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GIO N S: 01
cng bi ging in t cng sut
Ngh CN
BI 1: TNG QUAN V IN T CNG SUT1. Gii thiu chung v in t cng sut.
1.1. Khi nim:
in t cng sut l chuyn ngnh nghin cu cc phng php v cc thit b dng bin i v iu khin nng lng in.
Cc thit b sn xut ca chng ta s dng cc loi nng lng in khc nhau, c loi dng in mt chiu, c loi dng in xoay chiu, cc mc in p khc nhau, cc tn s khc nhau, v c bit l iu khin hot ng ca cc thit b , ta cn iu khin ngun nng lng in cp cho n. Nh vy, bin i v iu khin nng lng in l mt nhim v hng u trong t ng ho sn xut.
Vic bin i v iu khin nng lng in trong cng nghip trc y ch yu s dng cc relay (r-le), da vo vic ng m cc relay m c c ngun in nng theo mun. Tuy nhin, do yu cu ngy cng cao ca thc tin sn xut, km theo l s tin b ca cng ngh bn dn cho php ch to cc phn t ng ct bn dn (khng tip im) cng sut ln nhm thay th cc mch relay tip im >> ngnh in t cng sut.
Nh vy, theo ti, c th ni rng in t cng sut tc l dng cc thit b in t c cng sut ln vi cc thut ton iu khin nhm bin i v iu khin nng lng in.
1.2. Nguyn tc bin i tnh
Cc b bin i in t cng sut c phn loi da vo cc in xoay chiu >> in mt chiu: Cc b Chnh lu (Rectifier) iu khin (dng Thyristor) hoc khng iu khin (dng Diode) tu theo vic ta c cn iu khin gi tr ca dng in mt chiu u ra hay khng.
in mt chiu >> in xoay chiu: Cc b Nghch lu (Inverter). Cc b nghch lu c kh nng bin mt dng in mt chiu thnh mt dng in xoay chiu c gi tr in p v tn s thay i c tu vo lut ng m cc van bn dn.
in mt chiu >> in mt chiu: Cc b Bm xung mt chiu (cn c tn l iu p mt chiu, bin i in p mt chiu DC to DC converter, DC chopper). Cc b bin i ny bin dng in mt chiu c gi tr c nh thnh dng in mt chiu c gi tr in p, dng in iu khin c.
in xoay chiu >> in xoay chiu: Cc b Bin tn (Frequency Drive) trc tip (Cycloconverter) hoc gin tip (Inverter). Cc b bin tn c kh nng bin ngun in xoay chiu c gi tr dng in, in p v tn s c nh ca li in thnh dng in xoay chiu c gi tr dng, p v tn s iu khin c theo mun.
ch l s phn loi mang tnh cht c bn v rt chung chung, khi i su vo tng kha cnh ta s thy s phong ph ca cc loi thit b bin i in t cng sut.2. Cc linh kin chuyn mch dng trong in t cng sut (Diode, SCR, DIAC, TRIAC, IGBT, GTO).2.1. Diode.2.1.1. Cu to:
2.1.2. c tnh V-A:
Diode l tng gm 2 trng thi ng m
Hnh 2.1 c tnh ca Diode l tngDiode thc t:
Vi rR = : in tr ngc trong diodeUBR: in p nh thng
UTO: in p ri trn diode
rF = : in tr thun trong diodeCc thng s chnh ca diode:
Gi tr in p nh thng UBR Gi tr in p ngc lp li URRM.
Gi tr in p ngc khng lp li: URSM.
Dng in, nhit lm vic.
Gi tr trung bnh cc i dng in thun I F(AV)M.
Gi tr cc i dng in thun khng lp li I FSM.
2.1.3. o, Kim tra v kho st Diode2.1.3.1. o. Kim tra Diode
2.1.3.2. Kho st c tuyn Volt Ampe ca Diode
2.2. DIAC: (Diod Ac Semiconductor Switch)
- Cau tao, ky hieu.- Nguyen ly, ac tnh va cac thong so ky thuat.
Xet mach ien nh hnh 1.71a:
Vi nguon ien VDC co the ieu chnh c t thap len cao. Khi VDC = 0V th Diac khong dan, dong ien qua no bang khong. Khi tang VDC tr so nho th dong ien qua Diac ch la dong ien r co tr so nho. Neu ta tang VDC en mot tr so u ln th ien the tren Diac tang en gia tr VBO th ien the tren Diac lai giam xuong va dong ien qua diac bat au tang len nhanh. ien the nay goi la ien the ngap (Breakover) va dong ien tng ng vi no la dong ien ngap IBO. ien the VBO cua Diac co tr so trong khoang t 20V en 40V. Dong ien IBO co tr so khoang t vai chuc (A en vai tram (A. Khi oi chieu dong ien ngc lai va tang nguon VDC theo chieu am th Diac cung dan theo chieu ngc lai va ta ve c ac tuyen cua Diac nh hnh 1.71b. Nhn vao ac tnh Von Ampe cua Diac ta thay Diac giong nh hai diode zener au oi au nhau nh hnh 1.7a.
- ng dng: Dng trong cng ngip m hoc kch cho SCR, TRIAC iu khin cc mch ng c, qut xoay, n bn, n ngTHC HANH VE DIACNoi dung : o v kim tra tt hay xu, xc nh chn ca DIAC.
oc trc tiep tren than DIAC.
Nhn dng hnh dng DIAC.
2.2. Transistor lng cc cng cch ly- IGBT
(Insulated Gate Bipolar Transistor)
c tnh ng
Hnh 2.6
Hnh 2.7
IGBT thc t:
1MB-30-060- Fuji Electric
2.3. Thyristor
2.3.1. Trng thi:
M.
ng.
Kha.
2.3.2. K hiu: (Hnh 2.9)T: thun
D: kha
R: ngc
iu kin m Thyristor
UAK > 0
Xung iu khin a vo cc iu khin
iu kin ng : in p ngc t ln A-K
c tnh V-A
Thyristor l tng
Ba trng thi: ng- m- kha
Hnh 2.10
Thyristor thc t
UBR: in p ngc nh thng.
UBO: in p t m Th
UTO: in p ri trn Th
IH: dng duy tr (holding).IL: latching
2.3.3. c tuyn ca SCR
Hnh 2.11
c tnh iu khin Thyristor
Hnh 2.12 S iu khin Thyristor
Hnh 2.13 c tuyn V-A ca Thyristorc tnh ng:
M Thyristor
Hnh 2.14 c tuyn V-A khi m Thyristor
Kha Thyristor
Hnh 2.15 c tuyn V-A khi kha Thyristor
ng Thyristor
Hnh 2.16 c tuyn V-A khi ng Thyristor
Thyristor thc t;
Hnh 2.17 Thyristor thc t
2.2.4. KhO st c tuyn Volt Ampe ca SCR
2.4. GTO
(Gate Turn Off Thyristor)
Hnh 2.18 k hiu GTO
c tnh ng:
M GTO
Hnh 2.19 c tnh GTO khi m
ng GTO
Hnh 2.20 c tnh GTO khi ng
GTO thc t:
2.5. Triac
2.6.1. K hiu
Hnh 2.22 K hiu Triac
2.6.2. c tuyn ca Triac
Hnh 2.23 c tuyn V-A ca Triac
Triac thc t:
Hnh 2.24 Hnh dng ca Triac
2.6.3. Kho st c tuyn ca Triac
3. Cc tn hao trong mch in t cng sut.Yu t c trng cho hiu qu ca 1 thit b cng sut chnh l h s cng sut.
nh ngha:
H s cng sut l t s gia cng sut tch cc P v cng sut ton phn S
(3.1)i vi dng in v in p sin l tng th h s ny c dng n gin:
PF = cos
Trong l gc lch pha gia dng in v in p, hiu chnh h s cng sutchnh l hiu nchnh hay b cos. Trong thc t dng in v in p thng c dng sin khng l tng. H s cng sut theo cch hiu n gin khng cn ph hp v trong cc phn tch cng nh tnh ton phi xut pht t nh ngha chung (3.1). d tnh ton mi dng in thc t c coi l tng ca cc dng sin l tng, v mi dng sin thnh phn c gi l mt hi. Hi c tn s thp nht, bng tn s dng thc t, c gi l hi c bn, cc hi khc, c tn s cao hn, c gi l hi bc cao. Khi mc hay tnh cht sin ca mi dng in thc t c nh gi bng tng quan gia tng nng lng ca cc hi bc cao v nng lng ca hi c bn. Tng quan ny c gi l h s mo dng tng v thng c vit tt l TDH, l t s gia tr hiu dng ca tt c cc dng bc cao v tr hiu dng ca dng c bn:
(3.2)Dng in c h s ny cng ln th c dng cng khc nhiu so vi sin l tng, dng sin l tng c THD=0. in p thc t cng c biu din tng t nh biu din dng in trn.Trong cc ng dng thc t in p v dng in c coi nh sin l tng nu h s mo dng tng khng ln hn 3% [1], tuy nhin theo tiu chun ca hip hi k thut in th gi tr ny l 2% [2]. Nhn chung in p li tn s cng nghip chun, l trng hp c cp y, c coi nh c dng sin l tng. Khi , theo cch hiu hay nh ngha chung nht v h s cng sut th (1.1) c dng sau:
(3.3)trong h s Uhd, I1hd, tng ng l tr hiu dng ca in p ngun,
ca dng in c bn v gc lch pha gia dng in c bn v in p;
Kp =I1hd/Ihd v K = cos.
Quan h gia h s mo hi tng THD v h s Kp c dng:
(1.5) (3.4)Cui cng nhn c:
(1.6)(3.5)Biu thc trn cho thy h s cng sut ph thuc vo thnh phn hi bc cao, gc lch pha gia dng in c bn v in p. T d thy rng c h s cng sut ln th phi gim thiu hm lng cc hi bc cao trong thnh phn ca dng in vo ca cc thit b in t cng sut.Thc hnh lp rp mch cng tc DC dng BJT
Thc hnh mc mch cng tc xoay chiu s dng Triac
* Mch dung Triac nh hnh sau (Hnh 1.22):
a. Ln lt bt SW v v tr 1, 2, 3 quan st led v gii thch kt qu.
b. t SW v v tr 2 quan st ti, xong bt v v tr 1. Nhn xt gii thch.
c. i cc ca ngun VI, lp li cu a v b, gii thch kt qu.
Trnh t rp mch:
Bc 1: chun b dng c: km, ko, test-board, in tr, dy ni, ti, sw, led, ngun,..
Bc 2: rp mch theo hnh v: theo th t t trn xung di, t tri qua phi.
Bc 3: kim tra li s , chnh sa li ln cui cho chnh xc.
Bc 4: cp ngun cho mch chy.
Bc 5: nhn xt kt qu ca mch v bi hc kinh nghim c c.
Hng dn thng xuyn
Hc sinh thc hin vic lp rp mch
Gio vin hng dn, kim tra, nh gi
* Mch dng Triac nh hnh sau (Hnh 1.23):
a. Gii thch nguyn tc hot ng ca mch (nu r chc nng cc linh kin trong mch iu khin pha).
b. Chnh VR, quan st ti, v li dng sng hai u ti.
c. Th nu vi ng dng ca mch ny.
Hng dn thng xuyn
Hc sinh thc hin vic lp rp mch
Gio vin hng dn, kim tra, nh gi
Hnh 2.4
Transistor thc t
Hnh 2.5
3.1. Transistor lng cc cng cch ly- IGBT
(Insulated Gate Bipolar Transistor)
c tnh ng
Bi 2: CHNH LU
1. Khi qut chung.
1.1. Khi nim c bn
nh ngha: chnh lu l thit b bin i dng in (in p) xoay chiu thnh dng in (in p) mt chiu.
Cu trc nh hnh v
1.2. Phn loi
Theo s pha: mt pha, hai pha, ba pha, su pha..
Theo loi ngt in
Mch ch dng ton diode l chnh lu khng iu khin.
Mch ch dng ton Thyristor l chnh lu c iu khin.
Mt na chnh lu, mt na diode l chnh lu bn iu khin (chnh lu iu khin khng i xng)
Phn loi theo s mc.
Phn loi theo cng sut.1.3. Cc thng s c bn ca chnh lu.
