Previous Year Questions- Phase Controlled Rectifiers - 2 | Power Electronics - Electrical Engineering (EE) PDF Download

Q21: A phase-controlled, single-phase, full-bridge converter is supplying a highly inductive DC load. The converter is fed from a 230 V, 50 Hz, AC source. The fundamental frequency in Hz of the voltage ripple on the DC side is     (SET-2(2017))
(a) 25
(b) 50
(c) 100
(d) 300
Ans: (c)
Sol: All even harmonics are present.
Fundamental frequency = 2f_s = 2 x 50 =100Hz

Q22: In the circuit shown, the diodes are ideal, the inductance is small, and I_o ≠ 0. Which one of the following statements is true?     (SET-2 (2017))
(a) $𝐷1$D₁ conducts for greater than 180° and D₂ conducts for greater than 180°
(b) $𝐷2$D₂ conducts for more than 180° and D₁ conducts for 180°
(c) D₁ conducts for 180° and D₂ conducts for 180°
(d) D₁ conducts for more than 180° and D₂ conducts for 180°
Ans: (a)
Sol: Both diodes will conduct for more than 180°.

Q23: The figure below shows an uncontrolled diode bridge rectifier supplied form a 220 V, 50 Hz 1-phase ac source. The load draws a constant current I_o = 14A. The conduction angle of the diode D₁ in degrees is___________.         (SET-1 (2017))
(a) 180.25
(b) 44.64
(c) 224.17
(d) 28.48
Ans: (c)
Sol: Average reduction in output voltage due to source inductance = ΔV_d0

Q24: A single-phase bi-directional voltage source converter (VSC) is shown in the figure below. All devices are ideal. It is used to charge a battery at 400 V with power of 5 kW from a source V_s = 220 V (rms), 50 Hz sinusoidal AC mains at unity p.f. If its AC side interfacing inductor is 5 mH and the switches are operated at 20 kHz, then the phase shift (δ) between AC mains voltage (V_s) and fundamental AC rms VSC voltage (V_c1), in degree, is _________.      (SET-2 (2016))
(a) 2.6
(b) 6.8
(c) 6.8
(d) 8.7
Ans: (c)
Sol: 
Q25: A full-bridge converter supplying an RLE load is shown in figure. The firing angle of the bridge converter is 120°. The supply voltage v_m(t) = 200πsin(100πt)V, R = 20Ω, E = 800 V. The inductor L is large enough to make the output current I_L a smooth dc current. Switches are lossless. The real power fed back to the source, in kW, is __________.      (SET-2 (2016))
(a) 4
(b) 5
(c) 6
(d) 7
Ans: (c)
Sol:
Q26: A three-phase diode bridge rectifier is feeding a constant DC current of 100 A to a highly inductive load. If three-phase, 415 V, 50 Hz AC source is supplying to this bridge rectifier then the rms value of the current in each diode, in ampere, is _____________.       (SET-2 (2016))
(a) 24.6
(b) 36.4
(c) 57.7
(d) 92.4
Ans: (c)
Sol: In the 3 − ϕ diode bridge rectifier each diode conducts for 120° for one complete cycle.

Q27: A single-phase thyristor-bridge rectifier is fed from a 230 V, 50 Hz, single-phase AC mains. If it is delivering a constant DC current of 10 A, at firing angle of 30°, then value of the power factor at AC mains is       (SET-1 (2016))
(a) 0.87
(b) 0.9
(c) 0.78
(d) 0.45
Ans: (c)
Sol: Input power factor = power factor at ac mains = C.D.F. × D.F.

Q28: In the given rectifier, the delay angle of the thyristor T₁ measured from the positive going zero crossing of V_s is 30°. If the input voltage V_s is 100sin(100πt) V, the average voltage across R (in Volt) under steady-state is _______.          (SET-2 (2015))
(a) 32.32
(b) 45.82
(c) 61.53
(d) 82.45
Ans: (c)
Sol: 
Q29: In the following circuit, the input voltage V_in is 100sin(100 πt). For 100 π RC = 50, the average voltage across R (in Volts) under steady-state is nearest to         (SET-2 (2015))
(a) 100
(b) 31.8
(c) 200
(d) 63.6
Ans: (c)
Sol: Given, 
For one cycle ωτ = 2π rad = 6.28
[ωτ = 25]>>[ωτ = 2π rad = 6.28]
Each capacitor voltage is approximately equal to

Q30: A diode circuit feeds an ideal inductor as shown in the figure. Given v_s = 100sin(ωt)V, where ω = 100π rad/s, and L = 31.83mH. The initial value of inductor current is zero. Switch S is closed at t = 2.5ms. The peak value of inductor current i_L(in A) in the first cycle is _____.         (SET-3 (2014))
(a) 8.45
(b) 17.07
(c) 22.62
(d) 28.28
Ans: (b)
Sol: At t = 2.5 ms the circuit is as shown below
Integrating on both sides
when t = 0.0025 sec, at this instant i(t) = 0
The peak value of the inductor current will be at (ωt = π^c)

