Question Bank
Unit 3
Que1) For a series RLC circuit having R=10ohms, L= 0.15H, C=100
Sol: Impedance Z= 
Z= 
= 18.27ohm
Que2) For a series RLC circuit having R=12ohms, L= 0.2H, C=60
F. They are connected across 100v 50Hz supply. Calculate circuit current?
Sol: I= 
Z= 
Z= 
= 13.89ohm
I = 100/13.89 =7.2A
Que3) For a series RLC circuit having R=10ohms, L= 0.15H, C=100
F. They are connected across 100v 50Hz supply. Calculate power factor?
Sol: cosφ = 
Impedance Z= 
Z= 
= 18.27ohm
Cosφ = 
= 
φ = 56.81o lagging
Que4) A coil takes a current of 6A when connected to 24V dc supply. To obtain the same current with 50HZ ac, the voltage required was 30V. Calculate inductance and p.f of coil?
Sol: The coil will offer only resistance to dc voltage and impedance to ac voltage
R =24/6 = 4ohm
Z= 30/6 = 5ohm
XL = 
= 3ohm
Cosφ = 
= 4/5 = 0.8 lagging
Que5) The potential difference measured across a coil is 4.5V,when it carries a dc current of 8A. The same coil when carries ac current of 8A at 25Hz,the potential difference is 24V. Find current and power when supplied by 50V,50Hz supply?
Sol: R=V/I= 4.5/8 = 0.56ohm
At 25Hz Z= V/I=24/8 =3ohm
XL = 
= 2.93ohm
XL = 2
fL = 2
x 25x L = 2.93
L=0.0187ohm
At 50Hz
XL = 2x3 =6ohm
Z = 
= 5.97ohm
I= 50/5.97 = 8.37A
Power = I2R = 39.28W
Que6) A coil having inductance of 50mH an resistance 10ohmis connected in series with a 25
F capacitor across a 200V ac supply. Calculate resonant frequency and current flowing at resonance?
Sol: f0=
= 142.3Hz
I0 = V/R = 200/10 = 20A
Que7) A 15mH inductor is in series with a parallel combination of 80ohm resistor and 20
F capacitor. If the angular frequency of the applied voltage is 1000rad/s find admittance?
Sol: XL = 2
fL = 1000x15x10-3 = 15ohm
XL = 1/
C = 50ohm
Impedance of parallel combination Z = 80||-j50 = 22.5-j36
Total impedance = j15+22.5-j36 = 22.5-j21
Admittance Y= 1/Z = 0.023-j0.022 siemens
Que8)A circuit connected to a 100V, 50 Hz supply takes 0.8A at a p.f of 0.3 lagging. Calculate the resistance and inductance of the circuit when connected in series and parallel?
Sol: For series Z =100/0.8 = 125ohm
Cosφ = 
R = 0.3 x 125 = 37.5ohm
XL = 
= 119.2ohm
XL = 2
fL = 2
x 50x L
119.2 = 2
x 50x L
L= 0.38H
For parallel:
Active component of current = 0.8cosφ = 0.3x0.3 = 0.24A
R = 100/0.24 =416.7ohm
Quadrature component of current = 0.8 sinφ = 0.763
XL= 100/0.763 = 131.06ohm
L= 100/0.763x2
x50 = 0.417H
Que9) Calculate impedance of a parallel tuned RC circuit at a frequency of 400kHz and for a BW of operation equal to 20kHz. Given R=5ohm?
Sol: BW =R/2
L
L= 39
H
f0 = 
C = 2.6x10-9F
Z= L/RC = 3kohm
Que10) Explain principle of generation of three phase voltage?
Sol: In three phase the windings are separated by 1200 each. The voltage produced in those windings are 1200 apart from each other. Below shown is one coil RR’ and two more coils YY’ and BB’ each having phase shift of 1200.
The instantaneous value of voltages is given as
VRR’ = Vmsinωt
VYY’ = Vmsin(ωt-120)
VBB’ = Vmsin(ωt-240)
The three phase voltages are of same magnitude and frequency.
