Code:
AE26 Subject:
POWER ELECTRONICS
Time: 3 Hours Max. Marks: 100
NOTE: There are 9 Questions in all.
· Question 1 is compulsory and carries 20 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.
· Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.
· Any required data not explicitly given, may be suitably assumed and stated.
Q.1 Choose the correct or best alternative in the following: (2x10)
a. A triac can be turned on by applying
(A) a negative signal to gate.
(B) positive signal to gate.
(C) either positive or negative signal to gate.
(D) None of the above
b. An UJT exhibits negative resistance
(A) before peak point.
(B) between peak and valley point.
(C) any point between peak and valley.
(D) both (A) & (C)
c. The objective of connecting a resistance and capacitance across gate circuit is to protect the SCR against
(A) Over voltage (B) dv/dt
(C) Noise signals (D) Over currents
d. Secondary breakdown occurs in
(A) BJT (B) MOSFET
(C) BJT but not in MOSFET (D) both MOSFET and BJT
e. In a dc choppers, per unit ripple is maximum when the duty cycle is
(A) 0.2 (B) 0.5
(C) 0.7 (D) 0.9
f. A cycloconverter is
(A) ac-dc converter (B) dc-ac converter
(C) ac-ac converter (D) dc-dc converter
g. A converter with provision for both the output voltage and current be either positive or negative is called
(A) full converter. (B) dual converter .
(C) two quadrant converter. (D) semi converter.
h. A single phase converter
is fed by a source having an inductance L. If load current is 
the decrease in
output voltage due to overlap is
(A)
(B)
(C) 
(D)
i. The methods of voltage control in three phase and single phase inverters are the same.
(A) True (B) False
j. A full converter operates in four quadrants
(A) True (B) False
Answer any FIVE Questions out of EIGHT Questions.
Each question carries 16 marks.
Q.2 a. What is thyristor? Enumerate the various mechanisms by which thyristor can be triggered into conduction. What are the necessary conditions for turning on of an SCR? Discuss. (10)
b. It is required to operate 250A SCR in parallel with 350A SCR with their respective on state voltage drops of 1.6V and 1.2V. Calculate the value of resistance to be inserted in series with each SCR so that they share the total load of 600A in proportion to their current ratings. (6)
Q.3 a. Explain inverting mode of converters with suitable diagrams. (10)
b. A 230V, 1kW resistance load is connected across a fully controlled single phase bridge converter. If the load is to be delivered a power of 800W, find
(i) output voltage, (ii) firing angle and
(iii) load current (rms value). (6)
Q.4 a. Explain various steps to be carried out while designing chopper circuit. (7)
b. A dc chopper has an input of 200V and average output voltage of 150V. Load resistance is 10Ω. Find
(i) duty cycle,
(ii) average and rms load currents and
(iii) effective input resistance. (9)
Q.5 a. What is meant by commutation of SCR? How are commutation methods classified? (8)
b. What is unijunction transistor? Explain its configuration and characteristics. (8)
Q.6 a. Draw circuit diagram and wave shapes of input voltage, output voltage and load current in a single phase full wave ac regulator feeding purely resistive load? (10)
b. The single phase full
wave controller shown in Fig.1 supplies a resistive load of R=10Ω. The input
rms voltage Vs is 120V, 60Hz. The delay angles of thyristors are
equal: 
.
Determine
(i) the rms output voltage,
(ii) the input power factor
(iii) the average current of thyristor (6)
Q.7 a. Explain the principle of cycloconverter using simple diagram. (8)
b. Explain circulating current mode of cycloconverter. (8)
Q.8 a. Draw the diagram of a three phase bridge inverter circuit and explain its working. What are the two modes of operations? How these two modes are obtained? (10)
b. A single phase full bridge inverter has purely resistive load and its voltage is controlled by multiple pulse width modulation technique. The width of each pulse is 30º and each half cycle has 5 pulses. If input voltage is 200V find (i) rms value of output voltage,
(ii) pulse width to maintain the output power constant if input voltage is increased by 10%,
(iii) maximum possible input voltage if maximum possible pulse width is 33º. (6)
Q.9 a. Draw and explain block diagram for microprocessor based speed control of (i) dc motor and
(ii) induction motor. (8)
b. A 3-phase half controlled thyristor bridge with 400V, 3 phase 50 Hz supply is feeding a separately excited dc motor. The motor parameters are: rated current 50A, resistance 0.1Ω, back emf constant 0.3V/rpm.
(i) calculate no load speed if no load current is 5A and firing angle is 30º, (ii) also find firing angle to obtain a speed of 1600 rpm at rated current. (8)