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WorksheetsKỹ Thuật Điện Cuối Kỳ
Total questions: 147
Worksheet time: 2hrs 39mins
Given a branch as shown in the figure. Find the correct statement.
u_ab = R.i1 + L.di2/dt
u_ab = Ri1 - Li2
i1 + i2 + i3 = 0
i1 + i2 + i3 > 0
A linear inductor. If the current flowing through decreases by 2 times. Find the correct statement?
Magnetic flux increases by 4 times
Energy in the magnetic field decreases by 4 times
Energy in the magnetic field increases by 4 times
Energy in the electric field increases by 4 times
The voltage and current of an electrical circuit have the following values. Determine the property of the load. Find the incorrect statement?
U̇ = U.e^(j60°) (V); İ = I.e^(j30°) (A) → The load has inductive properties
U̇ = U.e^(-j60°) (V); İ = I.e^(-j30°) (A) → The load has capacitive properties
U̇ = 50 + 50j (V); İ = 5 - 5j (A) → The load has purely capacitive properties
U̇ = 40 - 30j (V); İ = 12 + 16j (A) → The load has purely capacitive properties
The current through an ideal coil with inductance L has the form: i_L(t) = I_m sin(ωt). Find the incorrect statement?
u_L(t) = L.di(t)/dt = L.I_m.d(sin(ωt))/dt
L.ω.I_m = U_m
u_L(t) = L.ω.I_m.sin(ωt - π/2)
X_L.I = U_L; X_L = ωL = 2πfL
Given u(t) = 100√2sin(314t + 80°) (V) and i(t) = 10√2sin(314t - 10°) (V). Find the total active power P of the circuit:
308W
616W
1000W
0W
Find I, knowing X_L = X_C = R = 20Ω; U = 200V
10A
15.4A
10√2 A
5.75A
For a 3-phase asynchronous motor with a wound rotor Y/Δ – 380/220V. R1 = 0.36Ω; X1 = 0.955Ω; R2' = 0.245Ω; X2' = 0.94Ω; p = 2; f = 50Hz; U1 = 380V. When starting, if the voltage drops by 20% U_dm, what is the maximum torque M_c?
M_c < 36.6Nm
M_c < 40Nm
M_c < 56.6Nm
Determine P, Q of the entire circuit and currents I1, I. Know that
What is the maximum value of M?
Mc <36.6Nm
Mc <40Nm
Mc <56.6Nm
Determine P, Q of the entire circuit and current I. Given Ud = 380V, symmetrical 3-phase load with P1 = 35540W, cosφ=0.6. Load Z2 has P2 = 10000W. What is the power factor of the entire circuit cosφ=?
P=35540W
I1=100A
Q=22056VAr
I=125A
Find the incorrect statement about static electric machines.
The windings do not have relative motion with each other.
Static electric machines are used to convert parameters of electrical energy.
Static electric machines are used to convert parameters of electrical energy and simultaneously convert energy forms (mechanical energy to electrical energy or vice versa).
The magnetic circuit of electric machines is usually made of electrical steel. Find the incorrect statement?
To reduce the no-load current of the machine.
To better relate the magnetic field between the windings.
To increase the magnetic flux of the machine.
How do the indicators of the meters change when we switch the key K from a to b? Given that the source voltage U remains constant. Find the incorrect statement?
I1 increases
I2 increases
The W meter index decreases
U2 increases
How does the current of the transformer change when we increase the number of turns W2? Given that the source voltage U1 remains constant. Find the correct statement?
I1, I2 both decrease
I1 decreases, I2 increases
I1, I2 both increase
The primary winding of a three-phase transformer should be Δ but is mistakenly connected as Y, while the secondary winding remains unchanged. How do the no-load current Io, secondary voltage U2, and power loss ΔP change? Find the correct statement?
Io does not change
U2 decreases by √3 times
ΔP increases
ΔP does not change
Determine the change of the currents after the switch K is opened. Find the incorrect statement?
IA decreases
IB increases
How does it change? Find the correct statement?
I does not change
U decreases 3 times
ΔP increases
ΔP does not change
Determine the change of the following lines when the switch K is interrupted. Find the incorrect statement?
I A decreases
I B increases
I C decreases
I does not change
Transformer 1 phase S rated =2kVA; U1 rated =380V; U2 rated =27V. When loaded I2 = I2 rated; cosφ=0.8 then efficiency is 0.97. Determine I1 rated, load power and losses in the transformer corresponding to the load. Find the incorrect statement?
