O CONG SUAT O IEN NANG

Chng 5O CNG SUT - O IN NNG

O H S CNG SUT 5.1 O CNG SUT IN MT CHIU 5.1.1 KHI NIM CHUNG

Cng sut v nng lng l cc i lng c bn ca h thng in . Cc i lng ny lin quan nhiu n nn kinh t quc dn . Do vic xc nh cng sut v nng lng l nhim v rt quan trng

Trong thc t , ngi ta phn thnh cc loi cng sut sau

Cng sut thc hay cn gi l cng sut hu ch P ( Watt )

Cng sut phn khng , cn gi l cng sut v cng Q ( VAR )

Cng sut biu kin hay cng sut danh nh S ( VA )

Tm o ca cng sut t 10-20 W n 1010 W v gii tn s t 0 n 109 Hz

i vi mch in mt chiu , cng sut thc c xc nh theo biu thc P = U . I = I2. R = eq \f( U2, R )

Vi mch in xoay chiu mt pha

P = eq \f( 1 , T ) = Nu dng v p c dng hnh sin , th

Cng sut thc c xc nh theo biu thc

P = U . I . cos (

Trong U , I l cc gi tr in p hiu dng , cng dng in hiu dng v cos ( l h s cng sut

Biu thc tnh cng sut phn khng l

Q = U . I . sin (

Biu thc tnh cng sut biu kin

S = eq \r ( P2 + Q2 ) Ngoi cc thng s cng sut in , cn c mt thng s quan trng l nng lng W . l cng sut tiu th ca mch in trong khong thi gian t1 (( t2

Nng lng c tnh theo biu thc

W = =

5.1.2 Phng php o gin tip

mch in mt chiu hoc mch in xoay chiu c ti l thun tr ta c th s dng ng h vn v ng h ampere xc nh cng sut

Phng php mc ampere trong phng php ny c s dng trong trng hp in tr cn o c gi tr ln hoc ni tr ca ampere k c gi tr nh

Phng php mc ampere ngoi phng php ny c s dng trong trng hp in tr cn o RX c gi tr nh hoc volt k c gi tr ln . Phng php ny c sai s tng i ln v khi ti thay i ta khng th cng lc c chnh xc tr s ampere v vn . Mt khc phi s dng cng thc tnh ton sau khi o

Cng sut in c xc nh theo biu thc sau

P = U . I = I2. R = eq \f( U2, R )

Phng php ny c sai s ln ( sai s volt k v ampe k ) v khi ti thay i , ta khng th quan st s thay i thng s c hai ng h o cng lc . Mc khc , phng php ny kh phc tp v phi s dng cng thc tnh ton sau khi o v cng gy ra sai s

Tht vy , ta thy cng sut ti PL c xc nh bi volt k v ampe k . Theo cch u dy nh s trn , tr s cng sut ca ti c xc nh bi gi tr in p v dng in

Chng hn s ampe k mc ngoi , ta c biu thc sau

I = IV + IL

Suy ra IL = I - IV

PL = UV . IL = UV . ( I - IV )

PL = UV . I + UV . IV Nh vy , sai s do cch mc ny ph thuc vo dng in IV i qua volt k , nu IV cng nh th php o cnh chnh xc

5.2 O CNG SUT XOAY CHIU MT PHA 5.2.1 Phng php o trc tip bng Watt k

Nh trnh by trn , phng php o gin tip c nhng hn ch nht nh . khc phc nhng nhc im trn , ngi ta s dng phng php o trc tip l dng Watt k

Watt k l dng c c in , dng o trc tip cng sut thc trong mch in mt chiu hoc xoay chiu mt pha

Watt k c ch to theo kiu in ng hoc st in ng , gm c 2 cun dy : mt cun dy di ng ( cun dy in p ) v mt cun dy tnh ( cun dy dng in )

Cun dy tnh c tit din ln nhng t vng dy ( nn c in tr nh ) c mc ni tip vi ti nn c gi l cun dng

Cun dy ng c tit din nh, nhiu vng dy (nn c in tr ln) c mc song song vi ti nn cn gi l cun p

Nh vy dng in I1 qua ph ti s i qua cun dng , cn in p t ln cun dy tnh t l vi dng in I2 i qua cun dy p

Khi c in p U t vo cun dy ng v c dng in chy qua cun dng ( dng in i qua ph ti ) di tc ng ca t trng in t , kim ca watt k lch i mt gc (m i vi ngun in mt chiu , gi tr ca gc lch (m ny c xc nh theo biu thc

( = eq \f( 1 , D) eq \f( UI , RU + RP ) eq \f ( dM12 , d() Gi s eq \f ( dM12 , d() = const th ( = K . U . I = K . P

Vi K = eq \f( 1 , D) eq \f( 1 , RU + RP ) eq \f ( dM12 , d() gi l h s ca watt k vi dng in mt chiu

i vi mch in xoay chiu , gi tr ca gc lch (m ny c xc nh theo biu thc

( = eq \f( 1 , D) I . IU . cos ( eq \f ( dM12 , d( )

Vi ( = ( - (Trong IU l dng in trong mch song song ca watt k

IU = eq \f ( U , RU + RP ) cos( Vi

eq \f (dM12 ,da) = const

T biu thc trn , ta nhn thy ch s ca watt k t l vi cng sut tiu th trn ph ti .

