# meepe 103 power converters

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Reg. No…………………….
Name……………………….
M.TECH DEGREE EXAMINATION
First Semester
Model Question Paper - I
Branch: Electrical and Electronics Engineering
Specialization: Power Electronics
MEEPE 103 POWER CONVERTERS
Time: Three hours
Maximum: 100 Marks
1. (a) A dc voltage of 100 V is switched on to a 23 Ω resistance in series with 2 mF capacitance.
Find the magnitude of current and capacitor voltage at t = 0.2 Seconds.
[6 Marks]
(b) A load comprises of resistance and inductance is fed from voltage source of
v = Vm Sin(wt) through a diode. A freewheeling diode is connected across the load. Analyze
the circuit assuming the output current is continuous and sketch the output current and output
voltage waveforms.
[12 Marks]
(c) An ideal capacitor of value C is connected to Vm Sin(314t) through one SCR. If SCR is
fired at firing angle α in the positive half cycle of input supply voltage, Will the SCR conduct
in the positive half cycle when fired at the same firing angle? Explain.
[7 Marks]
OR
2. (a) Explain in detail various thyristor specifications.
(b) Illustrates the limitations on di/dt and dv/dt of thyristors.
[15 Marks]
[10 Marks]
3. (a) A single phase semiconverter is feeding a resistive load. Derive an expression for average
[7 Marks]
(b) A three phase full converter is fires at 75 degrees. Sketch phase voltages, line voltages,
scheme of firing pulses and clearly indicate the output voltage.
OR
[18 Marks]
2 4. (a) A single phase full converter supplied from VmSin(wt) feeds RL load. A freewheeling
diode is connected across the load. Sketch voltage across and current through freewheeling
diode.
[7 Marks]
(b) A three phase semi converter is fired at 30 degrees. Sketch firing pulses, line and phase
voltages and output voltage.
[18 Marks]
5. With neat circuit diagram explain the operation of a buck boost converter in continuous and
discontinuous current modes.
OR
6. (a) Find the duty ratio of a cuk converter operating at 2 kHz to obtain an output voltage 200V.
The input dc voltage consist of two series connected 12V batteries. Also find the voltage
across the switch.
[8 Marks]
(b) In a step up converter, the duty ratio is adjusted to regulate the output voltage Vo at 48 V.
The input voltage varies in a wide range from 12 to 36V. The maximum power output is
120W. For stability reasons, it is required that the converter operate in a discontinuous –
current- conduction mode. The switching frequency is 50 kHz.
[8 Marks]
(c) In a buck-boost converter operating at 20 kHz, L=0.05mH. The output capacitor C is
sufficiently large and Vd = 15 V. The input is to be regulated at 10V and converter is
supplying a load of 10W. Calculate the duty ratio D.
7. (a) With neat circuits and V-I diagrams, classify dc chopper circuits.
[9 Marks]
[10 Marks]
(b) A type-A chopper is feeding a separately excited dc motor drive. If motor current is
discontinuous, derive an expression for the time at which motor current falls to zero during
OFF- period of chopper.
[15 Marks]
OR
8. (a) Briefly discuss various voltage control schemes in voltage source inverters.
[7 Marks]
(b) A three phase voltage source inverter is feeing balanced star connected resistive load and
operates in 180 degree mode of operation. Sketch firing signals and phase and line voltages.
Provide derivations.
[18 Marks]
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