I1 rated =5.26 (A)
Load power =1.6(kW)
ΔP=400(W)
Determine P,Q of the entire circuit and Qc of the capacitor bank. Given a balanced 3-phase circuit, Ud=380V; I1=100A; cosφ1=0.6. The overall power factor cosφ=0.8 (inductive). Find the correct statement?
P=35490W
Q=29618VAr
Qc=−35310Var
The asynchronous motor 3 phase has a Δ/Y connection – 380/220V working with the grid Ud=220V. When starting, the stator winding is connected in Y. Determine the starting phase current, starting line current and starting torque of the motor. Find the incorrect statement?
Starting phase current decreases √3 times; starting line current decreases 3 times
Starting phase current decreases 3 times; starting line current decreases 3 times
Starting torque decreases √3 times; starting phase current decreases √3 times
Starting line current decreases 3 times
The asynchronous motor with wound rotor, starting and starting resistance in the rotor circuit. The starting characteristic of the motor is presented in figure 1) is the natural characteristic 2) and 3) have starting resistance. Find the correct statement?
I_start2 = I_start3
I_start2 > I_start3; r_start2 < r_start3
I_start2 < I_start3; r_start2 < r_start3
I_start2 > I_start3; r_start2 > r_start3
In the following relationships between electrical power Pdt, electromagnetic torque Mdt, mechanical power Pco, losses in the rotor winding ΔPd2, useful mechanical power on the shaft P2 of the motor.
In the following relational expressions between electrical power Pdt, electrical moment Mdt, mechanical power Pco, loss in rotor winding ΔPd2, useful mechanical power on the axis P2 of the three-phase asynchronous motor, ignoring losses in the magnetic circuit ΔPst2. Find the false statement?
Pdt - Pco = ΔPd2 = 3I2'2.r2'2
Mdt.ω1.s = ΔPd2 = s.Pdt
s.Pdt = Pco
Determine the resistance connected in series to the armature circuit of a DC motor with parallel excitation to transfer the speed characteristic from curve a) to curve b). Armature resistance ru = 0.15Ω. Find the correct statement?
0.25Ω
0.45Ω
0.64Ω
0.36Ω
A DC generator with parallel excitation. Pdm = 1KW; Udm = 200V. Field circuit resistance Rkt = 200Ω, armature circuit resistance Rư = 1.5Ω. Find Eu?
209V
206V
203V
215V
For a branch as shown in the figure a and b. At time t: u>0; i>0. Find the correct statement?
Branch a generates power
Branch b generates power
Branch a receives power
How does the change in the readings of the meters occur? When f increases, knowing U=const. Find the false statement?
The wattmeter increases
The voltmeter decreases
I2 increases
I1 decreases
For the electrical circuit, knowing the vector graph. Find the false statement?
U = UR + UL + UC
UL < UC
UL = UC
UR < U
For the electrical circuit, knowing the vector graph. Find the false statement?
XL > XC
U > UR
UL > UC
XL < XC
To have induced electromotive force in the winding. Find the false statement?
Let the conductor move in a constant magnetic field
Let the constant magnetic field move cutting the stationary conductors
Let the varying magnetic field pass through the stationary loops
Let the constant magnetic field move synchronously with the conductor
Find I, knowing XL = XC = 20Ω; R = 10Ω; U = 100V
5A
5√2 A
Find I, knowing X_L = X_C = 20Ω; R = 10Ω; U = 100V
5A
5√2 A
10√2 A
10A
Given İ_1 = I_1√2sin(ωt); İ_2 = I_2√2sin(ωt−π/6). Represent I = I_1 + I_2. Find the incorrect statement?
Determine the characteristics of the armature reaction of synchronous machines. Find the correct statement?
Armature reaction is transverse
Armature reaction is longitudinal with auxiliary field
Armature reaction is longitudinal with demagnetizing
For a DC machine. Find the incorrect statement?
The field winding creates the main magnetic field
The auxiliary field winding is connected in series with the armature winding
The auxiliary field limits the spark between the brush and the commutator
The armature winding compensates for the armature reaction
The motor has P_dm = 1KW; U_dm = 200V; n_dm = 0.8. Find the rated current. Find the correct statement?
7.55A
6.25A
5.75A
Determine the current I_2, I, total circuit power Q, S in the figure. Knowing I_1 = 10A; R_1 = R_2 = X_1 = X_2 = X_C = 5Ω. Find the incorrect statement?
I_2 = 10A
I = 14.1A
Q = -1000VAr
S = 2000VA
Determine R and X_C. Knowing P = 90W; I = 5A; I_2 = 4A. Find the correct statement?