5.2.2 K hiu Watt k

K hiu * ch cc tnh ca cc cun dy dng v cun dy p . u dy no c k hiu * c qui nh l u u ca cun dy

Trong qu trnh lp Watt k vo mch in , ta cn lu cc tnh ca cun dy trnh lm cho kim quay ngc dn n h hng Watt k .

5.2.3 Cch mc dy watt k

Do watt k in ng c cc tnh , cho nn khi o pha ca mt trong hai cun dy , kim ca watt k s quay ngc v vy cc cun dy c nh du u u trnh lm kim watt k quay ngc . Khi u mch in , ta phi ni cc u dy c du * vi nhau

Watt k in ng thng c nhiu thang o theo dng v p . Gii hn o theo cng dng in l 5A v 10A , theo in p l 150V v 300 V . Gii tn t 0 n hng KHz , cp chnh xc c th t t 0.1 n 0.2% tn s thp hn 200Hz .

Do khi o mun c gi tr cho ng , ta cn phi xc nh hng s Watt k CW ( v Watt k c nhiu gii hn o ) ng thi phi ch n cc tnh ca cc cun dy

CW = eq \f( Um Im , (m ) ( W / vach )

Um

la c o ien ap a chon

Im

la c o dong ien a chon

(m

so vach chia gii han tren watt ke

Gia s khi o , kim ch vach th ( th cong suat mach la

PW = CW(

vi ( la so vach kim ch thc te tren ong ho o

Theo s u dy nh hnh trn , ch th ca c cu in ng c xc nh nh sau

( = K . IL . IA M IA = EQ \F( E , RS + R2) Trong

RS l in tr dng hn ch dng in qua cun dy in p ( cun dy ng )

R2 l in tr cun dy in p

Suy ra

T biu thc trn , ta nhn thy in tr ni ca cun dy dng in cng nh th sai s cng gim ( do tn hao trn cun dy ny gim ) . in tr RS dng gim bt in p trn cun dy in p v th nu in p vo cng ln th in tr RS phi cng ln .

hnh a : dng in ti bng dng in qua cun dy tnh

hnh b : dng in qua cun dy tnh bng tng dng in qua ti v dng in qua cun dy ng

Ta xt s mch in sau , khi in tr RS mc nh hnh bn , hiu in th gia u dy 2 v u dy 4 gn bng in ngun . Do c kh nng gy h hng cho Watt k v s cch in ca 2 cun dy . Ngoi ra khi mc nh th sai s ph s tng ln do nh hng tnh in ln nhau ca 2 cun dy Watt k . V th ta phi mc in tr shunt u dy 4 gim thiu sai s v tng tui th cho Watt k

Mun m rng thang o , ta c th

Phn on cun dy dng in

Ni in tr ph ni tip vi cun dy p

5.2.4 Dng watt k vi my bin p o lng

5.2.4.1 Dng Watt k vi my bin dng CTKhi dng in ph ti ln hn dng in nh mc ca Watt k , ta phi kt hp Watt k vi bin dng CT . S u dy nh sau

Cng sut cho bi Watt k l

PW = P2 = I2 U1 cos (2 = I2 U1 cos ( (1 - ( )Khi bit gi tr cng sut trn Watt k , ta xc nh c cng sut thc

Pthc = KI . I2 U1 cos ( (1 - ( )

Trong

Pthc l cng sut trn Watt k

KI l t s danh nh ca bin dng

Do goc ( nho nen ta co the xem (1 = (2 ( xem gian o vector dong va ap )V the ta co the viet Pthc = KI . I2 U1 cos(2

Nh vay , cong suat tieu thu cua tai bang tch so gia tr so oc c tren Watt ke vi ty so danh nh bien dong CT5.2.4.2 Dung Watt ke vi may bien dong CT va bien ien ap VT

Khi cn o cng sut ca ph ti c dng in v in p ln hn dng in v in p danh nh ca Watt k , ta phi s dng bin dng CT v bin in p VT tng thch vi dng in v in p ca Watt k

S u dy

Cun dy in p ca Watt k c mc hai u cun dy th cp ax ca VT ,hai u dy s cp ca VT c ni vi ngun in ( u song song vi ph ti )

Cun dy dng in ca Watt k c mc ni tip vi cun dy th cp ca CT ( thc hin tng t nh 6.4.1 ) m bo an ton , v ca CT v VT c ni t

