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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