Exciters
To supply exciting current for the fields of the alternators and to operate motors driving auxiliary apparatus, five 250-kilowatt direct current dynamos are provided. These deliver their current at a potential of 250 volts. Two of them are driven by 400 horse-power engines of the marine type, to which they are direct-connected, while the remaining three units are direct-connected to 365 horse-power tri-phase induction motors operating at 400 volts. A storage battery capable of furnishing 3,000 amperes for one hour is used in co-operation with the dynamos provided to excite the alternators. The five direct-current dynamos are connected to the organization of switching apparatus in such a way that each unit may be connected at will either to the exciting circuits or to the circuits through which auxiliary motors are supplied.
The alternators for which the new Interborough Power House are designed will deliver to the bus bars 100,000 electrical horse power. The current delivered by these alternators reverses its direction fifty times per second and in connecting dynamos just coming into service with those already in operation the allowable difference in phase relation at the instant the circuit is completed is, of course, but a fraction of the fiftieth of a second. Where the power to be controlled is so great, the potential so high, and the speed requirements in respect to synchronous operation so exacting, it is obvious that the perfection of control attained in some of our modern plants is not their least characteristic.
Switching Apparatus
The switch used for the 11,000-volt circuits is so constructed that the circuits are made and broken under oil, the switch being electrically operated. Two complete and independent sets of bus bars are used, and the connections are such that each alternator and each feeder may be connected to either of these sets of bus bars at the will of the operator. From alternators to bus bars the current passes, first, through the alternator switch, and then alternatively through one or the other of two selector switches which are connected, respectively, to the two sets of bus bars.
Provision is made for an ultimate total of twelve sub-stations, to each of which as many as eight feeders may be installed if the development of the company's business should require that number. But eight sub-stations are required at present, and to some of these not more than three feeders each are necessary. The aggregate number of feeders installed for the initial operation of the subway system is thirty-four.
Each feeder circuit is provided with a type H-oil switch arranged to be open and closed at will by the operator, and also to open automatically in the case of abnormal flow of current through the feeder. The feeders are arranged in groups, each group being supplied from a set of auxiliary bus bars, which in turn receives its supply from one or the other of the two sets of main bus bars; means for selection being provided as in the case of the alternator circuits by a pair of selector switches, in this case designated as group switches. The diagram on [page 93] illustrates the essential features of the organization and connections of the 11,000-volt circuits in the power house.