R = 90Ω; X_C = 22.5Ω
R = 7.5Ω; X_C = 10Ω
R = 10Ω; X_C = 7.5Ω
R = 22.5Ω; X_C = 90Ω
Given the circuit. When U = 100V; f = 50Hz then Q_L = 200Var; Q_C = -400Var. Determine Q_L and Q_C when U = 200V; f = 100Hz. Find the correct statement?
Q_L = 400VAr; Q_C = -3200VAr
Q_L = 800VAr; Q_C = -800VAr
Q_L = 100VAr; Q_C = -800VAr
Solve the circuit using the current method.
When U=200V; f=100Hz. Find the correct statement?
Q_L = 400VAr; Q_C = -3200VAr
Q_L = 800VAr; Q_C = -800VAr
Q_L = 100VAr; Q_C = -800VAr
Solve the circuit using the branch current method. Given R=10Ω; R1=9Ω; R2=4Ω; XL=8Ω; XC=12Ω; U=100V. Find the false statement?
İ - I1̇ - I2̇ = 0
10İ + (4-12j)I2̇ = -100
10İ + (9+8j)I1̇ = 100
A synchronous generator has p pole pairs, and the speed of the rotating magnetic field is n1. Determine the rotor speed n. Find the correct statement?
n = n1/p
n = p.n1
n = n1
n changes depending on the load (f=const)
Determine the characteristics of the armature reaction of a synchronous machine. Find the correct statement?
Axial armature reaction reduces magnetism
Radial armature reaction
Axial armature reaction assists magnetism
There are 3 identical single-phase transformers with parameters S_dm=100KVA; U1_dm=10000V; U2_dm=400V; P0=1050W; u_n=4%. Connect 3 single-phase transformers into a three-phase transformer group Δ/Y. Find R1, X1, R2', X2' of the transformer, knowing R1=R2'; X1=X2'. Find the correct statement?
R1=R2'=9.1Ω; X1=X2'=33.4Ω
R1=R2'=5.25Ω; X1=X2'=19.3Ω
R1=R2'=3.03Ω; X1=X2'=11.14Ω
R1=R2'=5.25Ω; X1=X2'=33.4Ω
The three-phase squirrel-cage induction motor has P_dm=22KW; Y/Δ−380/220V; cosφ=0.91; η=0.91; n=1461 rpm; p=2; f=50Hz; I_dm=40.36A; I_open/I_dm=2; M_open/M_dm=1.2; U_d=380V. Find I_open, M_open when the voltage drops by 5% U_dm; Calculate M_max when U_dc=U_dm. Find the false statement?
I_open=268.4A
M_open=155.7Nm
M_max=259.5Nm
Determine the characteristics of the armature reaction of a synchronous machine. Find the correct statement?
The armature reaction only has transverse components
The armature reaction includes transverse components and axial components that reduce magnetism
The armature reaction includes transverse components
What is the characteristic of the synchronous machine's armature reaction?
The armature reaction only has transverse components
The armature reaction consists of transverse and longitudinal components that demagnetize
The armature reaction consists of transverse and longitudinal components that assist in magnetization
A synchronous machine with hidden poles, neglecting armature resistance, has a vector diagram as shown. In which mode is the synchronous machine operating?
Generator mode with over-excitation
Motor mode with over-excitation
Motor mode with under-excitation
A synchronous machine with hidden poles, neglecting armature resistance, has a diagram as shown. In which mode is the synchronous machine operating?
Generator mode with under-excitation
Motor mode with under-excitation
Motor mode with over-excitation
A synchronous machine with hidden poles, neglecting armature resistance, has a diagram as shown. In which mode is the machine operating?
Generator mode with cosφ=1
Motor mode with cosφ=1
Motor mode consuming reactive power Q of the circuit
The external characteristic curve of the synchronous generator is shown in the figure. What is the correct statement?
Curve 3 when the load is purely resistive R
Curve 2 when the load is purely inductive L
Curve 1 when the load is purely capacitive C
Find the reading of the wattmeter. Given U_d = 220V; Z = 20 + 20j Ω. What is the correct answer?
1860W
1210W
2407W
403W
A branch has a complex impedance of 10.e^(-j36.9°); R1 = 10Ω; XL1 = 10Ω; R2 = R3 = 8Ω; XC = XL2 = 6Ω. What is the correct diagram?
Diagram A
Diagram B
Diagram C
Diagram D
What type of current is in the rotor winding of the synchronous machine?
Alternating current with the same frequency as the stator current
Alternating current with a frequency lower than the stator current
Direct current
Induced current with frequency f2 = s.f
Find the incorrect statement about alternating current.
