2 ALTERNATING CURRENTS 



today (1920) has a rating of about 50,000 kw. Owing to 

 commutation difficulties, direct-current generators cannot be 

 built in large units, particularly for high speeds. At 1,000 r.p.m., 

 it is difficult to build a direct-current generator having a rating of 

 even 1.000 k\v. On the other hand, 5,000-kw. alternators, oper- 

 ating at speeds of 3,600 r.p.m., are not uncommon. 



.For const ant -speed work, the alternating-current induction 

 motor is cheaper in first cost and in maintenance than the direct- 

 current motor. This is due to the fact that the induction motor 

 has no commutator. Therefore, it is occasionally desirable to 

 generate power as alternating current in order to be able to use 

 induction motors. 



The high transmission efficiencies obtainable with alternating 

 current make it possible to generate electrical energy in large 

 quantities in a single station and to distribute it over a compara- 

 tively large territory. The large boilers, automatic stokers, 

 superheaters, recording instruments, etc., which are possible 

 in large stations, result in high boiler-room efficiency. Large 

 turbines have an economy which may be three or four times as 

 good as that of the steam units in a small plant. The generator 

 has an efficiency of 95 to 96 per cent, in the larger sizes. Then 

 again, as the boilers and large turbo-units require few attendants 

 per kilowatt, the labor and superintendence charges per kilowatt 

 are small. 



For these reasons it is often more economical to generate power 

 with large units, to transmit it long distances and even to convert 

 it into direct current, than to generate the direct current at the 

 place where it is to be utilized. 



It must be remembered, however, that the reduced generating 

 costs may be balanced by the distribution costs resulting from 

 high investment charges in lines, cables, sub-stations, machin- 

 ery, etc., in addition to the labor and maintenance costs of this 

 distribution system. 



Alternating current owes its importance to the fact that it can 

 be generated economically with large units. Its voltage can be 

 readily raised and lowered, so that energy can be transmitted 

 economically for considerable distances. Alternating-current 

 motors for constant-speed work are usually preferable to direct- 

 current motors. 



