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. after twenty-four hours.

[Illustration:  THREE-CONDUCTOR NO. 000 CABLE FOR 11,000 VOLT DISTRIBUTION]

[Sidenote:  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.

[Sidenote:  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.