122 SYNCHRONOUS MOTORS 



in the expression e 2 i. But the equations are then no longer rigorous and 

 can only be used to predict the occurrence, in the neighborhood of syn- 

 chronous speed, of heavy fluctuations resulting in alternate accelera- 

 tions and retardations which are very marked and for which the inertia 

 of the system cannot compensate. As soon as one of these accelera- 

 tions becomes strong -enough, it -will, at one -bound, bring the speed to 

 its normal value, leaving the motor synchronized. 



The starting of motors of the Ganz type may thus be explained 

 approximately, without our being able to take exactly into account 

 -the effects of inertia. It is known', moreover, that synchronous motors 

 do not always run at synchronous speed, but at a speed near it, which 

 is periodically variable, -owing to the production of the alternating 

 field already mentioned. It is an extreme case of oscillations of load, 

 which we now proceed to discuss. 



Oscillations of Synchronous Motors. It is easily observable, by 

 means of a dead-beat ampere-meter connected in the armature-circuit, 

 that the operation of a synchronous motor is always accompanied by 

 fluctuations above and below the normal current, which are due to 

 oscillations above and below the synchronous speed. With a good 

 : motor, operating in a satisfactory manner, these oscillations are rapid 

 and slight, except when the load undergoes sudden variations. But, 

 with certain motors, these oscillations attain a considerable amplitude 

 and; they are complicated by slow variations of very high amplitude, 

 giving rise to exaggerated current-values like those due to a short- 

 circuit, which usually end by the motor falling out of step;, it may even 

 be that synchronous operation is altogether impossible. 



There are here two distinct phenomena, which will be examined 

 separately, i.e., short-period oscillations of the motor itself and long- 

 period parasite oscillations due generally to foreign causes. 



Short-Period Oscillations. Any variation of the speed of the gen- 

 erator or any sudden variation of the resisting effort at the motor-shaft 

 gives rise to an oscillating condition of motor-speed. If the generator, 

 for example, has taken the lead, the driving torque becomes stronger 

 than the resisting torque, and it causes the armature to accelerate 

 gradually; but, owing to inertia, the acceleration lasts longer than 

 would be necessary to attain the corresponding phase at the condition 

 of equilibrium, and the motor, in its turn, takes the lead in phase. 

 From that moment, as its driving torque diminishes, the excess of the 

 resisting torque tends to produce a retardation; the motor slackens 

 in speed, and its phase is soon behind what it should be; and so on. 



