WorksheetsUnit 06
Total questions: 63
Worksheet time: 52mins
An alternating current completes 100 cycles in 0.1 s. Its frequency is:
20 Hz
100 Hz
0.002 Hz
1 kHz
An A.C. circuit with a peak value of 100V and frequency of 50 Hz. Which of the following statements is false?
The periodic time is 20 ms
The r.m.s. value of the voltage is 70.7 V.
The Peak-to-Peak voltage value is 200 V
The Average voltage is 50 V.
For the diagram shown below, which one of the following statements is true?
V2 leads V1.
V1 leads V2.
V1 lags V2.
V1 in Phase with V2.
Which one of the following statements is true?
In a capacitor, current lags voltage by 90°
In a resistor, voltage lags current by 90°
In a capacitor, voltage lags current by 90°
In a resistor, current lags voltage by 90°
What is the angular frequency (ω) of a waveform whose frequency is 60 Hz?
377 rad/s
188 rad/s
9.55 rad/s
120 rad/s
Power Factor (Cos θ) =_________?
P/S
R/Z
The Cosine of angle between Current and voltage
All the Answers are correct.
What will be the value of a capacitor in farads when it has a capacitive reactance of 200Ω and is connected to a 50Hz supply?
15 μF
15.92 μF
62.8 KF
10 KF
What is the expression for impedance in a series circuit derived from the right angled triangle?
Z2 = R2 – X2
Z2 = R2 ÷ X2
Z2 = R2 × X2
Z2 = R2 + X2
Calculate the true power, reactive power and apparent power for a resistive/reactive load?
P=0 W, Q=169.25 VAR, S=238.73 VA
P=119.36 W, Q=119.99 VAR, S=169.25 VA
P=119.36 W, Q=238.73 VAR, S=238.73 VA
P=119.36 W, Q=119.99 VAR, S=119.99 VA
What is required to improve the power factor of an induction motor?
An inductor conducted in series.
A capacitor conducted in series.
A capacitor connected in parallel.
An inductor connected in parallel.
How are three phase voltages generated in one alternator?
By using three stator coils separated by 120º.
By using three stator coils separated by 60º.
By using three magnetic fields separated by 120º.
By using three magnetic fields separated by 60º.
The line currents in a three phase four-wire system are IBr, IBl and IG. What expression gives the current IN in the neutral conductor?
IN = IBr – IG + IBl
IN = IBr + IG – IBl
IN = IBr – IG – IBl
IN = IBr + IG + IBl
What is necessary for a balanced load in a three-phase system with star-connected loads?
Phases have different loads and the same power factor.
Phases have load currents in phase with their voltages.
Phases have the same current and power factor.
Phases have power factors that total zero.
What is the relationship between line current IL and phase current IP in a delta connected load in a three-phase system?
Iline=IPhase
Iline =√3*Iphase
IPhase=√3*Iline
Iline =√2*Iphase
The time required for the values to rise and fall through one complete cycle is called the ------------ of the waveform.
period
frequency
maximum value
wavelength
What is the instantaneous value of this waveform at 20 µs?
approximately 7 V
approximately 3.5 V
approximately 1.5 V
approximately 4 V
What is the peak-to-peak value of the triangle wave shown to the right?
60 Vpp
8 Vpp
4 Vpp
0 Vpp
To convert in the other direction (from rms values to peak values), use these equations:
Vp ≈ 1.414 × Vrms; Ip ≈ 1.414 × Irms
Vp ≈ 0.707 × Vrms; Ip ≈ 0.707 × Irms
Vp ≈ 2.414 × Vrms; Ip ≈ 2.414 × Irms
Vp ≈ 3.414 × Vrms; Ip ≈ 3.414 × Irms
One complete cycle of a sine wave is generated when a generator's rotor rotates through an angle of ----------.
360°
330°
300°
270°
What will be the phase angle relationship of a six pole, three phase, rotating field generator?
600
1200
1800
3600
For the diagram shown below, which one of the following statements is true?
Current lags voltage by 900 in phase
Voltage leads current by 00 in phase
Current leads voltage by 00 in phase
Voltage lags current by 900 in phase
Inductive reactance results in a current that
Leads the voltage by 90°
Is in phase with the voltage
Leads the voltage by π rad
Lags the voltage by π/2 rad
What is the ratio of voltage to current in an AC circuit with an iron cored coil of negligible resistance?
Impedance.
Self-inductance.
Mutual inductance.
Inductive reactance.
What is the inductive reactance of a coil with a reactance of 100 mH when the supply frequency is 50 Hz?
3.14 Ω
6.28 Ω
31.4 Ω
62.8 Ω
What is the inductive reactance of a 6.8 mH inductor when the current through it has a frequency of 96.2 kHz?
1.31 kΩ
243 µΩ
654 Ω
4.11 kΩ
What is the capacitive reactance of a 150 pF capacitor when the voltage across it has a frequency of 102 MHz?
65.4 Ω
10.4 Ω
96.1 mΩ
32.7 Ω
At which frequency would a 2.2μF Capacitor have a reactance value of 200Ω?
60 Hz
318 Hz
361.7 Hz
361.7 MHz
In RLC circuit, VL and VC are ---------- “out-of-phase” and in opposition to each other.
360o
180o
270o
90o
In RLC circuit, If R is the resistance, XL is the inductive reactance and XC is the capacitive reactance, the impedance Z of the circuit is:
In the RLC circuit, the impedance Z equal:
12 Ω
17.3 Ω
19.4 Ω
21.7Ω
What unit is used for measuring apparent power in an AC circuit?
kVA.
kVAr
kAh
kW.
