THE SYNCHRONOUS MOTOR 327 



the form of the V curves of Fig. 205. The value of field 

 current which gives cos cj> = 1 for light loads is not quite 

 sufficient to give cos 4> = 1 for the full load. 



87. Hunting of a Synchronous Motor. Suppose two a-c. 

 machines, one running as a generator and the other as a 

 synchronous motor, placed close together with their shafts 

 coaxial, so that by suitable experimental apparatus we 

 could compare the relative positions of the two armatures 

 as they both revolve at synchronous speed. 



Let us suppose that we could stand at some fixed point 

 on the periphery of the generator armature, as it revolves, 

 and look at some point on the armature of the motor. 

 Of course, this point would seem to us stationary, for 

 although it would be really revolving at high speed, we 

 (supposedly on the periphery of the generator armature) 

 would be revolving at the same speed. If, now, a load 

 should be put on the motor we would see the armature of the 

 motor slow up a little, for a fraction of a second, until the 

 point we were observing on the motor armature . had moved 

 backward a few degrees and then the motor armature would 

 regain its former speed. The point on the motor armature, 

 on which our gaze was fixed, would again appear stationary, 

 but it would be behind its former position. This change hi 

 its position would correspond to the angle a, in Fig. 213. 



A synchronous motor which has been properly designed 

 and which operates well, acts as we have just described. 

 For a given load any point on the motor armature has a 

 definite position when referred to a fixed point on the 

 generator armature. But it might be that the point on 

 the motor armature continually oscillated back and forth 

 about its proper mean position, so that at one instant the 

 motor armature would be ahead of its proper position and 

 the next instant, it would be behind its proper position. 

 While these oscillations were taking place, the armature 

 would be revolving on the average at synchronous speed. 

 This oscillation of the armature of a synchronous motor, 



