D. dong dien tire thoi trong mach bang tong cac dong dien tire thoi qua cac p h a n t i r sai vi d o n g d i e n t u c t h o i t r o n g m a c h k h o n g being t o n g cac d o n g
m Vi DU MAU:
Vi du 1: Dat di#n ap xoay chieu U = UQCOS c o t - ^
6 (V) vao hai dau dogn
mach noi tiep gom bien tro R, cupn cam thuan c6 dp tu cam L va tu di^n CO di#n dung C. Dieu chinh bien tro R den gia tri Ri thi dong di^n trong mach CO bieu thiic i = Ig sin cot + —
6 (A). Gia tri cua Ri bSng
2^0 h) h IQS
<Phdn tick m huang dan gidi
Dp lech pha giiia hi^u di^n the va cuong dp dong dien:
n n n
cp = cp,-cp^ = - - - - = - -
Vi theta c6: tancp = ^ = -yfs => Z^ -Z^= -^[3R•^
Tongtm: Z = ^R^ + (z^ - Z ^ ) ' = JR? + SR^ = 2R, = ^ =r> R^ = Ho
Chpn dap an A
Vl du 2: Dien ap giua hai dau mot doan mach RLC noi tiep som pha Tt/4 so
voi cuong dp dong dien. Phat bieu nao sau day la dung doi voi doan
mach nay?
A. Hieu so giua cam khang va dung khang bang dien tro thuan ciia doan mach.
B. Dien ap giOa hai dau di^n tro thuan som pha n/4 so voi dien ap giua
hai ban tu dien.
C. Tong tro ciia doan mach bang hai Ian di?n tro thuan ciia mach. v D. Tan so dong dien trong doan mach nho hon gia trj can dexay ra cpng huong.
^hdn tkh vd hucmg d&n gidi
Dien ap gii>a hai dau mot doan mach RLC noi tiep som pha 7t/4 so voi
cuong dp dong dien nen:
^ = ^ u - ^ i = ^ ^ t a n ( p = - t - ^ = l ^ Z L - Z c = R r ) Sau day la each chpn va khong chpn dap an ciia bai toan:
A. khi cp = (p^ - (pj = ^ nen theo (*) thi hifu so giua cam khang va dung khang bang dien tro thuan ciia doan mach vi the A diing
B. ta CO (p^j^ = (p. < cp^ nen di?n ap gii>a hai dau di?n tro thuan sem tre pha
Tt/4 so voi difn ap giua hai ban ty difn vi the'B sai
C. theo (*) ZL - Z c = R =^ Z = +{Z^-Z^f - VR^+R^ R^l nen tong tro
ciia doan mach bSng hai can hai Ian di^n tro thuan ciia mach vi the C sai. D. Tan so dong di^n de xay ra cpng huong la: COQ =
K h i Z , - Zf- = R <=> coL - — = R <=> co^LC -1 = RCco ^ ^ coC 2 <=> — = 1 + RCO) => COn = I Q COn Vl + RCco " ',>{ fj/i': CO
V o i R, C, CO la cac hang so' d u o n g nen 1 + RCco > 1 = > cOg = , < co V l + RCco
Vay tan so' dong dien trong doan mach fthe Ian han gia tri can de xay ra cong huong. Dap an nay ciing sai
Chpn dap an A
V l du 3: Trong doan mach RLC noi tiep dang xay ra cong h u o n g di^n. Ne'u tang dan tan so dong dien va g i i i nguyen cac thong so khac ciia mach thi
ke't luan nao sau day khong d u n g ?
A. He so cong suat ciia doan mach giam.
B. Di^n ap hieu d u n g giira hai dau dien tra giam. C. Cong suat tieu thu tren toan mach tang. D. Cuang do hieu d u n g ciia dong dien giam.
