Θ
(Mechanical) = θ (Electrical) / Pole pairs
= θ (Mechanical) × 2
/ P
Where P = Number of poles
Describe
the function of compensating winding? Where it is located?
Function of
compensating winding
The function of the compensating winding is to neutralize the cross magnetizing effect of the armature reaction. These windings are embedded in the slots of the pole shoes.
Why the
compensating winding is used in the DC machine which subject to fluctuation of
load?
Role of
compensation winding for variable load
When there
is large fluctuation of load, the flux may move backward or forward according
of change in load. The shifting of this flux set up statically induced emf in
the armature coils. The magnitude of this emf depends upon amount of change in
load. It may be so high to produce arc between consecutive commutator segment
and resulting armature may be short circuited. The compensating winding
neutralizes statically induced emf and reduces the flash over across commutator
segments.
What is
direction of current in the compensating winding with reference to armature winding?
The
direction of the current in the compensating winding is opposite to the
direction of the current flows in the armature winding.
Explain:
Commutation and commutation time
Commutation
The current
in the short circuit coil is reversed while it crosses the magnetic neutral
axis is called as commutation.
Commutation
time
The time
period during armature coils remain short circuited is called as commutation
time.
What is
meaning of the ideal commutation? What is its effect?
Ideal
commutation
The change of
current from + I to zero and then from zero to – I by the end of commutation
period is known as ideal commutation.
There are no sparking between brushes and commutator due to the ideal
commutation.
Why
sparking occurs on the brushes during commutation?
The main
reason for sparking on the brushes is due to reactance voltage in the armature
coil during commutation.
How to
decide commutation time in the DC Generator?
Commutation
time
The
commutation time in the DC Generator is given by
Tc =
( Wb – Wm ) / v
Where
Tc = Commutation time
Wb = Brush width in cm
Wm = Width of mica insulation in
cm
v = Velocity of commutator segment cm /
sec
Describe
the different methods to reduce sparking on the brushes.
There are
following methods to reduce sparking on the brushes
Resistance
commutation and
Emf
commutation
Describe
the advantages and disadvantages of carbon brushes over copper brushes.
Advantages
Self
lubricating
Help to
obtaining sparkless commutation
They damage
commutator less than that of copper brushes in the case of sparking.
Disadvantages
High
contact resistance
They are
not suitable for small machines due to higher (approx. 2 volt) voltage drop
across brushes.
Large brush
holder requires due to low current density as compared to copper brushes
The size of
commutator should be larger than that of copper brushes in order to dissipate
heat and prevent temperature rise.
Why carbon
brushes are not used in small rating DC Generator?
Carbon
Brushes
The carbon
brushes have high contact resistance as compared to copper brushes therefore it
has more voltage drop which is undesirable for small voltage rating DC
Generator. Therefore the carbon brushes are not used in the small rating DC
Generator.
How to
improve the commutation condition in the DC Generator?
The
commutation condition is improved in the DC Generator by
Emf
commutation method
Resistance
commutation
How the
reversing emf is produced in the armature of the DC Generator?
The
reversing emf is produced in the armature of the DC Generator by
Using inter
– poles and
Giving
brushes a forward lead such that to bring the short circuited coil under the
influence of the next pole of opposite polarity.
At which
place the inter – poles are located in the DC Generator?
The inter –
poles are located mid way between main poles at the geometrical neutral axis in
the DC Generator.
Describe
the functions of (a) Inter-poles (b) Compensating winding
Function of
inter – poles
The main
function of the inter-poles is neutralize the reactance voltage thereby making
sparkles commutation
The inter –
poles are used to neutralize cross – magnetizing effect of the armature
reaction.
Function of
compensating winding
The
function of the compensating winding is to neutralize the effect of armature
reaction.
At what
percentage of load, inter – poles neutralize the cross-magnetize effect of
armature reaction?
The inter –
poles neutralize the cross – magnetize effect of armature reaction at any load
i.e. 0% to 100% load
In which
winding equalizing connections are used? Lap winding or wave winding? Why?
Equalization connection
The
equalizing connections are used only in the lap winding.
The
characteristic of the lap winding is such that all conductors lie in any
parallel path under pair of the poles.
If the flux under all the poles is same, the
emf induced in each parallel path is also same and each path carries the same
current. There is always unbalance of emf in the two parallel paths due to
magnetic property of the material and slight variation of the air gap.
Therefore the conductors under stronger poles generate higher emf resulting
higher current than that of conductors under weaker poles. This will resulting
current distribution becomes unequal at the brushes and sparking occurs.
At which
place in the armature, the equalizer rings are connected?
The
equalizer connection is a copper ring connected at the armature back at the
equal potential point.
Describe
the function of equalizing ring in the lap winding.
Function of
equalizing ring
The
function of the equalizing ring is to avoid unequal distribution of the current
at the brushes and thereby help to improve sparking at the brushes and getting
the sparkles commutation.
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