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EEE : Three-Phase Systems in Power Applications - Quiz(MCQ)
1 .
In a ∆-connected source driving a ∆-connected load, the
A)
Load voltage and line voltage cancel for a given phase
B)
Load voltage and line voltage are one-third the source voltage for a given phase
C)
Load voltage and line voltage are two-thirds the source voltage for a given phase
D)
Load voltage, line voltage, and source phase voltage are all equal for a given phase

Correct Answer :   Load voltage, line voltage, and source phase voltage are all equal for a given phase

2 .
In a Y-Y source/load configuration, the
A)
Phase current, the line current and the load current are 120° out of phase
B)
Phase current, the line current and the load current are all equal in each phase
C)
Line current and the load current are in phase and both are out of phase with the phase current
D)
Phase current and the line current are in phase and both are 120° out of phase with the load current

Correct Answer :   Phase current, the line current and the load current are all equal in each phase

3 .
In a three-phase system, when the loads are perfectly balanced, the neutral current is
A)
Zero
B)
At maximum
C)
One-third of maximum
D)
Two-thirds of maximum

Correct Answer :   Zero

4 .
In a Y-connected source feeding a ∆-connected load,
A)
Each phase of the load has the full line voltage across it
B)
Each phase of the load has a voltage across it equal to √3
C)
Each phase of the load has two-thirds of the full line voltage across it
D)
Each phase of the load has one-third of the full line voltage across it

Correct Answer :   Each phase of the load has the full line voltage across it

5 .
The stator is the rotating assembly in a generator or motor.
A)
True
B)
False
C)
Can Not Say
D)
None of the above

Correct Answer :   False

6 .
In a Y-connected circuit, between each line voltage and the nearest phase voltage, there is a phase angle of
A)
0°
B)
30°
C)
60°
D)
120°

Correct Answer :   30°

7 .
Compare the total copper cross sections in terms of current-carrying capacity for a single-phase and a three-phase 120 V system with effective load resistance of 15 Ω.
A)
Single-phase 16 A; three-phase 0 A
B)
Single-phase 8 A; three-phase 4 A
C)
Single-phase 16 A; three-phase 8 A
D)
Single-phase 32 A; three-phase 16 A

Correct Answer :   Single-phase 16 A; three-phase 8 A

8 .
A three-phase ∆-connected generator is driving a balanced load such that each phase current is 12 A in magnitude. When Iθa = 12 ∠30° A, what are the polar expressions for the other phase currents?
A)
Iθb = 12 ∠120° A, Iθc = 12 ∠30° A
B)
Iθb = 12 ∠30° A, Iθc = 12 ∠120° A
C)
Iθb = 12∠ 90° A, Iθc = 12 ∠90° A
D)
Iθb = 12 ∠150° A, Iθc = 12 ∠-90° A

Correct Answer :   Iθb = 12 ∠150° A, Iθc = 12 ∠-90° A

9 .
In the transformer the function of a conservator is to
A)
Provide fresh air for cooling the transformer
B)
Supply cooling oil to transformer in time of need
C)
Protect the transformer from damage when oil expends due to heating
D)
None of the above

Correct Answer :   Protect the transformer from damage when oil expends due to heating

10 .
In a three-phase system, the voltages are separated by
A)
45°
B)
90°
C)
120°
D)
180°

Correct Answer :   120°

11 .
Polyphase generators produce simultaneous multiple sinusoidal voltages that are separated by
A)
Certain constant voltages
B)
Certain constant currents
C)
Certain constant frequencies
D)
Certain constant phase angles

Correct Answer :   Certain constant phase angles

12 .
In a certain three-wire Y-connected generator, the phase voltages are 2 kV. The magnitudes of the line voltages are
A)
6,000 V
B)
3,464 V
C)
2,000 V
D)
666 V

Correct Answer :   3,464 V

13 .
A simple two-phase generator consists of two conductive loops separated by 90°, rotating in a magnetic field.
A)
True
B)
False
C)
Can Not Say
D)
None of the above

Correct Answer :   True

14 .
In a balanced three-phase load, each phase has
A)
An equal amount of power
B)
One-third of total power
C)
Two-thirds of total power
D)
None of the above

