ac. - nhy cao,tuyn tnh trn tm rng. - o dng t mA n hng trm A. - Cch ly gia dng cm bin v mch o.

Nguyn l c bn cm bin dng hiu ng Hall nh hnh 2.27. Hnh 2.27: Nguyn l cm bin dng hiu ng Hall Mt li vng kn l vt liu t mm t gn hoc bao bc dy dn c dng in cn o

Ic.Khi dng Ic qua dy dn,li sinh ra t trng tc ng ln phn t Hall t trong khe h t ca li ,lm xut hin in p trn phn t Hall.in p ny rt b nn cn c khuch i thm.

Cm bin Hall c 2 loi: h mch(open loop) v vng kn(closed loop). Loi h mch c tuyn tnh ph thuc vo vt liu li v tri ph thuc vo phn t

Hall,mch khuch i.Gi thnh loi h mch thp. Hnh 2.28: Cm bin dng Hall vng kn Trong loi cm bin Hall vng kn,in p Hall c khuch i vc a v kch mch li

dng in cp cho cun dy b qun trn li.T trng cun dy to ra cng bin nhng ngc chiu vi t trng to bi dng s cp Ic.Kt qu l t thng tng trong li bng 0.iu ny lm tng nhy v tuyn tnh ca cm bin.

Gii thiu vi mch cm bin dng hiu ng Hall ACS712(xem data sheet) - Dng SOIC 8 chn - p ng ng ra 5s - BW=80Khz - Tng sai s ng ra 1,5%,Ta=25

0C


Cm bin

p/dngK

(chnh lu)So snh

iu khin

ng/ct

p tham chiu

Ti

+

-

+Vcc

R1

Dz

Q1

Q2 Vcc

R2

- in tr dn bn trong 1,2m - Cch in min 2,1KVrms - Cp ngun n 5V - nhy ng ra 66-185mV/A - in p ra t l vi dng dc/ac ng vo - Tm dng in vo max: 5 n 30A

Hnh 2.29: Mch ng dng tiu biu ACS712

2.3.2: Mch bo v qu p/dng: 1. S nguyn l:

Hnh 2.30: S khi nguyn l mch bo v qu p/dng

Hnh 2.30 minh ha s nguyn l chung ca mch bo v qu p/dng.Tn hiu t cm bin p/dng c khuch i tng nhy(c th chnh lu chnh xc trong trng hp cn chuyn sang dc),sau a vo mch so snh so snh vi tn hiu tham chiu.in p tham chiu xc nh mc/ngng qu p/dng cn tc ng.Ng ra mch so snh s pht tn hiu n mch iu khin ng ct ng ct mch cp ngun p/dng.Mch iu khin ng ct c th l BJT,FET cng sut lm vic ch giao hon hoc thyristor. nhy v p ng ca mch bo v c quyt nh bi mch khuch i,so snh v nht l mch iu khin ng ct,phn t kha cp ngun.

2. Mch bo v qu p cp ngun: Xem mch bo v qu p cp ngun nh hnh 2.31.

Hnh 2.31: Mch bo v qu p cp ngun tiu biu

Khi p cp ngun Vcc nh hn Vz+V1(V1: in p m Q1),Q2 dn bo ha,in p cp ra

ti bng Vcc-VCES2(VCES2: in p bo ha CE Q2) ccV


Khi p cp ngun Vcc vt qua Vz+V1,Q1 on lm gim dng IB2,Q2 dn yu lm tng VCE2,in p ra ti s gim.n khi Q1 bo ha,Q2 off ct ngun ra ti.

IC bo v qu p NCP346 a)Dng v ngoi NCP346 SOT-23-5

c) Mch tiu biu Hnh 2.32: IC bo v qu p NCP346

- Dng v ngoi SOT23 5 chn - in p chu ng cc i Vccmax=30V - in p ng vo ngng bo v: Vth=4,45V(danh nh) - Hot ng bnh thng: VinVth,CNTRL=0,OUT=1

hoc CNTRL=1,OUT=1 - Thi gian ON/OFF


Khi dng in qua cp BJT y ko,c th l Q2,vt qu gii hn,in p trn R2 vt qu V lm Q3 on.C1 x nhanh qua Q4 lm Q5,Q6 off ct r leK h mch ti. iu kin qu dng:

VIRRR

RV

RR

RV sBE

22

34

42

34

43 (2.14)

hay 4

34

2

2R

RR

R

VI

(2.15)

Thay cc gi tr vo:

AI 36,86,5

156,5

22,0

5,02

Bi tp chng 2 2.1: Phn tch cc trng hp qu p do giao hon bin p. 2.2: Phn tch cc trng hp qu p do ngun hoc ti c thnh phn L. 2.3: Phn tich trng hp qu p do ti l ng c. 2.4: Phn tch cc trng hp qu dng do bn thn mch in gy ra. 2.5: So snh hiu qu ca cc mch trit qu p s dng snubber,MOV,diode zener,diode

selenium. 2.6: iu g xy ra khi di/dt p vo phn t cng sut vt qu gii hn cho php?Lm cch

no gii hn di/dt trong mc cho php? 2.7: iu g xy ra khi dv/dt p vo phn t cng sut vt qu gii hn cho php?Lm cch

no gii hn dv/dt trong mc cho php? 2.8: Mch snubber dc nh hnh 2.11.Cho Edc=300V,L=10H,RL=10,SCR c

(dv/dt)max=600V/s. a) Tnh R,C mch snubber. b) V dng sng V trn SCR khi SCR tt. 2.9: Thit k mch snubber tha cc iu kin thng s nh sau:

Cng sut ngun cung cp 5KVA,in p th cp 120Vrms Tn s lm vic 400Hz in cm mch in 100H 75,0

2.10: a) Chng minh chi tit li cng thc (2.9a) b) Mt cu ch c thng s I2t =150A2s,thi gian ct dng 6ms.Tm gi tr dng nh s

c. 2.11: Thit k mch o p AC 50Hz,output t 0-5VDC,tng ng vi input t 0-250VAC. 2.12: Thit k mch o dng AC 50Hz,output t 0-5VDC,tng ng vi input t 0-100A AC. 2.13: S dng cm bin dng Hall ACS712,thit k mch o dng cho output t 0-5VDC,tng

ng input t 0-30A AC. 2.14: Thit k mch o v qu p,qu dng cho mng in 1 pha 220VAC 50Hz.

iu kin qu p: Vqa=240VAC iu kin qu dng: Iqd=100A AC

Mch s iu khin ct khi ng t(contactor) khi xy ra qu p hoc qu dng. 2.15: S dng IC NCP346,thit k mch bo v qu p cp ngun cho ti vi iu kin:

in p cp ngun danh nh 12V,khng c cao qu 10%. Dng cp ra ti 1A(chn MOSFET/BJT li ngoi thch hp)

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