[Illustration: DUCTS UNDER PASSENGER STATION PLATFORM
64 DUCTS]
The armature is stationary and exterior to the field. It consists of a
laminated ring with slots on its inner surface and supported by a
massive external cast-iron frame. The armature, as has been noted,
comprises seven segments, the topmost segment being in the form of a
small keystone. This may be removed readily, affording access to any
field coil, which in this way may be easily removed and replaced. The
armature winding consists of U-shaped copper bars in partially closed
slots. There are four bars per slot and three slots per phase per
pole. The bars in any slot may be removed from the armature without
removing the frame. The alternators, of course, are separately
excited, the potential of the exciting current used being 250 volts.
As regards regulation, the manufacturer's guarantee is that at 100 per
cent. power factor if full rated load be thrown off the e. m. f. will
rise 6 per cent. with constant speed and constant excitation. The
guarantee as to efficiency is as follows: On non-inductive load, the
alternators will have an efficiency of not less than 90.5 per cent. at
one-quarter load; 94.75 per cent. at one-half load; 96.25 per cent. at
three-quarters load; 97 per cent. at full load, and 97.25 per cent. at
one and one-quarter load. These figures refer, of course, to
electrical efficiency, and do not include windage and bearing
friction. The machines are designed to operate under their rated full
load with rise of temperature not exceeding 35 degrees C.
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