Nhng thng s c ngha quan trng nh gi chnh lu bao gm:
in p ti: Ud = ((t).d(t
Dng in ti: Id = Udc/Rd Dng in chy qua ngt in: IND = Id/m
in p ngc ca ngt in: UN= Umax
Cng sut bin p: SBA = = kad.Ud
S ln p mch trong mt chu k m
p mch (nhp nh) ca in p ti.1.4. Nguyn tc dn ca cc ngt in bn dn:
Nhm ngt in ni chung Kathode:
in p anode ca diode no dng hn th diode y dn. Khi in th im A bng in th anode dng nht
Nguyn tc dn v iu khin Thyristor:
Nhm ngt in ni chung anode:
in p cathode ngt in no m hn hn th diode y dn. Khi in th im K bng in th anode m nht.
Nguyn tc dn v iu khin Thyristor:
Ph thuc vo in th cc dng trn cc anode v tn hiu iu khin.2. Chnh lu mt na chu k.
2.1. Chnh lu khng iu khin bn k.
2.1.1. Chnh lu khng iu khin ti thun tr R
* S mch
* Cc thng s s :
in p ti: Ud = d(t = U2 = 0,45 U2 Dng in ti: Id = Udc/Rd Dng in chy qua diode: ID = Id in p ngc ca ngt in: Cng sut bin p: : SBA = = 3,09 Ud.Id
* Lp rp v kho st- in p ng vo
- in p ng ra
- Tnh ton cc thng s
2.1.2. Chn lu khng iu khin ti thun tr R, L* S mch
Do c tch ly v x nng lng ca cun dy, do dng in v in p c dng nh hnh v
* Cc thng s s :in p ti: Ud = d(t = 0,45 U2 Dng in ti: Id = Udc/Rd Dng in chy qua diode: ID = Id in p ngc ca ngt in:UN = Cng sut bin p: : SBA = = 3,09 Ud.Id* Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng s
2.2. Chnh lu c iu khin bn k.
2.2.1. Chnh lu c iu khin bn k ti thun tr.
* S mch
* Cc thng s s :in p ti c tnh:
Ud = d(t = 0,45 U2 * Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng s
2.2.2. Chnh lu c iu khin bn k ti R, L.* S mch
in p ti c tnh: Ud = d(t = 0,45 U2 * Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng s
2.2.3. Chnh lu c iu khin bn k ti R, L c diode x nng lng.* S mch
* Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng s
3. Chnh lu c chu k vi bin p c trung tnh.
3.1. Chnh lu khng iu khin.* S mch
* Thng s ca s :
in p, dng in chnh lu v ngt in.
Udtb =2. d(t = 2 U2 = 0,9 U2 Dng in ti: Id = Udc/RdI\Dtb = ; IDhd = UND = 2SBA = = 1,48 UdId* Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng s
3.2. Chnh lu c iu khin* S mch
3.2.1 Ti thun tr: Udtb =. d(t = 0,9 U2 3.2.2. Ti in cm: Ud = d(t = 0,9 U2 Khi dng lin tc: = 0
Ud = 0,9 U2
3.2.3. Chnh lu c diode x nng lng
* S v cc ng cong
* Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng s
3.2.4. Hin tng chuyn mch
Ch xt chuyn mch khi dng ti lin tc
4. Chnh lu cu 1 pha.
Chnh lu khng iu khin
Chnh lu iu khin i xng.
Chnh lu iu khin khng i xng.
4.1. Chnh lu khng iu khin
* S :
* Thng s ca s
in p v dng in ti c hnh dng ging nh chnh lu c chu k vi bin p trung tnh, do thng s ging nh trng hp trn
Ud =. d(t = 0,9 U2 Ud0 = Ud + (UBA + 2(UD + (Udn
SBA = 1,23 UdIdUn = * Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng s
4.2. Chnh lu cu iu khin i xng
* S v cc c tuyn
* c im iu khin ng thi hai SCR
* Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng s
4.3. Chnh lu iu khin khng i xng.
4.3.1. c im iu khin
Khc phc nhc im v iu khin ng thi hai SCR: ti mi thi im ch m 1 SCR
4.3.2. S
Ty theo cch mc SCR c 2 loi s :
4.3.3. Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng s
5. Chnh lu tia 3 pha.
Chnh lu khng iu khin.
Chnh lu c iu khin
Hin tng trng dn
5.1. Chnh lu khng iu khin
5.1.1. S chnh lu tia 3 pha khng iu khin
5.1.2. S v c tuyn
5.1.3. Thng s ca s
in p, dng in chnh lu v ngt in
Udtb = d(t = U2f = 1,17 U2f Dng in ti: Id = Udc/RdI\Dtb = ; IDhd = UND = 2,45f = (2,45/1,17)UdSBA = = UdId = 1,35 UdId
m = 35.1.4. Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng s5.2. Chnh lu c iu khin
5.2.1. S
nh ngha v gc thng t nhin
5.2.2. Nguyn tc iu khin
5.2.3. Hot ng ca s khi ti thun tr.
Thng s ca s
Khi ti thun tr gc m nh hn 300Udtb = d(t = U2f . = 1,17 U2fKhi gc m ngt in ln hn 300
Udtb = d(t = 1,17 U2f Cc thng s cn li nh chnh lu khng iu khin5.2.4. Hot ng ca s khi ti in cm
Thng s ca s
in p chnh lu Udtb = d(t = U2f . = 1,17 U2f5.2.5. Hot ng ca s khi c diode x nng lng.
Udtb = d(t = 1,17 U2f 5.3. Hin tng trng dn (tham kho thm ti liu)
Xt s c ti in cm ln cho dng in ti lin tc
5.4. Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng s6. Chnh lu cu 3 pha
6.1. Chnh lu khng iu khin
6.1.1. S
Hai nhm ngt in NA mc chung cathode cho in p dng, NK mc chung anode cho in p m.
6.1.2. Hot ng ca s .
6.1.3. Thng s ca s .
Udtb = d(t = U2f = 2,34 U2f Dng in ti: Id = Udc/RdI\Dtb = ; IDhd = UND = 2,45f = (2,45/234)UdSBA = 1,05 UdId
m = 6
6.1.4. Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng s6.2. Chnh lu iu khin i xng.
6.2.1. S
Ba cch cp xung iu khin
Cp 2 xung iu khin ng th t pha, 2 xung iu khin khng ng th t pha, ba xung iu khin nh bng di
6.2.2. Lp rp v kho st
- in p ng vo
- in p ng ra
- Tnh ton cc thng sBi 3: BIN I DC-DC (DC - DC CONVERTER)1. i cng v bin i DC - DC.
1.1. Khi qut v iu p mt chiu
nh ngha: l b iu khin dng in v in p mt chiu khi ngun cp l 1 chiu.
*Cc phng php iu p 1 chiu
iu khin bng cch mc ni tip vi ti mt in tr.
iu khin lin tc bng cch mc ni tip vi ti 1 Transistor.
iu khin bng bm p (xung p).
1.1.1. iu khin bng cch mc ni tip vi ti mt in tr
S
Dng in v in p c tnh:
Id= ; Ud= . RdNhc im ca phng php l hiu sut thp v khng iu chnh lin tc khi dng ti ln
1.1.2. iu khin lin tc bng cch mc ni tip vi ti 1 transistor
S v nguyn l iu khin
IC = IB ; (UT = U1 IC.Rdin p qua Rd:
Ud = Ic.Rd = IB Rd
Nhc im ca phng php l tn hao trn transistor ln, pht nhit nhiu lm transistor d hng.
1.1.3. iu khin bng bm p (bm xung).
Bm p mt chiu l b bin i in p mt chiu thnh xung in p. iu chnh rng xung in p, iu chnh c tr s trung bnh ca ti.
Cc b bm p mt chiu c th thc hin theo s mch ni tip) phn t ni tip ng ct vi ti) hoc theo s mch song song.1.2. Ngun cp trong bm xung 1 chiu
1.2.1. nh ngha v ngun dng v ngun p
1.2.1.1. Ngun p: l ngun m dng sng v gi tr in p ca n khng ph thuc vo dng in)k c gi tr cng nh tc bin thin).
c trng c bn ca ngun p l in p khng i v in tr trong nh st p trong ngun nh.
1.2.1.2. Ngun dng: l ngun m dng sng v gi tr dng in ca n khng ph thuc vo in p ca n (k c gi tr cng nh tc bin thin).
c trng c bn ca ngun dng l dng in khng i v in tr ln st dng trong ngun nh.
1.2.2. Tnh thun nghch ca ngun
- Ngun c tnh thun nghch
- in p c th khng o chiu (acquy) hay o chiu (my pht in mt chiu)
- Dng in thng c th o chiu
- Cng sut p = u.i c th i chiu khi mt trong hai i lng u, i o chiu.
1.2.3. Ci thin c tnh ca ngun
- Ngun p thng c Ro, Lo, khi c dng in R0i, L(di/dt) lm cho in p trn cc thay i. ci thin c tnh ca ngun p ngi ta thng mc song song vi ngun 1 t.
- Tng t, ngun dng c Z0 = . Khi c bin thin du/dt lm cho dng in thay i. ci thin c tnh ngun dng ngi ta mc ni tip vi ngun 1 in cm.
- Chuyn i ngun p thnh ngun dng v ngc li:
1.2.3. Quy tc ni cc ngun
1.2.3.1. i vi ngun p:
- Khng ni song song cc ngun c in p khc nhau
- Khng ngn mch ngun p.
- Cho php h mch ngun p.
1.2.3.2. i vi ngun dng:
- Khng mc ni tip cc ngun dng c dng in khc nhau.
- Khng h mch ngun dng.
- Cho php ngn mch ngun dng.
2. B n p.
2.1. S lc v l thuyt Mch n p mt chiu cn c gi l mch bin i DC-DC, y l mt mch bin i t in p mt chiu ny thnh in p mt chiu khc.
Thng thng, ngi ta chia n p mt chiu ra lm 2 loi:
- n p tuyn tnh,
- n p ngt m (switching regulator).
D l loi no, mt mch n p cng phi t 2 chc nng:
- n nh in p ng ra khi in p ng vo thay i v khi dng ti thay i.
- Gim n mc thp nht sng d ng ra.
2.2. n p tuyn tnh Trong bi th nghim ny ta kho st mch n p tuyn tnh dng ni tip. S khi nh sau:
- Cng sut n p (phn t iu khin): Thng l mt transistor cng sut ln, hot ng nh mt in tr thay i.
- So snh(d sai): So snh in th ly mu v in th chun to thnh in th iu khin VDK iu khin mch kch to dng kch cho cng sut.
- To in p chun: To in th chun Vref cho mch so snh (thng dng zener).
- Ly mu: Ly mt phn in th ng ra so snh vi in th chun (in th ly mu thay i theo in th ng ra vo).
Nguyn tc hot ng: vo=vi-AV Gi s khi vo thay i (v l do no ), in th ly mu thay i theo trong khi in th chun khng i nn ng ra VDK ca mch so snh thay i, in th VDK ny iu khin mch kch v cng sut thay i hot ng (chy mnh/chy yu) thay i AV sao cho vo n nh.
Mch cn bn c dng:
Q1: Cng sut ;
Q2: Thc (kiu darlington) ;
Q3: So snh
Zener D: to in th chun
R3, R4,VR: Ly mu
C3: Gim sng d ng ra
Do dng ti IL chy thng v thng trc qua Q1 nn Q1 phi c cng sut ln v phi c gii nhit cn thn. Mch thng c mt in tr R cng sut ln chia bt dng qua Q1 2.3. n p ngt m - Tuy c rt nhiu dng, nhng a s u dng phng php bin iu rng xung.
- Nguyn l chung vn nh mch n p tuyn tnh nhng thay mch khuch i kch bng mt mch dao ng to sng vung. Tn hiu ra ca mch dao ng kch vo transistor cng sut n p (thng l BJT hoc MOSFET cng sut ln), mc cao ca xung vung lm transistor bo ha, mc thp lm transistor ngng. Nh vy cng sut n p hot ng nh mt chuyn mch (switch).
- Dao ng to xung vung c th l a hi (cng sut c lp vi mch dao ng) hoc thng dng hn l dao ng blocking (cng sut tham gia vo mch dao ng) do cch ly c mass in.