Cng sut trn Watt k l

PW = U2 I2 cos(2 Cng sut thc ca ph ti la Pti = KI i2 KV u2 cos ( (1 + (V - (i )Nu gc lch (V v (1 nh , ta c th xem (2 = (1 Nh vy cng sut thc gn ng ca ph ti c xc nh bi biu thc

Pthc = KI . KU . PW = U1 I1 cos(1

Nh vy , cng sut tiu th ca ti bng tch s gia tr s c c trn Watt k vi t s danh nh bin dng CT v t s danh nh ca bin in p VT

Kt qu o c sai s l do t s bin dng , t s bin in p v gc lch pha do cun dng v cun p ca Watt k

S o cach mac watt ke co nhieu tam o5.3 O CNG SUT MCH IN BA PHA 5.3.1 Mch ba pha i xng

Mch ba pha 4 dy

i vi mch in ba pha c ti i xng , ta ch cn s dng mt watt k o cng sut mt pha , ri sau ta nhn vi 3 s c gi tr cng sut cho ton b mch in

Php o c thc hin nh sau

Cng sut mt pha

PW = Up Ip cos(Cng sut ba pha

P3p = 3 . PW = 3 Up Ip cos(Mch ba pha 3 dy

o cng sut in , ta to trung tnh gi bng cch s dng 2 in tr ph c gi tr bng gi tr ca cun dy p . S u dy theo s hnh a

Cng sut ba pha c xc nh theo biu thc

P3p = eq \r( 3) . PW

Ngoi ra , ta cng c th s dng 2 watt k xc nh cng sut in cho ton mch . S u dy nh hnh b . Cng sut ba pha c xc nh theo biu thc

P3p = PW1 + PW2Trong qu trnh o , nu mt trong hai watt k ch ngc th ta i cc tnh cun dy dng in hay cun dy in p . Lc kt qu nhn c t watt k s ly gi tr m .

V d nh watt k 2 quay ngc , sau khi i cc tnh cun dy th cng sut ton mch s la P3p = PW1 PW2

5.3.2 Mch in ba pha khng i xng ( ti khng cn bng )

Mch ba pha bn dy

xc nh cng sut in , ta phi s dng 3 watt k . Vic u dy cng tng t nh u dy Watt k mt pha i xng ( lu n dng in nh mc ca Watt k v in p danh nh ca cun p Watt k )

Cng sut ton mch c xc nh theo biu thc sau

P3p = PW1 + PW2 + PW3Mch ba pha ba dy

Thc hin tng t nh mch ba pha i xng

5.3.3 S dng watt k ba pha o ti ba pha khng cn bng

Watt k ba pha hai phn t

c cu to gm hai cun dy in p ( hai cun dy di ng ) c cng trc quay v hai cun dy dng in c nh . Phng php o s dng Watt k ny cng ging nh phng php o dng 2 Watt k mt pha o cng sut ti ba pha ba dy . V th cch mc cng ging nh cch mc 2 Watt k mt pha o cng sut ti ba pha ba dy

Cu to ca Watt k ba pha 2 phn t nh hnh v

Watt k ba pha hai phn t ri

Loi Watt k ny thng c s dng trong cng nghip . Watt k hai phn t ri c 2 cun dy p c cng trc quay v 3 cun dy dng in gm cun dy th 3 , mt na cun dy p (1) , mt na cun dy dy p ( 2 )

Phng php o v cch u dy ging nh Watt k ba pha , hai phn t cun dy p v loi ba phn t phn cun dy dng

5.3.4 o cng sut ba pha ca ti s dng bin dng v bin p

Cng ging nh Watt k mt pha , khi dng in ti ln hn dng in danh nh ca Watt k , s dng Watt k ny ta phi kt hp vi bin dng CT . Mch c u nh s trn

L lun tng t nh trnh by trn, ta xc nh c cng sut tiu th ca ph ti

Ppha A = KI1 . PW1

Ppha B = KI2 . PW2

Ppha C = KI3 . PW3

Pphu tai = Ppha A + Ppha B + Ppha C Thng ta s dng 3 CT c t s bin dng ging nhau . Do

Pphu tai = KI ( PW1 + PW2 + PW3 )

Khi ph ti c dng in v in p ln hn dng in v in p danh nh ca Watt k , ta phi s dng bin dng v bin in p .

S u mch nh hnh v

Lu

m bo an ton cho ngi v thit b , cc TI v TU phi c ni t

5.4 O IN NNG 5.4.1 Khi nim chung

in nng l sn phm chnh ca ngnh in vic o in nng c ngha rt ln v mt kinh t v k thut .

