TRANSMISSION OF POWER FOR SUBSTATIONS. 149 
mutator during the period of getting up to speed. ’The 
only device practically used in this country, however, is 
the rather obvious one of bringing the synchronous motor 
up to speed by means of an alternating induction motor, 
and then cutting out the latter, leaving the former to run 
in synchronism and take up its load from a clutch. Fig. 
85 shows this device as employed in the single phase 
transmission plant at Telluride, Colo., where a number 
of these machines have been in successful operation for 
several years, and have only recently been replaced by 
polyphase motors. When the main motor has been thrown 
into synchronous running the starting motor with its 
friction pulley can be withdrawn from service and the 
main clutch thrown in, starting the driving pulley with 
its load of dynamos or other machinery. 
This arrangement while perfectly applicable for sub- 
station work has been largely superseded for all purposes 
by polyphase motors, which start easily and unassisted 
under their own torque. 
The general principles of the polyphase systems are 
at the present time sufficiently well known to engineers to 
render detailed explanation here unnecessary. By poly- 
phase it is here intended to designate all alternating systems 
employing two or more alternating currents displaced in 
phase in a uniform and systematic way. Practically there 
are two species of this genus, one having two alternating 
currents go degs. apart in phase, the other having three 
currents 120 degs. apart in phase. ‘There are several 
varieties of each, but it may be stated broadly that for the 
practical purpose of transmitting power to substations for 
railway purposes, both the species and their varieties are 
substantially equivalent. From.an academic standpoint 
wide differences may be pointed out, and in certain branches 
of polyphase work the differences may be worth consid- 
ering. So far as the railway engineer is concerned these 
differences are practically negligible. 
Of course, polyphase apparatus is closely similar to 
that used for ordinary single phase work in general ar-