- Khi cha mc t lc ng ra, v0 c dng xung vi bin nh bng vi khi SW trng thi ON v v0=0 khi SW trng thi OFF. Tr trung bnh ca v0 l
Ta thy: thay i tr trung bnh ng ra vo ta c th:
- Thay i thi gian SW trng thi ON (Transistor dn bo ha)
- Thay i tn s ca mch dao ng (Tc thay i chu k T)
- Hoc thay i c hai
Thc t, tin vic thit k v kim sot, thng ngi ta gi nguyn tn s dao ng (thc t trong my thu hnh, monitor my tnh. Ngi ta dng xung qut ngang a v gi cho tn s dao ng bng vi tn s qut ngang), tn hiu ly mu ch lm thay i rng ca xung vung tc thay i thi gian dn-ngng ca transistor cng sut, tc Tx.
- n nh vo, th d khi vi cao ngi ta gim Tx, khi vi gim ngi ta tng Tx.
- Mch thng c thit k tn s kh cao (hn 10KHz) nn t lc ng ra khng cn ln m vn bo m c vic gim ti a sng d (vo gn l tng).
- to ra nhiu loi in p khc nhau, nht l cch ly c mass in v mass my (chng git), ngi ta thng thit k b ngun c bin p xung. Ty theo tn s hot ng ca mch v s vng cun s cp, th cp m ta c c cc in p khc nhau theo yu cu.
2.4. PHN THC HNH
2.4.1. n p tuyn tnh Mch dng thc tp c dng:
ZC
Vi vi c cp t mt ngun thay i bn ngoi a/ Gii thch vn tt nguyn l hot ng ca mch (khi vi v IL thay i)
b/ Cp vI = +18V, o in th ng ra vo , chnh VR theo hai chiu. Nhn xt v gii thch.
c/ Chnh VR vo=+12V, cho vi thay i t +15V +20V, o vo, lp bng theo mu sau v v th V0 = f(vi). Nhn xt th vo=f(vi). Nhn xt. d/ Cp vi=+18V, o vo khi thay i IL (bng cch thay i RL)
e/ Khng mc t C vo mch, quan st sng d ng ra. Lp li th nghim. Khi mc t C vo mch. Nhn xt v gii thch.
f/ Gi s khng mc Co vo mch, vo b nh hng g? Gii thch?
2.4.2. n p ngt m
3. B bm p (chopper).
3.1. Bm p ni tip
3.1.1. Nguyn l bm p mt chiu ni tip.
- S
S nguyn l bm p mt chiu ni tip gii thiu nh hnh 3.1. theo phn t chuyn mch to cc xung in p mc ni tip vi ti. in p mt chiu c iu khin bng cch iu khin thi gian ng kha K trong chu k ng ct. Trong khong thi gian 0t1 (hnh b) kha K ng in p ti bng in p ngun (Ud = U1), trong khong thi gian t1 t2 kha K m th in p ti bng 0.
Tr s trung bnh in p 1 chiu c tnh
Ud = = Nu coi = : Ud = .U1
f = 1/Tck3.1.2. Hot ng ca s vi ti in cm.
S in hnh c dng:
Dng in c xc nh bi phng trnh vi phn: U1 = Rd.i + Ld.Trong :
I: dng in ti.
Rd: in tr ti
Ld: in cm ti
Ib: dng in ban u ca chu k ang xt (m hay ng kha K);
I XL: dng in xc lp ca chu k ang xt.
Khi kha K ng ; khi kha K m IXL = 0
i = Ibd. + IXL.
Hng s thi gian in t ca mch: Td = nhp nh c tnh:
T biu thc thy rng, bin dao ng dng in ph thuc vo bn thng s: in p ngun cp (U1); rng xung in p (); in cm ti (Ld) v chu k chuyn mch kha K (Tck). Cc thng s: in p ngun cp, rng xung in p ph thuc vo yu cu iu khin in p ti, in cm ti Ld l thng s ca ti. Do ci thin cht lng dng in ti ( gim nh (I) c th tc ng vo Tck. Nh vy, nu chu k chuyn mch cng b (hay tn s chuyn mch cng ln ) th bin p mch dng in cng nh, cht lng dng in 1 chiu cng cao. Do b iu khin ny thng c thit k vi tn s cao hng chc khz.
C th minh ha bng gin dng in in p cho hai tn s khc nhau
3.1.3. lp rp v kho st.
3.2. Cc s ng lc bm p ni tip
Dng Thyristor:(hnh a) hoc dng Transistor
Dng Transistor trng (hnh c).
Dng IGBT (hnh d)
Bm p o chiu
S nh hnh v:
Theo chiu chy thun, iu khin T1, T3, dng in ti iT c chiu hng xung nh hnh v, UAB > 0. Theo chiu chy ngc, iu khin T2, T4, dng in ti iN c chiu di ln nh hnh v, UAB < 0.
3.3. Bm p song song
- Nguyn l bm p song song.
- Tn hao cng sut khi bm p song song.
- Bm p c hon tr nng lng v ngun.3.3.1. Nguyn l bm p song song
S :
Dng in v in p c tnh tng ng khi kha K ng: is = ; Ud = 0
V kha k h: iT = ; Ud = Rd
3.3.2. Tn hao cng sut khi bm p song song.
Trng hp tng qut
Khi iu chnh, chu k xung in p khng i. Khi , c tng t1 th gim t2 v ngc li. Khi cn gim in p ti, cn tng t1 v gim t2, cng sut tn hao trong biu thc tng
Do , bm p song song khng thch hp khi ti nhn nng lng t li.3.3.3. Bm p c hon tr nng lng v ngun
Trng hp ny ch xt khi ti c sc in ng (v d cp in mt chiu v ngun ti thun tr)
Xt trng hp khi ti in cm v c sc in ng (v d ng c lm vic ch h ti)
3.4. Bm p ni tip kt hp song song.
Trong trng hp ti lm vic c ch nhn nng lng v tr nng lng, s phi hp ni tip v song song c s dng.
Khi nhn nng lng v tr nng lng t li, iu khin KN.
Khi tr nng lng v li, iu khin KS.
2.5. Bm p tch ly nng lng
- Bm p tch ly in cm.
- Bm p tch ly in dung.
2.5.1. Bm p tch ly in cm
Khi b bm nm gia ngun p vi ti ngun p, phn tch ly nng lng phi l in cm
Hot ng
Khi T dn: iN = iT = iL , iR = iD = 0, UD = -(U+UR),
U = L.di/dt, iL tng tuyn tnh
Khi D dn: iN = iT = 0, iL = iR = iD, UD = -(U+UR),
UR = - L.di/dt, iL gim tuyn tnh.
Tr s trung bnh dng in ngun: IN = ILTr s trung bnh dng ti: IR = (1-)IL.
B qua tn hao ta c: UR.IR = UN.IN hay:
= = 2.5.2. Bm p tch ly in dung (B bm tng p)- S v hot ng.
- Cc biu thc c bn.
- S ng lc.
Trong khong 0t1 transistor dn c dng in iT chy qua cun dy; diode kha v chu mt in p bng in p ngun.
Trong khong thi gian t1t2 transistor kha, cun dy x nng lng qua ti bng dong iD. Dng in ny ng thi np cho t C.
Khi transistor dn li, t x qua ti duy tr dng in trn ti. Coi in dung ca t ln, dng in iC qua ti by gi gn nh khng i
Cc biu thc c bn
Khi T dn, diode chu mt in p:
UD = UN + UC = UN +UtiKhi T kha, n chu mt in p:
UT = UN + UC = UN +UtiCc gi tr dng in
iN = Id ; IL = IN + Id = IdUd = UC = E0 - ;
Nu coi R0 = 0, ta c:
Ud = E0; UN = E0 - Id3.6. iu khin mt chiu cc ngt bn dn
Nguyn l iu khin
S khi mch iu khin
Cc khu c bn
Mch v d3.6.1. Nguyn l iu khin
Mch iu khin bm p mt chiu c nhim v xc nh thi im m v kha van bn dn trong mt chu k chuyn mch. Nh bit trn, chu k ng ct van nn thit k c nh. in p ti khi iu khin c tnh
Uti = U1
3.6.2. S khi mch iu khin
S khi
- Khu to tn s; c nhim v to in p ta rng ca Urc vi tn s theo mun ngi thit k. Tn s ca cc b iu p mt chiu thng chn kh ln ( hng chc KHz). Tn s ny ln hay b l do kh nng chu tn s ca van bn dn. Nu van ng lc l Thyristor tn s ca khu to tn s khong 1-5 KHz. Nu van ng lc l transistor lng cc, trng, IGBT tn s c th hng chc kHz.
- Khu so snh: c nhim v xc nh thi im in p ta bng in p iu khin. Ti thi im in p ta bng in p iu khin th pht lnh m hoc kha van bn dn.
- Khu to xung, khuch i: c nhim v to xung ph hp m van bn dn. Mt xung ccoi l ph hp m van l xung c cng sut ( dng in v in p iu khin), cch ly gia mch iu khin vi mch ng lc c s dng.
Cc khu c bn3.6.2.1. Khu to tn s
3.6.2.2.To in p tam gic bng dao ng a hi
R1 =R2 = R ; T= 2RCl.ln3 = 2RC.1,1 = 2,2.RC3.6.2.3.To in p ta bng mch tch phn
3.6.3. Mch tch thot (UJT)
f=
3.6.4. Mch dao ng bng IC566
Mch dao ng bng IC 566
3.6.5. Mch ta in p bng 4046
3.6.5.1. Khu so snh
3.6.5.2. Khu khuch i
3.6.5.2.1. Mch khuch i cho iu p mt chiu bng Thyristor
S mch
S mch khuch i (tip)
3.6.5.2.2. Mch khuch i cho van ng lc l transistor
Bi 4: PHNG THC IU RNG XUNG (PWM) 4.1. PHNG PHAP IEU CHE O RONG XUNG SIN (SIN PWM)
4.1.1. S nguyn l 4.1.2. Nguyn l ca mchVe nguyen ly, phng phap thc hien da vao ky thuat analog. Gian o kch ong cong tac bo nghch lu da tren c s so sanh hai tn hieu c ban:
- Song mang up (carrier signal) tan so cao
V du: cong tac le c kch ong khi song ieu khien ln hn song mang (ur>up). Trong trng hp ngc lai, cong tac chan c kch ong.
Song mang up co the dang tam giac. Tan so song mang cang cao, lng song hai bac cao b kh bt cang nhieu. Tuy nhien, tan so ong ngat cao lam cho ton hao phat sinh do qua trnh ong ngat cac cong tac tang theo. Ngoai ra, cac linh kien oi hoi co thi gian ong ton, va ngat toff nhat nh. Cac yeu to nay lam han che viec chon tan so song mang.
Song ieu khien ur mang thong tin ve o ln tr hieu dung va tan so song hai c ban cua ien ap ngo ra. Trong trng hp bo nghch lu ap ba pha, ba song ieu khien cua ba pha phai c tao lech nhau ve pha 1/3 chu ky cua no. Trong trng hp bo nghch lu ap mot pha, tng ng vi hai pha tai tng tng hnh (H5.9), ta can tao hai song ieu khien lech pha nhau 1/2 chu ky (tc chung ngc pha nhau). e n gian mach kch hn na, ta co the s dung mot song ieu khien duy nhat e kch ong, v du: cap cong tac (S1S4) c kch ong theo quan he gia song ieu khien va song mang, con cap (S3S2) c kch ong ngc lai vi chung. Luc o, hnh thanh trang thai kch ong (S1S2) hoac (S3S4).
Goi mf la t so ieu che tan so (Frequency modulation ratio) : tria carrier
Viec tang gia tr mf se dan en viec tang gia tr tan so cac song hai xuat hien. iem bat li cua viec tang tan so song mang la van e ton hao do ong ngat ln.
Tng t, goi ma la t so ieu che bien o (Amplitude modulation ratio):
tri m carrier m Neu (bien o song sin nho hn bien o song mang) th quan he gia bien o thanh phan c ban cua ap ra va ap ieu khien la tuyen tnh.
4.2. PHNG PHAP IEU CHE O RONG XUNG CAI BIEN (MODIFIED SPWM)
Nhc iem cua phng phap ieu che o rong xung sin (SPWM) la kha nang ieu khien tuyen tnh ch thc hien c vi ch so ieu che m nam trong pham vi 0 m 0.785 (tng ng ch so ma 1). Luc o, bien o song hai c ban ien ap pha tai nam trong gii han (0,U/2).
e m rong pham vi ieu khien tuyen tnh, phng phap ieu che o rong xung sin cai bien co the c s dung.
