3 DIRECT CURRENT MOTORS. [Exp. 



Within the usual range of operation, this RI drop for a com- 

 mercial motor is only a few per cent, of the total line voltage. 

 Good design does not permit more, inasmuch as the output and 

 efficiency are decreased by the same percentage. 



The current which flows in the armature is seen to be 



If under running conditions the current 7 is not sufficient to 

 give the motor enough torque (which is proportional to current 

 and flux) to do its work at the speed at which it is running, the 

 motor will begin to slow down, thus decreasing the counter- 

 electromotive force E' (which is proportional to speed and flux). 

 As E' decreases / increases, until the torque is sufficient to meet 

 the demands upon the motor. The current accordingly increases 

 automatically with the load, and this increase can be continued 

 until the safe* limit, determined by heating, is reached. 



On the other hand, if the current / is more than is needed to 

 give the torque required for the load at a certain running speed, 

 the surplus torque will cause the armature to accelerate, thus 

 increasing E' and decreasing / to a value which gives the proper 

 torque for the load and speed. 



It will be seen that a small change in E' is sufficient to cause a large 

 change in 7 and therefore in the torque. As an example, suppose E' = 100, 

 =104; if an increase in speed causes E' to increase 2 per cent, that is 

 to 102, the current 7 will be reduced 50 per cent. 



5. Relations between Speed, Flux and Counter-electromo- 

 tive Force. Counter-electromotive force is proportional to speed 

 (S) and flux (<j>) ; that is 



E'v+S. (3) 



Hence, speed varies directly as the counter-electromotive force 

 and inversely with the flux; that is 



* A motor is usually rated so that it can be run for several hours at 

 25 per cent, over its rated load. 



