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D3827 Pages: 2

PART ^
Answer any two full questions, each carries 10 marks

With a neat sketch, explain the working of three-point starter. What is its main (10)
drawback? How this is eliminated in four point starters?
a) With supporting diagrams, show how the retardation test can be employed to find (6)
out the various losses occurring in a DC machine.

b) Explain the working principle of 10 transformers. (4)
Readings from O.C and S.C test on ೩ 8kVA, 400/200V, 50117 transformer are (10)
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

೩) Derive the condition for maximum efficiency for a transformer. (5) b Distinguish
between auto transformers and two winding transformers. Derive the (5) expression for
saving in copper in an auto transformer.
a) Derive the condition to be satisfied for zero voltage regulation and maximum (4)
voltage regulation for a transformer.

b) With neat circuit diagram, explain how a two-phase supply can be obtained from (6) a

three-phase supply.

Draw the connection diagram for T-T connection of transformers and explain the (10)
formation of three phase four wire system with two single phase transformers. Point out
its advantages and disadvantages

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