Phng phap nay cho phep thc hien ieu khien tuyen tnh ien ap tai vi ch so ieu che nam trong pham vi 0 m 0. 907, bien o song hai bac mot ien ap at gia tr cc ai bang va ch so ieu che luc o bang 0.907
Nguyen ly thc hien: gian o kch ong linh kien cung da vao ket qua so sanh cac tn hieu ieu khien va song mang (dang tam giac) tan so cao. Song ieu che (ur1,ur2,ur3) c tao thanh bang cach cong thanh phan tn hieu dang sin vi mot thanh phan song hai boi ba (thanh phan th t khong). Khi tang o ln song ieu khien e at ch so ieu che m ln hn 0,907, quan he ieu khien tr nen phi tuyen.
Song ieu che co the chon dang lien tuc hoac gian oan.
4.2.1.Trng hp song ieu che lien tuc di dang ham ieu hoa gom cac thanh phan ham ieu hoa bac 1 va ham ieu hoa bac boi ba nh sau, v du oi vi pha th nhat (xem o th ur1a, hnh H5.11a):
4.2.2.Trng hp song ieu che lien tuc dan giai t tng quan gia phng phap ieu che o rong xung lay mau (sampling PWM) va phng phap ieu che vector khong gian.
Ham mo ta song ieu khien ba pha oi vi pha th nhat co the viet di dang nh sau (xem o th ur1b, hnh H5.11b):
4.2.3.Trng hp ham ieu che gian oan: ton tai nhieu dang song ieu che dang khong lien tuc c a ra e thc hien phng phap ieu che o rong xung cai bien. Mot trong cac dang song ieu khien dang gian oan c mo ta bi ham sau ay oi vi pha th nhat: (xem hnh H5.11c):
u iem cua song ieu che dang gian oan la so lan chuyen mach trong mot chu kyb giam xuong, do o cong suat ton hao do qua trnh ong ngat cung giam theo. Do tn hieu song ieu che c thiet lap gia tr cc tr trong mot phan ba chu ky nen so lan chuyen mach se giam i mot phan ba so vi phng phap ieu che vi tn hieu lien tuc.
3. IEU CHE THEO MAU (REGULAR SAMPLING TECHNIQUES)
Nguyen ly cua phng phap ieu che o rong xung sin da vao ky thuat analog.
Viec ieu che o rong xung cung co the thc hien tren c s ky thuat so. Luc o, tn hieu ieu khien c so hoa trong tng chu ky lay mau. Mau tn hieu sau o c so sanh vi song rang ca v du thc hien bang mach em. Ky thuat lay mau co the thc hien oi xng hoac khong oi xng. Ky thuat oi xng c thc hien vi chu ky lay mau bang chu ky song tam giac (H5.12a), trng hp lay mau khong oi xng xay ra khi viec lay mau dien ra moi na chu ky song tam giac (H5.12b).
Khi ap dung phng phap lay mau oi xng, khong can thiet tao ra song tam giac nh tren hnh ve 5.12a. Goi T1, T2 la cac khoang thi gian (xem hnh H5.12a) dung e xac nh thi iem kch ong linh kien, T1,T2 co the xac nh trong thi gian thc (real time) bang phep tnh n gian (5.58), (5.59) nh sau:
2
Trong o, 2Ts la khoang thi gian cua chu ky lay mau, tsn, ts(n+1) la cac thi iem thc hien viec lay mau, u*a (ts ) la ham song ieu khien dang analog.
4. PHNG PHAP IEU CHE O RONG XUNG TOI U (OPTIMUM PWM)
Anh hng cua mot so song hai bac thap cha trong ap ra co the kh bo hoac han che bang phng phap ieu che o rong xung toi u. Gian o kch ong cac cong tac c thiet lap tren c s phan tch ham toi u theo cac bien la goc kch ong cac linh kien.
Trong trng hp ham toi u c thc hien bang cach triet tieu mot so song hai bac cao, phng phap tren c goi la phng phap triet tieu cac song hai chon loc (Selective Harmonic Elimination- SHE).
Bien o cac song hai co the xac nh qua khai trien chuoi Fourier dang song ap ra:
U = U ( , ...., )
U3=U3(1,2,...,n) (5.60)
U2k+1=U2k+1(1,2,...,n)
Vi SHE, gian o kch ong c chon se kh bo (n -1) song hai bac cao va ieu khien song hai c ban, ham toi u quan he gia cac goc 1,2,...,n c bieu dien qua he n phng trnh sau:
Giai he cac phng trnh xac nh goc kch 1,2,...,n ta se thiet lap c gian o kch ong cac cong tac.
Neu dang ien ap tai la ham le, he so bk trong phan tch chuoi Fourier se triet tieu va ta co:
Pham vi ieu khien ien ap cua phng phap SHE:
Trong pham vi ieu khien PWM tuyen tnh (m0.907), phng phap SHE chuyen sang pham vi ieu khien ieu che m rong (qua ieu che). Nghiem cua he phng trnh (5.61) khong the luon luon ton tai vi yeu cau triet tieu cac song hai cho trc. Do o,
vi yeu cau triet tieu mot so song hai chon loc, ton tai mot gii han toi a cua ch so ieu che mmax tng ng. o th tren hnh H5.14 minh hoa quan he gia ch so ieu che cc ai at c theo SHE va so song hai (n) c triet tieu kem theo.
Tai gia tr m=1, cac thanh phan song hai ton tai ay u nh cua trng hp ieu khien theo phng phap ieu khien sau bc.
V du 5.1: Thiet lap he phng trnh lng giac e tm nghiem la cac goc chuyen mach e ieu khien bien o song hai c ban va kh bo 4 song hai bac 5,7,11 va 13. Xac nh gia tr cu the cac goc chuyen mach khi ch so ieu che m=0.8.
Giai:
Ta can thc hien 5 lan chuyen mach (n=5) trong chu ky ap ra. He phng trnh xac nh goc chuyen mach se la:
Vi m=0.8, s dung phng phap Newton-Raphson va giai he phng trnh tren bang
may tnh, ta thu c he nghiem sau:
5. PHNG PHP IU CH RNG XUNG KHNG BINS mch:
Nguyn l: Phng php thc hin da vo k thut analog. Gin kch ng cng tc b nghch lu da trn c s so snh 2 tn hiu c bn:
Sng mangup (carrier signal) tn s cao.
Sng iu khin ur (reference signal) hoc sng iu ch (modulating signal) dng sin. V d: cng tc l s c kch ng khi sng iu khin ln hn sng mang (ur> u p). Trong trng hp cn li, cng tc chn s c kch ng.
Sng mang up c th dng tam gic. Tn s sng mang cng cao, lng sng hi bc cao b kh bt cng nhiu. Tuy nhin, tn s ng ct cao lm cho tn hao pht sinh do qu trnh ng ngt cc cng tc tng theo. Ngoi ra, cc linh kin i hi c thi gian ng ton v thi gian m toff nht nh. Cc yu t ny lm hn ch vic chn tn s sng mang.
Sng iu khin mang thng tin v ln tr hiu dng v tn s sng hi c bn ca in p ng ra. Trng hp b nghch lu p ba pha, 3 sng iu khin ca 3 pha phi c to lch nhau v pha 1/3 chu k ca n. Trong trng hp b nghch lu p 1 pha, ta cn to 2 dng sng iu khin lch pha nhau chu k (tc l chng ngc pha nhau). n gin mch kch hn na, ta c th s dng mt sng iu khin duy nht kch ng, v d: cp cng tc (s1s4) c kch ng theo quan h ca sng iu khin v sng mang, cn cp (s3s2) c kch ng ngc li vi chng. Lc hnh thnh trng thi kch ng (S1S2) hoc (S3S4).
Gi mf l t s iu ch tn s (frequency modulation ratio):
Vic tng gi tr mf s dn n vic tng gi tr tn s cc sng hi xut hin.im bt li ca vic tng tn s sng mang l vn tn hao ng ngt ln.
Tng t, gi ma l t s iu ch bin (Amplitude modulation ratio):
Nu ma 1 (bin sng sin nh hn bin sng mang) th quan h gia bin thnh phn c bn ca p ra v p iu khin l tuyn tnh.
i vi b nghch lu p 1 pha, bin sng hi c bn U t(1)m = ma.U/2
Khi gi tr ma > 1, bin tn hiu iu ch ln hn bin sng mang th bin hi c bn in p ra tng khng tuyn tnh theo bin ma. Lc ny, bt u xut hin lng sng hi bc cao tng dn cho n khi t mc gii hn cho bi phng php 6 bc. Trng hp ny cn c gi l qu iu ch (overmodulation) hoc iu ch m rng.
Trong trng hp b nghch lu p ba pha, cc thnh phn sng hi bc cao s c gim n cc tiu nu gi tr mf c chn bng s l bi ba
Nu n h thc tnh ch s iu ch, ta thy phng php SPWM t c ch s ln nht trong vng tuyn tnh khi bin sng iu ch bng bin sng mang. Lc , ta c:
Phn tch sng hi:
Vic nh gi cht lng sng hi xut hin trong in p ti c th c thc hin bng phn tch chui Fourier . y, chu k ly phn tch Fourier c chia thnh nhiu khong nh, vi cn ly tch phn ca tng khong c xc nh t cc giao im ca sng iu hin v sng mang dng tam gic.Bi 5: B BIN TN (CYCLO-CONVERTER)5.1. Khi nim v b bin tn
Bo bien tan dung e chuyen oi ien ap hoac dong ien xoay chieu au vao t mot tan so nay thanh ien ap hoac dong ien co mot tan so khac au ra.
ng dung: Bo bien tan thng c s dung e ieu khien van toc ong c xoay chieu theo phng phap ieu khien tan so, theo o tan so cua li nguon se thay oi thanh tan so bien thien. Ngoai viec thay oi tan so con co s thay oi tong so pha. T nguon li mot pha, vi s giup cua bo bien tan ta co the mac vao tai ong c ba pha. Bo bien tan con c s dung rong rai trong ky thuat nhiet ien. Bo bien tan trong trng hp nay cung cap nang lng cho lo cam ng.
5.1.1. Phn loi bin tn.
5.1.1.1. Bin tn quay.
Khi nim: bin tn quay l my pht in xoay chiu
Vi
Trong :
f: tn s in p tnh bng Hz;
n: tc quay tnh theo vng/pht;
p: s i cc my in
5.1.1.2. Bin tn tnh
Khi nim: l loi bin tn c ch to t cc linh kin bn dn cng sut. C 2 loi bin tn tnh thng gp:
Bin tn trc tip.
Bin tn c lp.
Bin tn trc tip: bin i trc tip ng cong li in xoay chiu cng nghip, n c dng:
Bin tn c lp: l loi bin tn i t in 1 chiu thnh in xoay chiu. in p ngun cp mt chiu nn khi bin i in p xoay chiu l nhng xung vung c dang nh hnh v sau:
5.1.1.3. Bin tn trc tip
Khi nim: ngi ta trc tip bin i ng cong li in cng nghip, khi ta thu c in p vi tn s khc tn s in li.
C th dng mch in vi ngun cp mt pha hay ngun cp 3 pha.
Bng cch ng m Thyristor theo quy lut no ta c c in p xoay chiu.
Bin tn trc tip ngun cp 1 pha
S mch in
Mt s khc ca bin tn 1 pha
5.1.1.4. Bin tn c lp 1 pha
Bin tn c lp ngun p dng bin p c trung tnh.
S nguyn l
Hot ng ca s c gii thch theo c tuyn nh hnh v sau:
Bin tn c lp ngun p dng na cu
S nguyn l:
Dng sng dng in v in p
5.1.1.5. Bin tn c lp ngun p dng cu
S nguyn l:
Dng sng dng in v in p
5.2. Bo bien tan gian tiep
5.2.1. Khai niem va ng dung Bo bien tan dung e chuyen oi ien ap hoac dong ien xoay chieu au vao t mot tan so nay thanh ien ap hoac dong ien co mot tan so khac au ra.
ng dung: Bo bien tan thng c s dung e ieu khien van toc ong c xoay chieu theo phng phap ieu khien tan so, theo o tan so cua li nguon se thay oi thanh tan so bien thien. Ngoai viec thay oi tan so con co s thay oi tong so pha. T nguon li mot pha, vi s giup cua bo bien tan ta co the mac vao tai ong c ba pha. Bo bien tan con c s dung rong rai trong ky thuat nhiet ien. Bo bien tan trong trng hp nay cung cap nang lng cho lo cam ng.
