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~.