DC Machines Objective test Questions

1.In DC Generator the critical resistance refers to:

a) Armature

b) Field

c) Load

d) Commutator

B
Remember:If resistance of field is beyond critical resistance Generator will not start

2.In DC Generator the residual magnetism will be of the order:

a) 2.5%

b) 10%

c) 15%

d) 20%

A
Remember:Residual magnetism is the magnetism left in field poles of dc generator. This helps in starting self excited dc generator

3.Sparkless commutation can be achieved by employing

a) Interpoles

b) Compensating winding

c) High resistance corbon brushes

d) All the above

D
Remember:All the above three methods will be used to reduce sparking due to commutation

4.Comutator in DC machine acts as:

a) Full wave rectifier

b) Half wave rectifier

c) Controlled full wave rectifier

d) Controlled half wave rectifier

A
Remember:Commutator converts pulsating AC (sinusoidal) to unidirectional pulsating dc

5. In DC machine, commuataor converts

a) dc to ac

b) ac to dc

c) both ac to dc and dc to ac

d) None of the above

C
Remember:Current flowing through the armature windings is ac. Therefore in dc motor commutator coverts dc supply to ac to armature and in dc generator comutator converts ac armature current to dc output

6. Which type of emf is induced in the dc generator

a) Dynamically

b) Static

c) Electrostatic

d) Magentic

A
Remember:In dc generator field produced by the magnets are stationary and moving conductors cut the stationary magnetic field.Static Induced emf: Transformer (eg) where conductors are stationary and moving magnetic field cuts stationary conductors

7. If a current carrying conductor is placed in the uniform magnetic field with its plane perpendicular to the direction of magnetic induction then,

a) The net force and torque on the coil are zero

b) net force is zero and torque is finite

c) net force is finite and torque is zero

d) the net force and torque are finite

A
Remember:Plane perpendicular to magnetic induction means angle theta = 0. Because angle theta is angle between conductor and line perpendicular to magentic axis. Hence induced emf e=BLV sin(theta)=0. No force exist hence the torque cancels out and zero.

8. In DC machine

a) The current and electromagentic force in armature conductors are alternating while those at the terminals are uni-directional

b) The current and emf in armature conductors are uni directional while those at the terminals are alternating

c) The current and the emf in armature conductors and at terminals are unidirectional

d) The emf in armature conductors and at the terminals is alternating while current is uni directional

A
Remember:emf and armature current is alternating in nature. However because of commutator they are uni directional at the machine terminals

9. The induced voltage in a single loop reverses:

a) Once in each revolution

b) Once in each half revolution

c) Once in each one quarter revolution

d) Once in each two revolutions

B
Remember:Induced emf will reverse for half cycle. This is the reason we get complete sine wave for one revolution of the conductor

10. The yoke of the dc machine is made of:____ magnetic material

a) soft

b) hard

c) non

d) very good

A
Remember:In DC machine yoke also serves purpose of allowing field magnetic flux. Therefore soft iron is used to reduce the losses