GROUP A- 2 MARKS

1. State the difference between ac voltage controller and cycloconverter.

Name the different types of cycloconverter.

2. A chopper fed by 230V dc mains remains on for 25ms and off for 10ms in
each cycle. Find the average load voltage.

3. Two SCRs connected back to back have a load resistance of 200 ohm and
input of 230V single phase ac. Il firing angle of each SCR is 60°, find rms
output voltage and average power.

4. The voltage at the input of an AC-DC rectifier is given by v(t) = 230√ 2 sin
wt where w = 2 × 50 rad/s. The input current drawn by the rectifier is
given by
i(t) = 10 sin (wt - π/3) + 4sin(3wt- π/6) + 3sin (5wt - π/3). Find the input
power factor.

5. A single-phase fully-controlled thyristor converter is used to obtain an

average voltage of 180 V with 10 A constant current to feed a DC load. It
is fed from single-phase AC supply of 230 V, 50 Hz. Neglect the source
impedance. Find the Power Factor of AC mains.

6. Define the Critical Inductance in Buck Converter and find its formula.

7. Define the Critical Capacitance in Buck Converter and find its formula.

8. Define Latching Current and Holding Current.

9. A single Phase half wave rectifier is operated from 120V, 50Hz supply.
Load resistance is 10 ohm if average output voltage is 25% of maximum
possible output voltage. Determine firing angle.

10. Define Chopper in Short.

11. Why is Circulating Mode preferred over non-Circulating current mode in

dual converter?
12.Discuss the turn on and turn off timing in a thyristor.

13. Why high frequency pulse triggering is required to turn on the SCR?

14. 3-φ fully controlled bridge converter with FD is fed from 400V, 50Hz ac
supply. The operating angle is 60 ° and in the load current is 10 A. find the
input power factor.

15. 3- φ full Convertor is used as a line commutated inverter to feed 50kw

power 420V dc to 3-φ 415V, 50Hz ac Supply. Consider Dc current to be
constant. The RMS line current of Thyristor will be:-

16. A single-phase controlled rectifier supplies RLE load. Does the value of E
affect the firing angle if Yes, what is the range of firing angle that can be
applied for RLE load?

17. Why gate triggering is so commonly used in thyristor? Why should high
dv/dt be able to trigger a thyristor into conduction?

18. Define single-phase half bridge inverter with Diagram.

19. Define Circulating Current in single-phase dual converter.

20. Write the advantages of FD (free wheeling diode) in 1-φ half wave
rectifier.

21. Derive the Expression to show that reverse recovery time of power
diode is dependent on storage charge.
GROUP-B (4 MARKS)
1. A voltage source v=100 Sin377t, supplies resistive load of 100 ohm
through a thyristor which perform half wave control rectification.
Calculate power in the load, if thyristor is fired at an angle 45° with
respect to supply voltage ?

2. A 230V, 50Hz pulse SCR controlled convertor is triggered at firing angle of

40° and the load current extinguishes at an angle of 120° . Find the circuit
turn off time, Average output Voltage, Average output Current, Average
RMS load voltage, and Average load Current for R=5ohm and L= 2mH.

3. Derive the expression for output voltage in terms of duty cycle for a buck
and boost converter with the help of voltage second balance equation.

4. A single-phase full bridge voltage source inverter (VSI) feeds a purely

inductive load. The inverter output voltage is a square wave in 180° in
conduction mode. The fundamental frequency of the output voltage is
50 Hz. If the DC input voltage of the inverter is 100 V and the value of the
load Inductance is 20mH. Find the peak-to-peak load current in amperes.

5. A single phase full bridge inverter is fed by a 150V dc battery. If load is

purely resistive with a resistance of 6 ohms, find (a) output power (b)
peak thyristor current (c) average thyristor current (d) PIV.

6. A stepup chopper is used to deliver load voltage of 400v from a 220v D.C
source. If the blocking period of thyristor is 60µ sec. Compute the
required pulse width.
7. Draw the circuit diagram of 3 phase bridge converter with RL load.
Explain its working principle at a=30°. Derive the rms voltage expression
for the same condition.

8. The circuit shown in the figure has reached steady state with thyristor
"T" in OFF condition. Assume that the latching and holding currents of
the thyristor are zero. The thyristor is turned ON at t = 0 sec. Find the
duration in microseconds for which the thyristor would conduct, before
it turns off.

9. A half-wave thyristor converter supplies a purely inductive load, as

shown in figure. If the triggering angle of the thyristor is 120". Find
the min two extinction angle.

10. A star connected load of 15 ohm per phase is fed from 420 V D.C supply
through a 3-phase bridge inverter of 180° mode.
a. RMS value of Load current
b. RMS value of thyristor current
c. Power
GROUP-C (10 MARKS)

1. What is the advantage of 120° conduction over 180° conduction for 3-

phase VSI? Explain the 180° conduction mode of three-phase VSI with
the help of waveform for line, phase, pole, and neutral voltages.
Deduce the phase voltage expressions. (2+8)

2. Draw the circuit diagram for the buck-boost chopper circuit and draw
the waveform for Inductor voltage, inductor current, and capacitor
current. Derive the expression for output voltage, and output current
in terms of duty ratio. Deduce the expression for calculating the
inductor, and capacitor value. (6+2+2)

3. The stepdown chopper has a resistive load of R = 102, and the input
voltage is Edc 200v. When the chopper is turned on the voltage drop
across the switch is VCH = 1v, and the chopping frequency is f = 1
KHZ. If the duty cycle is 40% determine. (2.5X4)
a) The average output voltage (Eo)avg.
b) The R.M.S output voltage (Eg)R.M.S.
c) Efficiency of the chopper
d) The effective input resistance (R) of chopper.

4. In an ideal type A chopper circuit, the supply voltage is 250

volts. chopping frequency 300Hz, duty cycle a= 0.5, load
resistance = 5ohms, load Inductance is 5mH. If the load has a
back emf of 100 volts, find the average output current of the
chopper and also find the maximum and minimum values of
steady-state output current. (3+3.5+3.5)

5. Explain the operating principle of a three-phase bridge inverter with a

suitable diagram when each semiconductor switch conducts for 120°
for a 3-phase star connected resistive load. Draw the phase voltage
and line voltage waveforms for same. (7+3=10)
6. Explain the working and operation of 1-phase Half Bridge Inverter.
Draw the waveforms of output voltages , output current and deduce
the equation of RMS output voltage. Also find distortion factor. (4+4+2)