Page 727 - Basic Electrical Engineering

14. A large capacity three-phase induction motor is started using a star–delta starter instead of
starting direct-on-line. The starting current

a. is increased three times
b. remains constant
c. is reduced to one-third its value
d. is reduced to half its value.

15. With rotor-circuit resistance, R of 2 Ω, the maximum torque of an induction motor is
2
developed at 10 percent slip. When the maximum torque is developed at 20 percent slip, the
value of R will be
2
a. 1 Ω
b. 2 Ω
c. 0.5 Ω
d. 4 Ω.

16. The speed of a three-phase slip-ring-type induction motor can be controlled by

a. changing the frequency of supply voltage to the motor
b. changing the rotor-circuit resistance
c. changing the number of poles
d. all the methods as in (a), (b) and (c).

17. The four no-load speeds of three-phase induction motors operating on 400 V, 50 Hz supply
are 576 rpm, 720 rpm, 1440 rpm, and 2880 rpm. The number of poles of the motor,
respectively, are

a. 8, 6, 4, 2
b. 10, 8, 4, 2
c. 12, 8, 4, 2
d. 16, 8, 4, 2.

18. The power factor of a three-phase induction motor on no load is found to be 0.15 lagging.
When the motor is fully loaded its power factor would be around

a. 0.05 lagging
b. 0.15 lagging
c. 0.85 lagging
d. 0.85 leading.

19. The three supply terminals R, Y, B when connected, respectively, to the three stator terminals
a, b, c of a three-phase induction motor, the motor rotates in the clockwise direction. The
motor will rotate in the reverse direction if

a. R is connected to c, Y is connected to a, and B is connected to b
b. R is connected to b, Y is connected to a, connection of B to c remains unchanged