394 ELEMENTS OF ELECTRICAL ENGINEERING. 



point P represents a possible running condition, but it is difficult 

 to bring about the state of affairs represented by the point P as 

 explained below, while the point P f represents an unstable and 

 therefore impossible condition of running. The instability of the 

 conditions represented by the point P may be shown as follows : 

 Suppose that the generator is in fact operating at the point P ', 

 and that a slight momentary increase in the speed of the gener- 

 ator takes place. This increases the generator voltage momen- 

 tarily and also the current and then, even if the generator comes 

 quickly back to its normal speed, it will continue to operate for a 

 very short interval of time at a point P" a short distance above 

 P ; but at the point P 1 the generator voltage is more than suffi- 

 cient to produce the current I" (greater than e -f R r l ff }> and the 

 excess of voltage will cause the current to go on increasing until 

 the generator reaches the point P. 



On the other hand a momentary drop in the speed of the gen- 

 erator will cause a momentary decrease of current and, even if 

 the generator comes quickly back to its normal speed, it will 

 continue to operate for a very short interval of time at a point 

 P" a short distance below P ; but at the point P" the generator 

 voltage is less than {e -f R r P" } , so that the current will continue 

 to fall off more and more. In this latter case (with the storage 

 battery in circuit) the current does not merely drop to zero, but a 

 reversed current is started by the battery. This current causes 

 the generator to build up in the opposite direction, and then both 

 generator and battery work together to produce current, the 

 battery being of course thereby discharged. The point P lv y Fig. 

 28, shows the operating conditions in this case. 



The difficulty in getting a series generator to operate at the 

 point P, Fig. 28, and so charge a storage battery, is due to the 

 fact that when the circuit is first closed (current then equal to 

 zero) the generator begins operating at the point 0, Fig. 28, and 

 the effect of the battery is to cause the generator always to build 

 up in such a direction as to carry the operating point to P IV , thus 

 discharging the battery instead of charging it. Under these con- 



