478 ELEMENTS OF ELECTRICAL ENGINEERING. 



allel with the series field coil of another similar generator which 

 is not in operation? Ans. 107.5 volts. 



NOTE. The effect, direct and indirect, of the full-load current in the series field 

 winding of this generator is to raise its voltage at full-load from 90 volts to 125 volts 

 and it may be roughly assumed that one half of the full-load current in the series field 

 winding will raise the full-load voltage half as much. 



128. A current of 200 amperes is to be delivered at 450 volts 

 to an extension of an electric railway at a " feeding point " which 

 is five miles from the power station. Two plans are considered 

 for accomplishing this result, namely: (i) To install a copper 

 feeder circuit (the resistance of the ground and rail return being 

 very uncertain, a return feeder wire is here assumed for the sake 

 of definiteness) which will deliver the prescribed current with a 

 drop of 100 volts, and (2) to install a feeder circuit of smaller 

 wire which will deliver the current with a drop of 300 volts, and 

 a 4O-kilowatt booster for raising the station voltage of 550 volts 

 up to 750 volts, which is required by this small feeder. 



The feeder required by plan (i) costs more than the feeder 

 required by plan (2) at the rate of 17^2 cents per pound for the 

 additional copper ; the cost of electrical energy at the switchboard 

 is 1.75 cents per kilowatt-hour ; the cost of the booster and its 

 accessories including the motor to drive it is $2,200 ; the efficiency 

 of the booster set, motor and generator combined, is 75 per cent .; 

 the depreciation of the booster is 8 per cent, per annum ; the 

 depreciation of the feeders is 5 per cent, per annum ; and the 

 interest and taxes on invested capital is 6 per cent, per annum. 



Find the maximum number of hours per day that the full-load 

 current of 200 amperes can be transmitted by plan (2) at a lower 

 actual cost than by plan (i). Ans. 5.97 hours. 



NOTE. The weight of bare copper wire in pounds is equal to 0.00000303 

 X length in feet X section in circular mils. 



The cost of large sizes of weather-proof insulated wire may be based on the weight 

 of the bare copper. Thus the high price of 17^ cents per pound for the bare copper 

 is intended to include the cost of the insulation. 



129, Given a direct reading ammeter (or voltmeter) of the 

 direct-current type of which the scale divisions are of equal length 