Alternating current has the same frequency as the stator current.
Alternating current has a frequency lower than the frequency of the stator current.
Direct current.
Induced current has frequency f.
Find the correct statement about synchronous motors compared to asynchronous squirrel-cage motors.
The excitation current can be adjusted to improve cosφ for the power grid.
The excitation current can be adjusted to provide reactive power for the power grid.
The excitation current can be adjusted to control the speed within a wide range.
Given a branch as shown in the figure, find the correct statement.
u = e1 + e2
u = e1 - e2
u = e2 - e1
u = e1 - e2 - i
The phenomenon of energy in ideal elements R, L, C is stated as follows. Find the correct statement.
Resistor R accumulates electrical energy.
Inductor L receives electrical energy and accumulates it as magnetic energy.
Capacitor C receives electrical energy and converts it into thermal energy.
Resistor R both accumulates electrical energy and converts it into thermal energy.
Load R, L has cosφ = 0.866; U = 380V; I = 24A; f = 50Hz. Find C of the compensation capacitor to improve cosφ = 1. Find the correct statement.
C = 50μF
C = 150μF
C = 100μF
C = 200μF
For the given circuit, how will the voltage across the element change if R remains constant, the frequency f in the circuit increases, and the current I remains constant? Find the correct statement.
UR remains constant, UL increases, UC decreases.
UR remains constant, UL decreases, UC increases.
UR decreases, UL increases, UC remains constant.
UR increases, UL remains constant, UC decreases.
Find the change in currents after closing K. Knowing XL = XC. Find the incorrect statement.
IA decreases.
IB remains constant.
IC remains constant.
IN increases.
DC motor Pdm = 8KW; Udm = 200V; ηdm = 0.8; ndm = 1500 rpm. Calculate the rated torque and the power consumed by the motor. Find the correct statement.
An electric motor with P rated = 8KW; U rated = 200V; η rated = 0.8; n rated = 1500 rpm. Calculate the rated torque and the power consumed by the motor. Find the correct answer?
10KW;63.7Nm
11.5KW;52.5Nm
6.4KW;45.5Nm
10KW;65Nm
A series excited motor has U rated = 220V; I rated = 25A. The resistance of the armature and the series excitation circuit = 0.15Ω. Find the starting resistance so that I start = 1.5 * I rated.
4.1Ω
5.7Ω
3.2Ω
4.2Ω
Determine the rated phase current of a three-phase asynchronous motor with: P rated = 10KW; Y/Δ - 380/220V; η rated = 0.75; cosφ rated = 0.84. Find the correct answer?
36A
40A
24A
46A
Determine the power P, Q_L, Q_C and the current I. Given u = 220√2sin(ωt); i1 = 22√2sin(ωt−45°); cosφ tm = 1. Find the incorrect answer?
P=3422W
Q_L=3422VAr
Q_C=−3422VAr
I=22A
Find the change in currents when closing switch K. Find the incorrect answer?
I_A increases
I_B remains unchanged
I_C remains unchanged
I_N remains unchanged
The curve 2) in the figure shows the natural characteristic of a three-phase asynchronous motor. Identify the basic characteristic of the motor when adding a supplementary resistance to the rotor circuit. Find the correct answer?
Characteristic number 1)
Characteristic number 3)
Characteristic number 4)
The figure shows the efficiency characteristic of a transformer corresponding to different power factor coefficients of the load. Find the relationship of the power factor coefficients corresponding to the curves a, b, c. Find the correct answer?
cosφ_a = cosφ_b = cosφ_c
cosφ_c > cosφ_b > cosφ_a
cosφ_a > cosφ_b > cosφ_c
For a balanced three-phase circuit, determine the line current I_d. Given U_d = 220V. Find the correct answer?
Cho mạch 3 pha đối xứng. Xác định dòng điện dây I d. Biết U d =220V. Tìm câu đúng?
2.220 √ 3.R
220 R + 220 2R √ 3
220 R √ 3 + 220 √ 3 R
220 2R √ 3
Nêu sự thay đổi của các dòng trong mạch khi ngắt khóa K. Tìm câu sai?
I B không đổi
I C không đổi
I N giảm
I N tăng
Cho mạch 3 pha đối xứng. Xác định I 3 ,P,Q,cosφ tm . Biết Z 1 =(6+6j) Ω;I 1 =26A;Z 2 =(4−12j) Ω;I 2 =30A. Tìm câu sai?
I 3 =52A
P=22970W
Q=44568VAr
cosφ tm =0,75
Về chức năng các bộ phận của máy điện. Tìm câu sai?
