TRANSMISSION OF POWER FOR SUBSTATIONS. 153 
variation in voltage with change of load almost negligible. 
The machines are primarily intended for direct coupling, 
as is generally the case with dynamos of so great output, 
and in this case they are connected to double horizontal 
turbines working under a head of about fifty feet. he 
shafts pass through close fitting sleeves to the exterior of 
the power house where the turbines are located. The 
transformers of the air ventilated type already described 
are located above the dynamo room and serve to raise the 
line pressure to nearly 11,000 volts for transmission twenty- 
three miles to Sacramento. The four generators are oper- 
ated in parallel, as is the case with many plants in which 
modern polyphase apparatus is employed. It should be 
noted that such parallel running is very easy if the gener- 
ators are so designed that they have good inherent regula- 
tion. 'The commercial efficiency of the Folsom machines 
is about ninety-six per cent at full load. 
Generally speaking these and other polyphase ma- 
chines are characterized by higher efficiency, lower induct- 
ance and enormously better regulation than is to be found 
in any of the older alternators. The value of the two 
latter characteristics for power transmission on a large 
scale can hardly be overestimated. 
The efficiency of these modern generators is subject to 
some variation according to size and speed, but between 
different makes of the same size, speed and voltage the 
differences are small. The following is about what can be 
counted on at full load from polyphase generators of vari- 
ous sizes: 
Output Efficiency 
in k. w. per. eent. 
50— 100 92—Q3 
100— 200 03—94 
200— 500 94—95 
500—I000 95—96 
T*his supposes moderate voltages, say, not exceeding 4000 
in the larger machines or 2000 in the smaller. It also 
supposes the speeds to be fairly high—not below 5001. p. m. 
for the sizes up to 200 k. w., and not below, say, 200