Correct Answer :   An equal amount of power

15 .
A balanced load is one in which all the impedances are equal.
A)
True
B)
False
C)
Can Not Say
D)
None of the above

Correct Answer :   True

16 .
The most common type of ac motor is the
A)
Single-phase induction motor
B)
Two-phase induction motor
C)
Three-phase induction motor
D)
Two-phase squirrel-cage motor

Correct Answer :   Three-phase induction motor

17 .
In a ∆-connected generator, all of the phase voltages are
A)
Zero
B)
One-third of total
C)
One-sixth of total
D)
Equal in magnitude

Correct Answer :   Equal in magnitude

18 .
In a Y-connected circuit, the magnitude of each line current is
A)
Zero
B)
One-third the phase current
C)
Equal to the corresponding phase current
D)
Three times the corresponding phase current

Correct Answer :   Equal to the corresponding phase current

19 .
In a certain Y-Y system, the source phase currents each have a magnitude of 9 A. The magnitude of each load current for a balanced load condition is
A)
3 A
B)
6 A
C)
9 A
D)
27 A

Correct Answer :   9 A

20 .
If in a Y-connected ac generator, each phase voltage has a magnitude of 90 VRMS, what is the magnitude of each line voltage?
A)
0 V
B)
90 V
C)
156 V
D)
180 V

Correct Answer :   156 V

21 .
An alternator is an electromechanical ac generator.
A)
True
B)
False
C)
Can Not Say
D)
None of the above

Correct Answer :   False

22 .
In a ∆-connected generator, there is a 120° difference between each line current and the nearest phase current.
A)
True
B)
False
C)
Can Not Say
D)
None of the above

Correct Answer :   False

23 .
In a Y-connected generator, there is a 120° difference between each line voltage and the nearest phase voltage.
A)
True
B)
False
C)
Can Not Say
D)
None of the above

Correct Answer :   False

24 .
A constant load power means a uniform conversion of
A)
Voltage to current
B)
Current to voltage
C)
Electrical to mechanical energy
D)
Mechanical to electrical energy

Correct Answer :   Electrical to mechanical energy

25 .
A single-phase sinusoidal voltage of 120 V is connected to a 90 Ω load. Current in the circuit is
A)
1.33 A
B)
6.2 A
C)
13.3 mA
D)
133 mA

Correct Answer :   1.33 A

26 .
The term 'squirrel-cage' applies to a type of three-phase ac generator.
A)
True
B)
False
C)
Can Not Say
D)
None of the above

Correct Answer :   True

27 .
A simple three-phase generator consists of three conductive loops separated by 120°.
A)
True
B)
False
C)
Can Not Say
D)
None of the above

Correct Answer :   True

28 .
Polyphase is characterized by two or more sinusoidal voltages, each having a different phase angle.
A)
True
B)
False
C)
Can Not Say
D)
None of the above

Correct Answer :   True

29 .
In a ∆-connected source feeding a Y-connected load
A)
Each phase voltage equals the corresponding load voltage
B)
Each phase voltage is one-third the corresponding load voltage
C)
Each phase voltage equals the difference of the corresponding load voltages
D)
Each phase voltage is 60° Ωut of phase with the corresponding load voltage

Correct Answer :   Each phase voltage equals the difference of the corresponding load voltages

30 .
Power is measured in a three-phase load by using an ammeter.
A)
True
B)
False
C)
Can Not Say
D)
None of the above

Correct Answer :   False

31 .
A two-phase generator is connected to two 90 Ω load resistors. Each coil generates 120 V ac. A common neutral line exists. How much current flows through the common neutral line?
A)
1.33 A
B)
1.77 A
C)
1.88 A
D)
2.66 A

Correct Answer :   1.77 A

32 .
A three-phase generator is connected to three 90 Ω load resistors. Each coil generates 120 V ac. A common neutral line exists. How much current flows through the common neutral line?
A)
0 A
B)
1.33 A
C)
2.66 A
D)
3.99 A

