328 ELECTRICAL MACHINERY 



about its proper mean position with respect to the generator 

 armature, is called hunting. 



Natural Period. Under special conditions this hunting 

 becomes so violent that the motor will not stay in synchron- 

 ism but pulls itself out of step with the line and opens 

 the circuit breakers. It cannot be proved here but must 

 be taken as a fact that the armature of a synchronous motor 

 has a natural period of oscillation just as a pendulum has. 

 If a pendulum bob is displaced and allowed to swing we know 

 that it always swings with a definite time for one oscillation; 

 the time taken for the pendulum, swinging freely, to com- 

 plete one oscillation is called the natural period of the 

 pendulum. The synchronous motor armature has a natural 

 period just as the pendulum has and, if any irregularity 

 occurs either in the line voltage or load, with a periodicity, 

 which is the same as the natural period of the motor, 

 violent oscillations will very likely be set up which result 

 in the motor pulling out of synchronism. 



Detection of Hunting. When a motor is hunting, the 

 electrical instruments showing the input oscillate back and 

 forth and the normal regular hum of the motor is changed 

 into a hum broken by rythmic beats. 



To Prevent Hunting. There are several ways to stop 

 the hunting of a synchronous machine. One method 

 consists in changing the natural period of the motor arma- 

 ture by the addition of a heavy flywheel; another way is 

 to fit the pole faces with very heavy damping grids. If 

 the hunting is started by some irregularity of the load, such 

 as would be produced by the flapping of a belt, this trouble 

 should first be removed and the hunting may cease with- 

 out further change. 



88. Mechanical Analogy of the Synchronous Motor. 

 Some of the operating characteristics of the synchronous 

 motor can easily be remembered by bearing in mind a suit- 

 able mechanical analogue. Fig. 214 gives such an analogue. 

 A source of power, such as the steam engine, shown at A, 



