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
Which
losses are considered as iron losses?
The hysteresis loss and eddy current loss are considered as iron loss.
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
hysteresis loss is due to reversal of the magnetism of the core material.
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.
State the
unit of Steinmetz hysteresis coefficient.
The unit of
Steinmetz hysteresis coefficient is joule / meter3.
Which
material has lowest Steinmetz hysteresis coefficient for same flux density?
Cast iron, cast steel or silicon steel
Silicon
steel
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.
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.
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.
Which
losses are known as stray losses (rotational losses)?
Stray loss
The core
loss and mechanical loss are collectively known as stray loss.
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.
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
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.
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.
Define:
Geometric neutral axis
Geometric
neutral axis
The axis
which is perpendicular to the magnetic flux lines passing through centre 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.
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.
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.
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.
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.
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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