DIRECT-CURRENT MACHINERY 



195 



across S% is found to be about double the terminal voltage, the 

 field of (2) must be reversed before closing switch >S 2 . After 

 closing $ 2 the field rheostat of (2) must be adjusted until (2) takes 

 its proper share of the load. If the voltage of (2) is the same 

 as the voltage of (1) when the switch is closed, (2) will not take 

 any load but will run idle. If the voltage of (2) is less than the 

 voltage of (1), machine (2) will run as a motor driving its prime 

 mover and will draw power from (1). If, however, the terminal 

 voltage of (2) is higher than that of (1), machine (2) will supply 

 part of the load and will relieve (1) until the voltages of the two 



Amperes 



(1) Generator (1) 



(2) Generator (2) 



(3) Station 



FIG. 161. Voltage characteristics. FIG. 162. Voltage characteristics. 



are the same. Fig. 161 represents the voltage characteristics of 

 the two machines plotted on the same base. If the terminal 

 voltage is E (1) supplies a current /i and (2) a current 7 2 and the 

 total current supplied by the station is I = /i + h- The machine 

 with the flatter characteristic will supply the greater amount of 

 power. If the two machines are rated at the same current out- 

 put, (2) can be made to take its share of the load by cutting out 

 resistance from its field rheostat and so raising its voltage char- 

 acteristic and inserting resistance in the field circuit of (1) and 

 lowering its characteristic as shown in Fig. 162. 



Shunt generators will operate in parallel and divide up the load 

 in proportion to their capacities if their voltage characteristics 

 are similar, that is, if their terminal voltage falls from no load to 

 full load by the same amount and in the same manner. If the 

 characteristics are different a proper division of load can be ob- 

 tained by regulating the field rheostats. 



119. Parallel Operation of Compound Generators. Fig. 163 

 shows two compound-wound generators connected in parallel and 

 supplying power to a load circuit. Their voltage characteristics 



