Types of Losses in DC Generator
There are four internal Types of Losses in DC Generator that decrease efficiency of a DC generator.
1) Copper losses
2) Eddy-current losses
3) Hysteresis losses
4) Mechanical losses
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| Types of Losses in DC Generator |
1) Copper Losses:
Copper loss is the power lost as heat in the windings; it is caused by the flow of current through
the coils of the DC armature or DC field. This loss varies directly with the square of the current
in the armature or field and the resistance of the armature or field coils.
Armature: Ia^2*Ra (armature curent square into armature resistance)
Field: If^2*Rf (field current square into field resistance)
2) Eddy-Current Losses:
As the armature rotates within the field, it cuts the lines of flux at the same time that the copper
coils of wire that are wound on the armature cut the lines of flux. Since the armature is made
of iron, an EMF is induced in the iron, which causes a current to flow. These circulating
currents within the iron core are called eddy-currents
How to reduce:
To reduce eddy-currents, the armature and field cores are constructed from laminated (layered)
steel sheets. The laminated sheets are insulated from one another so that current cannot flow
from one sheet to the other.
3) Hysteresis losses :
Hysteresis losses occur when the armature rotates in a magnetic field. The magnetic domains
of the armature are held in alignment with the field in varying numbers, dependent upon field
strength. The magnetic domains rotate, with respect to the particles not held in alignment, by
one complete turn during each rotation of the armature. This rotation of magnetic domains in
the iron causes friction and heat. The heat produced by this friction is called magnetic hysteresis
loss.
How to reduce:
To reduce hysteresis losses, most DC armatures are constructed of heat-treated silicon steel,
which has an inherently low hysteresis loss. After the heat-treated silicon steel is formed to the
desired shape, the laminations are heated to a dull red and then allowed to cool. This process,
known as annealing, reduces hysteresis losses to a very low value.
4) Mechanical losses :
Rotational or mechanical losses can be caused by bearing friction, brush friction on the
commutator, or air friction (called windage), which is caused by the air turbulence due to
armature rotation. Careful maintenance can be instrumental in keeping bearing friction to a
minimum. Clean bearings and proper lubrication are essential to the reduction of bearing friction.
Brush friction is reduced by assuring proper brush seating, using proper brushes, and maintaining
proper brush tension. A smooth and clean commutator also aids in the reduction of brush
friction.
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