158 POWER DISTRIBUTION FOR ELECTRIC RAILROADS, 
three or more, so spaced as to divide the windings in such 
wise that if the armature were worked as a dynamo it 
would deliver polyphase currents. 
Fig. go shows the connections of a two pole ring wind- 
ing tapped for three phase currents or for working as a 
three phase rotary converter. Here leads are simply 
taken off from three points on the winding 120 degs. apart 
and carried to the three collecting rings. In this case the 
machine will come up to speed as a three phase motor when 
the field is broken and current thrown on the rings. 
When at speed the field circuit, which is- connected like 
that of an ordinary shunt dynamo, is made, the armature 
falls into synchronism with the 
generator and continuous current 
may be drawn from the commuta- 
tor. For two phase currents the 
leads are taken off in a precisely 
similar way, but from four points 
90 degs. apart on the winding. 
There are still other and more complicated connections 
used for multipolar machines and for various practical reas- 
ons, but they all embody the same general principles. 
Asa matter of fact the rotary converter hasin efficiency 
or output an advantage over the same structure used as a 
dynamo since, as inspection of the winding will show, the 
average loss in the armature is lessened, because the current 
does not at all times have to traverse the full extent of 
the winding between ring and ring. 
An excellent example of modern practice in the rotary 
converter line is the Westinghouse two phase machine 
shown in Fig. g1, designed especially for railway substa- 
tion working. As a generator it can deliver either two 
phase or continuous currents or both, or when two phase 
current derived from reducing transformers is supplied 
to the collecting rings, continuous current at from 500 to 
550 volts can be withdrawn from the commutator. The 
striking similarity between this and an ordinary railway 
generator is at once apparent, and in practical properties