Question Bank
Unit 3
Question Bank
Unit 3
Question Bank
Unit 3
Que1) For a series RLC circuit having R=10ohms, L= 0.15H, C=100
Sol: Impedance Z= 
Z= 
= 18.27ohm
Que2) For a series RLC circuit having R=12ohms, L= 0.2H, C=60
F. They are connected across 100v 50Hz supply. Calculate circuit current?
Sol: I= 
Z= 
Z= 
= 13.89ohm
I = 100/13.89 =7.2A
Que3) For a series RLC circuit having R=10ohms, L= 0.15H, C=100
F. They are connected across 100v 50Hz supply. Calculate power factor?
Sol: cosφ = 
Impedance Z= 
Z= 
= 18.27ohm
Cosφ = 
= 
φ = 56.81o lagging
Que4) A coil takes a current of 6A when connected to 24V dc supply. To obtain the same current with 50HZ ac, the voltage required was 30V. Calculate inductance and p.f of coil?
Sol: The coil will offer only resistance to dc voltage and impedance to ac voltage
R =24/6 = 4ohm
Z= 30/6 = 5ohm
XL = 
= 3ohm
Cosφ = 
= 4/5 = 0.8 lagging
Que5) The potential difference measured across a coil is 4.5V,when it carries a dc current of 8A. The same coil when carries ac current of 8A at 25Hz,the potential difference is 24V. Find current and power when supplied by 50V,50Hz supply?
Sol: R=V/I= 4.5/8 = 0.56ohm
At 25Hz Z= V/I=24/8 =3ohm
XL = 
= 2.93ohm
XL = 2
fL = 2
x 25x L = 2.93
L=0.0187ohm
At 50Hz
XL = 2x3 =6ohm
Z = 
= 5.97ohm
I= 50/5.97 = 8.37A
Power = I2R = 39.28W
Que6) A coil having inductance of 50mH an resistance 10ohmis connected in series with a 25
F capacitor across a 200V ac supply. Calculate resonant frequency and current flowing at resonance?
Sol: f0=
= 142.3Hz
I0 = V/R = 200/10 = 20A
Que7) A 15mH inductor is in series with a parallel combination of 80ohm resistor and 20
F capacitor. If the angular frequency of the applied voltage is 1000rad/s find admittance?
Sol: XL = 2
fL = 1000x15x10-3 = 15ohm
XL = 1/
C = 50ohm
Impedance of parallel combination Z = 80||-j50 = 22.5-j36
Total impedance = j15+22.5-j36 = 22.5-j21
Admittance Y= 1/Z = 0.023-j0.022 siemens
Que8)A circuit connected to a 100V, 50 Hz supply takes 0.8A at a p.f of 0.3 lagging. Calculate the resistance and inductance of the circuit when connected in series and parallel?
Sol: For series Z =100/0.8 = 125ohm
Cosφ = 
R = 0.3 x 125 = 37.5ohm
XL = 
= 119.2ohm
XL = 2
fL = 2
x 50x L
119.2 = 2
x 50x L
L= 0.38H
For parallel:
Active component of current = 0.8cosφ = 0.3x0.3 = 0.24A
R = 100/0.24 =416.7ohm
Quadrature component of current = 0.8 sinφ = 0.763
XL= 100/0.763 = 131.06ohm
L= 100/0.763x2
x50 = 0.417H
Que9) Calculate impedance of a parallel tuned RC circuit at a frequency of 400kHz and for a BW of operation equal to 20kHz. Given R=5ohm?
Sol: BW =R/2
L
L= 39
H
f0 = 
C = 2.6x10-9F
Z= L/RC = 3kohm
Que10) Explain principle of generation of three phase voltage?
Sol: In three phase the windings are separated by 1200 each. The voltage produced in those windings are 1200 apart from each other. Below shown is one coil RR’ and two more coils YY’ and BB’ each having phase shift of 1200.
The instantaneous value of voltages is given as
VRR’ = Vmsinωt
VYY’ = Vmsin(ωt-120)
VBB’ = Vmsin(ωt-240)
The three phase voltages are of same magnitude and frequency.