Nng lng in trong mch in xoay chiu mt pha c xc nh theo biu thc

W = = = K . P . tTrong

P

l cng sut tiu th trn ph ti

t = t2 t1 l khong thi gian ph ti tiu th in K

l h s

5.4.2 Cu to cng t in

Cng t in c ch to da trn c cu ch th cm ng in t , gm hai phn chnh l phn tnh v phn ng

Phn tnh gm c

Cun dy dng in c mc ni tip vi ph ti nn cun dy c c dy ln nhng t vng dy

Cun dy in p mc song song vi ph ti , nn c s vng dy nhiu nhng c dy nh

Mt nam chm vnh cu hnh ch U t vung gc vi da nhm to ra moment hm

Phn ng gm

Mt a nhm D mng c ng knh t 80 n 100 mm b dy 0.5mm c gn vi trc , da nhm D c mp da nm trong khe h ca mch t cun dng v cun p . cho da nhm quay u , ngi ta gn thm nam chm m M v b c hc E c bnh rng n khp trc quay ca da nhm D

B hin th c ni lin vi b s c kh (h thng bnh rng ) hin th gi tr in nng tiu th

5.4.3 Nguyn tc hot ng ca cng t in

Khi c dng in chy qua cun dng v qua cun p , s sinh ra t thng (1 v (2

Cc t thng ny xuyn qua da nhm v cm ng trong da nhm cc sc in ng cm ng E1 v E2 , cc sc in ng to ra cc dng in xoy trn da nhm . Cc dng in xoy ny li nm trong t trng ca nam chm in to bi cun dy dng v cun dy p nn n chu lc tc dng v to thnh moment quay .

V cun dy p c nhiu vng dy nn h s t cm L ng k so vi cun dng nn c th xem in p lch pha 900 so vi dng in . Tc ng tng h gia t thng (1 v (2 vi cc dng in xoy to thnh moment lm quay da nhm

Moment quay Mq l tng cc moment thnh phn

Mq = K1 (1 I22 sin( + K2 (2 I12 sin( l gc lch pha gia (1 va (2

K1 , K2 l h s Nu dng in to ra t thng (1 v (2 l hnh sin v da nhm c cu to ng nht th dng in xoy I12 v I22 t l vi tn s f ca ngun in v t thng sinh ra chng

I12 = C3 f vI22 = C4 f (2Nh vy , ta c

Mq = C f (1(2 sin (Vi

C = C1 + C2 + C3 + C4

(1 = KI I

(2 = KU IU = KU eq \f( U , ZU) Trong U l in p t ln cun p

ZUl tng tr cun p

KI , KU l h s t l

Do cun dy p c in t thun nh so vi in khng nn ta c th xem

ZU = XU = 2( f LU

LU l in cm ca cun dy v f l tn s ngun Do (U = eq \f( KU U , 2( f LU ) = KU eq \f( U , f ) vi KU = eq \f( KU , 2( LU )

Nu ta xem (1 = (Iva (2 = (U th Mq = C f (I (U sin (

Do Mq = C KI KU U I sin (Ngoi ra , ta c Mq = K1 U I sin (Nu ( = 900 - ( th sin ( = sin( 900 - ( ) = cos(

Do biu thc moment quay c th vit li la Mq = K1 U I cos( = K1 P Di tc dng ca moment quay, da nhm s quay vi tc l n0 (vng / pht). Dng in xoy kt hp vi t trng do nam chm vnh cu to thnh moment hm da nhm MC = K2 n0 Khi da nhm quay u (trng thi cn bng ) , ta c

Mc = Mq hay K2 n0 = K1 P

Suy ra

P = eq \f( K2 , K1) n0

Trong khong thi gian t = t2 t1 , da nhm quay c N vng . V vy n0 = eq \f( N , t ) Nh vy , in nng tiu th c xc nh

W = P . t = eq \f( K2 , K1) n0 . t = eq \f( N ,C)

Hay C = eq \f( N , W)( vng /KWh )

Trong C = eq \f( K1 ,K2) l hng s thc ca cng t in , l s vng quay ca da nhm khi tiu th cng sut 1KW trong 1 gi

N = n0 . t l s vng quay thc t ca da nhm Kt lun

S vng quay ca da nhm trong khong thi gian t t l thun vi in nng tiu th

T s vng quay ca da nhm , ta xc nh c in nng tiu th

Mi cng t in c t trng bi cc thng s sau

nhy

Sm

( vong / KWh )

Hng s nh mc cng t

Cm = Sm

( vong / KWh )

Hng s thc t ca cng t C = eq \f( W , N ) ( KWh / vong )

in p nh mc ca cng t Um

Dng in nh mc ca cng t in Im 5.4.4 Kim tra hng s cng t

kim tra hng s cng t C , ngi ta iu chnh sao cho I = Im , U = Um v h s cng sut cos( = 1 . Khi P = Um . Im

