# Network Theory Questions and Answers – Polyphase System

This set of Network Theory Multiple Choice Questions & Answers (MCQs) focuses on “Polyphase System”.

1. The power generated by a machine increases _____________ percent from single phase to two phase.
a) 40.4
b) 41.4
c) 42.4
d) 43.4

Explanation: In an ac system it is possible to connect two or more number of individual circuits to a common poly phase source. The power generated by machine increases 41.4 percent from single phase to two phase. So, percentage increase = 41.4%.

2. The percentage of power increased from single phase to three phase is?
a) 50
b) 100
c) 150
d) 200

Explanation: The percentage of power increased from single phase to three phase is 50. So, percentage increase = 50%. Beyond three phase the maximum possible increase is only seven percent but the complications are many.

3. When the power factor is __________ the power becomes zero 100 times a second in a 50Hz supply.
a) 0
b) 1
c) 2
d) 3

Explanation: The power in a single phase circuit is pulsating. When the power factor is one, the power becomes zero 100 times a second in a 50Hz supply. Therefore single phase motors have a pulsating torque.

4. Which motors are called self-starting motors?
a) single phase
b) two phase
c) three phase
d) four phase

Explanation: Three phase motors are more easily started than single phase motors. Three phase motors are called self-starting motors. Single phase or two phase or four phase motors are not called self-starting motors.

5. In three phase system, the three voltages (currents) differ in phase by __________electrical degrees from each other in a particular sequence.
a) 30
b) 60
c) 90
d) 120

Explanation: In general a three phase system of voltages is merely a combination of three single phase systems of voltages. In three phase system, the three voltages (currents) differ in phase by 120⁰ from each other in a particular sequence.
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6. In a two phase generator, the armature has two distinct windings that are displaced __________ apart.
a) 45⁰
b) 90⁰
c) 135⁰
d) 180⁰

Explanation: Single phase voltages and currents are generated by single phase generators and the armature of such generator has only one winding or one set of coils. In a two phase generator, the armature has two distinct windings that are displaced 90⁰ apart. θ= 90⁰.

7. In three phase system at any given instant, the algebraic sum of three voltages must be?
a) 0
b) 1
c) 2
d) 3

Explanation: For three phase alternator the three voltages are of same magnitude and frequency. In three phase system at any given instant, the algebraic sum of three voltages must be zero. Algebraic sum of three voltages = 0.

8. Phase sequence depends on the _________________
a) field
b) rotation of the field
c) armature
d) rotation of the armature

Explanation: The sequence of voltages in the three phases undergo changes one after the other abd this is called phase sequence. Phase sequence depends on the rotation of the field not on rotation of armature or on field or on armature.

9. If RR, YY and BB constitutes three phase sequence if VRR = Vmsinωt its corresponding field magnets are in clockwise direction, then VYY =?
a) Vmsinωt
b) Vmsin(ωt+120⁰)
c) Vmsin(ωt-120⁰)
d) Vmsin(ωt-240⁰)

Explanation: If the field system is rotated in the anticlockwise direction, then the sequence of voltages in the three phases are in order RBY. The value of VYY is VYY = Vmsin(ωt-120⁰).

10. If RR, YY and BB constitutes three phase sequence if VRR = Vmsinωt its corresponding field magnets are in clockwise direction, then the value of VBB is?
a) Vmsin(ωt-240⁰)
b) Vmsin(ωt-120⁰)
c) Vmsin(ωt+240⁰)
d) Vmsinωt

Explanation: The value of VBB is VBB = Vmsin(ωt-240⁰). There are only two possible phase sequences; they are RBY and RYB.

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