CHAPIER VI 
TRANSMISSION OF POWER FOR SUBSTATIONS. 
The transmission of power for railway purposes is a 
comparatively simple matter so far as methods are con- 
cerned. Inasmuch as railway dynamos are already 
worked at a comparatively high voltage, as high, in fact, 
as any continuous current generators of large output, 
there islittle reason to consider continuous current methods 
for transmitting power. Iarge generators of this class 
cannot be built for voltage high enough above that already 
used in railway practice to make it worth while to trans- 
mit current for transformation by motor generators. Thus 
it is that for the purpose in hand one need only be con- 
cerned with the use of alternating currents, polyphase 
and other, and the transmission of power at high voltages, 
from 2000 volts up to 10,000 volts or more. 
To begin at the beginning, it should be understood 
that all generators from their constructional features are 
essentially fitted for the production of alternating currents. 
When continuous current is desired the current derived 
from the armature windings has to be commutated to re- 
duce it from its original alternating form to being unidi- 
rectional. Take, for example, a simple drum winding in- 
tended for continuous current, such as is shown in Fig. 72. 
Tracing out the direction of the currents as shown by the 
arrows one sees immediately that the two halves of the 
armature are in parallel between brush and brush. When 
the armature has turned through 180 degs. the coils that 
originally were under the - brush have come under the — 
brush and are generating E. M. F. in a direction opposite 
to the original one. All coils as they pass under a given 
brush are delivering current in the same direction, but