Mạch từ làm bằng thép kỹ thuật điện dùng để dẫn từ thông của máy
Dây quấn dùng để dẫn dòng điện
Cách điện để hạn chế dòng điện chạy trong dây quấn
Động cơ không đồng bộ 3 pha rôto, lồng sóc có M đm =140Nm,M mở /M đm =1,4;Y/Δ−380/220V; mômen cảm ban đầu khi mở máy M c =100Nm; điện áp lưới U 1 =220V. Tìm phương án đúng để mở máy động cơ?
Đổi nối Y→Δ
Dùng cuộn kháng mở máy với U mở =0,7U đm
Dùng máy biến áp tụ ngẫu với tỉ số biến đổi K=1,5
Dùng cuộn kháng mở máy với U mở =0,8U đm
Động cơ không đồng bộ 3 pha rôto lồng sóc có P đm =7,5KW;Y/Δ− 380/220V; η đm =0,8;cosφ đm =0,83. Điện áp lưới U d =380V. Tính dòng điện động cơ tiêu thụ từ lưới ở chế độ định mức. Tìm câu đúng?
17,2A
18,1A
29,7A
30,2A
Cho động cơ điện một chiều kích từ song song với U đm =200V;I u ̇ đm =73A;R u ̇ =0,15Ω;n đm =1200 vòng/phút. Để điều chỉnh tốc độ người ta giảm điện áp. Biết I u ̇ ,I kt ,U và tình trạng mạch từ không đ
A DC motor with field excitation in parallel has U_dm = 200V; I_u̇_dm = 73A; R_u̇ = 0.15Ω; n_dm = 1200 rpm. To adjust the speed, the voltage is reduced. Knowing I_u̇, I_kt, U, and the magnetic circuit condition is unchanged. Calculate the speed when the voltage U = 160V. Find the correct answer?
961 rpm
1100 rpm
700 rpm
946 rpm
The symmetrical 3-phase circuit shown has 10 light bulbs per phase, each consuming 100W, U_d = 380V. Find the current in each phase I?
4.56A
2.63A
3.15A
6.20A
The equivalent circuit of a capacitor consists of capacitance C and conductance g connected in parallel. A DC voltage U = 200V is applied to the capacitor. In steady state, the capacitor stores electric field energy W_E = 2.10^(-2) J and consumes power P = 4.10^(-2) W. Calculate capacitance C and conductance g. Find the correct answer?
C = 2.10^(-6) F
C = 1.10^(-6) F
g = 2.10^(-8) S
g = 1.10^(-8) S
Given the current and voltage of an electrical circuit İ = 40 - 30j = 50.e^(-j37°); U̇ = 60 - 80j = 100.e^(-j53°). Determine S, P, Q, cosφ of the circuit. Find the incorrect answer?
S = 5000VA
P = 2400W
Q = -1378VAr
cosφ = 0.96
The diagram shows the wiring of a 3-phase transformer. Find the secondary voltage U_2 corresponding to each diagram a, b, c, d. Knowing U_1 = 6000V; W_1 = 3000 turns; W_2 = 200 turns. Find the incorrect answer?
U_2 = 400V
U_2 = 400√3V
U_2 = 400√3V
U_2 = 400√3V
Two single-phase transformers have the same rated power, the same primary and secondary rated voltage, but transformer I has a higher short-circuit voltage u_nI% than transformer II. When both transformers operate in parallel, write the expression comparing the primary current of transformer I (I_1I) and the primary current of transformer II (I_1II).
Machine I has a short-circuit voltage u_nI % greater than u_nII % of machine II. For 2 machines working in parallel, write the expression comparing the primary current of machine I (I_1I) and the primary current of machine II (I_1II). Find the correct statement?
I_1I > I_1II
I_1I = I_1II
I_1I < I_1II
The asynchronous motor with a squirrel-cage rotor is commonly used in industry and domestic applications for the following reasons. Find the incorrect statement?
Uses common alternating current
Low cost
Convenient to use; high reliability
High power factor and good speed control
A 3-phase synchronous machine operates at a speed n, the magnetic field (rotor) is generated by the direct current excitation of the rotor winding. What type of magnetic field is it? Find the correct statement?
Stationary magnetic field
Pulsating magnetic field
Rotating magnetic field with constant amplitude
Rotating magnetic field with amplitude varying with the frequency of the stator current
Calculate the equivalent complex impedance of the circuit between points A and B. Given R1 = 13Ω; R2 = 3Ω; X_L1 = X_L2 = X_L3 = 18Ω; X_C1 = X_C2 = 6Ω. Find the correct statement?
