ALTERNATING MOTORS FOR RAILWAY WORK. 169 
ten or fifteen horse power should give somewhere be- 
tween eighty and eighty-five per cent efficiency, which is 
not materially different from that of continuous current 
motors of similar size. 
There are divers other means of starting an alternat- 
ing motor by means of a commutator. A commutated 
  
FIZ. 92. 
field in shunt to the armature can be made to give a power 
of starting sufficient to bring an unloaded motor up to syn- 
chronous speed, and in fact, an ordinary compound wound 
alternator can be made self starting by means of its com- 
pounding commutator. These devices do not permit of 
starting under anything much exceeding friction load and, 
hence, are inferior for severe work to the series starting 
device just described and various modifications of the same 
idea.