5.2.2. Phan loai
* Theo tong so pha, cac bo bien tan
- Mot pha
- Ba pha
- m pha
* Theo cau truc mach ien, cac bo bien tan
- Gian tiep (mach cha khau trung gian mot chieu), trong o ta phan biet bien tan dung bo nghch lu ap va bien tan dung bo nghch lu dong vi qua trnh chuyen mach phu thuoc mach nguon hoac vi qua trnh chuyen mach cng bc.
- Trc tiep (khong co mach trung gian mot chieu)- con goi la cycloconvertor. Bo
bien tan trc tiep co the hoat ong - Vi qua trnh chuyen mach phu thuoc ben ngoai: tn hieu ieu khien co dang hnh thang hoac dang ieu hoa:
- Voi qua trnh chuyen mach cng bc (t gap).
Trng hp qua trnh chuyen mach phu thuoc mach nguon co the chia lam hai trng hp: trng hp vi dong ien can bang va trng hp khong co dong ien can bang
5.2.3 S o bo bien tan gian tiep
Cau tao cua bo bien tan gian tiep gom co bo chnh lu vi chc nang chnh lu ien ap xoay chieu vi tan so co nh ngo vao va bo nghch lu thc hien viec chuyen oi ien ap (hoac dong ien) chnh lu sang dang ap hoac dong xoay chieu ngo ra.
Bang cau truc nh tren, ta co the ieu khien tan so ra mot cach oc lap khong phu thuoc tan so vao
Cac bo bien tan gian tiep thng hoat ong vi cong suat khoang t kW en vai tram kW. Pham vi hoat ong cua tan so khoang vai phan chuc Hz en vai tram Hz. Cong suat toi a cua chung co the len en vai MW va tan so toi a khoang vai chuc kHz (trong ky thuat nhiet ien - lo cao tan). Cau truc mach c ve tren hnh H5.56
Mach trung gian mot chieu: co cha tu loc vi ien dung kha ln Cf (khoang vai ngan F) mac vao ngo vao cua bo nghch lu. ieu nay giup cho mach trung gian hoat ong nh nguon ien ap. Tu ien cung vi cuon cam Lf cua mach trung gian tao thanh mach loc nan ien ap chnh lu. Cuon khang Lf co tac dung nan dong ien chnh lu.
Trong nhieu trng hp, cuon khang Lf khong xuat hien trong cau truc mach va tac dung nan dong cua no co the c thay the bang cam khang tan may bien ap cap nguon cho bo chnh lu. Do tac dung cua diode nghch ao bo nghch lu, ien ap at tren tu ch co the at cac gia tr dng. Tu ien con thc hien chc nang trao oi nang lng ao gia tai cua bo nghch lu va mach trung gian bang cach cho phep dong id2 thay oi chieu nhanh khong phu thuoc vao chieu cua dong id1.
Bo nghch lu ap: dang mot pha hoac ba pha. Qua trnh chuyen mach cua bo nghch lu ap thng la qua trnh chuyen oi cng bc. Trong trng hp ac biet bo nghch lu lam viec khong co qua trnh chuyen mach hoac vi qua trnh chuyen mach phu thuoc ben ngoai. T o, ta co hai trng hp bo bien tan vi qua trnh chuyen mach oc lap va qua trnh chuyen mach phu thuoc ben ngoai .
Bo chnh lu: co nhieu dang khac nhau, mach tia, mach cau mot pha hoac ba pha.
Thong thng ta gap mach cau ba pha. Neu nh bo chnh lu mot pha va bo nghch lu ba pha, bo bien tan thc hien ca chc nang bo bien oi tong so pha.
Khi ap dung phng phap ieu khien theo bien o cho ien ap tai xoay chieu ra bo chnh lu phai la bo chnh lu ieu khien.Thong thng, bo chnh lu co dang khong ieu khien, bao gom cac diode mac dang mach cau. o ln ien ap va tan so ap ra cua bo nghch lu con co the ieu khien thong qua phng phap ieu khien xung thc hien trc tiep ngay tren bo nghch lu. che o may phat cua tai (chang han khi ham ong c khong ong bo), nang lng ham c tra ngc ve mach mot chieu va nap cho tu loc Cf. Nang lng nap ve tren tu lam ien ap no tang len va co the at gia tr ln co the gay qua ap. e loai bo hien tng qua ien ap tren tu Cf, mot so bien phap sau ay co the thc hien. Phng phap n gian nhat la tac dung ong mach xa ien ap tren tu qua mot ien tr mac song song vi tu. Viec ong mach xa tu thc hien nh cong tac ban dan S- hnh H5.56 (chang han ieu khien ap tu gia hai gia tr bien) da theo ket qua so sanh tn hieu ien ap o c tren tu vi mot gia tr ien ap at trc cho phep Mot bien phap khac la thc hien a nang lng qua ap tren tu Cf ve nguon li ien xoay chieu. Trong trng hp nay, bo bien tan c trang b bo chnh lu kep (hnh H5.57). Kha nang bo chnh lu kep cho phep thc hien ao chieu dong ien qua bo chnh lu va bang cach nay, trong ieu kien chieu ien ap tu loc khong oi dau, nang lng c tra ve li ien xoay chieu qua bo chnh lu.
Xu hng nang cao chat lng ien nang bang cach s dung loai chnh lu ieu rong xung (boost PWM Rectifier) a cho phep thc hien tra cong suat ve nguon vi he so cong suat cao (gan nh bang mot) (hnh H5.58). Dong ien i qua nguon li xoay chieu co dang gan nh sin va cung pha vi ien ap xoay chieu.
5.3. Bo bien tan trc tiep (cycloconverter)
5.3.1. Khai niem
Bo bien tan trc tiep-Cycloconverter, tao nen ien ap xoay chieu ngo ra vi tr hieu dung va tan so ieu khien c. Nguon ien ap xoay chieu vi tan so va bien o khong oi cung cap nang lng cho bo bien tan nay.
Bo bien tan trc tiep dung e ieu khien truyen ong ong c ien xoay chieu.
Theo qua trnh chuyen mach, bo bien tan trc tiep c phan biet lam hai loai: bo bien tan co qua trnh chuyen mach phu thuoc va bo bien tan co qua trnh chuyen mach cng bc.
Bo bien tan trc tiep vi qua trnh chuyen mach cng bc cha cac linh kien t chuyen mach nh GTO, transistor. Chung c trnh bay ve nguyen ly hoat ong trong chng bo bien oi ma tran (Matrix conveter).
Bo bien tan vi qua trnh chuyen mach phu thuoc c s dung nhieu trong cong nghiep. Tnh phu thuoc ay bieu hien kha nang ngat dong ien qua linh kien thc hien nh tac dung cua ien ap nguon xoay chieu hoac sc ien ong xoay chieu cua tai. Do o, mach ch can trang b thyristor thong thng. Vi tai cong suat ln, viec s dung linh kien chuyen mach t nhien nh SCR co y ngha rat quan trong v hieu qua kinh te cua thiet b.
Do phu thuoc vao ien ap xoay chieu cua nguon nen tan so ien ap ngo ra b gii han mc thap hn tan so ien ap nguon. Bo bien tan nay c ng dung trong cac truyen ong ong c cong suat ln toc o cham.
5.3.2. Bo bien tan trc tiep mot pha
* Phan tch hoat ong bo bien tan trc tiep vi qua trnh chuyen mach phu thuoc ien ap nguon xoay chieu
S o cau tao va nguyen ly hoat ong.
Xet bo bien tan trc tiep mot pha tren hnh ve H5.61.
Bo bien tan co cau tao cua bo chnh lu kep. Do o, phan tch hoat ong va phng phap ieu khien bo bien tan giong nh bo chnh lu kep. ieu khac biet so vi chc nang cua bo chnh lu kep la bo bien tan co qua trnh ien ap tren tai - tc ien ap chnh lu oi dau mot cach lien tuc va tuan hoan.
S o ieu khien bo bien tan trc tiep theo phng phap ieu khien rieng c ve minh hoa tren hnh H5.63. Mach logic lien quan en cac tn hieu c mo ta kem theo bang B5. Qua trnh ien ap va dong ien cac phan t mach c ve tren hnh H5.62a.
Theo o, ton tai mot khoang thi gian dong tai bang zero khi no thc hien oi dau t dng sang am va ngc lai.
o th ien ap va dong ien tai cung nh o th cac dong ien thanh phan i qua cac bo chnh lu tia 3 xung trong trng hp ieu khien ong thi cac bo chnh lu c ve tren hnh H5.62b. ien ap tai xoay chieu tao thanh co the phan tch thanh song hai c ban co tan so bang tan so yeu cau cua ap tai va cac song bac cao vi tan so phu thuoc vao so xung ien ap chnh lu. Chat lng ien ap va dong ien tai c cai thien ro ret vi bo chnh lu nhieu xung, v du trng hp cycloconverter gom cac bo chnh lu cau ba pha tao nen co dang song ien ap va dong ien tai cho tren hnh H5.64.
Quan he gia ien ap ieu khien va ien ap tai.
ien ap ieu khien cho moi tai nghch lu co the chon dang sin (hoac dang ch nhat, dang hnh thang....). ien ap ieu khien va song hai c ban cua ap tai co tan so bang nhau va bien o t le vi nhau.
Ta gia thiet rang, chu ky ap ieu khien ln hn nhieu so vi chu ky ap chnh lu (s dung bo chnh lu 24,36....xung ). Do do, co the xem qua trnh ap ieu kh ien khong thay oi trong chu ky ang xet, tng t nh vay song hai c ban ien ap cua tai tao thanh co o ln khong oi trong chu ky ang xet va bang chnh tr trung bnh ien ap chnh lu.
ieu nay co ngha la tr tc thi cua song hai c ban cua ap ra la tr trung bnh ien ap chnh lu tai thi iem ang xet.
Goi: - ut(1) la song hai c ban cua ien ap tai
- uk la ham song ieu khien
- Ud0...tr trung bnh ien ap chnh lu khi goc ieu khien bang 0
- UpM bien o ien ap ong bo dang rang ca hoac dang cosin
Neu ap ieu khien dang sin co bien o la UkM va tan so : Uk = UkM.sin(t)
ien ap tai co song hai c ban thay oi theo he thc:
Khi song ieu khien co bien o bang bien o song rang ca, song hai c ban cua ap tai co bien o bang Ud0.
5.3.3. Bo bien tan trc tiep ba pha
Bo bien tan trc tiep co cac cau hnh dang ay u, oi xng (H5.65a,b), trong o phu thuoc kieu au cua nguon, ta phan biet cau truc s dung chung nguon t mot cuon th cap may bien ap va cau truc co nguon rieng cho tng pha tai. Cau truc co chung cuon th cap may bien ap oi hoi mach tai ba pha co iem trung tnh e h . Neu cac pha tai khong the phan cach oc lap, co the s dung cau truc mach bien tan trc tiep co nguon rieng (H5.65b). Vi cau truc mach bien tan hnh H5.65b s dung nguon chung, khi thc hien chuyen mach cac linh kien nhom na tren cua mach cau, hien tng ngan mach nguon co the xay ra.
Bo bien tan trc tiep ba pha co qua trnh chuyen mach phu thuoc ap nguon xoay chieu.
Cac cau truc tiet kiem linh kien se tao nen s khong oi xng cua cac nhanh linh kien. Hai dang bien tan trc tiep khong oi xng c ve minh hoa tren hnh H5.65c va H5.65d.
Tren hnh H5.66 ve s o mach cong suat bo bien tan trc tiep gom cac bo chnh lu tia ba pha vi nguon chung. Cac bo chnh lu kep co the c ieu khien theo phng phap rieng le hoac ong thi (co dong can bang). Tuy nhien, viec s dung chung nguon tren trong thc te se co nhng kho khan do qua trnh chuyen mach trong tng nhom bo chnh lu tao nen. Do o, vi yeu cau chat lng cao, cac bo chnh lu cho tng pha tai se c au vao tng pha nguon cach ly rieng (hnh H5.65b).
Tan so hai c ban cua c ban cua ien ap ra f2 cho bi he thc:
vi: l la tong so xung ap chnh lu cha trong na chu ky ap tai m la so pha f1 la tan so ap nguon xoay chieu Neu bo bien tan do cac bo chnh lu kep mach tia ba pha tao thanh, ta co: f2max 0,43 f1 (thc te s dung en gii han f1 / 3) va trong trng hp do cac bo chnh lu kep mach cau ba pha tao thanh, ta co: f2max=0,6 f1 (thc te khoang f1/2 )
u iem cua bo bien tan trc tiep vi qua trnh chuyen mach phu thuoc ien ap nguon la mach ch can thyristor thong thng. Neu so xung ap chnh lu ln, dang ap tren tai co dang gan nh sin. Ton hao phat sinh do ong ngat linh kien thap, bo bien tan khong can mach loc trung gian nen co hieu suat cao.
