1 86 ELEMENTS OF ELECTRICAL ENGINEERING. 



silver shunt 5 (not shown in Fig. 1 1 2) connected in parallel with 

 its series field coil as explained in Art. 50. 



(fr) The total resistances act! , bb' and cc' , Fig. 112, must be 

 inversely proportional to the full load current outputs of the 

 respective generators. This ensures the proper division of the 

 total current among the several series field coils. 



The use of the equalizing arrangement makes it possible to 

 operate in parallel compound generators which are very different 

 in size and in design. 



The instability of two or more compound generators connected 

 in parallel may be shown as follows : Consider two over-com- 

 pounded generators connected in parallel as shown in Fig. 1 1 1 . 

 Suppose that the machines A and B are running steadily and that 

 each is giving half of the total current that is being delivered to 

 the mains. If machine A then runs momentarily at a slightly 

 increased speed a momentary increase of its induced electromotive 

 force is produced, which causes an increase in the current which 

 it delivers and a corresponding decrease in the current delivered 

 by B.* The increase of load current in machine A causes a 

 further increase of the induced voltage of this machine, inasmuch 

 as it is understood to be over-compounded, and this increase of 

 induced electromotive force causes a further increase of load cur- 

 rent in A. At the same time the lessened current in B causes a 

 decrease of its induced electromotive force which tends to decrease 

 its current still more, and so on. This unequal distribution of 

 load between A and B when once started goes to an extreme and 

 leads to an excessive load current in machine A (or B) and a 

 negative load current in machine B (or A), that is one machine be- 

 comes a motor and takes po\^r from the other. 



Generator panels. The accessory apparatus belonging to each 

 one of a number of generators operating in parallel is mounted on 



* This is due to the fact that the two machines form together a closed circuit of 

 very low resistance, and a slight difference in the electromotive forces of the two ma- 

 chines causes a large current to flow around this circuit adding to the load current of 

 one machine and subtracting from the load current of the other. 



