DIRECT-CURRENT MACHINERY 



207 



Series Motor. In the series motor the flux increases almost in 

 direct proportion to the current, while the magnetic circuit is un- 

 saturated, and therefore the torque is proportional to the square 

 of the current; at heavy load, when the magnetic circuit becomes 

 saturated, the flux becomes almost constant and the torque then 

 increases in direct proportion to the current. Curve 2, Fig. 176, 

 is a typical torque-current curve for a series motor. 



(1) Shunt 



(2) Series 



(3) Compound 



(4) Differential Compound 



Load 



H Load H Load Full Load I'A Load 



Armature Current I 



FIG. 176. Motor torque characteristics. 



Compound Motor. In the compound motor the flux increases 

 with load but not in direct proportion to the current; thus, the 

 torque-current curve (curve 3) lies between those of the shunt and 

 the series motor. 



If the series winding is reversed, as in the differential compound 

 motor, the flux decreases with load and the torque-current curve 

 falls below that of the shunt motor. (Curve 4.) 



128. Starting of Motors. Fig. 177 shows the proper con- 

 nection for starting a shunt motor. Two conditions must be ful- 

 filled. First, the field current must be as large as possible and 

 therefore there must be no resistance in series with the field wind- 

 ing. This gives a large flux and therefore the required starting 



