AC MACHINES 1

AC MACHINES

1. Silicon content in iron lamination's is kept within 5% as it
a) increases hysteresis loss
b) increases the cost
c) makes the material brittle
d) reduces the curie point.
ans. c

2. If the iron core of a transformer is replaced by an air core, then the hysteresis losses in the transformer will
a) increase
b) decrease
c) remain unchanged
d) become zero
ans. d

3. Which of the following will improve the mutual coupling between primary and secondary circuits?
a) Transformer oil of high breakdown voltage
b) High reluctance magnetic core
c) Winding material of high resistivity
d) low reluctance magnetic core
ans. d

4. In order to reduce the hysteresis loss
a) core may be laminated.
b) silicon steel may be used as the core material
c) core may be constructed with any permanent magnet material such as Alnico.
d) core may be impregnated with varnish.
ans. b

5. Why is the core of the transformer built up of lamination?
a) To reduce eddy current loss.
b) For the convenience of fabrication.
c) No specific advantage.
d) For increasing the permeability.
ans. a

6. When are the eddy-current losses in a transformer reduced?
a) If laminations are thick.
b) If the number of turns in the primary winding is reduced.
c) If the number of turns in the secondary winding is reduced.
d) If laminations are thin.
ans.d

7. The primary and secondary winding's are interlaced for
a) easiness of coil making
b) reduced leakage
c) reduced cost
d) uniform heating.
ans. b

8. When a transformer winding suffers a short-circuit, the adjoining turns of the same winding experience
a) an attractive force
b) a repulsive force
c) no force
d) all the above
ans. a

9. Under short-circuit condition, the windings of a transformer having 5% impedance will experience a radial force
a) 5 times of full load value
b) 20 times of full load value
c) 25 times of full load value
d) 400 times of full load value
ans. d

10. The function of oil in a transformer is
a) to provide insulation and cooling.
b) to provide protection against lightning
c) to provide protection against short circuits.
d) to provide lubrication.
ans. a