THE CONTINUOUS CURRENT MOTOR 163 



be damaged. In electric railway installations the motor 

 is geared to the car axle so that there is never any possi- 

 bility of its running at excessive speeds. 



Development of Speed-load Formula. The reasons for 

 the different shapes of the speed load curves will now be 

 taken up. 



Let E = the voltage of the line to which the motor arma- 

 ture is connected; 



e = the voltage generated by the armature winding, 

 generally called the counter e.m.f. of the 

 motor; 



R s = ihe resistance of the series field; 



R a = ihe resistance of the armature; 



/ = the armature current; 



<j> s7l =rthe field flux produced by the shunt field; 



$> s = the field flux produced by the series field; 



$=the total field flux = cj> s /,+(j> s . 



A r = the speed of rotation in r.p.m.; 



The fundamental equation for determining the current 

 in such a circuit as a revolving armature is obtained by 

 equating the impressed force to the sum of the reacting 

 forces of the circuit. 



Shunt Motor. The counter e.m.f. and the resistance 

 reaction both act against the impressed e.m.f. of the motor 

 so we put 



E = e+IR a , (27) 



from which we get 



/ = fe ........ (28) 



Ka 



Now we know 



(29) 



