f = 50 (Hz) vao hai dau khong phan nhanh RLC trong do R bieh thien. Khi R = 50(Q) va R = 200 (Q) thi cong sua't tieu thu tren toan mach deu Khi R = 50(Q) va R = 200 (Q) thi cong sua't tieu thu tren toan mach deu bang nhau. Thay doi R de cong sua't toan mach dat cue dai la bao nhieu?
A. lOOW. B. 300W. C. 260W. D. 200W.
'Phan tkh vd huang ddn gidi
Khi R = R, va R = R2 thi Pj = Pj =^ R1R2 = (ZL -Z^f ^ - Z j = ^R^R^ (*) Cong sua't tieu thu tren toan mach , . Cong sua't tieu thu tren toan mach , .
P = R.-
R
Theo bat d^ng thiie Cosi thi P^^^ khi:
R = Z, - Zr- vaL (_ max P^,„
Bi quyet on luy$n thi d^i hgc d^t diem tot da Vat It, tap 1 - Le Van Vinh T u H v a r ) : P„ C h g n dap an D 200^ 27R1R2 2V5O.2OO = 2 0 0 W
V l d\f 6 : Dat mpt di^n ap xoay chieu u = 200\/2 cos(1007i t) vao hai dau do^n mach mac noi tie'p theo t h u t u cac phan t u g o m mot bien t r o R c6
1- 4
di^n tro thay d o i t u 0 den 00, t y di^n c6 di§n d u n g C = va mpt
cupn day khong thuan cam c6 r = 60Q va dp t u cam L = . Xac d i n h cong suat cue dai ciia mach.
A. 130 W . B. 393,4 W . C. 289 W . D. 400 W .
'Phan tkh vd huang ddn gidi
Di^n tro ciia cupn day: r = 60Q
Cam khang ciia cupn day: = coL = —. I O O T I = 150 Q 71
D u n g khang ciia t u : ZQ =
coC 10 lOOTt.—
-4 • = lOOQ
R bien thien de cong suat tieu thu tren mach cue dai k h i : R + r = Z L - Z C (1)
2 Z L - Z C 2(R+r) ( 2 )
Voi U va r de da cho m u o n c6 Pmax ta can xac d i n h R v i the t u (1) ta suy ra R = | Z L- Z c | - r = |l50-100|-60 = - 1 0 f i ma di?n tro thi khong c6 gia trj am nen bai toan tai day tam thoi be tat.
L u u y: khi r > Iz^ - Z(;-| thi ta khong the dp dung cdch gidi nhu a tren de'tim con^ suat tieu thu tren mach cue dqi ma ta can lap luqn tit cong thiec tong qudt ciia c6n^ suat tieu thu ciia mach nhu sau:
Cong suat tieu thu tren mach: P = - (R + r) + ( Z L - Z C ) ' -.(R + r ) : V a y P , .r = • 200^ 60^ +(150-100)2 -.60 = 393,4(W) C h p n dap an B CtyTNHHMTV m \ Khang Vi ft
V l d u 7 : Cho doan mach xoay chieu khong phan nhanh, cupn day c6 di^n t r a r = 1 5( Q) , d p t u c a m L = — ( H ) ^ r T
5n - K ^ r,
Va mpt bien tro R mac n h u h i n h ve. 2 Hi?u di^n the hai dau mach la: U = 80.cos(l007tt)(v)
Cong suat toa nhi?t tren toan mach dat gia trj cue dai la bao nhieu?
A. 89W B. 66W C. 80W D. 59W
i'hdn tick m huong ddn gidi (adsbygoogle = window.adsbygoogle || []).push({});
Cam khang cua cupn day: Z^ = coL = — .I O O T I = 20 Q
5n
Mach khuyet C nen cong suat tieu thu tren toan mach: P = l2.(r + R) = ^ . ( r + R) =
{r^RfUzJ - ( r . R ) = (r + R ) .
