DIRECT CURRENT GENERATORS. [Exp. 



voltage (flat compounding) or a definite percentage higher (over 

 compounding). In either case the regulation is the maximum 

 percentage deviation from the ideal straight line at any part of 

 the characteristic, rated full-load voltage being taken as 100 

 per cent. (See 17, and Standardization Rules.) 



22. If the field magnets of a compound generator are highly 

 saturated, the increase in field ampere-turns with load due to the 

 series winding cannot cause a corresponding increase in the mag- 

 netic flux and there will be considerable deviation from the ideal 

 straight line characteristic. A compound generator accordingly 

 gives better regulation when the iron is below saturation, which 

 is opposite to the conclusion reached for the shunt generator 



(19). 



In a compound generator there is less cause for sparking and 

 shifting of brushes than in a shunt generator, on account of 

 the strengthening of the field by the series winding under load. 

 For fluctuating loads, as railway service, the compound generator 

 is accordingly superior and generally used. 



Obviously, on account of the series winding, it is much worse 

 to overload or short circuit a compound than a shunt generator. 



23. Shunt for Adjusting Compounding. If the characteristic 

 of a compound generator rises more than is desired, there are 

 too many series ampere-turns. These can be reduced without 

 changing the number of turns by reducing the current which 

 flows through them. This is done by a shunt resistance in paral- 

 lel with the series winding. A generator is usually given more 

 series turns than are necessary, the desired amount of compound- 

 ing being obtained by adjusting the shunt resistance. This is 

 much easier than changing the number of series turns and makes 

 it possible to change the amount of compounding at any time, 

 even after the machine is in use. 



24. ( d) Differential Characteristic. The connections for 

 this are the same as for the compound characteristic (Fig. 5) 

 except that the series field winding is reversed so as to be in 