Trong khong thi gian t , cng t s quay c N vng

Ta c C = eq \f( N ,Um. Im.t ) = eq \f( N , Pm . t ) Hng s ny khng i i vi mi loi cng t v c ghi trn mt cng t in . Chng hn nh mt cng t in trn c ghi thng s 1KWh 1500vng , ngha l nu da nhm quay 1500 vng th mt hin th s nhy thm 1 n v cng sut . Nu hng s C khng bng gi tr nh mc ghi trn mt cng t in , ngi ta s iu chnh v tr nam chm vnh cu tng hoc gim moment cn MC cho n khi gi tr C t c gi tr nh mc th dng li . Sai s ca cng t in

(% = eq \f( Cm - Co , Co ) 100%

5.4.5 s u dy cng t in mt pha ba pha

Khi lp t cng t in , ta phi u cc u dy ng theo s hng dn ca hng sn xut . Mi loi cng t in c mt kiu mc dy ring nhng vn m bo nguyn tt Cun dng mc ni tip vi ph ti , cun dy p song song vi ph ti

5.4.5.1 Mc cng t in mt pha

Ty theo cch u cc u dy cun dng v cun p m ta c cch u dy khc nhau

S u dy thng c ghi trn np cng t in .

5.4.5.2 Cch mc cng t in ba pha

Cng t in ba pha c cu to tng t nh cng t in mt pha . Thng thng cng t in ba pha c cc loi sau Cng t in ba pha 2 phn t : loi cng t in ny c 2 cun dy dng in v 2 cun dy in p , cc u dy cun dng v cun p c u li vi nhau a ra ngoi 6 u dy . Cch mc cng t in loi ny tng t nh cch mc watt k 3 pha Cng t in ba pha 2 phn t c s dng o in nng trong h thng in ba pha 3 dy . S u dy nh hnh v Cng t in ba pha 3 phn t loi cng t in ny c 3 cun dy dng in v 3 cun dy in p , cc u dy cun dng v cun p c u li vi nhau a ra ngoi 7 , 8 hoc 11 u dy ( nu c s dng my bin dng ) . Cch mc cng t in loi ny tng t nh cch mc watt k 3 pha 3 phn t

Cng t in ba pha 3 phn t c s dng o in nng trong h thng in ba pha 4 dy ( c dy trung tnh ) . S u dy nh hnh v

Khi dng in i qua ph ti c tr s ln hn nhiu so vi dng in nh mc ca cng t in 3 pha , ta phi kt hp vi 3 my bin dng . Gi tr thc s l tch s ca t s my bin dng KI vi gi tr o c trn cng t in

Lu

Khi chn my bin dng , ta phi cn c vo cng sut ca ph ti chn dng in my bin dng cho ph hp

V d

Nu ph ti c dng in nh mc trong khong 100A > Ipt > 50A th ta chn CT c t s KI l 100/5 hoc 150/5

Nu ph ti c dng in nh mc trong khong 150A > I > 100A th ta chn CTc t s KI l 150/5 hoc 200/5

Cc thng s ghi trn cng t in mt pha v ba pha

cng t in mt pha , ngha cc thng s ln lt l in p s dng l 220V

Dng in nh mc l 10 A ( tuy nhin ta c th s dng ti c dng in l 20 A nhng trong thi gian ngn )

Tn s s dng l 50 Hz

S vng quay ca da nhm cho mi kilowatt gi l 600 vng ( Da nhm quay 600 vng th ch s kW.h s nhy 1 s )

cng t in ba pha , ngha cc thng s ln lt l in p s dng ngun in 3 pha 4 dy c in p dy l 220V hay 380 V

Dng in nh mc l 5A

Tn s s dng l 50 Hz

S vng quay ca da nhm cho mi kilowatt gi l 450 vng

S dng mi trng c nhit 270C

5.5 O H S CNG SUT DNG VN K V WATT K 5.5.1 o trc tip bng watt k phn khng mt pha ( VAR k )

Theo nh ngha , cng sut phn khng ca ti

Q = U . I . sin ( = U. I . cos ( 900 - ( ) Watt k phn khng c cu to tng t nh watt k in ng , nhng lm lch pha cun dy in p , ngi ta mc ni tip mt cun dy cm hay mt t in nh hnh v

Tng t nh watt k, gc quay ca kim cng t l vi dng in qua cun dy dng v cun dy p , ng thi cng ph thuc sin ca gc lch pha gia dng in v in p . Do gc quay t l vi cng sut phn khng

5.5.2 Cch o cng sut phn khng

Vic o cng sut phn khng tng t nh o cng sut hu ch c trnh by trn

Ngoi cch o cng sut bng watt k , ta cng c th o gin tip bng vn k v ampere k v Watt k . Nhng phng php ny cho kt qu khng chnh xc v s u dy phc tp nn t c s dng

