126 ELEMENTS OF ELECTRICAL ENGINEERING. 



can be explained as follows : When the motor speed is small, the 

 counter-electromotive force developed in the armature is small, 

 so that a large current flows through the series field winding and 

 through the armature. Therefore the field excitation is large, 

 the flux 4> is large, and the torque is large according to equation 

 (25). As the speed increases, the torque falls off on account of 

 decrease of current due to increase of counter-electromotive 

 force, and on account of the decrease of <l> due to the decrease 

 of current. If the torque required to turn the armature is small, 

 as it is when the load on the motor is zero, then the motor speed 

 will increase greatly. 



There is in the case of the series motor, no well-defined zero- 

 load speed as there is in the case of the shunt motor. Any 

 motor at zero-load approaches that speed for which its counter- 

 electromotive force, 3>Z'n, is equal to the electromotive force 

 of supply. A motor at zero-load approaches this limiting speed 

 more and more nearly the smaller the resistance of its arma- 

 ture, or, for a given armature resistance, the smaller its power 

 losses. In the shunt motor the armature flux 4> is nearly con- 

 stant irrespective of speed and of load, and therefore a perfectly 

 definite speed (the zero-load speed) gives a counter-electromotive 

 force equal to the electromotive force of supply. In the series 

 motor, on the other hand, the armature flux < approaches zero 

 with decrease of load on account of the decrease of current 

 with decrease of load, so that an indefinitely high speed would 

 be necessary at zero load, to make <I> Z'n equal to E the electro- 

 motive force of supply. 



