THE DYNAMO. 



45 



tain types of direct-current generators are usually excited by cur- 

 rent from an outside source ; thus the low voltage generators 

 used for electroplating are excited in this way. This method of 

 field excitation is called separate excitation. Direct-current gen- 

 erators, however, usually supply their own field current. This 

 method of field excitation is called self-excitation. 



The shunt generator. The winding on the field magnet of a 

 direct-current generator may consist of many turns of compara- 

 tively fine wire having considerable resistance. In this case the 

 terminals of the field winding are connected directly to the 

 brushes of the machine, and from 2 to 10 per cent, of the per- 

 missible current output of the generator is taken to magnetize or 

 excite the field, the remainder being available for use in the 

 external circuit. In this case the field winding and the outside 

 receiving circuit are in parallel with each other between the 

 brushes, so that the field winding is in the relation of a shunt to 

 the outside receiving circuit. A generator with its field windings 

 arranged in this way is called a shunt generator. 



Fig. 24. 



In the shunt generator an adjustable resistance (rheostat) is usu- 

 ally connected in the field circuit for controlling the value of the 

 field current and thereby controlling the voltage of the machine. 



