It can be run at unity power factor and minimum current input, or it can be over excited and thereby deliver leading current which compensates for the inductive load on other parts of the system. The synchronous condenser, therefore, can supply magnetizing current to the load on a system while the power component is supplied by the generators.
Fig. 2,479.—General Electric 400 kw., 550 volt, 600 R.P.M., synchronous condenser with direct connected exciter installed in sub-station No. 1 of the Colorado Light & Power Co., Cripple Creek, Colo. The machine is designed for alternating current starting by means of a compensator. The field is provided with a standard synchronous motor winding, and, in addition, an amortisseur winding which assists in starting and serves as a damping device to minimize hunting.
Fig. 2,480.—Diagram showing relative location of alternators and synchronous motors in plant of Witherbee Sherman & Co., Mineville N. Y. The distribution system of the Company is provided with three synchronous motors, as shown. The system includes two hydro-electric, one turbine driven, and one engine driven generator plants; from three of these, current is transmitted to the fourth, which is located in Mineville, at the point "A", the current being distributed to the motor circuits from the points "A" and "B." The transmission to the central station at Mineville is over three phase circuits at 6,600 volts. For operating the mine at Cheever, current is transmitted direct from the generating station at Port Henry. The distribution from "A" and "B" is all at 3,300 volts, being stepped down to 440 volts for the operation of the motors, which have a total rated capacity of 4,762 horse power. Excepting three synchronous motors, the load is practically all inductive, there being less than 10 kw. required for lighting. The actual power demand ranges from 60 to 65 per cent. of the rated motor capacity, and prior to the installation of the synchronous motors, the power factor was approximately 68 per cent., the condenser effect of these motors making it possible to maintain an average of about 90 per cent. power factor in spite of the fact that a considerable portion of the induction motor load is very widely distributed. The three synchronous motors are partially loaded, each motor driving an air compressor through belting. The 180 kva. motor at Cheever takes about 150 kw. for the operation of a 1,250 cubic foot compressor, while the two 360 kva. machines take about 300 kw. each, for the operation of two 2,500 cu. ft. sets. The operation of these compressors affords a method of utilizing a portion of the motor capacity mechanically, inasmuch as the load on the motors is practically constant during the time the mines are in operation, and thereby permit the motors to be run at approximately 80 per cent. power factor.
Effects of Low Lagging Power Factors.—Transformers are rated in kva. output; that is, a 100 kva. transformer is supposed to deliver 100 kw. at unity power factor at normal voltage and at normal temperatures; but, if the power factor should be, say .6 lagging, the rated energy output of the transformer would be only 60 kw. and yet the current and, consequently, the heating would be approximately the same as when delivering 100 kw. at unity power factor.
Fig. 2,481.—Field of a synchronous condenser. Note the amortisseur winding, erroneously called squirrel cage winding, consisting of two end rings which serve to short circuit spokes passing through the pole tips as shown. The amortisseur winding assists in starting and serves also as a damping device to minimize hunting.