28/03/2020

Design of DC Machines Interview Question Answer - 8



106
What value of flux density at the magnetic neutral axis does not generate emf?


Zero


107
Define : Commutation


Commutation
  • The commutation means reversal of current from one direction to other opposite direction.


108
Describe the different methods to reduce the armature reaction.


Methods of reduce armature reaction
  • Use of compensating winding
  • Use of inter poles
  • Increase the length of air gap at pole tips
  • Increase reluctance at pole tips


109
What is meaning of the chamfering of pole faces?


Chamfering pole tips
  • The length of air gap at pole tips is 1.5 to 2 times to that of pole centre. This is called as chamfering of pole tips.


110
What is direction of current in the compensating winding? Why?


Compensating winding
  • The direction of current in the compensating winding is opposite to that of armature current direction in order to cancel effect of armature reaction.
  • The compensating winding is connected is in series with armature winding in order to cancel armature mmf at all load condition.


111
At which place the compensating winding is located in the DC machine?


 The compensating winding is located in the axial slots of pole faces.

112
Whether the number of compensating winding ampere turns affects the armature reaction?


The compensating winding ampere turns are equal to armature winding ampere turns under the pole shoes in order to compensate completely armature mmf.

113
Where the inter poles or commutating poles are located in the DC machines?


Location of inter poles
  • The inter poles or commutating poles are located mid way between main poles at the geometric neutral axis.


114
Whether the inter poles neutralize armature mmf at the brush axis?


The inter poles neutralize the armature mmf completely at all load condition therefore the mmf required for the inter poles must be greater than the armature mmf per pole.

115
What should be polarity of the inter poles in the DC machines?


Polarity of inter poles
  • The polarity of the inter poles is main pole ahead in the case of DC generator and behind for DC motor.


116
Why the polarity of inter poles is main pole behind in the DC motor?


Inter poles polarity
  • The effect of armature reaction shifts the magnetic neutral axis and established a field corresponding to main pole ahead it.
  • The inter poles mmf should be equal and opposite to the armature mmf therefore the polarity of the inter poles should be main pole behind it.


117
Why the inter poles winding is connecting is in series with armature winding?


Inter poles winding
  • The armature reaction and reactance voltage is directly proportional to armature winding therefore the inter poles winding is connected is in series with armature winding in order to neutralize the cross magnetizing armature mmf and reactance voltage.


118
Compare lap winding and wave winding.


Lap winding – Wave winding
  • The number of parallel paths in the lap winding is equal to number of poles whereas the number of parallel path in the wave winding is equal to two.
  • The number of conductors in the simplex lap winding is p / 2 times to that of simplex wave winding but the cross section of each conductor in a lap winding is 2 / p times to that of each conductor in wave winding.
  • The current in each parallel path of the lap winding is 1 / p times the rated armature current whereas the current in each parallel path of the armature winding is 1 / 2 times the rated armature current.
  • The lap winding is identifies by large number of conductors with smaller cross section area whereas the wave winding is identifies by small number of conductors with large cross section area. 
  • The wave winding has smaller coils which reduces the cost of manufacture and repair.
  • The equalize connection is necessary in the lap winding whereas it is not requires in the wave winding.
  • The equalizing rings provide extra cost in the lap winding.
  • There is a commutation difficulty beyond 200 amp therefore the wave winding is used for current rating up to 400 amp whereas the lap winding employs beyond 440 amp. It does not mean that the lap winding is not used less than 440 amp.
  • The lap winding can short pitch which reduces its weight.
  • Each coil in the lap winding closes at the commutator end therefore the coils are easy to form than the wave winding.


119
Describe the limitation of wave winding.


Limitation of wave winding
  • If the current per parallel path exceed 200 amp, there is commutation difficulties are experienced. 
  • Therefore the wave winding is used for rated armature current up to 400 amp.


120
Explain : The maximum voltage between adjacent commutator segments should be high for small rating machines.


The maximum voltage between adjacent commutator segments should be high for small rating machines because of high internal resistance of small machines.



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