DC Machines Multiple Choice Questions on “Ways to Reduce Effects of Armature Reaction – 1”.
1. The armature reaction in a DC machine causes distortion in the main field flux. Effect of armature reaction can be reduced by_________
a) Increasing the length of air gap
b) Decreasing the length of air gap
c) Increasing the number of poles
d) Decreasing the number of poles
Answer: b
Clarification: Decreasing the air gap is simple to say but hard to achieve, due to various other limitations. But it is one of the way to reduce dropping down of armature mmf flux density distribution, which further reduces the distortion of resultant flux density.
2. In order to neutralize armature mmf perfectly under the pole shoe, the ampere-conductors of compensating winding must be (pole arc/pole pitch=1) _______
a) Not equal to the total armature ampere conductors under the pole shoe
b) Equal to the total armature ampere conductors under the pole shoe
c) Half of the total armature ampere conductors under the pole shoe
d) Twice of the total armature ampere conductors under the pole shoe
Answer: b
Clarification: The number of ampere-turns required for compensating winding is ATcw /pole = ATa (peak) *(pole arc/pole pitch) = [IaZ/(AP2)]*(pole arc/pole pitch). Here, pole arc= pole pitch, is given. Thus ampere-conductors required are equal to the total armature ampere conductors under the pole shoe.
3. What is the ampere turns per pole for compensating winding in DC machines?
a) (pole arc / pole pitch) * armature ampere turns per pole
b) (pole pitch / pole arc) * armature ampere turns per pole
c) (pole arc / pole pitch) * total ampere conductors per pole
d) Cannot be determined
Answer: a
Clarification: The number of ampere-turns required for this purpose is ATcw /pole = ATa (peak) *(pole arc/pole pitch) = [IaZ/(AP2)]*(pole arc/pole pitch). The compensating winding neutralizes the armature mmf directly under the pole while in the interpolar region, there is incomplete neutralization.
4. Where are no equalizer rings connected?
a) Only wave winding
b) Only lap winding
c) Both wave winding and lap winding
d) Cannot be determined
Answer: a
Clarification: The armature coils forming each of the two parallel paths are under the influence of all pole-pairs so that the effect of the magnetic circuit asymmetry is equally present in both the parallel paths resulting in equal parallel-path voltages. Thus, equalizer rings are not needed in a wave winding.
5. Inequality in brush arm currents caused due to different emf induced in different parallel paths may give rise to copper losses. These effects can be avoided by using _________
a) Compensating windings
b) Interpoles
c) Equalizer rings
d) Cannot be determined
Answer: c
Clarification: The armature coil in lap windings have equal to or more than 2 parallel paths. Under the influence of pole pairs there may arise asymmetry in various paths resulting in different parallel path voltages. So, equalizer rings are connected in order to achieve symmetry again.
6. Which of the following method will completely neutralize the armature reaction in a DC machine?
a) Only compensating winding
b) Only interpoles
c) Compensating winding and interpoles
d) Cannot be determined
Answer: c
Clarification: To neutralize the cross magnetizing effect of armature reaction, a compensating winding is used. The compensating windings consist of a series of coils embedded in slots of the pole faces. Interpoles are designed to overcome the effects of the armature reactance and the self-induction of the machine.
7. What is the exact location of compensating winding?
a) Across armature
b) In series with armature
c) Across armature or in series
d) Cannot be determined
Answer: b
Clarification: By adding a compensating winding in the pole face plate which carries armature current in the opposite direction of current in the adjacent armature windings, the position of the flux at the pole face plate can be restored to the position it would have with zero armature current.
8. Exact purpose of compensating winding in a DC generator is _________
a) Mainly to reduce the eddy currents by providing local short-circuits
b) To provide path for the circulation of cooling air
c) To neutralise the cross-magnetising effect of the armature reaction
d) Cannot be determined
Answer: c
Clarification: Armature reaction can be reduced with the help of compensating winding. To neutralize the cross-magnetizing effect of armature reaction in a DC machine, a compensating winding is used. The compensating windings consist of a series of coils embedded in slots of the pole faces.
9. Function of interpole flux is_________
a) Neutralise the commutating self-induced emf
b) Neutralise the armature reaction flux
c) Neutralise both the armature reaction flux as well as commutating emf induced in the coil
d) Perform none of the above functions
Answer: c
Clarification: Interpoles are designed to overcome the effects of the armature reactance and the self-induction of the machine i.e. to neutralise both the armature reaction flux as well as commutating emf induced in the coil.
10. Why dummy coils are connected in DC machine?
a) To reduce eddy current losses
b) To enhance flux density
c) To amplify voltage
d) To provide mechanical balance for the rotor