148 ELECTRICAL MACHINERY 



than No. 2, will force current through the armature of No. 

 2 besides furnishing the current to the feeders. The final 

 result is that No. 1 operates at some extreme point on its 

 external characteristic, furnishing all of the 130 amperes for 

 the load and in addition enough to run machine No. 2 as a 

 motor. 



If the action had started the other way (i.e., machine 

 No. 1 had for some reason, decreased its share of the load 

 from 50 amperes to 40 amperes) then the decrease in the 

 load of No. 1 would have continued until No. 2 was furnish- 

 ing all of the load current and enough to run No. 1 as a motor. 

 The operation of two such machines is therefore unstable; 

 any action which starts a re-distribution of the load con- 

 tinues until one machine is carrying all of the load and the 

 other has become a motor. 



Action of the Equalizer Bus. It will be noticed that this 

 instability is due to the action of the series field; that 

 machine which momentarily increases its load at the same 

 time increases its terminal voltage and the terminal voltage 

 of the other machine must fall as part of its load is taken 

 away by the first machine. Now the function of the 

 equalizer bus is to get rid of this instability and it does this 

 by maintaining the IR drop over the two series fields equal 

 at all loads. As the resistance of the equalizer is prac- 

 tically zero, as is that of the main bus-bar, it is evident 

 that the drop in voltage from A to B must be equal to 

 that from C to D (Fig. 93).* 



Suppose No. 1 tries to increase its share of the load; 

 the IR drop from C to D must increase and therefore so 

 must the IR drop from A to B increase. But the IR drop 

 from A to B can only increase if the current through the 



* It will be noticed that the resistance of the cables connecting 

 the machines to the switch board must be treated as part of the series 

 field circuit. Evidently the drop in voltage from A to B (or C to D) 

 is equal to the drop in the series field proper plus the drop in the con- 

 necting cables. 



