the direct-connected type as can be
constructed by the engine builders unless more than two bearings be
used--an alternative deemed inadvisable by the engineers of the
company. The adoption of a smaller unit would be less economical of
floor space and would tend to produce extreme complication in so large
an installation, and, in view of the rapid changes in load which in
urban railway service of this character occur in the morning and again
late in the afternoon, would be extremely difficult to operate.

The experience of the Manhattan plant has shown, as was anticipated in
the installation of less output than this, the alternators must be put
in service at intervals of twenty minutes to meet the load upon the
station while it is rising to the maximum attained during rush hours.

After careful consideration of the possible use of steam turbines as
prime-movers to drive the alternators, the company's engineers decided
in favor of reciprocating engines. This decision was made three years
ago and, while the steam turbine since that time has made material
progress, those responsible for the decision are confirmed in their
opinion that it was wise.

[Illustration: PART OF BUS BAR COMPARTMENTS--MAIN POWER STATION]

[Sidenote: _Alternators_]

The alternators closely resemble those installed by the Manhattan
Railway Company (now the Manhattan division of the Interborough Rapid
Transit Company) in its plant on the East River, between 74th Street
and 75th Street. They differ, however, in having the stationary
armature divided into seven castings instead of six, and in respect to
details of the armature winding. They are three-phase machines,
delivering twenty-five cycle alternating currents at an effective
potential of 11,000 volts. They are 42 feet in height, the diameter
of the revolving part is 32 feet, its weight, 332,000 pounds, and the
aggregate weight of the machine, 889,000 pounds. The design of the
engine dynamo unit eliminates the auxiliary fly wheel generally used
in the construction of large direct-connected units prior to the
erection of the Manhattan plant, the weight and dimensions of the
revolving alternator field being such with reference to the turning
moment of the engine as to secure close uniformity of rotation, while
at the same time this construction results in narrowing the engine and
reducing the engine shafts between bearings.

[Illustration: REAR VIEW OF BUS BAR COMPARTMENTS--MAIN POWER STATION]