Polyphase AC Circuit MCQs-1
In this post, polyphase ac circuit mcqs are given. It includes voltage in polyphase circuit, phase, phase sequence, current – voltage in star load, current – voltage in delta load, phase voltage in star connection, phase voltage in delta connection, active power, reactive power, apparent power, star connection, power consumption in delta connection, current coil of wattmeter, pressure coil of wattmeter, connection of current coil and pressure coil in wattmeter are given.
Polyphase AC Circuit MCQs: 1 to 5
1 The voltage induced in the polyphase winding is displaced by _____. (Where n is number of phases)
( a )180o / n
( b )360o / n
( c )180o × n
( d )360o × n
Correct Answer ( b ): 360o / n
2 The resultant induced emf in the three-phase circuit if three phase supply is to given to it
( a )Maximum
( b )Minimum
( c )430 Voltage
( d )Zero
Correct Answer ( d ): Zero
ER + EY + EB = 0
3 The order in which the three phases attain the maximum value is called as
( a )Phase sequence
( b )Phase
( c )Phase difference
( d )None of the above
Correct Answer ( a ): Phase sequence
4 Which of the following relation is true for star connected load?
( a )IL = Iph / √ 2
( b )Vph = VL
( c )Vph = VL / √ 3
( d )Iph = IL / √ 3
Correct Answer ( c ): Vph = VL / √ 3
For star connection Vph = VL / √ 3 and Iph = IL
5 Which of the following relation is true for delta connected load?
( a )Vph = VL / √ 3
( b )VL = Vph / √ 3
( c )Iph = IL
( d )Iph = IL / √ 3
Correct Answer ( d ): Iph = IL / √ 3
For delta connection Vph = VL and Iph = IL / √ 3
Polyphase AC Circuit MCQs: 6 to 10
6 The neutral point is possible in the
( a )Mesh connection
( b )Delta connection
( c )Star connection
( d )None of the above
Correct Answer ( c ): Star connection
Neutral point is only possible in the star connection.
7 In a star connected transformer, if the line voltage is 11 kV, the phase voltage is
( a )11 kV
( b )√ 3 × 11 kV
( c )11 / √ 3 kV
( d )√ 2 × 11 kV
Correct Answer ( c ): 11 / √ 3 kV
For star connection Vph = VL / √ 3
Vph = 11 / √ 3 kVL
8 Which of the following condition is true for three phase star connected system?
( a ) IL = √3 × Iph
( b )IR + IY + IB = 0
( c )Vph = √ 3 × VL
( d )Iph = √ 3 × IL
Correct Answer ( b ): IR + IY + IB = 0
In the star connection balanced load system, IR + IY + IB = 0
9 The line voltage leads phase voltage in the star connection by
( a )0o
( b )30o
( c )60o
( d )120o
Correct Answer ( b ): 30o
10 The active power of the three-phase star / delta connected system is
( a )VLIL Cos Ф
( b )√ 2 ( VLIL Cos Ф )
( c )√ 3 ( VLIL Cos Ф )
( d )3VLIL Cos Ф
Correct Answer ( c ): √ 3 ( VLIL Cos Ф )
P = 3 VLIL Cos Ф
In the star connection Vph = VL / √ 3 and Iph = IL
Therefore P = 3 VphIph Cos Ф
P = 3 (VL / √ 3)(IL) Cos Ф
P = √ 3VL IL Cos Ф
In the delta connection Vph = VL and Iph = IL / √ 3
Therefore P = 3 VphIph Cos Ф
P = 3 (VL )( IL / √ 3) Cos Ф
P = √ 3VL IL Cos Ф
In both cases, three phase power P = √ 3VL IL Cos Ф
Polyphase AC Circuit MCQs: 11 to 15
11 The reactive power of a capacitor is generally taken as
( a )Positive
( b )Negative
( c )Zero
( d )None of the above
Correct Answer ( b ): Negative
According to power triangle, reactive power of capacitor is taken as negative.
12 The apparent power of the three-phase circuit is given by
( a )S = √ ( P2 + Q2 )
( b )S = ( P2 + Q2 )
( c )S = √ ( P2 – Q2 )
( d )S = √ ( P2 ± Q2 )
Correct Answer ( a ): S = √ ( P2 + Q2 )
13 All the starting end/finishing ends are joined together in the three phases, it is ______ connection.
( a )Delta connection
( b )Mesh connection
( c )Star connection
( d )None of the above
Correct Answer ( c ): Star connection
14 The line current in the delta connection is ______ times to that of in star connection for the same circuit parameters.
( a )2
( b )√ 3
( c )3
( d )None of the above
Correct Answer ( c ): 3
15 For the same circuit parameters, the power consumption in the delta connection is ______ times to that of in star connection for the same circuit parameter.
( a )2
( b )√ 3
( c )3
( d )None of the above
Correct Answer ( c ): 3
Polyphase AC Circuit MCQs: 11 to 15
16 The current coil of the wattmeter has
( a )Zero resistance
( b )Low resistance
( c )High resistance
( d )None of the above
Correct Answer ( b ): Low resistance
The current coil has less number of turns and thick wire. The thick wire has small resistance so current easily flows through it.
17 The pressure coil of the wattmeter has
( a )Zero resistance
( b )Low resistance
( c )High resistance
( d )None of the above
Correct Answer ( c ): High resistance
The current coil has higher number of fine turns. It has thin wire therefore higher resistance.
18 The pressure coil of the wattmeter is connected in
( a )Series
( b )Parallel
( c )Series – parallel
( d )Zig zag
Correct Answer ( b ): Parallel
The pressure coil is also called as voltage coil and it is always connected in parallel. It is used to measure voltage across terminals or load. If it is connected in series, there is higher voltage drop due to higher resistance of voltage coil.
19 The pressure coil of the wattmeter is connected between______ in the three-phase power measurement by two wattmeter methods.
( a )Lines
( b )Line and neutral
( c )Either of the above
( d )None of the above
Correct Answer ( b ): Line and neutral
The potential coil is always connected between line to neutral or line to line to measure voltage across terminals.
20 The three-phase power measurement is possible for balanced load by
( a )One wattmeter
( b )Two wattmeter
( c )Three wattmeter
( d )All of the above
Correct Answer ( d ): All of the above
The three-phase power measurement is done by one wattmeter, two wattmeter and three watt meter.