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Rajshahi University of Engineering & Technology
Department of Electrical & Computer Engineering
B.Sc. Engineering 2nd Year Odd Semester Examination, 2020
Course No: ECE 2107
Course Title: Electrical Machines I
Marks: 72
Time: 03 Hours
N.B.
- Answer SIX questions taking any THREE from each section.
- Figures in the margin indicate full marks.
- Use separate answer script for each section.
SECTION - A
Question 1
(a) What is the purpose of laminating the core in a transformer? [02]
(b) Derive the equivalent circuit of a single phase two winding transformer. [04]
(c) With neat sketch explain the working of transformer under no load. [03]
(d) A single phase transformer takes 10A on no load at p.f. of 0.2 lagging. The turn ration is 4:1 (step down). If the load on the secondary is 200A at a p.f. of 0.85 lagging. Find the primary current and the p.f. [03]
Question 2
(a) Compare two winding transformer and auto transformer. [03]
(b) Prove that amount of copper saved in auto transformer is (1−k) times that of an ordinary transformer. [04]
(c) Draw the circuit diagrams for conducting OC and SC tests on a single phase transformer. [02]
(d) In no load test of a single-phase transformer the following test data were obtained: [03]
- Primary voltage: 220V
- Secondary voltage: 110V
- Primary current: 0.5 A
- Power input: 30W
Find the following:
- (i) The magnetizing component of no-load current
- (ii) Its working(loss) component
- (iii) The iron loss
Question 3
(a) Why the transformers are rated in KVA? [02]
(b) What do you mean by an auto-transformer? Show that, using an auto-transformer can save the amount of copper used in transformer winding as compared to an ordinary transformer. [05]
(c) Step by step draw the equivalent circuit of a transformer referred to its primary side. [05]
Question 4
(a) What are the fields of application of an auto-transformer? [03]
(b) Is it possible to maintain the continuity of supply if one phase of the three phase transformer is burn out? If yes, then what are the possible ways? Show that open-delta connection can bear 57.7% of original load without exceeding its ratings. [05]
(c) Two 25-KVA transformers are connected in open Δ to supply a 220-V balanced three-phase load. Then: [04]
- (i) What is the total load that can be supplied without overloading either transformer?
- (ii) When the Δ is closed by the addition of a third 25-KVA transformer, what total load can now be supplied?
SECTION - B
Question 5
(a) Classify A.C. motors. [02]
(b) Why an asynchronous motor is treated as rotating transformer? [04]
(c) How the equivalent circuit model of an induction motor can be obtained? [06]
Question 6
(a) How the direction of rotation of a three-phase induction motor can be reversed. [02]
(b) Starting from equivalent circuit, derive the various power equation of an induction motor. [04]
(c) Why is a synchronous motor not self starting? [02]
(d) The power input to the rotor of a 400V, 50-Hz, 6-pole, 3-Φ induction motor is 75KW. The rotor electromotive force is observed to make 100 complete alternation per minute. Calculate: [04]
- (i) Slip
- (ii) Rotor speed
- (iii) Rotor copper losses per phase
- (iv) Mechanical power developed
Question 7
(a) Describe any one method for making single phase induction motor self-starting. [03]
(b) Show that, the maximum torque of an induction motor varies proportionally with the square of the supplied voltage when the motor is running. [04]
(c) Prove that, the star-delta switch starter is equivalent to an auto-transformer of ratio 1/3 or 58%. [05]
Question 8
(a) Define: [02]
- (i) Plugging
- (ii) Slip
(b) Derive torque-slip characteristics of 3-phase induction motor and explain. [03]
(c) Explain the double field revolving theory for operation of single phase induction motor. [04]
(d) Discuss in detail how V curves is obtained for a synchronous motor. [03]