5.5.3 o cng sut phn khng ca ti ba pha

o cng sut phn khng trong h thng in ba pha bn dy

Nh ta bit in p dy UBC , UAC v UAB tr pha 900 so vi in p pha UA , UB v UC . V th ta c th s dng Watt k mt pha o cng sut phn khng Q

S nh hnh v

Cng sut pha A :

PA = IA . UBC . cos ( 900 - ( )

PA = eq \r(3) IA UA sin( = eq \r(3) QA Ngha la QA = eq \f(PA , ) Gi tr PA c c trn Watt k

Tng t i vi pha B v pha C , cng sut phn khng ca ph ti ba pha s bng tng cng sut phn khng cc pha

Qba pha = QA + QB + QC Qba pha = eq \f( PW1 + PW2 + PW3 , \R(3)) o cng sut phn khng trong h thng in ba pha ba dy

Nu ti cn bng v in p ngun i xng

Trong trng hp ny , ta c th s dng 2 Watt k mt pha hoc 1 Watt k ba pha hai phn t

Gi s , ta s dng 2 Watt k mt pha v mc mch nh hnh v

Cng sut o bng 2 Watt k mt pha cho kt qu l

PW = IA . UBC . cos ( 900 - ( ) + IA . UBC . cos ( 900 - ( )Do ti cn bng v in p i xng , nn UBC = UCA v IB = IA V vy cng sut tc dng l

PW = 2 IA UBC sin( = 2 eq \r(3) IA UA sin( M Q = IA UA sin( Do PW = 2 eq \r(3) Q

Suy ra

Q = eq \f( 1,2.\r(3)) PW

Kt lun

Mun chuyn Watt k thnh VAR k th kt qu c trn thang o Watt k nhn vi h s t l v n v VAR ( hoc KVAR )

Neu ien ap oi xng va phu tai khong can bang Trong trng hp ny , ta phi s dng 3 Watt k v c u mch nh hnh bn . Cng sut phn khng mi pha bng kt qu c trn cng sut tc dng chia cho eq \r(3) QA = eq \f( PW1 , \R(3)) , QB = eq \f( PW2 , \R(3)) va QC = eq \f( PW3 , \R(3)) Q = QA + QB + QC = eq \f( PW1 + PW2 + PW3 , \R(3))

Cng vi cch mc trn , nu ti cn bng th kt qu cng sut phn khng s l tch s gia kt qu c trn Watt k vi eq \r(3) Q = eq \r(3) PWNu s dng Watt k hai phn t hoc hai Watt k mt pha , mch in c mc nh hnh v . Mch s dng 3 in tr mc ni tip to ra mch sao cn bng , nh vy in p pha C c p vo mch th nht v in p pha A c p vo mch th hai

oi vi pha B , ta co

PW = PW1 + PW2 = IAUC cos( 600 - ( ) + ICUA cos( 1200 - ( )

PW = Ipha Upha [ 0.5 cos( + ( eq \r(3) / 2) sin( - 0.5cos( + ( eq \r(3) / 2) sin( ]

PW = eq \r(3) Ipha Upha sin( = eq \r(3) Qpha Suy ra eq \r(3) PW = 3 Qpha Kt lun

Cng sut phn khng ca ti ba pha khng cn bng tr s c c ca hai Watt k nhn vi eq \r(3) 5.6 O H S CNG SUT DNG C CU O IN NG Dng in v in p l hai tn hiu in khi i qua ti s lch pha nhau mt gc ( gc lch ny ty thuc vo tnh cht ca ti .

Nu ti l thun tr th ( = 0 hay cos( = 1

Nu ti l thun cm th in p s nhanh pha so vi dng in mt gc l ( = eq \f( ( , 2)

Nu ti l thun dung th in p s chm pha so vi dng in mt gc l ( = - eq \f( ( , 2)

i lng th hin s lch pha gia hai tn hiu ny l cos( , i lng ny gi l h s cng sut cos( . i lng ny dng lm ch tiu nh gi cht lng cung cp in nng . H s cng sut cos( cng ln ( cng gn bng 1 ) th hiu qu s dng in cng cao v qu trnh truyn ti in nng cng hiu qu

Vic nng cao h s cng sut s tn dng tt cng sut ngun .

V d mt my pht in c cng sut nh mc Sm = 10000kVA

Nu h s cng sut ca ph ti l cos( = 0.5 th cng sut tc dng ca my pht cung cp cho ph ti l

P = Sm . cos( = 10000 x 0.5 = 5000 kW

Nu h s cng sut ca ph ti l 0.9 th cng sut tc dng ca my pht cung cp cho ph ti l

P= Sm . cos( = 10000 x 0.9 = 9000 kW

Nh vy h s cng sut l i lng c ngha rt ln v kinh t cng nh cht lng cung cp in . Do , trong lnh vc o lng in , ta cn phi bit h s cng sut cos(. T c bin php nng cao h s cng sut cos( nu h s cng sut thp hn qui nh nhm ci thin cht lng li in

