AC MACHINES 3

1. Which of the following is a correct statement about eddy current?
a) Eddy currents improve the efficiency of a motor.
b) Eddy currents do not influence the movement.
c) Eddy current heat up the metal parts
d) Eddy currents are used for are welding.
ans. c

2. The steel for construction of transformer core is made to have
a) high permeability and low hysteresis loss
b) low permeability and high hysteresis loss
c) low permeability and low hysteresis loss
d) high permeability and high hysteresis loss
ans. a

3. As compared to an amplifier , a transformer cannot
a) increase the output voltage
b) increase the output current
c) increase output power
d) none of these
ans. c

4. In an ideal transformer on no-load, the primary applied voltage is balanced by
a) the secondary voltage
b) the drop across resistances and reactance
c) the secondary induced emf
d) the primary induced emf
ans. c

5. A sinusoidal emf
a) lags the flux inducing it by 180 degrees
a) lags the flux inducing it by 90 degrees
a) leads the flux inducing it by 90 degrees
a) leads the flux inducing it by 180 degrees
ans. b

6. The magnitude of mutual flux in a transformer is
a) low at low loads and high and high loads
b) high at low loads and low at high loads
c) same at all loads
d) varies at low loads and constant at high loads
ans. c

7. At every instant the direction of secondary current in a transformer must be such as to oppose any change in flux . This is in accordance with
a) Faraday's laws
b) Joule's law
c) Lenz's law
d) Coulomb's law
ans. c

8. If in a transformer the secondary turns are doubled at the same time the primary voltage is reduced by half, then the secondary voltage will
a) be halved
b) be four times as high
c) not change
d) be reduced to quarter
ans. c

9. Transformers are rated in
a) kw
b) kV
c) kWh
d) kVA
ans. d

10. Which is the common method of cooling a power transformer ?
a) Air-cooling
b) Air-blast cooling
c) Oil cooling
d) natural cooling
ans. c