21
|
Which are the different losses taken
places in the DC Generator?
|
|
Losses in the DC Generator
- Iron loss : ( 1 ) Hysteresis loss and ( 2 ) Eddy current loss
- Armature copper loss
- Field copper loss
- Mechanical losses : ( 1 ) Friction loss and ( 2 ) Windage loss
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22
|
Which losses are considered as iron
losses?
|
|
The hysteresis loss and eddy current loss are considered as iron loss.
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23
|
Why the hysteresis loss occurs in the
core of the armature?
|
|
Hysteresis loss
- When the armature core rotates in the magnetic flux set up by the
field poles, the portion of the armature core passes under the N and S field
poles respectively, thereby attaining S and N poles respectively.
- Its magnetism reverses after passing through
pair of field poles.
- Therefore the hysteresis loss is due to reversal of the
magnetism of the core material.
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24
|
Which parameters affect the value of
hysteresis loss?
|
|
The hysteresis loss is given by
- Wh α Bmax2
f V watt
- Wh = η Bmax2
f V watt
Therefore its value is affected by ( a ) type of the core material ( b
) maximum value of flux density and ( c ) frequency.
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25
|
State the unit of Steinmetz hysteresis
coefficient.
|
|
The unit of steinmetz hysteresis coefficient is joule / meter3.
|
26
|
Which material has lowest steinmetz
hysteresis coefficient for same flux density? cast iron, cast steel or
silicon steel
|
|
Silicon steel
|
27
|
How to reduce hysteresis loss?
|
|
Hysteresis loss
- The hysteresis loss depends upon the B-H curve of the material. ( The
B – H loop is a measure of hysteresis loss per unit volume per unit cycle. ) Therefore the core material should be
selected such that area enclosed by the core material is as small as
possible.
- The silicon steel has small B – H loop therefore it is used as core
material to reduce hysteresis loss.
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28
|
What is eddy current?
|
|
Eddy current
- When the armature rotates in the magnetic field, an emf is induced in
it according to the Faradays’ law of electromagnetic induction.
- As the core
resistance is very small, it sets up large current in the core. This current
is known as eddy current.
|
29
|
How to reduce eddy current loss?
|
|
The eddy current loss is given by
- We α Bmax2 f 2
t2 V2 watt
Therefore the eddy current loss is reduced by laminating the armature
core at right angle to its axis.
|
30
|
Which losses are known as stray losses
( rotational losses )?
|
|
Stray loss
- The core loss and mechanical loss are collectively known as stray
loss.
|
31
|
What are constant losses?
|
|
Constant losses
- The combined iron and mechanical losses are known as constant losses.
- The shunt field copper loss is constant in the DC shunt and compound motor.
|
32
|
What is condition for maximum
efficiency in the DC Generator?
|
|
Condition for maximum efficiency
- The variable loss is equal to the constant loss is the condition for
the maximum efficiency in the DC Generator.
Variable loss = Constant loss
|
33
|
What is armature reaction? Describe
its effects.
|
|
Armature reaction
- The effect of the armature flux on the main ( field ) flux is called
as armature reaction in the DC machine.
Effect of armature reaction
- The armature flux weakens the main flux and
- It distorts the main flux.
|
34
|
Define : Magnetic neutral axis
|
|
Magnetic neutral axis
- The axis along which no emf is produced in the armature conductor is
called as magnetic neutral axis.
OR
- The axis which is perpendicular to the armature flux is called as
magnetic neutral axis.
|
35
|
Define : Geometric neutral axis
|
|
Geometric neutral axis
- The axis which is perpendicular to the magnetic flux lines passing
through center of the armature is known geometric neutral axis.
- The armature conductor
along this axis does not cut flux therefore no emf is produced in the
armature conductors.
|
36
|
Why the magnetic neutral axis is
called as the ‘axis of commutation’ ?
|
|
Axis of commutation
- The magnetic neutral axis is called as axis of commutation because
brushes are always placed on this axis and reversal of current takes place on
this axis.
|
37
|
What is direction of brush shift due
to armature reaction?
|
|
Brush Shift
- The direction of brush shift always is in the same direction as that
of direction of armature rotation in the case of generator whereas the
direction of brush shift is opposite to that of direction of rotation of the
motor.
|
38
|
What do you mean by the leading pole
tips and trailing pole tips?
|
|
Leading and trailing pole tips
- The pole tips which is first met during the rotation of the armature
is known as leading pole tips and other pole tips is known as trailing pole
tips.
|
39
|
What is direction of flux for
demagnetizing conductors in the DC Generator?
|
|
- The direction of flux for demagnetizing conductors is opposite to that
of main flux or field flux.
|
40
|
What is direction of flux for cross -
magnetizing conductors in the DC Generator?
|
|
The direction of flux for cross – magnetizing conductors is vertically
downward right angles to the main flux if main flux flows left to the right.
|
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