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.
|
No comments:
Post a comment