THE ALTERNATING CURRENT GENERATOR 261 



stations as many as ten or more alternators have been 

 installed to supply the load and they all operate in parallel. 



Number and Capacity of Machines. The considerations 

 which would govern the selection of the number and size 

 of machines for any given station are the same as those 

 discussed in Chapter III in the study of c-c. machines, 

 operating in parallel, but they may be reviewed here. In 

 general it is best to have the machines in a station of the same 

 size and type, because only a few spare parts need be kept 

 in stock for making repairs. Coils are likely to burn out at 

 any time and a few should always be kept in the station 

 for making quick repairs in case of necessity. If all machines 

 are of the same size and type then only one set of coils need 

 be kept on hand but if several sizes are installed in a station 

 a proportionally greater number would have to be kept in 

 stock. Also if the same size machines are installed there 

 always exists the possibility of interchanging parts from 

 different machines if necessary. 



In any ordinary station there should be, however, at 

 least one small sized machine for carrying the load during 

 the early hours of the morning when it is very light. 



Parallel Connection of Alternators Always Used. Having 

 several alternators, then, in a station, to be operated on 

 the same bus bars, the question arises as to how they shall 

 be connected together, in series or parallel. Practically 

 all power and lighting systems are designed for operation 

 at constant voltage irrespective of the load. If the alterna- 

 tors were connected for series operation it is evident that 

 the voltage on the busses would vary directly with the 

 number of alternators operating. If for example three 

 machines were operating in series, the bus-bar voltage would 

 be three times as high as if one only machine were operat- 

 ing. Hence it is evident that series connection of alternators 

 would not satisfy the requirement of constant voltage. But 

 even if it were all right to use the series connection in so 

 far as the load was concerned it could not easily be 



