190 POWER DISTRIBUTION FOR ELECTRIC RAILROADS.
difficulties in the way of maintenance that, while probably
surmountable, are serious in its application to railway con-
ditions.
M. Korda’s device dispenses with the condenser and
initially splits up the primary monophase currént into two
components 6o degs. apart by inductance alone and re-
combines these so as to give three phase resultants. It
gives a somewhat less stable phase relation and power fac-
tor than the method just described employing a con-
denser. | :
Second in the list of motors for monophase circuits
comes that class which employs a split phase current at
starting to obtain a simultaneous transformer and motor
action, but in running is purely monophase. Motors of
this kind have been considerably developed abroad, but are
only used tentatively in this country. Asat present made
they all start either with very poor torque, or if with better
torque demand an enormous starting current, which lags
badly. When once up to specd, however, they perform
well although never with as high output as a polyphase
motor of the same dimensions and efficiency. There are a
large number of ways of getting the phase difference at
starting, some of them requiring modifications of the motor
structure, others merely special connections. A consider-
able variety of phase splitting devices were devised by
Tesla as corollaries to his pioneer polyphase work and di-
vers others have been added to the list. Variations of cap-
acity and inductance in branches of the main circuit exter-
nal to the motor are most often used.
In construction and appearance these monophase
motors are closely similar to the polyphase ones already
described. Indeed most polyphase motors can be worked
as monophase motors with very trifling changes. When
carefully designed, these machines give a high efficiency
and a high power factor when once at speed. Fig. 105
gives the curvesof efficiency and power factor for a fifteen
horse power, Brown, monophase, asynchronous motor de-
signed for a speed of about 850 r. p. m. at g0~.