The hub of the revolving field is of
cast steel, and the rim is carried not by the usual spokes but by two
wedges of rolled steel. The construction of the revolving field is
illustrated on pages 91 and 92. The angular velocity of the
revolving field is remarkably uniform. This result is due primarily to
the fact that the turning movement of the four-cylinder engine is far
more uniform than is the case, for example, with an ordinary
two-cylinder engine. The large fly-wheel capacity of the rotating
element of the machine also contributes materially to secure
uniformity of rotation.
[Illustration: MAIN CONTROLLING BOARD IN POWER STATION]
[Illustration: CONTROL AND INSTRUMENT BOARD--MAIN POWER STATION]
The alternators have forty field poles and operates at seventy-five
revolutions per minute. The field magnets constitute the periphery of
the revolving field, the poles and rim of the field being built up by
steel plates which are dovetailed to the driving spider. The heavy
steel end plates are bolted together, the laminations breaking joints
in the middle of the pole. The field coils are secured by copper
wedges, which are subjected to shearing strains only. In the body of
the poles, at intervals of approximately three inches, ventilating
spaces are provided, these spaces registering with corresponding air
ducts in the external armature. The field winding consists of copper
strap on edge, one layer deep, with fibrous material cemented in place
between turns, the edges of the strap being exposed.
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