15 January 2011

Electrical Objective Questions: DC Machines (Set 1)

Electrical Objective Questions With Answers:

1.The nature of the current flowing in the armature of the DC machine is:
a) ac
b) dc
c) pulsating
d) dc superimposed on ac

2.The Commutator in a dc machine can be convert
a) ac to dc
b) dc to ac
c) both a and b
d) None of the above

3.The emf produced in the dc generator is ____ induced emf
a) Statically
b) Dynamically
c) magnetically
d) Electrostatically

4.DC generator operates on the principle of
a) electro magnetic induction
b) Lenz's law
c) Biov Savart's law
d) none of these

5.Reluctance torque in rotating machines is present when:
a) airgap is not uniform
b) Reluctance seen by stator mmf varies
c) Reluctance seen by rotor mmf varies
d) Reluctance seen by working mmf varies

6.A Conductor is rotating within a magnetic field. At which position do the peak voltage occur:
a) At the right angles to the axis of magnetic field
b) Along the axis of the magnetic field
c) At 45 degree angles to the axis of magnetic field
d) Anywhere

7.Fleming's left hand rule is applicable for:
a) dc generator
b) dc motor
c) alternator
d) Transformer

8.EMF induced in a coil rotating in a uniform magnetic field will be maximum when the:
a) flux linking with the coil is maximum
b) rate of change of flux linkage is minimum
c) rate of change of flux linkage is maximum
d) rate of cutting flux by the coil sides is minimum

Answers:
(1) a (2) c (3) b (4) a
(5) d (6) b (7) b (8) c

Explanations for the Objective Questions: 
1. Answer (a).In the armature of the DC machine the current flow direction will be alternating manner. This alternating current is made to uni directional with the help of Commutator. This is the main use of the commutator in the DC machine.
2. Answer (c). Commutator is used to convert the ac current or voltage to dc and dc variables in to ac. This is interesting, in DC generator the current flow direction in armature winding is ac in nature, so commutator helps in rectifying ac to dc. In DC motor the current we supply to the motor is dc, however for armature of dc motor ac current is required to be supplied to produce alternative flux for generating rotating torque. Hence commutator in DC motor converts dc to ac.
3. Answer (b). The emf produced in the dc generator is dynamically induced emf. This can be explained with the help of faradays law electro magnetic induction.
4. Answer (a). DC generator works on the principle of faraday's law of electro magnetic induction
7. Answer (b). Fleming's left hand rule is applicable for DC Motors.


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