ISO ELEMENTS OF ELECTRICAL ENGINEERING. 



will decrease and thus counteract the momentary increase of 

 speed due for example to a sudden decrease of motor load on 

 machine B. That is to say, the region between s and M is 

 the region of stable running of machine B as a synchronous 

 motor. 



The region between M and s', Fig. 129, is a region of unstable 

 running, for, when the point P is in this region, a momentary 

 decrease of speed of machine B, due for example to a sudden 

 increase of its motor load, causes machine B to take in a decreased 

 amount of power so that the decrease of speed will not only persist 

 but it will become more and more pronounced until the point P, 

 Fig. 1 29 is carried beyond s r ; then machine B not only has a 

 belt load as a motor but it actually gives out power as a generator, 

 so that its speed will fall very rapidly indeed and carry the point 

 P round and round the circle until the momentum of the rotating 

 armature of machine B is exhausted and the machine comes to a 

 dead stop. 



If machine B is operating steadily with the point P in the 

 stable region sM, then any increase of load which carries the 

 point P beyond M will cause the machine to come to a dead 

 stop as above described. 



From Fig. 1 29 it is evident that there are two positions of the 

 point P for a given value of motor intake, one position is in the 

 region sM of stable running and the other position is in the region 

 Ms' of unstable running. The position for stable running has 

 the smaller value of E and therefore the smaller value of /. 



If machine B is operating steadily with the point P in the 

 stable region sM, then if the motor load on B were reduced to 

 zero, the speed of B would be momentarily increased and the 

 point P would move to s. When P is at s, Os is the resultant 

 electromotive force E y the current vector /, which is behind 

 E in phase, is vertically upwards, the angle (BI) is 90, and P" 

 is zero according to equation (iv) Art. 68. 



72. Hunting action of the synchronous motor. When the load 

 on a synchronous motor is suddenly increased, the motor speed 