5.6.1 o cos( bng volt k

kim tra hoc ly chun cho cos( k hoc o s lch pha ( gia hai tn hiu

e1 = E1 cost v e2 = E2 cos ( t + ( )

Ta c th s dng phng php sau

Dng volt k o S l tng in p ca tn hiu e1 v e2 o D l hiu s gia hai tn hiu

S2 = E12 + E22 + 2E1 E2 cos(

D2 = E12 + E22 - 2E1 E2 cos(Nh vy cos( = eq \f( S2 - D2 , 4E1E2 ) Tng t ta cng c th xc nh h s cng sut cos( ca ti khng bng cch s dng 3 volt k S u mch nh hnh v

cos( = eq \f( V3 2 - V12 - V22 , 2 V1 V2 )

Trong

V1 l in p trn ti

V2 l in p trn in tr thun

V3 l in trn in tr thun v ph ti

Phng php ny c sai s t 50 n 100 nu gc lch pha khong 900 v s sai s ny cng ph thuc vo cp chnh xc ca volt k V1 , V2 v V3

5.6.2 Phng php o gin tip

o h s cng sut cos( mch in mt pha

Dng vn k , ampere k v watt k . Phng php ny n gin nhng s mc dy phc tp v c sai s ln . Watt k cho bit cng sut hiu dng ca ti , vn k v ampere k cho bit gi tr in p v cng dng in .

S mc mch nh hnh v

H s cng sut c xc nh theo biu thc cos( = eq \f( P , U. I ) T biu thc trn , ta nhn thy , nu bit cc gi tr cng sut hu ch P , in p U v cng dng in I ta s tnh c cos(o h s cng sut cos( mch in ba pha i xng

Trong trng hp ny gc lch pha c o gia dng in ca mt dy pha vi in p gia cc dy pha ca ti ba pha

P = Ud Id cos(Suy ra

cos( = eq \f( P , Ud Id ) 5.6.3 Phng php trc tip

Cos( k in ng mt pha

C cu o kiu t s k in ng hoc st in ng , phn tnh v phn ng chia lm hai cun dy mc ni tip nhau . c bit cun dy 1 c mc ni tip vi in tr R v cun dy 2 c mc ni tip vi cun khng L sao cho dng in IR v IL lch pha nhau mt gc 900 . Do 2 cun dy c t vung gc vi nhau s to ra hai moment quay c xc nh theo biu thc

M1 = M . cos(

M2 = M . sin(Trong

M1 l h s h cm gia cun dy di ng 1 vi cun dy c nh

M2 l h s h cm gia cun dy di ng 2 vi cun dy c nh

M l h s h cm ln nht khi cc cun dy di ng c t thng ( do cun dy c nh to ra

Gi s tng tr ca cun dy rt nh so vi gi tr in tr R v XL ca cun dy L

Ta c uti = U eq \r( 2) cos t

l in p ca ti

iti = U eq \r( 2) cos ( t - ( ) l dng in ca ti

Khi dng in qua cun dy 1 v cun dy 2 l

ir = eq \f( U , R )

eq \r( 2) cos t v iL = eq \f( U , (L )

eq \r( 2) cos ( t - ( )

Do moment quay ca ca cun dy 1 v cun dy 2 l

Mq1 = eq \f( UI , R)

eq\f( UI ,

eq \f( dM1 , d() cos( v Mq2 = eq \f( UI , (L)

eq\f( UI ,

eq \f( dM2 , d() sin(Ti tr s (i ca cun dy di ng Mq1 = Mq2 , hai cun dy di ng ng yn ( cn bng ) . Lc ny ta xc nh c gc lch pha ( gia hai tn hiu p v dng

tg(= eq \f( (L , R) tg(i

Nu nh cun dy v mch in c ch to sa cho R = L th ti v tr ca ch th ca 2 cun dy ta c (i = (

Trn cos( k c khc theo tr s cos ca gc lch pha ( c tr s 0 gia ( tng ng vi gi tr cos( = 1 )

Nu kim cos( k lch v pha trn ( hay pha phi ) th in p nhanh pha hn dng in ( Lead )

Nu kim cos( k lch v pha di ( hay pha tri ) th in p chm pha hn so vi dng in ( Lag )

Vic ly chun cho cos( k ph thuc vo tn s ca tn hiu o . hn ch nh hng ca tn s ta s dng cun dy 2 gm 2 phn t bng nhau , phn t 1 ni vi L v phn t 2 ni t in nh hnh . gi tr ca L v C c chn sao cho LC2 = 1 khi o gc lch pha ca cc tn hiu (Cng hng tn s ca tn hiu o) Cos( k ba pha

