I70 POWER DISTRIBUTION FOR ELECTRIC RAILROADS.
II. Synchronous motors of the polyphase type are
capable of starting fairly well asinduction motors, the field
poles serving as armature. When the starting torque is
obtained merely from eddy currents in the pole pieces, as
in most synchronous motors and rotary converters, the
torque is weak and the starting current abnormally large.
To secure a quarter of the full load running torque, fully
twice the full load current would be ordinarily required,
or proportionally less if the motor is starting under merely
friction load!
It is quite possible, however, to construct a specialized
field with inductive windings in the pole faces, so that the
the motor will give its full normal torque at starting on a
current not greatly in excess of its full load current, and
will be capable of shifting over to synchronous run-
ning when up to speed. ;
In a similar way a monophase motor could be arranged
to be self starting as an induction motor and then trans-
formed to the synchronous type.
For starting under load these forms are probably in-
ferior to those starting as series motors by commutation,
but they are simpler and sufficient for starting unloaded.
To ordinary street railway service with constant stop-
ping and starting under all sorts of unfavorable conditions,
these essentially synchronous motors are inapplicable,
since they do not start well enough and are incapable of
speed variation when running in synchronism. Neverthe-
less, they are not to be despised for certain classes of rail-
way work.
For long lines with stops only at stated stations such
motors can even now be made available. If starting by
clutch be considered inadvisable there is now no serious dif-
ficulty in the way of a commutating start quite good enough
to bring a train up to speed. Once in synchronism the
motors would drive steadily ahead up grade and down at a
uniform speed until the next station was reached. The
longer the line and the fewer the stops the better would
be the operation of the system.