Q31: A three-phase fully controlled bridge converter is fed through star-delta transformer as shown in the figure.
The converter is operated at a firing angle of 30°. Assuming the load current (I₀) to be virtually constant at 1 p.u. and transformer to be an ideal one, the input phase current waveform is          (SET-3 (2014))
(a) (b) (c) (d) Ans: (b)
Sol: Let us assume all the impedance are Z and to flow I₀ only closed path is required. So, in three phases one phase is open circuited.
According to correct division,
The transformer ratio is given as 1 : K
According to 3 phase source waveform,

Q32: A fully controlled converter bridge feeds a highly inductive load with ripple free load current. The input supply (V_s) to the bridge is a sinusoidal source. Triggering angle of the bridge converter is α = 30°. The input power factor of the bridge is      (SET-2  (2014)
(a) 0.25
(b) 0.50
(c) 0.78
(d) 0.98
Ans: (c)
Sol: The input power factor (PF) is defined as

Q33: The figure shows the circuit diagram of a rectifier. The load consists of a resistance 10 Ω and an inductance 0.05 H connected in series. Assuming ideal thyristor and ideal diode, the thyristor firing angle (in degree) needed to obtain an average load voltage of 70 V is ____.     (SET-1 (2014))
(a) 7
(b) 70
(c) 140
(d) 1400
Ans: (b)
Sol: 
Q34: The figure shows the circuit of a rectifier fed from a 230 V (rms), 50-Hz sinusoidal voltage source. If we want to replace the current source with a resistor so that the rms value of the current supplied by the voltage source remains unchanged, the value of the resistance (in ohms) is _____. (Assume diodes to be ideal.)       (SET-1 (2014))
(a) 23
(b) 46
(c) 13
(d) 79
Ans: (a)
Sol: 
Q35: Thyristor T in the figure below is initially off and is triggered with a single pulse of width 10 μs. It is given that L = (100/π)μH and  C = (100/π)μF Assuming latching and holding currents of the thyristor are both zero and the initial charge on C is zero, T conducts for        (2013)
(a) 10 μs
(b) 50 μs
(c) 100 μs
(d) 200 μs
Ans: (c)
Sol: Conduction time,

Q36: A half-controlled single-phase bridge rectifier is supplying an R-L load. It is operated at a firing angle α and the load current is continuous. The fraction of cycle that the freewheeling diode conducts is       (2012)
(a) 1/2
(b) $(1-\alpha /\pi )$(1−α/π)
(c) α/2π
(d) α/π
Ans: (d)
Sol: Freewheeling diode conducts for 2α over complete cycle

Q37: The fully controlled thyristor converter in the figure is fed from a single-phase source. When the firing angle is 0°, the dc output voltage of the converter is 300 V. What will be the output voltage for a firing angle of 60°, assuming continuous conduction       (2010)
(a) 150 V
(b) 210 V
(c) 300 V
(d) 100 V
Ans: (a)
Sol: For firing angle, α, average output voltage of the converter is given by

Q38: A three phase fully controlled bridge converter is feeding a load drawing a constant and ripple free load current of 10 A at a firing angle of 30°. The approximate Total harmonic Distortion (%THD) and the rms value of fundamental component of input current will respectively be       (2008)
(a) 31% and 6.8A
(b) 31% and 7.8A
(c) 66% and 6.8A
(d) 66% and 7.8A
Ans: (b)
Sol: Load current = I₀ = 10A
RMS value of total source current
Supply current i_s can be expressed by fourier series,
RMS value of fundamental current,

Q39: A single-phase half controlled converter shown in the figure feeding power to highly inductive load. The converter is operating at a firing angle of 60°.
If the firing pulses are suddenly removed, the steady state voltage (v₀) waveform of the converter will become        (2008)
(a) (b) (c) (d) Ans: (a)
Sol: Single phase half controlled converter.
As load is highly inductive, it means laod current is continuous and almost constant.
During,
Now, if firing pulses are removed after T₁ is triggered at 2π + α.
At ωt = 3π + α, no firing pulse will be available to trigger T₂. So, load current will flow through T₁D₂ as load current is continuos. So during  3π + α < ωt < 4π,  T₁D₂ will continue to conduct and V₀ = 0.
At ωt = 4π, D₁ will become forward biassed and D₂ will become reverse biased. So, D₁ will conduct and T₁ is already conducting.
So, load current will flow through T₁D₁ and V₀ = V_s.

Q40: A single phase fully controlled bridge converter supplies a load drawing constant and ripple free load current, if the triggering angle is 30°, the input power factor will be        (2008)
(a) 0.65
(b) 0.78
(c) 0.85
(d) 0.866
Ans: (b)
Sol: Average output voltage
Average output current = I₀ (constant)
RMS value of supply current  = I_s = I₀
RMS value of supply voltage =
Input power factor,