HUNTING OF SYNCHRONOUS MOTOR 135 



73. Hunting of Synchronous Motor 



A trouble which sometimes arises in connection with synchronous 

 motors is that of hunting, or phase-sivinging. By this are meant the 

 persistent periodic fluctuations in the speed and armature current of 

 the motor which are observed under certain conditions. It is not 

 difficult to see how such fluctuations may be started. Let us suppose 

 that the generator supplying the p.d. runs at an absolutely uniform 

 speed, so that the vector 0V of Fig. 97 has a perfectly constant speed. 

 Let the motor be also running at a steady speed under a constant 

 load, the angle remaining constant. Suppose now that a sudden 

 change of load takes place say an increase. A momentary retarda- 

 tion of the motor results, and this retardation will go on until the 

 increase in the driving torque (or the driving power), consequent on 

 the decrease of (see power curves, Fig. 99), becomes equal to the 

 increase in the resisting torque due to the larger load. When the 

 retardation * ceases, however, the motor is running at a lower speed 

 than that of synchronism. Hence will go on decreasing ; the driving 

 torque will accordingly increase above the value required to overcome 

 the resisting torque, acceleration will take place, and the speed of the 

 motor will increase ; when the speed of synchronism is reached, 61 will 

 cease to decrease,! and will then begin to increase, since acceleration 

 is still taking place. The vector OE (Fig. 97) now gains on 0V. As 

 increases, the driving torque decreases, and at a certain stage 

 becomes equal to the resisting torque. Acceleration now ceases ; but 

 at this point the motor is running at a speed above synchronism, so 

 that 6 will go on increasing, and the driving torque decreasing : 

 retardation takes place and synchronous speed is reached, when 6 

 becomes a maximum. Further retardation now takes place, begins 

 to decrease, and so on. 



We see, then, that any sudden change of load will cause the speed 

 of the motor and the current taken by it to undergo fluctuations. 

 Such fluctuations would be indicated by an ammeter in the arma- 

 ture circuit. The irregular motion of rotation which causes the 

 fluctuations may be regarded as consisting of a uniform motion 

 of rotation at- synchronous speed, combined with a to-and-fro or 

 pendular motion. The pendular motion is, in the vector diagram of 

 Fig. 97, represented by the swaying to and fro of the vector OE 

 relatively to the vector 0V. Under ordinary circumstances, the 

 pendular motion will die out after a time, owing to the resistances 



* Retardation = rate of decrease of angular velocity. 



t OV and OE(Fig. 97) now having the same angular velocity that of synchronism 

 and hence becoming momentarily constant. 



