Any tendency to leakage of magnetic lines will
therefore be within the machine, the iron acting as a shield. This
build of field--shown in Fig. 3A--is also advantageous as a mechanical
shield to the parts of the machine most likely to suffer from rough
handling in transport, and it will be seen that the field coils are
easily slipped on before the armature is mounted in its bearings.
[Illustration: FIG. 3A]
The winding is compound, and in such a direction that the two opposite
horizontal poles have the same polarity; it follows from this that
there will be two consequent poles in the iron, these being opposite
in name to the horizontal poles and at right angles to them, viz.,
above and below the armature. Opposite sections of the commutator are
connected together internally as in most four-pole machines, so that
only two brushes are necessary, at 90 deg. apart.
The section of iron in the field is 60 square inches and rectangular
in form, and the whole machine measures 4 ft. 3 in. in length, and 2
ft. in height, without including the height of the bed plate. The
armature is 17 in. in length and the same in diameter, measured over
the winding, and develops at the machine terminals 70 volts and 200
amperes at 480 revolutions. The moving parts of the engine are well
balanced, and run remarkably well and without noise at this high rate
of speed.
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