Neu s co xay ra trong qua trnh chuyen mach mot pha nao o, bo bien tan van tiep tuc hoat ong bnh thng du li ien b bien dang. Bo bien tan co kha nang lam viec tan so thap.
Khuyet iem cua mach la so linh kien s dung nhieu va do o mach ieu khien phc tap. Do bo bien tan lam viec tren nguyen ly ieu khien pha cua bo chnh lu nen he so cong suat thap.
5.3.4. Bo bien tan trc tiep vi qua trnh chuyen mach phu thuoc vao ap tai
S o mach cong suat giong nh tren hnh H5.65, tuy nhien phng cach ieu khien linh kien SCR khac biet. Bo bien tan trc tiep vi qua trnh chuyen mach phu thuoc co the cho ien ap tai vi tan so cao hn tan so nguon ap xoay chieu neu ien ap chuyen mach lay t ien ap cua tai. Trong trng hp nay, tai co tnh dung khang, chang han ong c ong bo kch t d.
Cac bo chnh lu kep A,B,C c ieu khien e tao dong ien qua tai dang 6 bc. Do o, tai moi thi iem, ch ton tai dong ien qua hai nhanh thyristor trong cac bo chnh lu, chang han dong qua mot thyristor nhom A+ va mot thyristor nhom B - Trang thai (A+ B-) . S thay oi vai tro dan ien gia cac nhom tao nen cac trang thai mach Thi iem chuyen mach gia cac nhanh chang han t (A+ C-) sang (B+ C-) c chon sao cho ap khoa ton tai tren thyristor can ong. V du khi A1 ong, ta co ap tren B1 bang:
UVB1 = Ut1 - Ut2 (gia s bo qua ap tren cac cuon khang can bang).
Khi UVB1 > 0 : xung kch ong se lam B1 ong.
Do ien ap chuyen mach cua A1 cung chnh la ap khoa tren thyristor B1 phu thuoc vao ien ap cac pha tai nen qua trnh chuyen mach ay c goi la qua trnh chuyen mach phu thuoc vao ap tai.
Khi ien ap tren tai nho (chang han ong c chay vi van toc cham), ien ap chuyen mach khong u tin cay, viec kch ong va ngat thyristor co the da vao ien ap nguon xoay chieu. Luc o, qua trnh chuyen mach phu thuoc ap nguon xoay chieu. V du e ngat dong ien ang dan qua thyristor A1 cua nhom A+ va ong thyristor B3 cua nhom B+ , xung kch a vao B3 tai thi iem ma ien ap tren UB3 dng tc la:
UB3 = U3 - U1 + Ut1 - Ut2 U3 - U1 > 0 (do Ut1, Ut2 nho ) 5.3.3. So sanh bo bien tan trc tiep va bo bien tan gian tiep
Pham vi hoat ong cua ien ap ra: tng oi nho trong trng hp bo bien tan trc tiep vi qua trnh chuyen mach phu thuoc ben ngoai (0 en 25Hz) va kha cao trong trng hp bo bien tan gian tiep ( vai phan chuc cua Hz en vai ngan Hz)
Dang cua song ien ap ra: thuan li hn trong trng hp bo bien tan trc tiep, oi vi chung, ta co the dung cac mach ieu khien n gian e at c dang dong ien tai gan nh hnh sin.
Phng phap chuyen mach: rat thuan tien trong trng hp bo bien tan trc tiep.
Do tac dung cua qua trnh chuyen mach phu thuoc ben ngoai trong cac bo bien tan trc tiep vi qua trnh chuyen mach phu thuoc, ta co the at cong suat rat ln hang chuc MW so vi trng hp bo bien tan gian tiep vi qua trnh chuyen mach oc lap (khoang n v MW). Qua trnh chuyen mach oc lap cua cac bo bien tan gian tiep oi hoi t chi tiet ban dan so vi trng hp bo bien tan trc tiep. Chang han, bo bien tan gian tiep ba pha gom 12 thyristor chnh va cac bo chuyen mach. Bo bien tan trc tiep ba pha gom cac bo chnh lu 6 xung oi hoi en 36 thyristor He so cong suat: tot nhat trong bo bien tan gian tiep s dung phng phap ieu khien o xung rong cua ien ap ra (PWM).
Bi 6: B nghch lu (inverter) 6.1. Gii thiu v ngun in p nghch lu.
Bo nghch lu co nhiem vu chuyen oi nang lng t nguon ien mot chieu khong oi sang dang nang lng ien xoay chieu e cung cap cho tai xoay chieu.
ai lng c ieu khien ngo ra la ien ap hoac dong ien. Trong trng hp au, bo nghch lu c goi la bo nghch lu ap va trng hp sau la bo nghch lu dong.
Nguon mot chieu cung cap cho bo nghch lu ap co tnh chat nguon ien ap va nguon cho bo nghch lu dong co tnh nguon dong ien. Cac bo nghch lu tng ng c goi la bo nghch lu ap nguon ap va bo nghch lu dong nguon dong hoac goi tat la bo nghch lu ap va bo nghch lu dong.
Trong trng hp nguon ien au vao va ai lng ngo ra khong giong nhau, v du bo nghch lu cung cap dong ien xoay chieu t nguon ien ap mot chieu, ta goi chung la bo nghch lu ieu khien dong ien t nguon ien ap hoac bo nghch lu dong nguon ap.
Cac bo nghch lu tao thanh bo phan chu yeu trong cau tao cua bo bien tan. ng dung quan trong va tng oi rong rai cua chung nham vao lnh vc truyen ong ien ong c xoay chieu vi o chnh xac cao. Trong lnh vc tan so cao, bo nghch lu c dung trong cac thiet b lo cam ng trung tan, thiet b han trung tan. Bo nghch lu con c dung lam nguon ien xoay chieu cho nhu cau gia nh, lam nguon ien lien tuc UPS, ieu khien chieu sang, bo nghch lu con c ng dung vao lnh vc bu nhuyen cong suat phan khang.
Cac tai xoay chieu thng mang tnh cam khang (v du ong c khong ong bo, lo cam ng), dong ien qua cac linh kien khong the ngat bang qua trnh chuyen mach t nhien.
Do o, mach bo nghch lu thng cha linh kien t kch ngat e co the ieu khien qua trnh ngat dong ien.
Trong cac trng hp ac biet nh mach tai cong hng, tai mang tnh chat dung khang (ong c ong bo kch t d ), dong ien qua cac linh kien co the b ngat do qua trnh chuyen mach t nhien phu thuoc vao ien ap nguon hoac phu thuoc vao ien ap mach tai.
Khi o, linh kien ban dan co the chon la thyristor (SCR). 6.2. Bo nghch lu ap
Bo nghch lu ap cung cap va ieu khien ien ap xoay chieu ngo ra. Trong cac trng hp khao sat di ay ta xet bo nghch lu ap vi qua trnh chuyen mach cng bc s dung linh kien co kha nang ieu khien ngat dong ien.
Nguon ien ap mot chieu co the dang n gian nh acquy, pin ien hoac dang phc tap gom ien ap xoay chieu c chnh lu va loc phang.
Linh kien trong bo nghch lu ap co kha nang kch ong va kch ngat dong ien qua no, tc ong vai tro mot cong tac. Trong cac ng dung cong suat nho va va, co the s dung transistor BJT, MOSFET, IGBT lam cong tac va pham vi cong suat ln co the s dung GTO, IGCT hoac SCR ket hp vi bo chuyen mach. Vi tai tong quat, moi cong tac con trang b mot diode mac oi song vi no. Cac diode mac oi song nay tao thanh mach chnh lu cau khong ieu khien co chieu dan ien ngc lai vi chieu dan ien cua cac cong tac. Nhiem vu cua bo chnh lu cau diode la tao ieu kien thuan li cho qua trnh trao oi cong suat ao gia nguon mot chieu va tai xoay chieu, qua o han che qua ien ap phat sinh khi kch ngat cac cong tac.
6.2.1. Bo nghch lu ap mot pha
Bo nghch lu ap mot pha dang mach cau (con goi la bo nghch lu dang ch H) (hnh H5.1a) cha 4 cong tac va 4 diode mac oi song.
Gian o kch ong cac cong tac va o th ap tai c ve tren hnh 5.1b.
Bo nghch lu cung co the mac di dang mach tia (hnh H5.2).
Mach gom hai cong tac va hai diode mac oi song vi chung. Mach tai va ngo ra cua bo nghch lu cach ly qua may bien ap vi cuon s cap phan chia. Pha Trong trng hp khong s dung may bien ap cach ly pha tai, nguon ien ap mot chieu can thiet ke vi nut phan the gia (hnh H5.3), ay la dang mach nghch lu ap na cau.
6.2.2 Bo nghch lu ap ba phaTrong thc te mach bo nghch lu ap ba pha ch gap dang mach cau (hnh H5.4a)
Mach cha 6 cong tac S1,S2....S6 va 6 diode oi song D1,D2....D6.Tai ba pha co the mac dang hnh sao (H5.4b) hoac tam giac (H5.4c).
6.2.3. Bo nghch lu ap a bac (Multi-level Voltage source Inverter)
Cac bo nghch lu va c mo ta phan 5.1.1 va 5.1.2 cha 2 khoa ban dan (IGBT) tren moi nhanh pha tai. Chung c goi chung la loai nghch lu ap 2 bac (two- level VSI), c ap dung rong rai trong pham vi cong suat va va nho. Khai niem hai bac xuat phat t qua trnh ien ap gia au mot pha tai (v tr 1,2,3) en mot iem ien the chuan tren mach dc (iem 0) (pole to phase voltage) thay oi gia hai bac gia tr khac nhau, v du khi chon iem co ien the chuan la tam nguon dc th ien ap t pha tai en tam nguon thay oi gia (+U/2) va (-U/2) trong qua trnh ong ngat cac linh kien. Bo nghch lu ap 2 bac co nhc iem la tao ien ap cung cap cho cuon day ong c vi o doc (dV/dt) kha ln va gay ra mot so van e kho khan bi ton tai trang thai khac zero cua tong ien the t cac pha en tam nguon dc (common-mode voltage) (xem dang ien ap uNO). Bo nghch lu ap a bac c phat trien e giai quyet cac van e gay ra neu tren cua bo nghch lu ap 2 bac va thng c s dung cho cac ng dung ien ap cao va cong suat ln.
u iem cua bo nghch lu ap a bac: cong suat cua bo nghch lu ap tang len; ien ap at len cac linh kien b giam xuong nen cong suat ton hao do qua trnh ong ngat cua linh kien cung giam theo; vi cung tan so ong ngat, cac thanh phan song hai bac cao cua ien ap ra giam nho hn so vi trng hp bo nghch lu ap hai bac.
oi vi tai cong suat ln, ien ap cung cap cho cac tai co the at gia tr tng oi ln,
Cac cau hnh c ban cua bo nghch lu ap a bac:
Cau hnh dang cascade (Cascade inverter):[28],[48] -hnh H5.5b, s dung cac nguon dc rieng, thch hp s dung trong trng hp nguon dc co san v du di dang acquy, battery. Cascade inverter gom nhieu bo nghch lu ap cau mot pha ghep noi tiep, cac bo nghch lu ap dang cau mot pha nay co cac nguon DC rieng.Bang cach kch ong cac linh kien trong moi bo nghch lu ap mot pha, 3 mc ien ap (-U,0,U) c tao thanh. S ket hp hoat ong cua n bo nghch lu ap tren mot nhanh pha tai se tao nen n kha nang mc ien ap theo chieu am (-U,-2U,-3U,..,-nU), n kha nang mc ien ap theo chieu dng (U,2U,3U,..,nU) va mc ien ap 0. Nh vay, bo nghch lu ap dang cascade gom n bo nghch lu ap mot pha tren moi nhanh se tao thanh bo nghch lu (2n+1) bac.
Tan so ong ngat trong moi modul cua dang mach nay co the giam i n lan va dv/dt cung giam i nh vay. ien ap tren ap at len cac linh kien giam i 0,57n lan. Cho phep s dung linh kien IGBT ien ap thap.
