A DC generator is the same as an AC generator except in the manner of collecting the generated current. Figure 12 shows a simple DC generator consisting of a magnetic field, one loop, and a collector ring split into two parts. These parts,
called "segments," are insulated from the shaft and from each other. One segment is
connected to one end and the second segment to the other end of the loop. This type of
collector is called a "commutator." As in the case of an AC generator, brushes are
mounted so as to make a wiping contact with the split ring and permit current to
be drawn from the generator as it rotates. The brushes are mounted so that
they change contact from one segment to the other when the loop is at right angles
to the lines of force as shown in (b) of Figure 12. It will be remembered from
the discussion of the alternating current generator that, when the loop is in this
position, there is no current generated in it. Therefore, shorting of the loop, caused
by the brushes making contact with both segments at the same time, will do no
damage because there will be no current flowing. When the loop is in the position
shown in (a) the current will flow in at the left-hand brush and out at the right
hand brush. When the loop is in position (b) no current will flow because no voltage
is generated in the loop. When the loop is in position (c) the current again flows
in at the left-hand brush and out at the right-hand brush. The current that flows
in the loop changes in direction as the loop changes from position (a) to position
(c) as it did in the AC generator, but the brush connections also change when the
loop is rotated, that is, side "A" of the loop is connected through the commutator
to the left-hand brush when the loop is in position (a), and is connected to the right-hand brush when the loop is in position (c), so that the net result is that the current always flows in at the left-hand brush and out at the right-hand brush.
The current flowing in the load circuit of such an arrangement just described,
with only one loop and two segments, is pulsating (continually varying in magnitude), but flowing always in the same direction. In practice, a large number of
loops and segments are used so as to give a fairly constant DC voltage. The loops
are connected in series in such a way that the generated voltage is the sum of the
voltage generated in nearly all of the loops. The brushes are of such size as to short
circuit two or three of the segments, which short-circuits two or three loops. These
loops occupy a position as shown in (b) of Figure 12 when they are short-circuited,
and very little, if any, voltage is generated in them. However, the current through
the loops must change in direction as they pass under the brushes. and an arc will
form at the brushes unless they make a firm, even contact. For this reason it is important that the
brushes fit the commutator snugly, and that the commutator is kept clean.
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