'Phdn tich vd hu&ng ddn gidi
Tong tra ciia mach: Z = y R j + (Z^ - ZQ )^
K h i mach cong h u o n g thi: Z^ - Z,-- = 0 => Z = Z^^^ = R
K h i tang tan so'thi:
Z L = coL t ; Zc = Z L - Z c > 0 => Z = ^R^ +{Z^-Z^f > R :^ Z t
Cuang do dong d i ^ n hieu d u n g trong mach: I = => I i Sau day la each chon va khong chon dap an ciia bai toan:
A. H$ so'cong suat ciia doan mach: coscp = coscp i vay A diing B. Dien ap hieu d u n g giiia hai dau dien tra: Uj^ = RI iz:> i nen B diing B. Dien ap hieu d u n g giiia hai dau dien tra: Uj^ = RI iz:> i nen B diing C. Cong suat tieu thu tren toan mach: P = RI^ 4'=> P i nen C sai
D. C u a n g do hieu dung ciia dong dien giam. D i i n g n h u lap luan tren. C h p n dap an C
fi du 4: Dat dien ap u = Uocos lOOrrt vao hai dau doan mach dien tro thuan R noi tiep voi cuon day thuan cam c6 do t u cam L thay d o i dugc. K h i L = L i = ^ H thi i lech pha (j)^ so vai u, khi L = L2 = — H t h i i l^ch
371 71
pha ^2 so voi u . Biet cpj + cpj = --^. Gia tri ciia R la
200V3
A. Q . B. 100 Q . c.5on.
Theo bai ra, ta c6:
71
^hdn tich vd hu&ng ddn gidi
+ ^ 2 =-"2 ^
• tancpj =
•9i = - V2 + 2 71
tancp. - <=> tancpj .tan(p2 = 1 cj> • tancpj = tan Z, 2 'L2 R R V2 + 2 = 1 = COtg(p2 ^ R = . / Z L J . Z L 2 = J ^ . I O O V S = lOOQ Chon dap an B
Vl du 5: Tren doan mach xoay chieu khong phan nhanh c6 bon diem theo diing t h i i t u A, M , N va B. Giiia hai diem A va M chi c6 dien tra thuan, giua hai diem M va N chi c6 tu dien, giua hai diem N va B chi c6 cuon day. Dat vao hai dau doan mach mpt dien ap xoay chieu 240V - 50 H z thi
" M B va u^f^ l^ch pha nhau ^ , va u ^ g lech pha nhau - . D i e n ap hi^u d u n g tren R la
A.60V3(V) B. 80(V). C. 80V3(V) D. 60 (V).
'Phdn tich vd huong ddn gidi
Ta CO gian do vecto trugt n h u hinh ve:tu' hinh ve de dang thay:
A B M = - ; A M B = —
6 3
A p d u n g djnh ly ham sin trong tarn giac A M B
A M A B i . 71 s i n - 6 . 271 sin 3
Bi quyet on luyfu thi dai hoc liat diem toi da Vat It, tap 1 - Le Van Vinh A M = A B = 4S = ^ = 80V3 => U R = 80V3V sin 3 Vay chpn dap an C Nhan xet:
Cdc bdi todn vepha trong dong dien xoay chieu noi chung la rat pho Men trong dc thi dai hoc. Co hai cdch giai dang todn nay Id giai thco Men dot dai sovd giai theo hinh hoc I'mg dung cua cdch ve vecto trugt. Trong cdch giai thco vecto trugt, true quan vd de hieu han, cdc ban co gang nam that chdc de tiet kiem thai gian trong qud trinh Idm bdi thi.
V i d u 6: Dat dien ap u = UocoslOOTtt (V) vao hai dau doan mach AB gom hai doan mach A M va MB mac noi tiep. Doan mach A M gom dien tro thuan IOON/SQ m§c noi tiep voi cuon cam thuan c6 dp tu cam L . Doan
lO""*
mach MB chi c6 tu dien c6 di?n dung ——F . Biet di?n ap giira hai dau
2n
doan mach A M lech pha - so voi dien ap giiia hai dau doan mach AB. 3
Gia tri cua L bang