POWER ELECTRONICS Model question paper EEE fifth semester

Time : 3 Hrs Max. Marks: 100
PART - A (10 x 2 = 20 Marks)
1. What are the differences between Transistor & SCR?
2. Explain latching current and holding current of thyristor.
3. What is natural or line commutation?
4. Why is power factor of semiconverter better than full converter?
5. What are the differences between free wheeling diode and feedback diode?
6. What are the main differences between voltage source inverter and current source
7. What are the advantages and disadvantages of AC voltage controllers?
8. Explain time ratio control of choppers.
9. Draw the load voltage waveform for a = 150o.
10. Explain the principle of operation of step up chopper.
PART - B (5 x 16 = 80 Marks)
11. A dc battery is charged through a resistor R as shown in fig.1. Derive an expression
for the average value of charging current in terms of Vm, E, R on the assumption
that SCR is fired continuously.
a. For an ac source voltage of 230V, 50Hz find the value of average charging
current for R = 8O and E = 150V
b. Find the power supplied to battery and dissipated in the resistor.
c. Calculate supply PF.
Fig. 1
12.a) For three phase thyristor controlled half wave rectifier feeding load R as shown in
fig. 2. Show that the average output voltages are given by
Vo = (3v3 Vm Cos a) / 2p for 0 = a = p/6
Vo = (3/2p) Vm [1+cos (a + p/6)] for p/6 = a = 5p/6
Where Vm is the maximum value of phase voltage and a is the firing angle delay.
Fig. 2
12.b) A half controlled 3 phase bridge rectifier is supplied at 220v from a source of
reactance 0.24O/phase. Neglecting resistance and device volt drops determine
mean load voltage for level load current of 40A at a firing delay angle of 45º and
13.a) Explain the operation of single phase modified MC Murray half bridge inverter
13.b)i) Explain the operation of modified series inverter with neat diagram. Derive an
expression for the output frequency of the inverter.
ii) What is the difference between series and parallel inverter?
14.a) Explain the operation of voltage commutated chopper with neat diagram and
waveforms. Derive expressions for commutating capacitor (C) and commutating
inductor (L).
14.b) For a current commutated chopper peak commutating current is twice the maximum
possible load current. The source voltage is 230V dc and main SCR turn of time is
30µsec. The circuit turn off time is twice device turn off time. For maximum load
current of 200A calculate.
(i) Values of the commutating inductor and capacitor
(ii) Maximum capacitor voltage
(iii) Peak commutating current
15.a) Explain the operation of multistage sequence control of ac voltage controllers with
neat diagram.
15.b) Explain the operation of single phase to single phase step down cyclo converter
with voltage and current waveforms for
(i) Continuous load current
(ii) Discontinuous load current

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