Correct Answer :   0 A

33 .
The torque – slip characteristics for an Induction motor is a curve because
A)
For small values of slip, torque is directly proportional
B)
For larger values of slip, torque is inversely proportional
C)
Both (A) and (B)
D)
None of the above

Correct Answer :   Both (A) and (B)

34 .
For an 8-pole induction motor supplied with power by a 6 pole Alternator at 1200 revolutions per minute, the value of motor speed at slip of 3% is –
A)
400 RPM
B)
800 RPM
C)
873 RPM
D)
900 RPM

Correct Answer :   873 RPM

Explanation :

Speed of Alternator, N = 1200 RPM

Number of Alternator poles, P = 6

Supply frequency, f = PN/120 = 60 Hz

Hence, for number of 8 poles in Induction motor, the synchronous speed or speed of rotating field is: Ns = 120f/P = 900 RPM

Let Nr is actual speed of motor.

Thus, Percentage Slip, S % = ((Ns – Nr)/Ns) x 100

Substituting the values, we get: Nr = Ns (1-S/100) = 873 RPM.

35 .
In a squirrel cage induction motor, the rotor slots are_______
A)
Parallel to the rotor shaft
B)
Skewed at a certain angle to the rotor shaft
C)
Placed in line with the rotor shaft
D)
None of the above

Correct Answer :   Skewed at a certain angle to the rotor shaft

Explanation : A squirrel cage rotor consists of a laminated steel cylinder with slots in which either Aluminium conductors are die-cast of copper bars are used. These bars are shorted at both ends by heavy end rings of same materials. To reduce the magnetic noise and hence produce a more uniform torque and to prevent possible magnetic locking of the rotor with the stator, the rotor slots are skewed at a certain angle to the rotor shaft.

36 .
Slip of an induction motor lies between_______
A)
0 to 5 %
B)
0 to 10 %
C)
0 to 400 RPM
D)
0 to 4000 RPM

Correct Answer :   0 to 5 %

Explanation : Slip is the difference between the speed of rotating field and actual speed of rotor. The speed of the rotor must be less than the speed of the rotating field, else there would be no relative motion and the rotary motion would cease to exist.

The speed of the rotor should be such that the magnitude of the rotor current is enough to exert the necessary torque. The slip speed denotes the speed of rotor relative to that of the field. Though it is measured in revolutions per minute, more commonly it is denoted in percentage and ranges from 0 to 5%.

37 .
An induction motor is_______
A)
Requires external supply
B)
Self-starting
C)
Both (A) and (B)
D)
None of the above

Correct Answer :   Self-starting

Explanation : As the rotation of the rotor conductors are initiated normally due to the force produced because of interaction between induced current and rotor magnetic field, no external supply is required and the Induction motor is self-starting.

38 .
For a balanced three phase, three wire system with input power of 10kW, at 0.9 power factor, the readings on both wattmeter are ________ respectively
A)
7kW, 3kW
B)
5000W, 5000W
C)
6350W, 3650W
D)
7600W, 1200W

Correct Answer :   6350W, 3650W

Explanation :

Let reading of one Wattmeter = W1

Reading of second Wattmeter = W2

Input Power, P = W1+W2 = √3VLILcosφ = 10000………………1

Power Factor, cosφ = 0.9

Phase angle, φ = 25.8 degrees

Therefore, W1 = VLILcos(30-φ) = 0.99VLIL = 6350W

W2 = VLILcos(30+φ) = 0.56VLIL = 3650W

39 .
For a three-phase delta connected load, fed from a star connected network, the power transferred to the load is_
A)
3 KW
B)
4.7 KW
C)
5 KW
D)
7KW

Correct Answer :   4.7 KW

Explaination :

40 .
In a three phase, delta connection
A)
Line current is equal to phase current
B)
Line voltage and line current is zero
C)
Line voltage is equal to phase voltage
D)
None of the above

Correct Answer :   Line voltage is equal to phase voltage

Explanation : A delta connected AC circuit is achieved by connecting the start end of a winding to the finish end of another winding such that all three windings form a mesh. Since each end of the windings forms the line connection, voltage across each winding is equal to the potential difference between the corresponding lines taken from that winding. Hence the phase voltage is equal to the line voltage.