Khi o h s cng sut cos( ba pha ngha l ta o gc lch pha ( gia dng in mt dy pha vi in p gia cc dy pha ca ti ba pha . Theo hnh v , ta thy cun dy c nh ca pha k c mc ni tip vi ti mt trn ba pha in ca ti , cn 2 khung dy 1 v 2 c mc gia cc pha ca ti qua trung gian cc in tr R c tr s ln v in cm ca cun dy c tr s khng ng k Moment quay trung bnh ca khung quay 1 v 2 l

Mq1 = K1 UAB IA cos ( 1200 + ( )

Mq2 = K2 UAC IA cos ( 1200 - ( )

Ti v tr cn bng , ta c Mq1 = Mq2

Hay eq \f ( K1 , K2 ) = eq \f ( cos ( 1200 + ( ) , cos ( 1200 - ( ) )

Do cos( = G(()

Ngc vi mt pha , pha k 3 pha khng nh hng bi tn s nu nh tn s ca tn hiu khng cao

Ngoi ra ta cng c th o lch pha bng dao ng k ( my hin sng hay my Oscillocope cho cc tn hiu c bin nh )

CU HI N TP

Cu 1Nu cu to ca watt k tc dng ? Ti sao watt c cc tnh ?

Cu 2Nu tc dng ca cc cun dy trong watt k tc dng ?

Cu 3Trnh by cch mc watt k ?

Cu 4Trnh by cch o cng sut tc dng mch in ba pha c ti i xng ? ( mng in ba pha 3 dy v ba pha 4 dy )

Cu 5Nu cu to watt k phn khng ? So sng vi watt k in ng ?

Cu 6C bao nhiu cch o cng sut phn khng ? Trong cc cch , cch no l thng dng ?

Cu 7Trnh by cu to ca cng t in mt pha ?

Cu 8Trnh by nguyn l lm vic ca cng t in mt pha ?

Cu 9V cc s u cng t in mt pha ?

Cu 10Trnh by cch xc nh cng t in quay nhanh hay quay chm ? nu cch khc phc ?

Cu 11V cc s u dy cng t in ba pha ? Nu im khc nhau ca cc loi cng t in ny ?

Cu 12Lm th no o nng lng in khi dng in ph ti ln hn dng in nh mc ca cng t in ?

Cu 13Tnh cng sut in tiu th ca mt ph ti c gn vo cng t in trn c ghi 600 vng / kWh . Bit rng ph ti c s dng trong thi gian 15 pht v cng t in quay c 50 vng

Cu 14Mt bp in c cng sut 1000W 220V

Tnh dng in v gi tr in tr ca dy may xo dng lm bp in

Tnh s vng quay ca cng t in nu ta s dng bp in trn trong khong thi gian l 20 pht . Bit rng trn cng t in c ghi 450 vng/kWh

Cu 15M t cch xc nh cng sut in ca mt ng b mt nhn ? ( ta c th s dng vn k ampe k , watt k v cos( k xc nh )

Cu 16Trong qu trnh o cng sut tc dng , nu kim ca 1 trong 3 watt k b quay ngc th ta x l nh th no ?

Cu 17Nu ngha cc thng s ghi trn cng t in mt pha v ba pha ?

(((

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_1196306265.vsdPhu tai

*

*

W

A

V

x

I2

I1

S o mac Watt ke , bien dong TI va bien ien ap TU

a

X

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_1196341089.vsdUA

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_1196937119.vsdR

L1'

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L

TAI

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_1218289967.vsdkW.h

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9

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5

IEN NANG KE MOT PHA

DD28 220V 10( 20)A 50 Hz

600r/kW.h 1996 No 64268

San xuat tai cong ty thiet b o ien

kW.h

EMIC

0

8

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CONG T IEN 3 PHA 4 DAY HU CONG

MV3E4

So SX 05

San xuat tai cong ty thiet b o ien

185642

3x 220 /380V

3 x 5A

50 Hz

450vong /kWh

270C

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C

150V

1

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300V

2

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_1218290874.vsdB

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+

+

D

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_1218056154.vsd*

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W

W1

W2

W3

PHU TAI

TI1

TI2

TI3

_1196899979.vsdj

E2

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D

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_1196307016.unknown

_1196340312.unknown

_1196340955.vsd

_1196306914.unknown

_1196139326.vsdR

RP

RU

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IU

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_1196261767.vsdVA

IA

VBC

VB

VC

jA

900 - jA

_1196274931.vsdVA

IA

VBC

VB

VC

jA

900 - jA

600

_1196280376.vsdA

B

C

*

*

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*

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W1

_1196262387.vsd*

*

W1

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TAI BA PHA

_1196255459.vsd*

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W3

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( 2 )

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VA

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A

V

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U1

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-I2

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dv

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IA

_1156448712.unknown

_1196109085.vsdA

V

RT

RA

RV

A

V

RT

RA

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Ampere ke mac ngoai

_1196108502.vsdG

W

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*

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_1156448582.unknown