CHARACTERISTIC CURVES. 



413 



istic of a shunt motor is shown in Fig. 46. The equation to this 

 curve may be obtained by eliminating I a between equations (23*:) 

 and (2$), Chapter IV.; 4> being assumed constant, the resulting 

 equation is the equation to a straight line. The effect of the 

 demagnetizing action of the armature current is to cause the 

 speed to fall off less with increasing torque than would other- 

 wise be the case, as explained in Art. 60, Chapter IV. 



($) Shunt motor started by a starting rheostat. As explained 

 in Art. 59, Chapter IV., the shunt motor (supplied with cur- 

 rent from constant voltage mains) is started by first connecting 



Torque 



O 



Starting carve 



Torque 



Fig. 46. 



Fig. 47. 



its field winding direct to the mains, and then connecting its 

 armature to the mains through an auxiliary resistance which is 

 slowly cut out as the motor speeds up. The ideally correct 

 manipulation of the starting rheostat would be that which keeps 

 the armature current constant until full speed is reached. This 

 would produce a constant torque from the beginning to the end 

 of the starting process, inasmuch as the field excitation of the 

 motor is invariable. The vertical straight line in Fig. 47 repre- 

 sents the ideal starting characteristic of a shunt motor. The start- 

 ing and accelerating torque T may be made large or small at will. 

 Two rigidly coupled shunt motors in series. Two similar shunt 

 motors coupled together mechanically operate satisfactorily and 

 divide a load equally when their armatures are connected in 