Do tir so la U khong doi nen P Ion nhat k h i mau so be nhat. ^ 2
NghTa la: y = r + R + — ^ be nhat. r + R
A p d u n g bat dang thuc cosi cho hai so khong am ta c6:
y2 r + R L _ y = r + R + — I- > 2 J( r + R). r + R ' r + R = 2Z, Dau b a n g x a y r a k h i r + R = Z^ - > R = Z ^ - r = 2 0 - 1 5 = 5(n)
Cong suat eye dai: P^ ( 4 0 ^ 2 ) ^ = 8 0 ( W ) " 2( r + R f 2(15 + 5)
Chpn dap an C
K h i l a m t r i e n g h i ^ m ta giai n h a n h n h u sau: 1 Cong suat tieu thu tren toan mach cue dai k h i d i f n tro thay doi thoa man:
R + r = | Z L - Z c | = Z L ( Z c = 0 ) 2(R + r) R = Z L- r = 20-15 = 5n (40>/2) = 80(W) 2(R + r) 2(5 + 15)
Cdch gidi tren cd le la hay nhat diing khong cdc ban! Vdo phdng thi, cdc ban khong thedi chitng minh cong thiec de dung ma tot nhat la hay chitng mink & nhd devdo phdng thi ma dung.
Bi quyet on luyfti thi dai hgc dat diem to'i da Vat It, tap 1 - Le Van Vinh
Vi dtf 8: Cho doan mach xoay chieu khong phan nhanh, cuon day c6 dien
1 10"* t r o r = 25Q, do t u cam L = - H , t u dien c6 dien d u n g C = — — F va mpt t r o r = 25Q, do t u cam L = - H , t u dien c6 dien d u n g C = — — F va mpt
• • 71 271
bien t r o R c6 gia t r i thay d o i . Hieu dien the hai dau mach la: u = 220V2cosl007rt(V). Cong suat toa nhi^t tren bien t r o dat gia t r j cue dai khi bien tro c6 gia tri nao va gia trj cua cong suat la bao nhieu
A. 189W B. 126W C. lOOW D . 219W
<Phdn tick vd hu&ng dan gidi
Cam khang cua cupn day: = mL = -^.10071 = 100 Q
D u n g khang cvia t u :
(oC 10 lOOTt.
-4 = 200Q
271
Cong suat toa nhiet tren bien tro R la :
P = I2R = - -R = (R + r ) % ( Z L - Z c ) ' (R + r ) % ( Z i ^ - Z c ) ^ R U2 2r + R + r ^ + ( Z , - Z c ) ' R PRmax « 2r + R + r ^ + ( Z L - Z c ) R 2^ o R = R R 2 = r 2 + ( Z L - Z c ) ^ ^ R = ^r^ + ( Z L - Z c )^ = ^25^ + (lOO - 200) =1030
Cong suat cue dai tren R la: , 220^
2r + 2R 2(25 + 103)
Vay chpn dap an A (adsbygoogle = window.adsbygoogle || []).push({});
K h i lam trie nghi^m, ta lam nhu sau:
= 189(W)
R bien thien de PR^ax '^h'-
R 2 = r 2 + ( Z L - Z c ) '
^Rmax 2(r + R )
Cty TNHH MTV DWH Khang Viet R = + ( Z L -Z^f = yJ25^ + (lOO - 200)^ = ] 0 3 Q
220^
2(r + R) 2(25 + 103) = 189(W)
IVl du 9: Cho mach di^n xoay chieu gom cuon day c6 dien tro r =15Q va do t u cam L = — H noi tiep voi bien tro R. Dat vao hai dau ciia doan mach
57r
mot dien ap: u = SO.coslOOTX.t (V). K h i dieh chuyen bien tro den gia tri Ro thi cong suat tren bien tro cue dai. Tim Ro va gia tri P max.
A. 19W B. 26W C. 40W D . 29W
^hdn tich vd huang dan gidi
Theo bai ra, ta c6: r = 15 , Z L = — . I O O T I = 20 Q , U = 40 V2 V
571
Chu y: dei/(}u cau tim P trcn R cue dai phai phan Met vai cong suat tren toan mach gom (r + R). Bai toan gom R bien thien not tiep v&i cuon day co r. Khi do tir cong thitc da chimg minh, ta duge:
P tren R max khi va chi k h i : R^ = Z^ + r^ c=> R = ^20^ +15^ = 25.Q Gia tri cong suat cue dai tren R la: P[^^ - ^ - ^^'^^^
Chgn dap an C
2(R + r) 2(25 + 15) -=40 W
Vl du 10: (Dai hQc2012):
Dat dien ap u = 400cosl007tt (u tinh bang V, t ti'nh bang s) vao hai dau doan mach A B gom dien tro thuan 50 Q mac noi tiep v o i doan mach X. C u o n g do dong dien hieu dung qua doan mach la 2 A. Biet 6 thai diem t,
1 di^n ap tuc thoi gii>a hai dau AB eo gia trj 400 V; 6 thoi diem t
400 euong do dong dien tuc thoi qua doan maeh bang khong va dang giam. Cong suat tieu thu dien ciia doan mach X la
