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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.


Chapter Three Pages
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[1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15]

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