DIRECT-CURRENT MACHINERY 



199 



ing. The compensating winding opposes the action of the po- 

 tential winding on the main control magnet so that as load 

 increases a higher potential is necessary at the generator terminals 

 in order to close the main contacts and open the shunt across the 

 field rheostat. Thus the generator voltage rises with load. The 

 condenser connected across the relay contacts serves to reduce 

 the sparking when the circuit is opened. 



Field 

 ^Rheostat 



Rheostat 



FIG. 167. Shunt 

 motor. 



Source 



of 

 Power 



FIG. 1G8. Series motor. 



FIG. 1G9. Compound 

 motor. 



121. Electric Motors. In generators mechanical power is 

 supplied and electrical power is generated. The speed is fixed by 

 the prime mover and is constant. The terminal voltage is ap- 

 proximately constant in the shunt generator and flat-compound 

 generator and increases with load in the over-compound generator 

 and the series generator. The generated voltage is always greater 

 than the terminal voltage by the drop in the armature resistance; 

 it is 



& = E + Ir. 



In motors electrical power is supplied and mechanical power 

 is generated. The impressed e.m.f. is fixed by the supply~circuit 

 and is constant. The speed is either approximately constant as 

 in the shunt motor or decreases with load as in the compound motor 

 and series motor. The voltage generated in the armature has 

 the same equation as the voltage in a generator, but it is a back 

 voltage and opposes the current; the impressed voltage E is greater 

 than the back-generated voltage by the armature resistance 

 drop; thus, 



E = g + 7r, (213) 



or 



E - Ir. 



(214) 



122. Types of Motors. There are three types of direct-cur- 

 rent motors corresponding to the three types of generators, shunt, 

 compound and series. The shunt motor has its field circuit con- 