Calculate the true power, reactive power and apparent power for a purely reactive load?
P=0 W, Q=238.73 VAR, S=238.73 VA
P=0 W, Q=0 VAR, S=238.73 VA
P=238.73 W, Q=238.73 VAR, S=238.73 VA
P=100 W, Q=238.73 VAR, S=238.73 VA
How is the power factor of installation improved?
Connecting capacitors in parallel with the load
Connecting inductors in parallel with the load
Connecting capacitors in series with the load
Connecting inductors in series with the load
What is power factor in a circuit where the phase angle between current and voltage is ɸ?
Sin ɸ
Cos ɸ
Tan ɸ
ArcCos ɸ
What is the purpose of measuring the power factor of ac power systems?
To use more power.
To increase the reactive power.
To identify the phase angle between true power and apparent power.
To increase the phase angle between true power and apparent power.
For a given real power output, what is the effect of low power factor?
low voltage
low current
high current
high voltage
What is the relationship for power factor in a single phase circuit?
Pf = I / V
Pf = kW / kVA
Pf = V / I
Pf = kVA / kW
With improving the power factor, the voltage level in the electrical system will:
decrease
increase
sometimes increase and others decrease
not get affected
What is one condition for resonance in a parallel ac circuit containing resistance (R) inductance (L) and capacitance (C)?
IL = IC
IL = IR
IL = 0
IC = 0
In the series RLC circuit, the resonant frequency ( ƒr ) occurs when:
XL = XC
XL > XC
XL < XC
XL ≠ XC
In the parallel RLC circuit, the resonant frequency (ƒr) occurs when:
XL ≠ XC
XL > XC
XL < XC
XL = XC
In the series RLC circuit, the resonant frequency ƒr is given by the formula:
In a balanced three-phase system-delta load, if we assume the line voltage is VRY = V∠0⁰ as a reference phasor. Then the source voltage VYB is?
V∠0⁰
V∠-120⁰
V∠120⁰
V∠-240⁰
What does the term "lag" means when voltage V1 lags V2 by 90 degrees?
V2 reaches its maximum value a half cycle after V1
V1 reaches its maximum value a half cycle after V2
V2 reaches its maximum value one quarter after V1
V1 reaches its maximum value one quarter after V2
What is the correct phase sequence of a three phase generator when V1 reaches maximum voltage before V2 and after V3?
V3, V1, V2
V1, V3, V2
V2, V1, V3
V3, V2, V1
What is the RMS value of a sinusoidal alternating voltage with a peak value of 100 V?
63.7 V
70.7 V
141.4 V
157.2 V
The elementary 3-phase 2-pole synchronous generator has a stator equipped with --------- coils displaced -------- from each other.
1 coil, 90o
2 coils, 120o
3 coils, 120o
4 coils, 150o
What is the relationship between phase voltage (VP) and line voltage (VL) in a star connected three-phase alternator?
VL = √3*VP
Vp = Z*VL
VL = Z*VP
Vp = √3*VL
What is the relationship between phase current IP and line current IL in a balanced star-connected load in a three phase circuit?
IL = IP
IL = IP ÷ 1.73
IL = IP + 1.73
IL = IP – 1.73
The effect of high impedance in the neutral conductor of a three-phase four-wire system supplying an unbalanced load.
the impedances of the loads will increase
the impedances of the loads will decrease
the voltages will increase
the voltages will decrease
In the figure below, the value of the current passing in the neutral conductor is given by the equation:
IN= IR+IY−IB
IN= IR−IY+IB
IN= IR−IY−IB
IN= IR+IY+IB
In case of delta connection,
Line current = Phase current and Line voltage = √ 3 × Phase voltage
Line current = Phase current and Line voltage = phase voltage
Line voltage = Phase voltage and Line current = √3 × Phase current
Line current = Phase current and phase voltage = √3 × line voltage
Phase voltages of a star connected alternator are VR = 240∠ 0° V, VY = 240∠ -120° V & VB = 240 ∠ + 120° V. What is the phase sequence of the system ?
RYB.
RBY.
YBR.
BYR
What is the effect of an inductive load on an AC supply?
power factor lag
power factor unity
no effect
power factor lead
A 3-phase 4-wire balanced load is connected to a 3-phase 4-wire supply. Each phase draws 65A from the supply. Calculate the neutral current?
195 A
65 A
6.15 A
0 A
A circuit is supplied at 230v 50Hz and draws 10A from the supply with a power-factor = 0.8. Calculate the 'true power' for the circuit
zero Watts
1840W
184W
2300W
A 30 ohm resistance and a coil with inductance of 40 ohms are connected in series to a 100V 50Hz supply. Calculate the 'Apparent power' for the circuit.
120VA
80VA
200VA
140VA
What component can be added to large inductive loads to improve the 'power factor'?
Inductors
Reactors
Resistors
Capacitors
An AC voltage has a peak voltage of 325V. Calculate the RMS voltage.
189.8v
206.9v
229.8v
165.6v
What is calculated with this formula "VICosΘ"
3-phase power
Ac apparent power
single phase power
power factor
What effect does an inductor have on an AC circuit?
sets up opposing current
decreases its resistance
converts it to DC
no effect
The current-carrying conductor arrangement shown in the figure below is
Single-phase 3-wire phase angle 0°
Two-phase 3-wire phase angle 90°
Three-phase 3-wire star connected
Three-phase 3-wire Delta connected
A 30 ohm resistance and a coil with inductance of 40 ohms are connected in series to a 100V 50Hz supply, where the power factor is CosƟ=0.6. Calculate the 'True power' for the circuit.
200 W
120 W
333.33 W
70 W
