268 ALTERNATING CURRENTS 



If the load requires a lagging current, this machine cannot 

 supply it. This is illustrated by Fig. 249. A certain load re- 

 quires a current 7, lagging a degrees behind the terminal voltage 

 V. It is desired to supply as much of this current as possible 

 by means of an induction generator and to allow an alternator 

 to supply the remainder. Resolve the load current 7 into two 

 components, an energy component ii and a lagging quadrature 

 component 2 2 . The induction generator can by proper speed 

 adjustment supply the energy current ii. However, its leading 

 exciting current 7 is fixed, as has already been demonstrated. 

 Therefore, I a , the resultant of ?'i and 7 , is the total induction 

 generator current at this load. 



Obviously the alternator must supply that part of the load 

 current which the induction generator cannot supply. That is, 



FIG. 249. Currents supplied by an alternator and by an induction generator 



in parallel. 



the alternator must supply the difference between the load 

 current and the induction generator current. To obtain the 

 difference between two vectors, reverse one and add (page 12, 

 Par. 7). As I g is subtracted, it is reversed and the resulting 

 alternator current is I 8 , which is equal in magnitude to the arith- 

 metical sum of i z and 7 . It will be observed that the alternator 

 in this case supplies no power. Its entire current is lagging quad- 

 rature current and is equal to the exciting current of the induction 

 generator plus the lagging quadrature current of the load. 



If the load were such as to require a leading current, the quad- 

 rature component of which was just equal to 7 , theoretically 

 the induction generator could of itself supply the entire load. 

 Even then it would be necessary to have synchronous apparatus 

 on the system to secure satisfactory operation. 



