102 ELEMENTS OF ELECTRICAL ENGINEERING. 



are small. Therefore, neglecting R a l a , equation (23^) may be 

 written : 





 n = -j-i at zero load (26) 



Inasmuch as QZ'n is the counter electromotive force of the 

 motor, we see from equation (26) that a shunt motor at zero load 

 runs at a speed such as to make its counter electromotive force, E a1 

 sensibly equal to the impressed electromotive force, E x . 



Shunt motor loaded. When a load is thrown on a shunt 

 motor which has been previously running at zero load, its tend- 

 ency is to lower the speed slightly and thereby to decrease the 

 counter electromotive force, 3>Z'n. As the counter electromo- 

 tive force decreases, the armature current increases according 

 to equation (23^), and the torque increases according to equation 

 (25), thus enabling the motor to carry its load. The drop in 

 speed of a shunt motor, from zero load to full load, ranges from 

 2 per cent, of zero load speed for large-sized motors, to 10 per 

 cent, or more for small motors. 



Tendency, in a shunt motor, for the demagnetizing action of the 

 armature current to counteract the tendency of the speed to decrease 

 with increase of load. When a shunt motor is loaded it is neces- 

 sary for the counter electromotive force to decrease sufficiently 

 to permit enough current to flow through the armature to de- 

 velop the torque required to carry the load. Now, if < were 

 invariable, the necessary decrease of counter electromotive force 

 could be produced only by a drop in speed, but as a matter of 

 fact the flux <> is decreased slightly by the demagnetizing ac- 

 tion of the current in the armature, and consequently the neces- 

 sary decrease of counter electromotive force (3>Z'ii) is brought 

 about in part by this decrease of <, and in part by a decrease in 

 speed, so that the actual decrease in speed is less than it would 

 be if <l> were invariable. 



The dependence of the speed of a shunt motor upon the value 

 of the armature flux, <I>, is strikingly shown by the following ex- 



