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APJ ABDUL KALAM TECHNOLOGICAL UNIVERSITY
THIRD SEMESTER B.TECH DEGREE EXAMINATION, APRIL 2018
Course Code: EE205
Course Name: DC MACHINES AND TRANSFORMERS
Max. Marks: 100 Duration: 3 Hours
PART A
Answer all questions, each carries 5 marks Marks
1 Point out the necessity of equalizer rings in a lap wound dc machine. Why this is
not applicable in wave wound machines

2 What is armature reaction and how it is eliminated in DC machines
3 Compare the electrical and mechanical characteristics of a DC shunt motor with
those of a DC series motor. Based on this point out the application areas of these
motors.

4 What is the principle of operation of a transformer? Show how the flux is
balanced when the transformer is supplying a load.

5 With supporting phasor diagrams, derive the expression for secondary side
voltage regulation of a transformer for lagging and leading power factor loads.

6 What are the necessary and desirable conditions to be satisfied for parallel
operation of two single phase transformers?

7 Distinguish the vector groupings Yy0, Dd0,Dy1,Yd11 in three phase transformer
connections

8 Show how three phase power is obtained by using two single phase transformers
connected in open delta.

PART B
Answer any two full questions, each carries 10 marks
9 For a 6 pole DC armature with 16 slots having two coil sides per slot and single
turn coils, calculate the relevant pitches for a wave winding and draw the
developed winding diagram.

10 Draw the magnetisation characteristics of a DC shunt machine. Point out the
conditions for voltage build- up of a DC shunt generator. Give the significance of
the terms critical resistance and critical speed as applicable to a DC shunt
generator.

11 A short shunt compound DC generator supplies a current of 100A at 220V. If
theresistance of the shunt field is 50 of the series field is 0.02 of the
armatureis 0.05 the iron and friction losses amount to 1KW. Find:
The generated emf ii) The copper losses
iii) Output power of theprime-mover driving the generator
iv) The generator efficiency.

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PART C
Answer any two full questions, each carries 10 marks
12 With a neat sketch, explain the working of three-point starter. What is its main
drawback? How this is eliminated in four point starters?

13 With supporting diagrams, show how the retardation test can be employed to find
out the various losses occurring in a DC machine.

Explain the working principle of transformers.
14 Readings from O.C and S.C test on a 8kVA, 400/200V, 50Hz transformer are
OC Test 200V, 2A, 80W ;meters on low voltage side
SC Test 10V, 20A, 120W; meters on high voltage side
Compute equivalent circuit of the transformer as referred to high voltage side.

PART D
Answer any two full questions, each carries 10 marks
15 Derive the condition for maximum efficiency for a transformer.
b Distinguish between auto transformers and two winding transformers. Derive the
expression for saving in copper in an auto transformer.

16 Derive the condition to be satisfied for zero voltage regulation and maximum
voltage regulation for a transformer.

With neat circuit diagram, explain how a two-phase supply can be obtained from
a three-phase supply.

17 Draw the connection diagram for T-T connection of transformers and explain the
formation of three phase four wire system with two single phase transformers.
Point out its advantages and disadvantages