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WorksheetsMEC 303 Final Exam
Total questions: 40
Worksheet time: 37mins
An alternating current completes 100 cycles in 0.1 s. Its frequency is:
20 Hz
100 Hz
0.002 Hz
1000 Hz
An A.C. circuit with a peak value of 100V and frequency of 50 Hz. Which of the following statements is TRUE?
The periodic time is 16.7 ms
The r.m.s. value of the voltage is 70.7 V.
The Peak-to-Peak voltage value is 400 V
The Average voltage is 50 V.
For the diagram shown below, which one of the following statements is true?
V2 leads V1.
V2 lags V1.
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 50 Hz?
377 rad/s
188 rad/s
314.16 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 the capacitive reactance in ohms when it has a capacitance of 15.92 microfarards and is connected to a 50Hz supply?
200Ω
150Ω
100Ω
50Ω
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 reactive power for a resistive/reactive load?
Q=169.25 VAR
Q=119.99 VAR
Q=238.73 VAR
Q=119.99 VAR
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.
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
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 ≈ 1.414 × Vpp; Ip ≈ 1.414 × Vpp
Vp ≈ 0.707 × Ipp; Ip ≈ 0.707 × Ipp
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 leads voltage by 900 in phase
Voltage leads current by 00 in phase
Current leads voltage by 00 in phase
Voltage leads 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 60 Hz?
37.68 Ω
6.28 Ω
31.4 Ω
62.8 Ω
What is the inductance in mH of an inductor with an inductive reactance of 4.11 kΩ when the current through it has a frequency of 96.2 kHz?
1.31 mH
4.11 mH
654 mH
6.8 mH
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, 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 inductive reactance XL is equal to:
12 Ω
47.1 Ω
19.4 Ω
21.7Ω
What unit is used for measuring TRUE or REAL Power in an AC circuit?
kVA.
kVAr
kAh
kW.
Calculate the true power for a purely reactive load?
P=0 W
P=10 W
P=238.73 W
P=100 W
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.
What is the relationship for power factor in a single phase circuit?
Pf = I / V
Pf = kW / kVA
Pf = V / I
Pf = kVA / kW
In the series 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:
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 peak-to-peak value of a sinusoidal alternating voltage with a peak value of 100 V?
200 V
70.7 V
141.4 V
157.2 V
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 DELTA-connected load in a three phase circuit?
VL = VP
VL = VP ÷ 1.73
VL = VP x 1.73
VL = VP – 1.73
What is the effect of a capacitive load on an AC supply?
power factor lag
power factor unity
no effect
power factor lead
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
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
