

DETAILED STUDY OF OPERATION WITH NORMAL LOAD 51 



From this we get 



_ F i- 2 cos r 



- r 2max. --2 ~ 



=E 2 



. . . . (22) 



The power-value will be higher in proportion as the impedance Z and 



/' F \ 



the resistance jR are smaller ( provided that cos r <- -=^ } . 



\ 2 &2/ 



The formula (22), can also be written directly from the diagram 



(Fig. 29) corresponding to the load which is at the limit of stability, 

 because, we have, from this diagram, 



ZI w =Ei E2 cos 7-, 



and also P^^E^Iw- 



In well-constructed modern motors (having low resistance and low 

 impedance), there is usually no tendency of the machine to fall 

 out of step from gradual overloading, when it is supplied from a con- 

 stant potential circuit, because, as Fig. 28 shows clearly, the current 

 becomes excessive before the power-outputs approach the limit of 

 stability just indicated. For example, the Labour synchronous motors, 

 which can be cited as excellent existing examples of this class of appa- 

 ratus, are only limited, in regard to load, by the heating of the armature, 

 and not by the tendency to fall out of synchronism. It is the circle of 

 maximum current, drawn as we have already seen, which limits long- 

 continued loads. 



But loss of synchronism may result from anything which can 

 increase the impedance and diminish the reactance-factor, and, con- 

 sequently the tendency to fall out of step is increased by the resistance 

 of the line in long-distance transmission. 



It is therefore necessary to be able to foresee, under certain cir- 

 cumstances, whether the operation will be stable or not. 



Means of Determining the Practical Stability of Synchronous 

 Motors. In practice, owing to oscillations of speed, the load is restricted 

 to a value which is less than the maximum power just determined. 

 It can be conceded, in all cases, that, during these speed-oscillations, 



