DC Generators
DC GENERATOR THEORY
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.
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.
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.
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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