AITERNATING MOTORS FOR RAILWAY WORK. 73 
Their general properties are very little influenced by 
the number of phases for which they are wound. There 
is supposed to be a slight increase of output with increase 
in the number of phases, but as in the case of multipolar 
continuous current machines the increased output is more 
a matter of finesse in design than it is dependent on any 
theoretical considerations. 
At the present time all polyphase induction motors are 
strikingly alike in structural features. With very few ex- 
ceptions they consist of two concentric annular masses of 
laminated iron, of which the inner one is supported on the 
shaft and is free to rotate, while the outer one is carried by 
the frame of the machine. ‘The outer face of the inner 
ring and the inner face of the outer ring are provided with 
slots or holes to receive the windings. Fig. 93 shows the 
character and relation of these rings. The slots or holes 
are various in number and shape, but those in the two 
members are different in number to keep the magnetic re- 
lations constant irrespective of the position of the rotating 
member. ’The teeth are very seldom developed into any- 
thing approaching projecting pole pieces, unless in small 
motors, as it is desirable to distribute the windings as uni- 
formly as possible. In American motors, the slots are usu- 
ally open, in European types they are frequently closed as 
shown. 
Both rings are supported in a suitable frame. In one 
set of slots is wound the primary inducing winding, in the 
other the secondary or induced current winding. Some- 
times one winding rotates, sometimes the other. Conven- 
tionally we call the primary member the field and the sec- 
ondary member the armature. 
Fig. 94 shows a fifty horse power, two phase induction 
motor of the latest type and gives an admirable idea of the 
way in which such a machine is constructed. In this case 
the field revolves, while the armature is stationary. The 
working current is led into the field through the three col- 
lecting rings just outside the bearing, the two phases being 
given a lead in common at the motor. This revolving field