Question Paper: Electronics and Electrical Engineering : Question Paper Dec 2013 - Mechanical Engineering (Semester 4) | Pune University (PU)
0

## Electronics and Electrical Engineering - Dec 2013

### Mechanical Engg (Semester 4)

TOTAL MARKS: 100
TOTAL TIME: 3 HOURS
(1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks.

### Answer any one question from Q1 and Q2

1 (a) Derive the relation between power factor angle and the two wattmeter readings W1 and W2 for a balanced three-phase inductive load.(6 marks) 1 (b) What is tariff? Discuss HT and LT tariff.(6 marks) 1 (c) Explain the construction and working of single-phase energy meter with the help of neat diagram.(6 marks) 10 (a) For TRIAC (Bidirectional Triode SCR):
(i) Draw structure and symbol
(ii) Draw only static V-I characteristic
(iii) State only two applications.
(8 marks)
10 (b) For DIAC: (i) Draw only structure and symbol
(ii) Draw only V-I characteristic
(iii) State any two applications.
(8 marks)

### Answer any one question from Q11 and Q12

11 (a) State and explain various advantages of electrical drives over other conventional drive system, also state factors to be considered while selecting a motor for a drive.(8 marks) 11 (b) What is chopper ? With the help of diagram explain two quadrant chopper circuit (Class C Chopper).(8 marks) 12 (a) State and explain advantages and disadvantages of group drive, also state any two field of applications of group(8 marks) 12 (b) Explain V/F control of three-phase induction motor.(8 marks) 2 (a) Explain one wattmeter method for measurement of reactive power in a three-phase balanced load with the help of connection diagram and phasors.(6 marks) 2 (b) What are demerits of low power factor? State power factor improvement methods and explain any one of them.(6 marks) 2 (c) Two wattmeters connected to measure the input to a balanced three-phase circuit indicate 2500 W and 500 W respectively. Find the power factor of the circuit, when:
(i) Both readings are positive
(ii) The later reading is obtained after reversing the connections of current coil.
(6 marks)

### Answer any one question from Q3 and Q4

3 (a) Discuss the typical layout of distribution transformer substation with the help of single line diagram. Mention specifications of transformer therein.(8 marks) 3 (b) A 6-pole, 50 Hz, 3-phase induction motor runs at 960 RPM when the torque on the shaft is 200 N-m. If the stator losses are 1500 W and friction and windage losses are 500 W, find:
(i) Rotor copper loss and
(ii) Efficiency of the motor.
(8 marks)
4 (a) Discuss three-phase transformer connections with the help of suitable diagrams. Mention possible applications of these connections.(8 marks) 4 (b) State the expression for torque developed in a three-phase induction motor. Discuss the relationship between torque and slip with the help of characteristic curve.(8 marks)

### Answer any one question from Q5 and Q6

5 (a) Why is a single-phase induction motor not self-starting? How is it made self-started ? State its types and applications.(8 marks) 5 (b) Derive equation of induced E.M.F. in an alternator. Mention typical specifications of an alternator of your choice.(8 marks) 6 (a) Explain the construction and working of a shaded pole induction motor with the help of neat sketches. Mention its applications.(8 marks) 6 (b) A 1200 kVA, 3300 V, 50 Hz three-phase star connected alternator has armature resistance of 0.25 ohm per phase. A field current of 40 A produces a short circuit current of 200 A and an open circuit voltage of 1100 V line to line. Find the voltage regulation on:
(i) Full load 0.8 power factor lagging and
(ii) Full load 0.8 power factor leading.
(8 marks)

### Answer any one question from Q7 and Q8

7 (a) Draw and explain electrical characteristics (T Vs Ia and N Vs Ia) of the following d.c. motors:
(i) D.C. shunt motor
(ii) D.C. series motor
(iii) D.C. cumulative compound motor.
(9 marks)
7 (b) A d.c. motor runs at 1200 RPM by taking 100 A current from 230 V d.c. supply. If armature and series field winding resistances are 0.12 Ω and 0.03 Ω respectively, calculate the speed of motor if motor current has fallen to 50 A. (Note : Assume flux is directly proportional to the field current).(9 marks) 8 (a) State and explain with neat construction diagram construction, working, characteristic features and industrial applications of Universal Motor.(9 marks) 8 (b) A 230 V d.c. shunt motor runs at 2000 RPM and takes 5 A current from the d.c. supply. If armature and field winding resistances are 0.25 Ω and 230 Ω respectively, determine the drop in the speed if motor is loaded in such a way that it takes current from the supply 41A. (Note : Neglect armature reaction).(9 marks)

### Answer any one question from Q9 and Q10

9 (a) Explain with neat diagram constructional details and V-I characteristics of SCR.(8 marks) 9 (b) Explain the following for n-channel enhancement mode MOSFET:
(i) Output V-I characteristics
(ii) Transfer characteristics
(iii) Features (any four).
(8 marks)