Important MCQs of AC Circuits
In this post, important mcqs of ac circuits are given. It includes equation of inductive reactance, voltage vector in series RL circuit, true power in RL circuit, definition of power factor, wattful component, watt less component, active power, reactive power, apparent power, power factor is ratio of, equation of active power, equation of reactive power, reciprocal of power factor, Q factor and power factor of RLC circuit.
AC Circuit MCQs: 101 to 105
101 The inductive reactance is given by
( a )ωC
( b )πfL
( c )2πfL
( d )L / 2πf
Correct Answer ( c ): 2πfL
The inductive reactance is given by XL = 2πfL
102 The voltage vector ______ current vector in the series RL circuit.
( a )Leads
( b )Lags
( c )Is in phase with
( d )Any of the above
Correct Answer ( b ): Lags
The series RL circuit is inductive circuit due to presence of inductance L so it is lagging circuit.
103 The true power in the series R – L circuit is given by
( a )VI × Load factor
( b )VI × Power factor
( c )VI × Form factor
( d )None of the above
Correct Answer ( b ): VI × Power factor
The true power or useful power is given by VI × Power factor whereas the wattless component or reactive power is given by VI × Sin Ф.
104 The cosine of angle between voltage vector and current vector is known as
( a )Power factor
( b )Load factor
( c )Form factor
( d )Peak factor
Correct Answer ( a ): Power factor
The power factor is defined as the cosine of angle between voltage vector and current vector.
105 The ratio of true power to the apparent power is known as
( a )Power factor
( b )Load factor
( c )Form factor
( d )Peak factor
Correct Answer ( a ): Power factor
Power factor = VI Cos Ф / VI
= Active power / Apparent power
AC Circuit MCQs: 106 to 110
106 The power factor is a ratio of
( a )R / Z
( b )X / Z
( c )R2 / Z
( d )Y / Z
Correct Answer ( a ): R / Z
If R, X and Z are component of triangle where Z = impedance of circuit, X = reactance of circuit and R = Resistance of circuit
Cos Ф = R / Z
107 The active power is better known as
( a )Idle component
( b )Wattless component
( c )Wattful component
( d )None of the above
Correct Answer ( c ): Wattful component
108 Which of the following component is responsible for power factor?
( a )VI
( b )VI Cos Ф
( c )VI Sin Ф
( d )VI tan Ф
Correct Answer ( c ): VI Sin Ф
The reactive power is responsible for power factor. If the reactive power increases, power factor decreases and vice versa. E.g.
Let Ф = 0 degree
Active power = VI Cos 0 = VI
Reactive power = VI Sin 0 = 0
Let Ф = 45 degree
Active power = VI Cos 45 = VI / √2
Reactive power = VI Sin 45 = VI / √2
Let Ф = 90 degree
Active power = VI Cos 90 = 0
Reactive power = VI Sin 45 = 1
From the above, as the reactive power increases, active power increases or power factor decreases considering product of VI remains constant.
109 The reactive power is better known as
( a )VI
( b )VI Cos Ф
( c )VI Sin Ф
( d )kVA
Correct Answer ( c ): VI Sin Ф
110 The product of the RMS voltage and RMS current is known as
( a )Reactive power
( b )Active power
( c )Apparent power
( d )Real power
Correct Answer ( c ): Apparent power
Active power = VI Cos Ф
Reactive power = VI Sin Ф
Apparent power = √ ( active power )2 + ( Reactive power )2 = VI
AC Circuit MCQs: 111 to 115
111 The power developed in the inductive reactance of the series R – L circuit is known as
( a )Apparent power
( b )Active power
( c )Reactive power
( d )DC power
Correct Answer ( c ): Reactive power
112 The power dissipated in the series R – L circuit is better known as
( a )Active power
( b )Reactive power
( c )Apparent power
( d )Idle power
Correct Answer ( a ): Active power
113 The reciprocal of the power factor is
( a )Load factor
( b )Real factor
( c )P – factor
( d )Q factor
Correct Answer ( d ): Q factor
Power factor Cos Ф = R/Z
Reciprocal of power factor is known as quality factor Q = Z/R
114 The Q factor of the series R – L circuit is
( a )XL / R
( b )Z / R
( c )1 / R
( d )L / R
Correct Answer ( b ): Z / R
The impedance of series RL circuit Z = R + j XL
Q factor of series RL / RC circuit is always Z/R
115 The current vector leads voltage vector in the series
( a )R – L circuit
( b )L – C circuit
( c )R – C circuit
( d )R – L – C circuit
Correct Answer ( c ): R – C circuit
The current leads voltage vector in the series RC circuit whereas it legs in the series RL circuit.
AC Circuit MCQs: 116 to 120
116 The power factor of the series R – C circuit is
( a )Unity
( b )Lagging
( c )Leading
( d )Zero
Correct Answer ( c ): Leading
Due to capacitance of the circuit, power factor is leading
117 The power factor of the series R – L – C circuit is
( a )Unity
( b )Lagging
( c )Leading
( d )Any of the above
Correct Answer ( d ): Any of the above
Two parameter affects the power factor of series RLC circuit: XL and XC
Inductive reactance = XL
Capacitive reactance = XC
If
XL > XC = Inductive circuit, lagging power factor
XL < XC = Capacitive circuit, leading power factor
XL = XC = Resistive circuit, Unity, leading power factor
118 The net reactance of the series R – L – C circuit is
( a )R
( b )XL + XC
( c )XL~XC
( d )XL – XC
Correct Answer ( c ): XL ~ XC
Net reactance of series RLC circuit is XL ~ XC
XL > XC = Inductive circuit, lagging power factor
XL < XC = Capacitive circuit, leading power factor
119 The current leads voltage in the series R – L – C circuit when
( a )XL = XC
( b )XL>XC
( c )XC>XL
( d )R = 0
Correct Answer ( c ): XC > XL
Current leads voltage vector means capacitive circuit so XC > XL
If current legs voltage vector, inductive circuit so XL > XC
120 The voltage leads current in the series R – L – C circuit when
( a )Z = 0
( b )XL = XC
( c )XC > XL
( d )XL > XC
Correct Answer ( d ): XL > XC
Voltage leads current, it means that current lagging therefore it is inductive circuit. In the inductive circuit, XL > XC
AC Circuit MCQs PDF
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