A. 400W. B. 200W. C. 160W. D . lOOW.
^hdn tich v>d hu&ng ddn gidi
• U = 200V2 V ; I = 2A
6 thoi diem t, u = 400V
(Py = lOOTtt = k27r
1 6 thoi diem t + , i = 0, dang giam
Biliuxicl <;f luucu III' </<" /"'< ''"'-'in toi da Vat li, tap 1 - Le Van VtnfT (adsbygoogle = window.adsbygoogle || []).push({});
=> (p'j = lOOTt 1
400 = - + kin <=> lOOTit + - = - + k27t => (pj = - + k 2 7 i tai thoi d i e m t pha cua dong di?n: — + k27t
D Q l?ch pha giiia u va i : 9 = <Pu ~ = Cong suat: P = UIcoscp = 200V2.2.cos
Chpn dap an A
= 400W
Vi du 11: Doan mach xoay chieu AB gom dien t r o R noi tiep cuQn day
thuan cam c6 L thay doi dugc, di?n ap hai dau cuon cam dugc do bSng mot von k e c o dien tro rat Ion. K h i L = Li thi von ke chi V i , d p l^ch pha giCra di#n ap hai dau doan mach v o i dong dien la (pi, cong suat cua mach la Pi. K h i L = L2 th'i v o n k e c h i V2, dp l#ch pha giua dien ap hai dau doan mach va d o n g dien la (p2, cong suat ciia mach la P2.
Biet (pi + (p2 = 71/2 va V i = 2V2. Ti so P1/P2 la:
A. 1/4 B.6 C.5 D. 8
^hdn tick vd hu&ng dan gidi
Theo gia thiet: L = L j => tan(pj - •
L = L2 => tan(p2 = •
R
tan(pj .tan(p2 = 1 <=>
R R R ^1
Gpi U la dien ap hieu d u n g dat vao hai dau doan mach, k h i do:
u u u I, =• I, =• u ^2 ^ p T ^ ; J Z , ^ ( Z L , - Z L 2 ) U.Z,. V , = U , . = I I . Z L . = U.Z,. V2=U. =I2.Z. = •2
Theo bai ra: V, = => ^ = 2 ^ ^ Z^^ = 4 Z L
p j2j^ j2 2
Theo yeu cau de can t i m : — = —— = = — = —
P2 I^R l2 Z L ^ 4
CtyTNHHMrv nw !l KhangViet
Chpn dap an A
Vl du 12: Doan mach AB gom hai doan mach A M va M B mac noi tiep.
Doan A M g o m di^n tro thuan R = lOOQ mac noi tiep v o i cupn cam thuan L = 1 / 7 i ( H ) . Doan M B la tu dien c6 di?n d u n g C. Bieu thuc di^n ap tren doan mach A M va M B Ian lupt la: u ^ j ^ =10oV2cos(1007rt + 7i/4)(V) va
" M B = 200cos(1007tt - T: / 2)(V). so cong suat cua doan mach AB la: A. cos(p = V 2 / 2 B. cos(p = V3/2 C. 0,5 D . 0,75.
'Phdn tick vu huong ddn gidi
Cam khang: Z L = coL = lOOn.- = lOOQ
Tong tro doan AM: Z^,^ = ^R^ + Z^ = VlOO^+lOO^ = IQOV'IQ
Bieu thuc cuang dp dong di?n: (adsbygoogle = window.adsbygoogle || []).push({});
; _ " A M casio ^ 4 i ^ • i nrv ^
I'Tong tro tinh theo so phiic:
,, , „ 100x/2Z- + 2 0 0 Z - -
Z = ^ = ^ i A M ^ ^ ^ 2 ^;ioo_ jooj bam tiep Shift 23
I I 1IZO /n r
= 141,4Z -1 Bam tiep Shift 21 = cuoi ciing bam cos(ans) = ^
^ 7t _ n V2
=>(p = — costp =
4 ^ 4 ^ 2
Chpn dap an A
Vl du 13: M o t mach di,en xoay chieu A B gom doan mach A N chua cupn day thuan cam va d i f n tro thuan mSc noi tiep, doan mach N B chua tu
dien. Dat vao hai dau doan mach AB mot dien ap UAB = 15 N/2 coslOOnt (V) thi dien ap hieu d u n g hai dau A N va NB Ian lupt bSng UAN = 20V va
UN'B = 25V. so cong suat doan mach la