Ngoai dang mach gom cac bo nghch lu ap mot pha, mach nghch lu ap a bac con co dang ghep t ngo ra cua cac bo nghch lu ap 3 pha (H5.5c). Cau truc nay cho phep giam dv/dt va va tan so ong ngat con 1/3. Mach cho phep s dung cac cau hnh nghch lu ap ba pha chuan. Mach nghch lu at c s can bang ien ap cac nguon dc, khong ton tai dong can bang gia cac module. Tuy nhien, cau tao mach oi hoi s dung cac may bien ap ngo ra.
Cau hnh nghch lu cha cap diode kep: (Neutral point Clamped Multilevel Inverter (NPC) hoac- diode clamped multilevel inverter):-hnh H5.5a, s dung thch hp khi cac nguon dc tao nen t he thong ien ac. Bo nghch lu a bac cha cac cap diode kem co mot mach nguon DC c phan chia thanh mot so cap ien ap nho hn nh chuoi cac tu ien mac noi tiep.
Gia s nhanh mach dc gom n nguon co o ln bang nhau mac noi tiep. ien ap pha nguon dc co the at c (n+1) gia tr khac nhau va t o bo nghch lu c goi la bo nghch lu ap (n+1) bac. V du chon mc ien the 0 cuoi day nguon, cac mc ien ap co the at c gom (0,U,2U,.,nU). ien ap t mot pha tai (v du pha a) thong en mot v tr bat ky tren mach dc (v du M) nh cap diode kep tai iem o (v du D1, D1). e ien ap pha- nguon dc at c mc ien ap neu tren (ua0=U), tat ca cac linh kien b kep gia hai diode (D1, D1) gom n linh kien noi tiep lien tuc ke nhau, phai c kch ong (v du S1, S5,S4,S3,S2), cac linh kien con lai se b khoa theo qui tac kch oi nghch. Tng ng vi 6 trng hp kch ong linh kien b kep gia 6 cap diode (hai cap diode kep hai v tr bien la trng hp ac biet), ta thu c 6 mc ien ap pha- nguon dc : 0,U,2U,..,5U. V co kha nang tao ra 6 mc ien ap pha- nguon dc nen mach nghch lu tren hnh H5.5a con goi la bo nghch lu 6 bac.
Dang mach nghch lu ap a bac dung cap diode kep cai tien dang song ien ap tai va giam shock ien ap tren linh kien n lan. Vi bo nghch lu ba bac, dv/dt tren linh kien va tan so ong ngat giam i mot na. Tuy nhien vi n>3, mc o chu gai ap tren cac diode se khac nhau. Ngoai ra, can bang ien ap gia cac nguon dc (ap tren tu) tr nen kho khan, ac biet khi so bac ln.
6.3. Phan tch bo nghch lu ap
6.3.1. Phan tch ien ap bo nghch lu ap 3 pha
Gia thiet tai ba pha oi xng thoa man he thc:ut1 + ut2 + ut3 = 0 Ta tng tng nguon ap U c phan chia lam hai na bang nhau vi iem nut phan the O (mot cach tong quat, iem phan the 0 co the chon v tr bat ky tren mach nguon DC).
Goi N la iem nut cua tai ba pha dang sao. ien ap pha tai ut1,ut2,ut3. Ta co: Ut1= U10- UNO
Ut2 = U20- UNO ut3 = u30-uNO
u10, u20 , u30 c goi la cac ien ap pha -tam nguon cua cac pha 1,2,3. Cac ien ap ut1, ut2, ut3; u10, u20, u30 va uNO co chieu dng qui c ve tren hnh H5.4a
Cong cac he thc tren va e y rang ut1+ut2+ut3=0, ta co:
0 = U10 +U20 +U30 3.UNO
T o: Thay uNO vao bieu thc tnh ien ap moi pha tai, ta co: ien ap day tren tai:
* He qua: Qua trnh ien ap ( va do o qua trnh dong ien) ngo ra cua bo nghch lu ap ba pha se c xac nh khi ta xac nh c cac ien ap trung gian u10, u20, u30.
* Xac nh ien ap pha - tam nguon cho bo nghch lu ap. Cap cong tac cung pha: gom hai cong tac cung mac chung vao mot pha tai, v du (S1S4), (S3,S6) va (S5,S2) la cac cap cong tac cung pha.
Qui tac kch ong oi nghch: cap cong tac cung pha c kch ong theo qui tac oi nghch neu nh hai cong tac trong cap luon trang thai mot c kch ong va mot c kch ngat.
Trang thai ca hai cung kch ong (trang thai ngan mach ien ap nguon ) hoac cung kch ngat khong c phep.Neu bieu dien trang thai c kch cua linh kien bang gia tr 1 va trang thai khoa kch bang 0, ta co the viet phng trnh trang thai kch cua cac linh kien trong mach nghch lu ap 3 pha nh sau:
* Qui tac: Gia thiet bo nghch lu ap ba pha co cau tao mach va chieu ien the cua cac phan t trong mach cho nh hnh ve H5.4. Gia thiet cac cong tac cung pha c kch ong theo qui tac oi nghch va gia thiet dong ien cua cac pha tai co kha nang oi dau.
ien ap pha tai en tam nguon cua mot pha nguon nao o co gia tr + neu cong tac le cua pha c kch ong va - neu cong tac chan c kch khong phu thuoc trang thai dong ien.* He qua:
1/- ien ap tren tai c xac nh hoan toan neu ta biet c gian o kch ong cac cong tac va ien ap nguon. Do o, ta co the ieu khien ien ap ngo ra cua bo nghch lu ap bang cach ieu khien gian o xung kch ong cac cong tac.
2/- Neu cac cap cong tac cung pha khong c kch ong theo qui tac oi nghch, dang ien ap tai se thay oi phu thuoc vao trang thai dong ien tai (va tham so tai ). ay la trng hp kch
ong do y muon oi vi tai dang cong hng. Dong ien co the trang thai lien tuc hoac gian oan.
Ta can chu y rang, mot cong tac c kch ong khong co ngha la no se dan ien. Phu thuoc vao chieu dong ien dan qua tai co the xay ra trng hp cong tac kch ong khong dan ien ma dong ien lai dan qua diode mac oi song vi cong tac c kch ong.
3/- Dang dong ien c xac nh da tren phng trnh mach tai. V du oi vi tai oi xng ba pha gom RL mac noi tiep, ta co phng trnh dong ien ba pha tai it1, it2, it3.
Thi gian chet (dead- time): la khoang thi gian can thiet ap at trong gian o ong ngat cap linh kien cung pha tai, trong khoang thi gian nay hai cong tac cung pha tai se b khoa kch (v du S1,S4). Thi gian chet bat au qua trnh chuyen mach cua hai cong tac cung pha tai e tranh xay ra hien tng ngan mach nguon. Do thi gian chet nho khong ang ke, trong qua trnh phan tch hoat ong mach, ta thng gia thiet bo qua giai oan nay.
6.3.2. Phan tch bo nghch lu ap mot pha Ta co the phan tch ien ap tai cua bo nghch lu ap mot pha dang mach cau tng t nh bo nghch lu ap ba pha. Hai cap cong tac (S1,S4) va (S2,S3) tng ng vi he thong hai pha tai oi xng tng tng (hnh H5.6).
Ro rang :
ut =ut1/2=-ut2/2=u10 - u20 (5.10)
Neu cac cong tac c kch theo qui tac oi nghch, ta co the xac nh dang ap tren tai da tren gian o kch cong tac va ien ap nguon.
Phan tch ien ap tai cua bo nghch lu ap mot pha dang na cau: ien ap bang vi ien ap pha tai - tam nguon, bai toan tr nen n gian.
Phan tch ien ap tai cua bo nghch lu ap mot pha dang cau: Qua trnh ien ap va dong ien c ve tren hnh (H5.8)
Xet qua trnh cac ai lng trong mot chu ky hoat ong che o xac lap. Gia thiet rang tai thi iem t=0, thc hien ong S1 va S2, ngat S3 va S4. ien ap tai bang U, dong ien tai chay qua mach (U-S1-S2) tang len theo phng trnh:
Nghiem dong ien co dang:
A la hang so, =L/R la hang so thi gian. Tai thi iem t=T/2, thc hien ngat S1,S2 va ong S3,S4. ien ap xuat hien tren tai bang U, dong ien qua mach (U,RL,S3,S4) giam theo phng trnh:
trang thai xac lap, dong ien bien oi theo dang xoay chieu, tuan hoan. Cac hang so A,B co the xac nh t ieu kien dong ien tai tai cac thi iem t=0, t=T/2 va t=T.
Luc o, tai thi iem t=0:
Tai thi iem t=T/2:
Tai thi iem t=T:
Nh vay, qua trnh dong tai trong mot chu ky hoat ong se co the bieu dien nh sau:
Gia tr Imin va Imax co the xac nh t qua trnh oi xng cua hai na chu ky ien ap va dong ien tai, t o suy ra rang Imax=-Imin. Ap dung quan he tren vao cac he thc tnh I, ta thu c:
Cong suat tai:
Cong suat tieu thu tren tai R-L co the xac nh theo he thc R.It2 vi It la tr hieu dung dong ien qua tai c tnh theo bieu thc:
Cong suat tai co the xac nh theo tr trung bnh dong qua nguon dc Is neu ta bo qua ton hao cua linh kien bo nghch lu:
Phan tch song hai:
Qua trnh ien ap tai qua phep phan tch Fourier co dang:
Ap tai ch cha cac thanh phan hai bac le.
o meo dang ien ap c tnh theo he thc sau:
De dang suy ra rang:
o meo dang ien ap cua bo nghch lu cau 1 pha kha ln trong trng hp ap ra dang vuong nen co tac dung khong tot. ieu nay giai thch v sao loai ien ap nay khong c s dung pho bien trong thc tien.
o meo dang dong ien phu thuoc vao tai va xac nh theo he thc:
6.3.3. Phan tch ien ap bo nghch lu ap a bac Xet bo nghch lu ap 6 bac dang cha cap diode kep (NPC) tren hnh ve H5.5a.
Goi U la o ln ien ap tren moi tu rieng le. Phu thuoc o ln ien ap pha nguon dc can thiet lap, cac linh kien b kep gia cap diode noi en mot ien the tren mach dc can thiet lap se trang thai kch. ien ap pha-tam nguon dc (phase -to- pole voltage), tnh t iem au day cua pha tai en mot ien the tren mach dc, trong trng hp tren hnh ve la iem 0, co the at cac gia tr cho trong bang B5.1 sau ay:
Vi x=1,2,3.Trang thai ong ngat cua cac khoa ban dan tren mot nhanh tai cua cac pha a,b,c thoa man ieu kien kch oi nghch:
S1j+S1j=1 ; S2j+S2j=1 ; S3j+S3j=1 ; j=1,2,3,4,5 (5.28)
ien ap pha tai trong trng hp 3 pha tai oi xng au dang Y co the thiet lap tng t nh trng hp bo nghch lu ap hai bac:
Trong trng hp 3 pha tai dang tam giac, ien ap pha tai bang ien ap day do bo nghch lu cung cap:
ut12 =u10 - u20; ut23 = u20 - u30; ut31 = u30 - u10 (5.30)
6.3.4. Cac phng phap ieu khien bo nghch lu ap a bac Cac phng phap ieu khien ap dung cho bo nghch lu ap hai bac nh phng phap ieu che o rong xung va cac dang cai bien cua no, phng phap ieu khien vector, phng phap kh song hai toi u, cac phng phap ieu khien dong ien (hay vector dong ien). co the c ieu chnh e co the ap dung cho bo nghch lu ap a bac. Bo nghch lu ap a bac co pham vi hoat ong chu yeu oi vi tai cong suat ln. Do o, van e giam bt tan so ong ngat va giam shock ien ap dv/dt tren linh kien cong suat co y ngha quan trong. Cac thuat toan ieu khien co gang thc hien duy tr trang thai can bang cac nguon ien ap dc va kh bo hien tng common-mode voltage, nguyen nhan gay ra mot so hien tng lam sm lao hoa ong c.
Phng phap ieu bien tuy khong phat huy c ng dung trong ieu khien bo nghch lu ap hai bac nhng lai to ra hieu qua trong ng dung bo nghch lu ap a bac cho tai cong suat ln, nhat la trong cac he thong ieu khien nang lng ien ac.
