In a synchronous machine, for open circuit characteristics, the final value of generated EMF Ef should be about __________ of the rated voltage.

Question

Question

This question was previously asked in

SSC JE EE Previous Paper 8 (Held on: 28 Oct 2020 Morning)

Answer 1

Concept:

Open Circuit Test:

This test is used for determining the synchronous impedance.
The alternator is running at the rated synchronous speed, and the load terminals are kept open.
The excitation current may be increased to get 25% more than the rated voltage i.e. up to 125% of the rated voltage.
A graph is drawn between the open circuit phase voltage E_g and the field current I_f. The curve so obtained is called Open Circuit Characteristic (O.C.C).
The linear portion of the O.C.C is extended to form an air gap line.

The Open Circuit Characteristic (O.C.C) and the air gap line is shown in the figure below

Additional information:

Short circuit test:

The machine is driven at rated synchronous speed and armature terminals are short-circuited through an ammeter.
Now, field current gradually increased from zero, until the short-circuit armature current reached its safe maximum value, equal to about 125 to 150% of the rated current.
Latter readings may be taken in a short time to avoid armature overheating.
A graph is drawn between short-circuit current i_sc and field current i_f as shown in the figure.