Important MCQs of AC Circuit
In this post, some of the important MCQs of AC circuit is given. It includes series LC circuit, lagging power factor, leading power factor, series resonance mcq, power factor of series resonance, impedance at the resonance, relation between resonance frequency and off resonance frequency, power factor at series resonance, condition for series resonance, impedance at series resonance and series resonance as accepter circuit.
AC Circuit MCQs: 121 – 125
121 The series L – C circuit becomes inductive when
( a )VL = VC
( b )VL> VC
( c )VC> VL
( d )VR = 0
Correct Answer ( b ): VL > VC
When the voltage across inductance is higher than that of capacitor, it is called as lagging circuit or inductive circuit.
122 The power factor of the series L – C circuit becomes leading when
( a )VC > VL
( b )VC< VL
( c )VC = VL
( d )VR = VL
Correct Answer ( a ): VC > VL
When the voltage across capacitor is higher than that of inductance, it is called as leading circuit or capacitive circuit.
123 The power factor of the series L – C circuit becomes unity when
( a )VC > VL
( b )VC < VL
( c )VC = VL
( d )VR = VC
Correct Answer ( c ): VC = VL
When the voltage drop across inductor is equal to voltage drop across capacitor, it is called as series resonance circuit and under such condition Z = R therefore power factor is unity.
124 The current leads voltage in the series L – C circuit only when
( a )XL = XC
( b )XL>XC
( c )XC>XL
( d )XL = XR
Correct Answer ( c ): XC > XL
Whatever the condition, when current leads voltage vector, it is capacitive circuit or leading power factor circuit.
125 The impedance of the series R – L – C circuit is
( a )R + j XL
( b )R – j XC
( c )R + j ( XL – XC )
( d )R + j ( XL ~ XC )
Correct Answer ( d ): R + j ( XL ~ XC )
The impedance of the series RLC circuit is given by Z = R + j ( XL ~ XC )
When XL > XC resulting XL – XC
Similarly, when XC > XL resulting XC – XL
AC Circuit MCQs: 126 – 130
126 The unit of impedance is
( a )Mho
( b )Ohm
( c )Siemens
( d )Ohm –1
Correct Answer ( b ): Ohm
Unit of impedance is ohm
127 The losses in the ideal 60 µf capacitor is
( a )60 Watt
( b )600 Watt
( c )60 / √ 2 Watt
( d )Zero watt
Correct Answer ( d ): Zero watt
It is ideal capacitor therefore power factor is zero.
128 The condition for series resonance is
( a )XL – XC > 0
( b )XL – XC < 0
( c )XL< 1
( d )XL– XC = 0
Correct Answer ( d ): XL – XC = 0
The series resonance of the given circuit is possible when the capacitive reactance is equal to inductive reactance.
129 The power factor of the series resonance circuit is
( a )0.8 lagging
( b )0.8 leading
( c )Unity
( d )Zero
Correct Answer ( c ): Unity
At series resonance, Z = R or we can say that impedance is equal to resistance therefore power factor of the series resonance is unity.
130 The series resonance circuit requires
( a )Constant voltage, Constant frequency
( b )Variable voltage, Constant frequency
( c )Constant voltage, Variable frequency
( d )Variable voltage, Variable frequency
Correct Answer ( c ): Constant voltage, Variable frequency
The series resonance required constant voltage from 0 – 10 V ac supply and variable frequency supply of 500 Hz to 500 kHz.
AC Circuit MCQs: 131 – 135
131 The impedance of the series resonance circuit at resonance frequency is
( a )R
( b )R + j XL
( c )R + j ( XL – XC)
( d )R – j XC
Correct Answer ( a ): R
At series resonance XL = XC therefore voltage drop across inductor is equal to voltage drop across capacitor VC = VL
Impedance Z = √ R2 + ( XL – XC )2
Therefore Z = R
More Details About Series Resonance
132 Which of the following condition is true condition at series resonance?
( a )VL> VC
( b )VL< VC
( c )VC = VL
( d )VR= VC
Correct Answer ( c ): VC = VL
At series resonance XL = XC therefore voltage drop across inductor is equal to voltage drop across capacitor VC = VL
133 The series resonance frequency is
( a )1 / √ ( LC )
( b )1 / 2π √ ( LC )
( c )2π / √ ( LC )
( d )1 / π √ ( LC )
Correct Answer ( b ): 1 / 2π √ ( LC )
At the series resonance XL = XC
2πfL = 1/2πfL
f2 = 1/ 4π2LC
f = 1/2π√LC
134 Which of the following parameter becomes minimum at series resonance?
( a )Voltage
( b )Impedance
( c )Power factor
( d )Power loss
Correct Answer ( b ): Impedance
At the series resonance, inductive reactive is equal to capacitive reactance therefore impedance becomes minimum at resonance condition.
Z = R because XL = XC
135 The series resonance current is limited by
( a )XL – XC
( b )XL
( c )R
( d )XC
Correct Answer ( c ): R
At series resonance Z = R therefore current is limited by the resistance of the circuit.
AC Circuit MCQs: 136 – 140
136 The current vector______ with the supply voltage vector at the resonance in the series R – L – C circuit.
( a )Lags
( b )Leads
( c )Is in phase
( d )Any of the above
Correct Answer ( c ): Is in phase
At the resonance condition, impedance of RLC circuit is equal to resistance of the circuit therefore voltage vector is in phase with current vector at the resonance.
Z = R at resonance condition
XC > XL at lower frequency and less than resonance frequency
XL > XC at higher frequency and greater than resonance frequency
137 The series resonance circuit is also called as
( a )Current resonance
( b )Voltage resonance
( c )Power resonance
( d )Power factor resonance
Correct Answer ( b ): Voltage resonance
At the resonance, voltage drop across inductor and capacitor may be higher than the supply voltage therefore series resonance circuit is also called as voltage resonance.
138 The series resonance circuit is a
( a )Acceptor circuit
( b )Rejector circuit
( c )Either ( a ) or ( b)
( d )None of the above
Correct Answer ( a ): Acceptor circuit
As the current is maximum in the series resonance circuit, , it produces large voltage across inductor and capacitor but these voltage drops are equal and opposites cancel each other. If R is not present in the circuit, LC circuit act like short circuit to current at the resonance frequency. Hence the series resonance circuit is called as resonance circuit or referred as voltage resonance.
139 The relation between resonance frequency ( fo ) and off resonance frequency ( f ) is _______. ( where fo is resonance frequency and f is off resonance frequency )
( a )fo = f √ ( XL / XC )
( b )fo = f √ ( XC / XL )
( c )fo = f √ ( XC XL)
( d )f = fo √ ( XC / XL )
Correct Answer ( b ): fo= f √ ( XC / XL )
140 Which of the following parameter becomes maximum at series resonance?
( a )Admittance
( b )Current
( c )Impedance
( d )Both ( a ) and ( b )
Correct Answer ( d ): Both ( a ) and ( b )
The impedance is minimum at the series resonance therefore current and admittance is maximum at the series resonance.
I = V / Zmin
Y = 1 / Zmin
Where Y is admittance and I is current at resonance
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AC Circuit MCQ PDF