182 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
of the motors was as good as could be desired, and the 
abolition of the commutator is a very material gain, since 
collecting rings give decidedly less trouble. Change of 
armature resistance gave opportunity to pass smoothly 
from one field connection to another without jerking the 
car. Asappears from curves C II and C III, apparatus of 
this kind has the very considerable advantage of fairly 
constant speed over a wide range of drawbar pull. Al- 
though polyphase induction motors are termed asynchron 
ous they have so strong a tendency to run near synchronous 
speed that they have the power of driving ahead regard- 
less of grades unless grossly overloaded. 
None of the methods of regulation as yet devised is 
quite the equivalent of the series parallel control so exten- 
sively used in continuous current practice, so far as effi- 
ciency is concerned. It is possible to get, however, as 
complete control of the speed and nearly as good efficiency 
at all except the lowest speeds. In the line of work for 
which alternating motors are most needed, i.e., long inter- 
urban and similar lines the need of highly efficient control 
at very low speeds is not so great as in ordinary street 
railway work, since by far the largest aggregate output is 
at the higher speeds. 
The weak points of polyphase induction motors for 
railway work are as follows: 
I. Necessity for at least two trolley wires. 
II. Lagging current. 
Inasmuch as all true polyphase systems require at least three 
working conductors, the best that can be done in supplying 
polyphase current is to utilize the rails for one conductor 
and provide separate trolley wires for the other two. In 
rare instances it might be possible to use a third rail and a 
single trolley wire or even to utilize the two track rails as 
separate conductors, but such cases are likely always to be 
exceptional. In conduit work, of course, two working 
conductors are available without much difficulty, but for 
general purposes the burden of two trolleys is difficult to 
avoid.