DIRECT-CURRENT MACHINERY 191 



= E. of is the voltage which must be generated in the armature 

 when supplying full-load current 7 = og. df is the increase in 

 generated e.m.f. required to overcome the armature resistance 

 drop IT ; this requires an increase in field current ab = x. The other 

 component of field current be = y is required to overcome the 

 effect of the armature m.m.f . 



The total increase in field current required to maintain constant 

 terminal voltage is 



ac = x + y = i. (Figs. 153 and 154.) 



If N is the number of turns on the field winding the increase in 

 field m.m.f. is Ni ampere turns. 



The increase of- field current under load is obtained by gradually 

 cutting out resistance from the field rheostat R c in series with the 

 field winding. 



This regulation must be done by hand and cannot take care of 

 sudden changes in load. 



116. Compound Generator. Increase of field m. m. f. under 

 load can be obtained automatically by placing a series winding on 

 the field poles in addition to the shunt winding. The series wind- 

 ing carries the load current and so the field m.m.f. increases under 

 load. 



Such a generator is called a compound-wound generator. If 

 the series winding is designed so that the terminal voltage is the 

 same at full load as at no load the generator is flat-compounded; 

 if the terminal voltage at full load is higher than at no load the 

 generator is over-compounded. 



In Fig. 155 are shown the regulation curves or voltage charac- 

 teristics of a machine (1) self-excited, (2) separately excited, (3) 

 flat-compounded and (4) over-compounded. If a generator is flat- 

 compounded so that it gives the same terminal voltage at full load 

 as at no load it will be slightly over-compounded below full load. 



If the field characteristic of a self-excited generator is known 

 (Fig. 153) and the number of turns on the field winding is N, the 

 number of turns required on the series field to produce flat-com- 

 pounding can easily be calculated. The increase in field m.m.f. 

 at full load is Ni ampere turns where i is the increase in field cur- 

 rent from no load to full load. Since this m.m.f. is to be produced 



Ni 

 by the load current 7, the number of turns required is N 8 = -j- 



