DC GENERATOR THEORY
DC Generator Ratings
A DC generator contains four ratings.
Voltage rating of a machine is based on the insulation type and design of
The current rating is based on the size of the conductor and the amount of
heat that can be dissipated in the generator.
The power rating is based on the mechanical limitations of the device that
is used to turn the generator and on the thermal limits of conductors,
bearings, and other components of the generator.
Speed rating, at the upper limit, is determined by the speed at which
mechanical damage is done to the machine. The lower speed rating is
based on the limit for field current (as speed increases, a higher field
current is necessary to produce the same voltage).
There are four internal losses that contribute to lower efficiency of a DC generator.
Each of these is described in the paragraphs that follow.
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.
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.