> 00000EE205121903 Pages: 2

Reg No.: Name:
APJ ABDUL KALAM TECHNOLOGICAL UNIVERSITY

Third semester B.Tech examinations (S) September 2020

Course Code: EE205
Course Name: DC MACHINES AND TRANSFORMERS

Max. Marks: 100 Duration: 3 Hours
Graph sheets shall be provided
PARTA
Answer all questions, each carries 5 marks. Marks
1 Explain the phenomenon of electromechanical energy conversion in the case of (5)

a DC generator. What are the torques involved?

2 Derive the expression for generated emf in DC generator. (5)
3 Explain significance of back emf? (5)
4 Explain different methods of cooling of a transformer. (5)
5 Derive the condition for maximum efficiency of a single-phase transformer. (5)
6 What is the difference between commercial efficiency and all day efficiency? (5)
7 What are the necessary conditions to be satisfied for parallel operation of a three (5)
phase transformer?
8 What are the advantages and disadvantages of delta-delta connection? (5)
PART ‏تا‎
‎Answer any two full questions, each carries 10 marks.
9 ஐ Explain construction of DC machine with the help of neat diagram (10)
10 a) Equalizer ring is not needed for wave winding of a dc machine. Give reason. (5)
b) An 8 pole lap wound armature having 40 slots with 12 conductors/ slot (5)
generates 50017 . Determine speed at which machine is running if the flux per
pole is 50 mWb.
11 A shunt generator gave the following open circuit characteristics: (10)
Field current (A) 0.5

ன

The armature and field resistances are 0.1Q and 800 respectively. Calculate :

i) The voltage to which the machine will excite when run as a

generator at the same speed.

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