This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: 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
View Answer
Explanation: Dummy coils are connected in armature winding of type wave winding. In a lap winding Yc= +/-1 irrespective of the number of armature coils so that coils can always be chosen to completely fill all the slots (C =1/2 US). In a wave winding the number of coils must fulfil the condition C =P/2 Yc +- 1 while at the same time C must also be governed by C =1/2 US.
11. The most likely cause(s) of sparking at the brushes in a DC machine is /are ____________
a) Open coil in the armature
b) Defective interpoles
c) Incorrect brush spring pressure
d) Open coil in armature, defective interpoles and incorrect brush spring pressure
View Answer
Explanation: Brushes are the point of contacts between rotating and non-rotating parts. So, if point of contact is rough then brushes will face damage and will lead to sparking in them. Defective interpoles will contribute in irregular commutation ultimately it will lead to sparking.
12. Each of the following statements regarding interpoles is true except______________________
a) They are small yoke-fixed poles spaced in between the main poles
b) They are connected in parallel with the armature so that they carry part of the armature current
c) Their polarity, in the case of generators is the same as that of the main pole ahead
d) They automatically neutralize not only reactance voltage but cross-magnetisation as well
View Answer
Explanation: Interpoles are located in interpolar regions hence, they are called as interpoles. They are small narrow poles which speed up the commutation process (Also called commutating poles/ compoles).
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