4 + 3jΩ
4 - 3jΩ
13 + 6jΩ
10 + 21jΩ
Find the incorrect formula?
M = 3p.U1^2.r2' s.u.[(r1 + r2' s)^2 + (X1 - X2')^2]
M_open = 3.I2'2.r2' ω1
M = 3.I2'2.r2' s.ω1
P_dt = 3.I2'2.r2' s.(ω = 2πf; ω1 - speed of the rotating field)
Determine I1; I2; P; Q for the entire circuit in the figure. Given U = 100V; R1 = 5Ω; R2 = 4Ω; X2 = 3Ω. Find the incorrect statement?
I1 = 20A
I2 = 20A
P = 4800W
Q = 1200VAr
Determine the total power Q for the entire circuit in the figure. If I = 15.4A; I2 = 15A; X_C = 15Ω; X_L = 5Ω; X_L1 = 8Ω. Find the correct statement?
Q = 1485.8VAr
Q = 4485.8VAr
Q = 685.8VAr
For a branch as shown in the figure. Find the correct statement?
u = e - L.di/dt
u = e + L.di/dt
u = -e - L.di/dt
1 = 8Ω. Find the correct statement?
Q=1485.8VAr
Q=4485.8VAr
Q=685.8VAr
Given a branch as shown in the figure. Find the correct statement?
u=e−L.di/dt
u=e+L.di/dt
u=−e−L.di/dt
Write Kirchhoff's equation 2 for the following loop circuit. Find the correct statement?
R.i1 - Li2 + (1/C) ∫ i3 dt from 0 to t = e
R.i1 - L.di2/dt + (1/C) ∫ i3 dt from 0 to t = e
R.i1 - L.di2/dt + (1/C) ∫ i3 dt from 0 to t = -e
R.i1 - Li2 + (1/C) i3 = e
Knowing the short circuit test results of a single-phase transformer: Un = 20V; I1n = 100A; Pn = 800W. Find rn = r1 + r2'; xn = x1 + x2'. Find the correct statement?
rn = 0.2Ω
rn = 0.8Ω
xn = 0.196Ω
xn = 0.183Ω
Given I1 = 40.e^(j160°); I2 = 30.e^(j40°). Find I3. Find the correct statement?
70A
10A
36A
25A
For the given circuit, knowing the vector graph. Find the correct statement?
UR = U; UL = UC
UR < U; UL > UC
UR > U; UL < UC
XL > XC
Find P of the entire circuit, knowing I1 = 3A; I2 = 4A; I = 5A; R = 10Ω. Find the correct statement?
150W
1000W
288W
500W
Determine the power factor of a three-phase asynchronous motor with the following data: Pdm = 40KW; Y/Δ−380/220V; Idm = 91/157A; ηdm = 0.86. Find the correct statement?
0.78
0.875
0.8
0.76
The no-load current i0% of the asynchronous motor is greater than i0% of the transformer due to the following reasons. Find the correct statement?
The magnetic circuit of the motor has a larger air gap
Because the magnetic field of the motor is rotating
Because the magnetic field in the transformer is a pulsed magnetic field
Find R and P knowing XL = 30Ω; I = 4A; u = 200V. Find the correct statement?
R=40Ω; P=640W
R=20Ω; P=320W
R=50Ω; P=800W
R=40Ω; P=840W
Find R and P knowing X_L = 30Ω; I = 4A; U = 200V. Find the correct answer?
R=40Ω;P=640W
R=20Ω;P=320W
R=50Ω;P=800W
R=40Ω;P=840W
Given 4 asynchronous 3-phase motors with the data: a) p=4; f=50Hz; s_dm=0.04 b) p=1; f=500Hz; s_dm=0.03 c) p=2; f=1000Hz; s_dm=0.03 d) p=1; f=400Hz; s_dm=0.04 Determine the synchronous speed n_1 and the rated speed n_dm of the motors. Find the correct answer?
n_1=750 rpm; n_dm=730 rpm
n_1=30000 rpm; n_dm=29100 rpm
n_1=60000 rpm; n_dm=58200 rpm
n_1=24000 rpm; n_dm=23940 rpm
For the electrical circuit, knowing i(t) = I_m.sin(ωt). Find the incorrect expression?
u(t) = u_R(t) + u_L(t) + u_C(t)
u(t) = R.I_m.sin(ωt) + X_L.I_m.sin(ωt + π/2) + X_C.I_m.sin(ωt - π/2)
U = U_R + U_L + U_C
I = U/√(R^2 + (X_L - X_C)^2); z = √(R^2 + (X_L - X_C)^2)
The DC motor has P_dm = 8KW; U_dm = 200V; η_dm = 0.8; n_dm = 1500 rpm. Calculate the rated torque and the power consumed by the motor. Find the correct answer?
