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Figure 6 Commutation in a DC Generator | Up
Electrical Science Volume 2 of 4 | Next
DC Generator Ratings |
DC Generators
DC GENERATOR THEORY
Field Excitation
The magnetic fields in DC generators are most commonly provided by electromagnets. A current
must flow through the electromagnet conductors to produce a magnetic field. In order for a DC
generator to operate properly, the magnetic field must always be in the same direction.
Therefore, the current through the field winding must be direct current. This current is known
as the field excitation current and can be supplied to the field winding in one of two ways. It
can come from a separate DC source external to the generator (e.g., a separately excited
generator) or it can come directly from the output of the generator, in which case it is called a
self-excited generator.
In a self-excited generator, the field winding is connected directly to the generator output. The
field may be connected in series with the output, in parallel with the output, or a combination
of the two.
Separate excitation requires an external source, such as a battery or another DC source. It is
generally more expensive than a self-excited generator. Separately excited generators are,
therefore, used only where self-excitation is not satisfactory. They would be used in cases where
the generator must respond quickly to an external control source or where the generated voltage
must be varied over a wide range during normal operations.
Terminal Voltage
Figure 7 Varying Generator Terminal Voltage
DC generator output voltage is
dependent on three factors (recall
equation 5-1): (1) the number of
conductor loops in series in the
armature, (2) armature speed, and
(3) magnetic field strength.
In
order to change the generator
output, one of these three factors
must be varied. The number of
conductors in the armature cannot
be
changed
in
a
normally
operating generator, and it is
usually impractical to change the
speed at which the armature
rotates.
The strength of the
magnetic field, however, can be
changed quite easily by varying
the current through the field winding. This is the most widely used method for regulating the
output voltage of a DC generator (Figure 7).
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