28/03/2020

Design of DC Machines Interview Question Answer - 7



91
What should be width of radial ventilating ducts for DC machine?


10 mm

92
Which are the factors to be considered before selecting the length of air gap in the DC machine?


There are following factors to be considered before selecting the length of air gap
  • Armature reaction
  • Pole face losses
  • Noise
  • Cooling
  • Circulating current


93
Describe the effect of air gap on the armature reaction in the DC machine.


Air gap – Armature reaction
  • The field mmf must be made large as compared to armature mmf in order to reduce effect of armature reaction.
  • If the length of air gap must large in order to get large field mmf.
  • Therefore the distorting effect of armature reaction is reduced by increasing the length of air gap in the DC machine.


94
Describe the effect of air gap on the pole face losses in the DC machine.


Air gap – Pole face losses
  • The variation in the air gap flux density due to slotting is very small if the length of air gap is large.
  • This will result in pulsating loss in the pole face of the DC machine reduces.


95
Describe the effect of air gap on the circulating current of the DC machine.


The slight irregularity in the air gap cause flow of large circulating current in the DC machine therefore the length of air gap in the DC machine should be large.

96
What should be value of armature mmf at pole center and center of inter polar axis?


The armature mmf is zero at pole center and maximum at the center of the inter polar axis.

97
Why the flux density at the inter polar axis is small?


The flux density in the inter polar axis is small because the reluctance in this region is large due to large air gap.

98
What should be wave shape of armature mmf?


Symmetrical triangular

99
What should be flux density at pole tips in the case of motor and generator?


Flux density at pole tips
  • The flux density at the leading pole tips increases whereas it decreases at trailing pole tips in the case of DC motor.
  • Similarly the flux density in the leading pole tips decreases whereas it increases in the trailing pole tips in the DC generator.


100
What is value of flux density in the magnetic neutral axis?


Zero

101
State the other name of inter polar axis.


Geometrical neutral axis

102
Explain : The effect of armature reaction is responsible delay commutation in the DC machine.


Armature reaction – Delay commutation
  • The coils undergo commutation should not generate any emf for good commutation condition.
  • This is only possible if the flux density at the inter polar axis is zero.
  • The flux density has some finite value at the inter polar axis due to armature reaction.
  • This flux density generated emf ( reactance voltage ) which is responsible for delay commutation.


103
Describe the effect of armature reaction in the DC machine.


Effect of armature reaction
  • Increases in iron loss
  • Delay commutation
  • Sparking between adjacent commutator segments


104
Why the iron loss increases due to effect of armature reaction in the DC machine?


Armature reaction – Iron loss
  • The iron losses in the teeth and pole shoes are determined by maximum value of flux density.
  • The maximum value of flux density at load increases considerably than at no load due to field distorting effect of armature reaction. 
  • The flux density at load is approximately 1.3 times that of no load therefore the iron losses at load are 1.5 times to that of no load.


105
What should be voltage limit between adjacent segment of commutator?


30 V




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