6.3.4.1. Phng phap ieu rong Ap dung cho cac bo nghch lu a bac co cap diode kep en nguon va bo nghch lu ap a bac cascade.
Xet bo nghch lu ap ba bac tren hnh H5.41, viec ieu khien ien ap tai co the thc hien theo gian o tren hnh H5.35 oi vi pha a bang cach thay oi o rong goc . ien ap pha- tam nguon:
T ien ap pha tam nguon, s dung he thc (5.5), ta co the dan giai qua trnh ien ap cac pha tai. ien ap tai co dang 12 bc. Khi goc ieu khien giam en gia tr zero, ta thu c dang ap 6 bc nh trong bo nghch lu ap hai bac.
Phng phap ieu khien 12 bc co he so meo dang ln khi ch so ieu che thap, khi o cac phng phap ieu che o rong xung co dang chat lng hn.
6.3.4.2. Phng phap ieu bien Tng t nh phng phap ieu bien bo nghch lu ap hai bac, ien ap nguon dc c ieu khien thay oi o ln. ien ap ngo ra c ieu khien theo dang bac thang nhieu bc (H5.35b). Goc chuyen mach cua cac linh kien c tnh toan e song hai c han che toi a. Cac gia tr goc chuyen mach co the thiet lap san va xung kch ong thc hien theo dang tra bang (look-up table). Phng phap ieu khien nay n gian, tan so ong ngat rat thap. Phng phap nay c s dung chu yeu trong he thong bu nhuyen cong suat phan khang cho li (static var compensator). S mat can bang cac ien ap nguon dc khong con la van e tr ngai trong ng dung va neu.
6.3.4.3 Phng phap ieu che o rong xung Phng phap con co ten Subharmonic PWM (SH-PWM), Multilevel carrier based PWM. e thc hien tao gian o kch ong cac linh kien trong cung mot pha tai, ta s dung mot so song mang (dang tam giac) va mot tn hieu ieu khien (dang sin). oi vi bo nghch lu ap m bac, so song mang c s dung la (m-1). Chung co cung tan so fc va cung bien o nh-nh Ac. Song ieu khien (hay song ieu che) co bien o nh- nh bang Am va tan so fm va dang song cua no thay oi chung quanh truc tam cua he thong (m-1) song mang. Neu song ieu khien ln hn song mang nao o th linh kien tng ng song mang o se c kch ong, trong trng hp song ieu khien nho hn song mang tng ng cua no, linh kien tren se b khoa kch.
Bo nghch lu a bac dang mach b diode kem
Chang han, xet pha a cua bo nghch lu ap ba bac tren hnh H5.41. Xung kch cho cac linh kien Sa1,Sa2,Sa3,Sa4 c thiet lap tren c s so sanh song ieu khien ura cua pha a vi song mang up1 (oi vi xung kch cho cap Sa1 va Sa3) va up2 (oi vi xung kch cho cap Sa2 va S ). Cu the la:
T gian o thiet lap tren, ien ap pha- tam nguon c xac nh, v du cho pha a:
T o, s dung cac he thc (5.5), (5.6) e xac nh cac ien ap tai.
Ch so bien o ma va t so tan so mf trong bo nghch lu a bac c nh ngha nh sau:
Ba dang phoi hp gia cac song mang thng c s dung gom (xem hnh H5.36a):
a. Hai song mang ke can lien tiep nhau se b dch 180 o APOD (Alternative Phase Opposition Disposition)
b. Bo tr oi xng qua truc zero (POD- Phase opposition Disposition). Cac song mang nam tren truc 0 se cung pha nhau, ngc lai, cac song mang nam di truc 0 se b dch i 180 o.
c. Bo tr cung pha (DP- In phase disposition): tat ca cac song mang eu cung pha.
Trong cac phng phap bo tr song mang, phng phap bo tr cac song mang a bac cung pha cho o meo dang ap day nho nhat. oi vi bo nghch lu ap ba bac, phng phap POD va APOD cho cung ket qua dang song mang.
Bo nghch lu a bac dang cascade: phng phap ieu che o rong xung s dung song mang dch pha (Phase Shifted Carrier PWM-PSCPWM) la phng phap ieu che c ban c s dung. Theo o, moi bo nghch lu ap cau mot pha trong mach cascade co nguyen ly ieu che PWM giong nhau cua mot bo nghch lu cau mot pha. Cac song mang cua cac bo nghch lu co cung bien o va tan so va o dch pha gia cac song mang nay bang /m, m la so bo nghch lu cau mot pha. Tren hnh ve H5.37 la bo nghch lu dang cascade 5 bac. moi nhanh pha, co 4 song mang lech pha nhau mot goc /2 c s dung. Gian o kch cac linh kien c thiet lap tren c s so sanh song mang va tn hieu ap ieu khien (dang sin) va qui tac kch oi nghch. ien ap tao thanh ngo ra cua moi bo nghch lu ap mot pha (ua,ua) co dang ba bac. Chung ket hp tao thanh ien ap pha- tam nguon (uao) dang 5 bac.
6.3.4.4. Phng phap ieu che o rong xung cai bien (Modified PWM hoac Switching Frequency Optimal PWM method- SFO-PWM)) Co ky thuat thc hien phng phap ieu che o rong xung va trnh bay. iem khac biet la song ieu che c cai bien. Theo o, moi song ieu che c cong them tn hieu th t khong (song hai boi ba). Ton tai nhieu kha nang tao nen thanh phan th t khong, Mot trong cac tn hieu th t khong co the chon bang tr trung bnh cua gia tr tn hieu ln nhat trong 3 tn hieu ieu che vi tn hieu nho nhat trong 3 tn hieu ieu che- phng phap SFO-PWM.
Goi Va*,Vb*,Vc* la cac tn hieu ieu khien cua phng phap ieu che PWM. Tn hieu ieu khien theo phng phap SFO-PWM va c mo ta co the bieu dien di dang toan hoc nh sau:
S o thc hien tao tn hieu ieu che cai bien t tn hieu ieu che dang sin c ve tren hnh H5.39. Tn hieu ien ap ieu che va ien ap ngo ra bo nghch lu c ve tren hnh H5.40.Phng phap ieu che o rong xung cai bien ch c s dung cho tai ba pha 3 day. He qua cua phng phap la ien ap pha tai c tang len khoang 15% so vi phng phap ieu che o rong xung sin phan tren, trc khi at en pham vi ieu che m rong (overmodulation).
6.3.4.5. Phng phap ieu che vector khong gian Vector khong gian cua bo nghich lu ap a bac:
Qua trnh ong ngat cac linh kien tao ra ien ap ba pha tai vi vector khong gian cua no thay oi nhay cap tren hnh luc giac a bac. V du, oi vi bo nghch lu ap ba bac, kha nang ieu khien kch dan linh kien tao nen 27 trang thai khac nhau, moi trang thai c minh hoa bi to hp (kakbkc),vi ka=0,1,2 ;kb=0,1,2; kc=0,1,2 .la he so trang thai tng ng cua cac pha a,b,c. V du: xet he so ka cua pha a ta co:
Trong qua trnh kch dan, qui luat sau c tuan thu:
T o, chang han, trang thai (200) co ngha la: Sa1=Sa2=1; Sb3=Sb4=1;Sc3=Sc4=1.
Theo nh ngha vector khong gian, tng ng 27 trang thai kch dan linh kien ta thu c 19 v tr vector khong gian cua vector ien ap tao thanh, bao gom 12 vector nam tren nh va trung iem cua hnh luc giac ln bao ngoai, 6 vector ien ap nam tren 6 nh cua hnh luc giac ben trong va vector khong tai tam cua hnh luc giac (hnh H5.41). oi vi cac vector nam tai nh cua hnh luc giac ben trong, ton tai hai trang thai kch dan khac nhau cua linh kien nhng lai co cung chung v tr vector khong gian. Ngoai ra, ton tai 3 trang thai kch dan khac nhau cho cung v tr vector khong.
ieu che vector khong gian cua bo nghch lu ap a bac:
Ve nguyen ly, phng phap ieu che vector khong gian oi vi bo nghch lu ap a bac c thc hien tng t nh oi vi bo nghch lu ap hai bac. e tao vector trung bnh tng ng vi mot vector v cho trc, trc het can xet xem vector v nam trong v tr nao cua nh luc giac (hexagon). e thuan tien, thong thng dien tch hnh luc giac c chia nho anh cac hnh tam giac con. V du, goc phan sau th nhat cua hnh luc giac gii han bi ba vector v0, v 2 va v 6 c chia nho thanh cac dien tch (1), (2),(3) va (4) nh hnh ve H5.42.
Vector v ang khao sat can ieu khien e at c co v tr nam tren phan dien tch (2). Bc tiep theo, ta xac nh cac vector khong gian can thiet- goi la cac vector c ban, can s dung e tao nen vector trung bnh nam trong dien tch (2). Ro rang, trng hp nay, cac vector c ban tng ng la v 1 , v 2 va v 3. Nh vay, vector tng ng vi vector v co the thc hien bang cach ieu khien duy tr tac dung theo trnh t vector v 1 trong thi gian T1, vector v 2 trong thi gian T2 va vector v 3 trong thi gian T3 theo he thc:
Van e con lai la xac nh thi gian tac dung T1,T2 va T3 cua cac vector c ban. Neu ta biet c vector v di dang cac thanh phan vuong goc V ,V trong he toa ong yen (stationary frame), quan he gia cac thanh phan vector V ,V vi thi gian duy tr trang thai vector v 1 , v 2 va v 3 co the bieu dien di dang ma tran sau:
Vi V1 ,V2 ,V3 ,V1 ,V2 ,V3 la cac thanh phan theo truc toa o va cua cac vector tren hnh luc giac v 1 , v 2 va v 3 T o, thi gian c xac nh (ap dung ma tran ngc): hay dang thi gian tng oi: dj=Tj/Ts; j=1,2,3
Ap dung cu the vao 4 dien tch hnh tam giac trong goc phan sau th nhat cua hnh luc giac, chu y en vector c ban trong moi dien tch tren, ta thu c ket qua sau (xem hnh H5.42):
Trong dien tch (1), vector c ban v0,v1 va v4
Neu vector nam goc phan sau th i so vi goc phan sau th nhat cua hnh luc giac tnh t v tr truc thc , ta co the qui oi no ve goc phan sau th nhat e xac nh thi gian tac ong cua cac vector c ban theo he thc:
Thc hien ky thuat ieu che vector khong gian cho bo nghch lu ap ba bac:Gian o kch dan cac linh kien bo nghch lu ap ba bac c minh hoa tren hnh H5.43, ap dung cho goc phan sau th nhat cua hnh luc giac. Chu y do trang thai kch dan cac linh kien tren cung nhanh pha tai cho bi qui luat oi nghch nen tren gian o ch can trnh bay trang thai cua Sx1 va Sx2, x=a,b,c. T gian o ta thay cac trang thai kch dan tng ng 3 vector c ban V 1 , V 3 va V 4 . Thi gian kch dan cua cac vector nay co the suy ra t bieu thc tnh toan T1,T2,T3 hoac tren ky thuat ieu che o rong xung da vao song mang nh tren hnh ve H5.43.
N P N
T2
T1
Hnh 1.70a: Cau tao
Hnh 1.70b: Ky hieu, hnh dang
T1
T2
Hnh 1.71a
T1
T2
R
VDC
IBO
VBO
V
I
-VBO
-IBO
T1
T2
T1
T2
Hnh 1.71b: ac tnh
n p
Vi
V0 RL
R
UV
Phan t ieu khien
Uo
Phan t lay mau
Tao ien ap chuan
Phan t so sanh
Vo
Vi
Phan t lay mau
Tao ien ap chuan
Phan t so sanh
Phan t
iu khin
ieu khien
R
Hnh 2.14b: S o khoi mach on ap song song
Hnh 2.14a: S o khoi mach on ap noi tiep
Ghi ch quan trng:
* Trong mch c 2 mass, mt mass in v 1 mass my. Sinh vin khi lm thc tp phi tht cn thn, trnh b in git.
* Trong bi thc tp dng SCR v E-MOSTFET knh N vi s chn nh sau:
Ghi ch quan trng:
* Trong mch c 2 mass, mt mass in v 1 mass my. Sinh vin khi lm thc tp phi tht cn thn, trnh b in git.
* Trong bi thc tp dng SCR v E-MOSTFET knh N vi s chn nh sau:
Hnh 2.8
Hnh 2.9
1
_1412363195.unknown
_1412363972.unknown
_1412495025.unknown
_1412495049.unknown
_1412488794.unknown
_1412363345.unknown
_1412363159.unknown
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