Jkp Constable Telecommunication Telecommunication Mcqs Jkp Constable (1089 MCQs) | JKSSB & SSC Quiz

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Practice Questions

1089 Total
Question 61 Discuss
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A 4-pole lap wound DC generator has 600 armature conductors and rotates at 1000 RPM. If the flux per pole is 0.02 Wb, calculate the generated EMF.

A
100 V
B
150 V
C
200 V
D
400 V

Answer & Explanation

Correct Option: C

EMF = (P * Phi * Z * N) / (60 * A). For lap winding, A=P. So EMF = (0.02 * 600 * 1000) / 60 = 200 V.
Question 62 Discuss
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In a loaded DC generator, the cross-magnetizing effect of armature reaction causes:

A
Complete loss of residual magnetism
B
A shift in the Magnetic Neutral Axis (MNA)
C
Increase in the main field flux
D
Reduction in brush sparking

Answer & Explanation

Correct Option: B

Cross-magnetization distorts the main field, shifting the MNA in the direction of rotation, leading to poor commutation.
Question 63 Discuss
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A DC shunt generator fails to build up voltage. Which of the following is NOT a valid reason?

A
Lack of residual magnetism
B
Field circuit resistance is higher than critical resistance
C
Speed is above critical speed
D
Direction of rotation is reversed

Answer & Explanation

Correct Option: C

The generator must run ABOVE the critical speed to build up voltage. Running below it would cause failure.
Question 64 Discuss
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A 4-pole, 50 Hz induction motor runs at 1440 RPM. What is its percentage slip?

A
2%
B
4%
C
5%
D
8%

Answer & Explanation

Correct Option: B

Synchronous speed Ns = 120f/P = 120*50/4 = 1500 RPM. Slip = (1500 - 1440)/1500 * 100 = 60/1500 * 100 = 4%.
Question 65 Discuss
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If an induction motor is operating with a slip of 0.05 on a 60 Hz supply, calculate the frequency of the rotor current.

A
3 Hz
B
6 Hz
C
30 Hz
D
60 Hz

Answer & Explanation

Correct Option: A

Rotor frequency (fr) = slip (s) * supply frequency (f). fr = 0.05 * 60 = 3 Hz.
Question 66 Discuss
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The phenomenon of 'cogging' in a 3-phase induction motor refers to:

A
Running at a stable low speed due to 7th harmonic
B
The magnetic locking of stator and rotor teeth at starting
C
Excessive humming noise under full load
D
Overheating due to unbalanced voltage

Answer & Explanation

Correct Option: B

Cogging occurs when the number of stator and rotor teeth are equal or an integral multiple, causing them to lock magnetically and fail to start.
Question 67 Discuss
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Under what condition does a 3-phase induction motor produce maximum torque?

A
When rotor resistance equals rotor reactance
B
When rotor resistance is zero
C
When slip is zero
D
When slip is equal to 1

Answer & Explanation

Correct Option: A

Maximum torque is produced when the rotor resistance (R2) equals the slip times the standstill rotor reactance (s*X2).
Question 68 Discuss
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In a DC machine, a wave winding is generally preferred over a lap winding for applications requiring:

A
High voltage and high current
B
High voltage and low current
C
Low voltage and high current
D
Low voltage and low current

Answer & Explanation

Correct Option: B

Wave winding has only two parallel paths regardless of poles, making it suitable for generating high voltage with lower current ratings.
Question 69 Discuss
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What is the primary function of the commutator in a DC motor?

A
To act as a mechanical rectifier converting AC to DC
B
To convert the supplied DC into alternating current in the armature conductors
C
To reduce armature reaction
D
To provide physical support to the armature windings

Answer & Explanation

Correct Option: B

In a DC motor, the commutator acts as a mechanical inverter, converting external DC supply into AC to maintain unidirectional torque.
Question 70 Discuss
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A star-delta starter reduces the starting current of a 3-phase induction motor by a factor of:

A
1/2
B
1/sqrt(3)
C
1/3
D
1/4

Answer & Explanation

Correct Option: C

By connecting the stator in star during starting, the voltage per phase is reduced by 1/sqrt(3), which reduces the starting current and torque to 1/3 of the delta values.
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