THE SYNCHRONOUS MOTOR. 151 



decreases momentarily and the motor falls behind the generator 

 in phase. By the time the motor has fallen behind the generator 

 sufficiently to take in power enough to enable it to carry its load, 

 its speed has become a minimum because up to this instant it has 

 been taking less than enough power to carry its load. Therefore 

 the motor continues to fall behind the generator, taking in more 

 than enough power to carry its load, and this excess of power 

 quickly brings the motor up to the generator speed. At this 

 instant the motor is farthest behind the generator in phase because 

 up to this instant its speed has been less than the speed of the 

 generator. Then, the motor as it continues to take in an excess 

 of power begins to exceed the generator in speed and gains on 

 the generator until it takes in less than enough power to carry 

 its load, when its speed again falls below that of generator and 

 so on. This oscillation of a synchronous motor backwards and 

 forwards through a certain mean position is called hunting and it 

 is similar to the hunting of a badly governed steam engine. 



The hunting oscillations of a synchronous motor are always 

 accompanied by oscillations of the driving generator, the motor 

 and generator oscillations being of the same rhythm and related 

 to each other like the oscillations of the two prongs of a tuning 

 fork. 



The hunting of a synchronous motor (or rotary converter) is 

 often very troublesome, making it impossible to operate the 

 motor even with a fairly light load, on account of the carrying of 

 the point P, Fig. 1 29, so far into the region Ms' of unstable run- 

 ning that it cannot return (motor intake being less than motor 

 load), but goes on beyond s f ', and the motor comes to a dead stop 

 as described in Art. 71. 



In general the hunting oscillations of a synchronous motor 

 when once started tend to go on increasing in amplitude, and the 

 prevention of troublesome hunting depends upon some action 

 which tends to decrease the amplitude of the oscillations. Two 

 such actions are available, namely, (a) The use of a fly-wheel 

 loosely attached to the synchronous motor shaft so as to slip as 



