THE CONTINUOUS CURRENT MOTOR 



159 



of the current and thus the torque-current curve tends to be- 

 come a straight line, similar to that of a shunt motor. In 

 Fig. 98 the results of a test are shown. The magnetization 

 curve of the machine is given for reference, and it is seen 

 that so long as the magnetization curve is a straight line 

 the torque-current curve is parabolic in form but that for 

 currents which begin to saturate the field (shown to be about 

 14 amperes in Fig. 98), the curve becomes less steep and 

 approaches a straight line in form. 



18 



1C 



Field 



strength 



Torque 



10 12 14 16 18 20 22 

 Armature current 



FIG. 98. Current-torque Curve for Series Motor. 



Compound Motor. The compound motor gives a cur- 

 rent-torque curve the shape of which is intermediate 

 between those of the shunt and series types. The curves 

 for such a motor are given in Fig. 99; the field strength 

 at zero armature current is given by the shunt coils only, 

 but as the armature current increases the field strength 

 increases somewhat, due to the m.m.f. of the series coils. 



Comparison of Different Motors. The current-torque 

 curves for three motors, all of the same rating, are given 