10KW; 63.7Nm
11.5KW; 52.5Nm
9.5KW; 45.5Nm
10KW; 51Nm
Determine the inductance L of the coil knowing the experimental results: - When connecting the coil to a DC source U=220V, the current through the coil is I=5A - When connecting the coil to an AC source U=200V; f=50Hz, the current through the coil is I=4A. Find the correct answer?
0.2H
0.4H
1H
0.095H
The single-phase transformer has parameters: transformation ratio K=2; R_1=0.4Ω; R_2' = 0.4Ω; X_1=0.2Ω; X_2' = 0.2Ω. The secondary winding is connected to the load, the equivalent load impedance calculated is Z' = R' + j.X' = 36.8 + j38.4Ω. The primary winding is connected to the source U_1 =
Find the voltage U2.
216.5V
185V
196.9V
220V
Find the incorrect statement about synchronous motors working in over-excitation mode.
To generate reactive power for the power grid
To make the current lag the voltage (phi > 0)
To improve the power factor for the power grid
Find the incorrect reasons why a DC generator in parallel excitation cannot establish voltage.
No residual magnetism
Residual magnetism is opposite to the magnetic field created by the excitation current
The resistance of the excitation circuit is greater than the critical resistance
The generator is not connected to the load
Find the correct measurement of the ammeter. Given u = 180sin(ωt); XC1 = XC2 = XC3 = 3Ω; XL = 9Ω.
20A
60A
0A
14.1A
Find the incorrect statement about iron losses in the steel core of electrical machines.
Eddy currents and hysteresis phenomena cause iron losses
To reduce losses due to hysteresis, the steel core is made of electrical steel
Iron losses depend on the amplitude of the magnetic flux in the steel core
Find the correct iron losses and copper losses when β = I2/I2dm = 0.5.
ΔPst = 110W
ΔPđ = 500W
ΔPst = 250W
ΔPđ = 250W
Find the incorrect statement about the efficiency of electrical machines.
The efficiency of an electrical machine is the ratio of output power P2 to input power P1
The efficiency of a generator is the ratio of power
Find the incorrect statement about electric machines?
The efficiency of an electric machine is the ratio of output power P2 to input power P1.
The efficiency of a generator is the ratio of apparent output power to the mechanical power of the primary motor.
The efficiency of a motor is the ratio of useful mechanical power to the electrical power consumed by the motor.
For a balanced three-phase circuit, determine I1, I2, total power P, Q. Given Ud=380V; Z=(24+18j)Ω. The motor consumes power P1=9.85KW and has a power factor cosφ=0.8. Find the correct statement?
I1=22A
I2=22A
Q=-1325VAr
P=17362W
The three-phase asynchronous motor with wound rotor has the following data: Y/Δ-380V/220V. Core loss and stray loss ΔPc=700W; I2'=50A; r2'=0.1Ω; synchronous speed n=1440 rpm; f=50Hz; p=2. Determine the rated slip Sdm; electrical power Pe; mechanical power Pm; electromagnetic torque Md; output power P2. Find the correct statement?
Sdm=0.04; Pm=35280
Pe=18750W
Md=212Nm
P2=34580W
Determine the reactance Xc of the circuit knowing: R=6Ω; XL=8Ω; the reactive power of the circuit Q=0. Find the correct statement?
18Ω
14Ω
10Ω
12.5Ω
For the given circuit diagram, find the correct statement?
u=-R.i-1/C∫idt
u=R.i+1/C∫idt
u=-R.i+1/C∫idt
u=R.i+C.i
A source is replaced by an ideal electromotive force e. Find the correct statement?
u does not depend on I
u depends on I
u=-e
u=e-i
For a DC motor, find the incorrect statement?
M=p.N/2πa.φ.Iu
Eu=KE.n.φ
U=Eu-Iu.Ru
Determine I, I1, I2, I3 of the electric circuit.
For a DC motor, find the incorrect statement?
M = p.N / (2πa) . φ . I / u
E / u = K . E . n . φ
U = E / u - I / u . R
Determine I, I1, I2, I3 of the circuit. Given U=100V; X1=4Ω; X2=X3=10Ω; X=10Ω; R1=3Ω; R=5Ω. Find the incorrect statement?
I3 = 5A
I = 10A
I2 = 5A
I1 = 5A
For the circuit shown, given: u(t) = 100√2sin(314t+80°) V, i(t) = 10√2sin(314t+20°) A. Calculate the total power P, Q, S of the circuit. Find the correct statement?
