270 ELECTRICAL MACHINERY 



for the other. When No. 2 is over-excited (i.e., excited 

 to give a voltage higher than that generated by No. 1) 

 the current 01' tends to demagnetize No. 2 and magnetize 

 No. 1 while current 01" tends to magnetize No. 2 and 

 demagnetize No. 1. 



This circulating current of course heats the armatures 

 and so decreases the safe current which the machines can 

 furnish to the load circuit because the total armature cur- 

 rent is made up of the vector sum of the load current and the 

 circulating current. 



Summing up these ideas about circulating current we 

 may say, that if the voltage of one generator (operating in 

 parallel with others) is altered it does not affect the load 

 division between the different machines but produces a cir- 

 culating current which unnecessarily heats the armatures 

 and tends at the same time to so react on the fields as to equalize 

 the generated voltages of all machines. 



| 67. Division of Load. The next question which arises 

 is this if the load division between two alternators (opera- 

 ting in parallel on the same bus bars) is not affected by a 

 variation of the field excitation how can the load be shifted 

 from one machine to the other? It may be shown both 

 experimentally and theoretically that if two machines are 

 sharing a load equally and it is desired to have machine 

 No. 2 take more load this can be accomplished only by in- 

 creasing the torque of the steam engine (or turbine) driving 

 machine No. 2. 



To analyze the question let us suppose that two alter- 

 nators are operating in parallel, supplying power to some 

 outside load and that the load is equally divided between 

 the two machines. Under such conditions the ejn.fs. 

 of the two machines are exactly opposite in phase. We 

 further assume that the two e.m.fs. are equal so that there 

 will be no reactive circulating current between the two 

 machines. The two e.m.fs. are shown by OE\ and 

 in Fig. 166, 



