4 8 



ELEMENTS OF ELECTRICAL ENGINEERING. 



is a combination of the two previous methods, and a generator with 

 its field windings so arranged is called a compound generator. 



Fig. 28 shows the arrangement of a compound generator 

 with a rheostat in circuit with the shunt field winding, Fig. 29 

 shows the conventional diagram of connections of a " long shunt " 

 compound generator, and Fig. 30 shows the conventional diagram 



main 



6hunt fic7d 



K/w^/"5 



Fig. 29. 



Fig. 30. 



of connections of the "short-shunt" compound generator. In 

 practice it is a matter of but little consequence whether a com- 

 pound generator is connected " long shunt " or " short shunt." 

 The calculations of currents, electromotive forces, losses, and effi- 

 ciencies are however slightly different in the two cases, as ex- 

 plained in Art. 65. 



Shunt and compound generators are always used for generating 

 direct current for lighting and power purposes when the current 

 is to be delivered to lamps and motors at approximately constant 

 voltage, which is the most usual condition of supply. The series 

 generator, on the other hand, is seldom used except for constant 

 current, series, arc lighting and for long distance direct-current 

 power transmission as exemplified by a few European plants. 



Field excitation of motors. Inasmuch as a motor always 

 receives current from an outside source there is no such thing as 

 self-excitation of a motor, the field excitation being necessarily 

 what is called separate excitation. A shunt motor has a fine wire 

 field winding which is connected in parallel with the armature to 



