ALTERNATING MOTORS FOR RAILWAY WORK, I9I 
These results are nearly as good as can be obtained 
from a polyphase motor of similar output, but since most 
of these monophase motors are built with exceedingly 
small clearance for the armature, down to less than ' in., 
there is little likelihood of approximating closely the fig- 
ures just given with a motor fit for railway work. Nor is 
it possible to get effective speed regulation in momnophase 
motors by a resistance in the secondary or any other simple 
means. 
Summing up the present state of the art, we find that 
the only alternating motors yet constructed, of properties 
100 
  
   
  
Per cent efficlency and Power Factor 
  
  
H. P. out put Street Ry.Journal 
FIC. 105. 
immediately suitable for railway service, are the polyphase 
induction motors, which while often weak in power factor, 
are of sufficient efficiency and general excellence to replace 
existing continuous current motors. It is certain too, that 
the lag factor trouble can be overcome by careful design 
particularly if the frequency is kept low, say, 30~ to 40~. 
The synchronous motors, both monophase and poly- 
phase, have excellent properties when up to speed, but do 
not start will except at the cost of considerable complica- 
tion. The commutating start appears to give the best 
torque, but this is not comparable with the best that 
can be done by polyphase induction motors. The whole