Page 665 - Basic Electrical Engineering
P. 665
d. dc separately excited generator.
16. A dc motor when connected directly to the supply would draw a very heavy current because
a. the back EMF at starting is zero
b. the back EMF at starting is maximum
c. the back EMF is opposing the supply voltage
d. torque required at starting is high.
17. The speed of a dc shunt motor can be reduced by
a. decreasing the field current
b. increasing the supply voltage to the motor
c. decreasing the supply voltage to the motor
d. by decreasing the supply voltage to the motor and simultaneously decreasing the
field current.
18. The relationship between torque, T and armature current, Ia for a dc series motor is
a. T ∝ I a
b.
c.
d. .
19. The relationship between torque, T and armature current, Ia for a dc shunt motor is
a. T ∝ I a
b.
c. T ∝ I 2
a
d. .
20. Efficiency of a dc machine is less than that of an equivalent transformer because
a. there is friction and windage losses in dc machines
b. core losses are more in dc machines than in transformers
