SYNCHRONOUS GENERATORS 



315 



1 



2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 

 Generator Capacity iu Thousand Kv.A. 



FIG. 185. Approximate Efficiencies of Polyphase Water-wheel-driven 



Alternators. 



The curves in Fig. 185 represent approximate efficiencies of 

 polyphase water-driven alternators, the ratings being maximum 

 continuous. 



Speed. The speed of water-wheel-driven generators is deter- 

 mined by the frequency of the system and by the hydraulic con- 

 dition, that is the speed of the wheel, which, in turn, is governed 

 by the size of the unit and the head. 



With a fixed frequency the number of poles must be increased 

 in inverse proportion to a reduction in the speed. To accommo- 

 date this increased number of poles the diameter must necessarily 

 be larger and with this follows also an increased amount of material 

 and labor. The cost of slow-speed machines must, therefore, 

 necessarily be much higher than fcr machines of higher speeds. 



Table XLV shows what has actually been the practice in re- 

 gard to the speed of hydro-electric units. This table covers a 

 number of years' manufacture of wheels and generators, and can 

 hardly be said to represent latest practice, and future speeds may 

 be considerably higher than these, particularly in the smaller units 

 on the higher heads. It is seen that the speeds range from as low 

 as 55 R.P.M. to as high as 600 R.P.M., these figures, of course 

 referring to direct-connected units. 



