AMIETE – ET (OLD SCHEME)

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 the best alternative in the following: (210)

a. The following power device switch requires a continuous signal to turn the switch on

(A) GTO (B) Thyristor

(C) IGBT (D) none of the above

b. In a three phase six pulse rectifier, each diode conducts for

(A) (B)

(C) (D)

c. The following Fig.1 represents

Fig.1

(A) an SMPS (B) a linear regulated power supply

(C) a buck converter (D) a CuK converter

d. Fall in 4% of supply voltage, the torque of an induction motor will be decreased by.

(A) 2% (B) 8%

(C) 16% (D) 4%

e. A resistor connected across gate and cathode of an SCR, increases its

(A) dv/dt rating

(B) holding current.

(C) noise immunity

(D) all of these

f. Over current protection is provided by use of

(A) saturable di/dt (B) snubber circuit

(C) circuit breaker and fuse (D) heat sink

g. For UJT, has the values

(A) between 0.5 and 0.8 (B) 0.5

(C) not defined (D) any value.

h. Three phase full wave a.c. regulator uses

(A) three diodes (B) three thyristors

(C) three diodes , three thyristors (D) six thyristors

i. A type-A chopper requires always

(A) forced commutation (B) natural commutation

(C) impulse commutation (D) line commutation

j. Among the following provide less harmonic in output

(A) single PWM (B) multi PWM

(C) multi-pulse PWM (D) six-pulse PWM

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2 a. Draw thyristor circuit and explain the V-I characteristics of the SCR. Also, discuss significance of latching current and holding current. (10)

b. Compare the salient features of power transistors and thyristors. (6)

Q.3 a. Differentiate between natural commutation and forced commutation by giving example of each. (6)

b. Describe the principle of resonant pulse commutation and obtain the expression for the circuit turn off time. (10)

Q.4 a.

A single phase full-wave ac regulator is shown in Fig.2, has a resistive load of R=10 ohms and the input voltage is V_s=120 V (rms), 60 Hz. The delay angle of thyristors are . Determine (8)

(i) the rms output voltage V_o,

(ii) the input power factor (PF),

(iii) the average current of thyristors,

(iv) the rms current of thyristors.

b. Give the circuit and describe the operation of a three phase inverter with 180 degree conduction for star connected resistive load. (8)

Q.5 a. Enlist the commonly used techniques for voltage control of a single phase inverter. Explain the single pulse-width modulation method. (8)

b. Explain single-phase semi converter and single phase full-converter drives. (8)

Q.6 a. What is cycloconverter? With waveforms, explain the operation of single phase cycloconverter. (8)

b. Draw and explain torque- speed characteristics of induction motor. (8)

Q.7 a. Give the circuit arrangement of a buck-boost regulator and explain its operation with necessary waveforms. (8)

b. Explain the working of three phase to three phase cycloconverter. (8)

Q.8 a. Explain single-phase full converter circuit with RL load with waveforms. Also, derive equation to find average output voltage. (8)

b. With a neat schematic diagram describe the speed control of a d.c. drive using micro computer control. (8)

Q.9 a. Write short note on any TWO of the following: (10)

(i) single phase inverter

(ii) cycloconverter with intergroup reactor

(iii) IGBT

b. Explain the operation of a three phase full-wave a.c. voltage controller with resistive load. (6)