194 



ELECTRICAL ENGINEERING 



there is no current in the field winding and the only voltage gen- 

 erated is that due to the residual magnetism. It is shown as ok. 

 As the load current increases the flux increases and the generated 

 e.m.f. increases until the magnetic circuit becomes saturated and 

 the decreasing permeability and increasing leakage flux cause the 

 generated e.m.f. to fall off. This is shown in curve (1) which is 

 the internal characteristic. The ordinates of the external charac- 

 teristic or regulation curve (2) are less than those of (1) by the 

 resistance drop in the armature and series field. 



The terminal voltage of a series generator can be varied by con- 

 necting a resistance in shunt to the field winding. 



118. Parallel Operation. In power houses in which the load 

 varies at different hours of the day a number of generators are 

 usually installed. When the load is light one generator is operated 

 and supplies the demand and when the load increases a second ma- 

 chine is started up and connected in parallel with the first and its 

 excitation is adjusted until it takes its proper share of the load. 



Load 



S 2 



(2) 

 & ^ 



FIG. 160. Parallel operation of shunt generators. 



Fig. 160 shows two shunt generators (1) supplying power and 

 (2) ready to be connected in parallel with it. Before closing the 

 switches Si and 82 which connect the second machine to the load 

 it is necessary that its polarity be correct and that its terminal 

 voltage be the same or a little higher than that of (1). 



If the field rheostat of (2) is so adjusted that the voltage of (2) 

 is the same as the voltage of (1) and switch S\ is closed, then, if 

 there is no voltage across $2, it may be closed. But if the voltage 



