61

Comment : The conductor area is directly
proportional to square of armature diameter.


Conductor
area – Armature diameter
 Let we assume
that conductor area is directly proportional to square of armature diameter.
 This is true
if depth and width of slot increases linearly with armature diameter.
 The width of
slot increases with diameter of armature but depth of the slot cannot make to
vary directly with armature diameter.
 As the deep
depth of slot result in high leakage reactance giving rise to high reactance
voltage which greatly affects the armature reaction.
 Therefore the
conductor area is not directly proportional to square of armature diameter
because it is limited by depth of slot.

62

Describe the relation between ampere conductor
per meter and armature diameter.


Ampere
conductor – Armature diameter
 There is
linear relation between ampere conductor and armature diameter only if there
is no change in the flux density in teeth.

63

Describe the effect of ampere conductors on the
armature reaction of the DC machine.


Ampere
conductors – Armature reaction
 As the ampere
conductors increase, the armature flux becomes stronger than the field flux.
 This will
result in distortion of field flux increases.
 However the
distortion effect is reduced by increasing the field flux.
 The field flux
increases by increasing the field turns which increase the cost of machine.
 If the ampere
conductors of the armature increase, the cost of machine increases due to
effect of armature reaction.

64

Describe the effect of high value of ampere
conductors on the commutation of DC machine.


Ampere
conductors – Commutation
 The ampere
conductors per meter for DC machine is given by = IZ / πD
 The ampere
conductors per meter are increased by increasing number of armature
conductors or decreasing armature diameter D.
 The inductance
of the coil is directly proportional to square of armature turns. Therefore
the high value of ampere conductors per meter results in high inductance.
 If the
diameter of the armature decreases, it is not possible to use wider slots.
 If
the slots are become wider, the thickness of teeth becomes smaller giving
rise to high flux density in them.
 Therefore the
deeper slots are used to accommodate armature conductors which also increase
the inductance of coil.
 The inductance
of the coil increases in both of the above cases. The reactance voltage of
coils is directly proportional to inductance of coil.
 Therefore the high
value of reactance voltage delays commutation condition.
 It is finally
conclude that the higher value of ampere conductors per meter adversely
affects the commutation condition.

65

State the advantages and disadvantages of high
value of electrical loading.


Advantages
 Volume and
size of machine is reduced
 Weight is
reduced
 Reduce cost
Disadvantages
 Armature and
field copper loss increases
 Reactance
voltage increases which worsens commutation condition
 Temperature of
machine increases

66

State the advantages and disadvantages of high
value of magnetic loading.


Advantages
 Volume and
size of machine is reduced
 Weight is
reduced
 Reduce cost
Disadvantages
 No load
current increases
 Field copper
losses increases
 Iron loss
increases
 Noise
increases
 Flux density
in the tooth increases
 Magnetic
saturation of iron parts increases

67

Explain : ‘ The specific electrical loading and
specific magnetic loading are interdependence ’.


Specific
electrical loading – Specific magnetic loading
 The output of
the DC machine is directly proportional to product of specific electrical
loading and specific magnetic loading.
 The value of
specific electrical loading and specific magnetic loading are interdependent
for same output rating.
 The specific electrical
loading of the machine increases if the lower value of the specific magnetic
loading is chosen and vice versa.

68

Comment : The tooth width affect the specific
magnetic loading of the DC machine.


Tooth
width – Specific magnetic loading
 The
flux density in any part of the tooth does not allow increasing beyond 2.2
weber / m^{2 }in the DC machines.
 The
specific magnetic loading of the DC machines increases if the tooth width
allows to increases but keeping the slot width same as earlier.
 Therefore
we conclude that higher value of specific magnetic loading allows for wide
tooth and vice versa.

69

Comment : The tooth width affect the specific
electrical loading of the DC machine.


Tooth width – Specific electrical
loading
 The
flux density in any part of the tooth does not allow increasing beyond 2.2
weber / meter^{2 }in the DC machines.
 If
the tooth width increases for specific slot pitch, the width of slot
decreases. Therefore there is small space left to accommodate armature
conductors. This is only possible by smaller value of ampere turns per meter.
 Therefore we conclude that as the tooth width
increases, the specific electrical loading of machine reduces.

70

Describe the effect of yoke area on the number of
poles in the DC machine.


Yoke
area – Number of poles
 The flux
carries by the yoke is inversely proportional to number of poles.
 As the
number of poles increases, the yoke area decreases and vice versa.

71

Why the iron losses do not occur in the yoke of
the DC machine?


Iron
losses in the yoke
 The yoke
carries steady flux therefore there is no iron losses in the yoke of the DC
machine.

72

Describe the effect of number of poles on iron
losses of the core and weight of armature core.


Number
of poles – Iron losses, Weight of armature core
 As the number
of poles increases, the iron losses of the core increase due to increase
frequency of flux reversals.
 The weight of
armature core decreases as the number of poles increases.

73

Describe the effect of number of poles on the
eddy current loss in the DC machine.


As the number
of poles in the DC machine increases, the eddy current loss does not affected
for the same core area.

74

Describe the effect of number of poles on the
hysteresis loss in the DC machine.


As the number
of poles in the DC machine increases, the hysteresis loss decreases for the
same core area.

75

Define : Active conductors, Inactive conductors


Active
conductors
 The conductors
which are responsible for production of induced emf or torque production in
the DC machines is called as active conductors.
 The conductors
in side slots area are called as active conductors.
Inactive
conductor
 The overhang
portion of the conductor is called as inactive conductors.
 The overhang
portion of the conductor provides connection between active conductors.
 The inactive
conductors do not contribute any emf.

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