P = 380W
P = 500W
Q = -886VAr
P = 0W; Q = 500VAr
For the circuit shown, knowing that the source voltage U is constant. When K is open P=640W; UC=60V; UR=80V. Find UR, UC, I, P after K is closed. Find the correct statement?
P = 640W
UC = 135V
I = 9.36A
UR = 110V
Determine the voltage across the inductor of the AC circuit consisting of R and L in series. Given R=30Ω; XL=40Ω; U=100V. Find the correct statement?
100V
40V
80V
90V
For a DC motor, Pdm = 1KW; Udm = 200V; ηdm = 0.8. Find the total losses in the machine. Find the correct statement?
0.35KW
0.15KW
0.25KW
For the circuit, knowing that the ammeter reads 10A. Calculate the reading of the voltmeter, given XL2=1Ω; XC=1Ω; R1=1.4Ω; R2=10Ω; XL1=1Ω. Find the correct statement?
15V
10V
2.1V
22.2V
Using the stacking method to calculate the current i. Given u = 100 + 200√2sin(ωt); 1/ωC = 10Ω; R = 10Ω. Find the correct statement?
i = 10 + 20√2sin(ωt) + 20√2sin(ωt - π/2)
i = 10 + 20√2sin(ωt - π/4)
i = 10 + 40sin(ωt + π/4)
Find the correct statement?
i=10+20√2sin(ωt)+20√2sin(ωt−π/2)
i=10+20√2sin(ωt−π/4)
i=10+40sin(ωt+π/4)
Determine the characteristics of the armature reaction of the synchronous machine. Find the correct statement?
Armature reaction is transverse to the axis
Armature reaction is longitudinal to the axis with demagnetizing effect
Armature reaction is longitudinal to the axis with aiding effect
A synchronous generator supplies power to a purely resistive load. When the load increases, the frequency of the stator current decreases. To maintain a constant frequency, adjustments are made as follows. Find the correct statement?
Increase the rotor speed n
Decrease the rotor speed n
Decrease the excitation current of the rotor
How do the currents change after closing switch K? Given XL=R. Find the incorrect statement?
IA increases
IB remains unchanged
IC increases
I remains unchanged
Given U=220V; R=10Ω; XL=XC=20Ω. Calculate UR, UL, UC, I. Find the correct statement?
UR=200V
UL=440V
UC=400V
I=3.33A
Why is the magnetic circuit of the transformer made of electrical steel? Find the incorrect statement?
To reduce no-load current
To increase the coupling coefficient between windings
To facilitate the installation of windings
To reduce no-load losses of the windings
A single-phase transformer, primary winding turns W1=800 turns; electromotive force E1=400V; f=50Hz; magnetic circuit cross-section S=18cm². Calculate the self-inductance Bm. Find the correct statement?
0.125T
1.25T
5.5T
Given the results of the no-load test of a single-phase transformer: U1dm=400V; U2dm=36V; ΔPst=200W; I0=1.2A. Calculate rth, xth and the transformation ratio K of the transformer. Find the incorrect statement?
rth=129Ω
xth=...
Calculate the resistance r_th, reactance x_th and transformation ratio K of the transformer. Find the incorrect statement?
r_th = 129Ω
x_th = 303Ω
K = 11.1
To start a synchronous motor, the following methods are available. Find the incorrect statement?
Use a helper motor connected to the rotor
Use a starting winding placed in the rotor slot
Use a starting winding placed in the stator slot
A current transformer has a primary winding W1 = 1 vòng. When the primary current is 400A, the secondary current is 5A. Find the number of turns W2? Find the correct statement?
W2 = 500 vòng
W2 = 80 vòng
W2 = 100 vòng
A DC motor has P_dm = 2KW; U_dm = 200V; η_dm = 0.8. Calculate the rated current. Find the correct statement?
12.5A
6.25A
5.75A
A synchronous generator operates with a load that has a reactive armature as shown in the figure. Determine the nature of the load? Find the correct statement?
Pure resistive load R
Pure capacitive load C
Pure inductive load L
What are the disadvantages of synchronous motors compared to asynchronous squirrel-cage motors? Find the incorrect statement?
For the same power, synchronous motors are more expensive
Operating synchronous motors is more complex
The power factor cosφ of synchronous motors is lower
Using the two-node voltage method, calculate the voltage U_AB and the branch current I through the resistor R. Given E1 = 100.e^(j90°); E2 = 200.e^(-j90°); XL = 10Ω; XC = 10Ω;
