28/03/2020

Design of DC Machines Interview Question Answer - 6



76
Describe the effect of poles on the inactive conductor of the armature.


Number of poles – Inactive conductor
  • The pole pitch of the armature is inversely proportional to number of poles for the constant diameter of machine.
  • Therefore the length of conductor in overhand portion decreases as the number of poles increases.


77
Describe the effect of number of poles on height of the DC machine.


Number of poles – Height of machine
  • The mmf developed by the field coil is inversely proportional to number of poles.
  • The winding height and height of the pole decreases as the number of poles increases for the same depth of winding.


78
Describe the effect of number of poles on overall diameter of the DC machine.

                                                
Number of poles – Overall diameter
  • As the number of poles increases, the height of pole decreases for same depth of winding.
  • This will result in overall diameter of the DC machine also decreases. Therefore the weight of iron or machine decreases as the number of poles increases.


79
Describe the effect of number of poles on overall length of armature conductor.


Number of poles – Length of armature conductor
  • As the number of poles increases, the weight of copper in the armature and overhang portion of the winding reduces therefore the overall length of the conductor reduces.


80
Describe the effect of number of poles on overall length of field conductor.


Number of poles – Length of field conductor
  • The field mmf per pole varies inversely as the number of poles therefore the weight of field copper reduces as the number of poles increases and vice versa.
  • We finally conclude that the weight of armature copper and field copper reduces as the number of poles increases.


81
Describe the effect of number of poles on brush size and overall size of the machine.


Number of poles – Brush size
  • The collection of current by each brush is inversely proportional to number of poles.
  • As the number of poles increases, the area of brush in each arm decreases.
  • The brush area is reduced decreasing the length of brushes.
  • This will result in decrease the length of commutator and overall length of machine also.


82
Describe the effect of number of poles on the flash over between brush arms.


Number of poles – Brush arms   
  • As the number of poles increases, the distance between adjacent brush arms decreases by considering same diameter of the commutator.
  • This will result in possibility of flash over between adjacent brush arm increases.
  • The diameter of the commutator will increase in order to reduce flash over between brush arms.


83
Describe the effect of number of poles on commutation condition of the DC machine.


Number of poles – Commutation
  • The armature mmf of the DC machine is inversely proportional to number of poles.
  • As the number of poles decreases, the armature mmf increases resulting distortion of field flux under load condition.
  • The distortion in field flux causes poor commutation and sparking. As the flux reduces, the generated emf also reduces.
  • Lower number of poles, poor commutation.


84
Describe the advantages and disadvantages of number of poles in the DC machines.


There are following advantages and disadvantages of large number of poles.
Advantages
  • Weight of armature and yoke reduces
  • Overall machine diameter reduces
  • Weight of copper for armature and field conductors reduces
  • Length of commutator reduces

Disadvantages
  • Iron losses increases due to frequency increases
  • Flash over between brush arm increases
  • Labour charges increases


85
Why the peripheral speed of the DC machine is limited?


Peripheral speed
  • The peripheral speed of the DC machine is limited to 30 meter / second.
  • If the peripheral speed exceed above 30 meter/sec, there is strong tendency to fly out overhand portion of the winding due to excessive centrifugal force.
  • Therefore the peripheral speed of the DC machine is limited.


86
Why the cross section of the pole should be kept circular?


Cross section of pole
  • As the circular cross section of the pole gives minimum periphery for a given area.
  • Therefore length of mean turn of field winding is minimum and cost of copper and copper loss reduces.


87
Which type of cross section of the field poles results in minimum cost of field winding?


Circular

88
Why the solid pole structure is not used for static SCR driven DC machine?


Disadvantages of solid pole structure
  • Higher core losses
  • Commutation problem
  • Slow response


89
How the pole faces affects the commutation condition in the DC machine?


Shape of pole face
  • The flux density in the air gap must decrease gradually from maximum value at the center of pole to zero value at the inter polar axis in order to good commutation.
  • The length of air gap should not uniform under pole faces in order to achieve good field form.
  • Therefore the pole face must be designed such that air gap at the pole tips is kept 1.5 to 2 times to that of length of air gap at the center of pole.


90
Why should the air gap at the pole tips are kept 1.5 to 2 times to that of length of air gap at the center?


In order to achieve good commutation


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