104 ELEMENTS OF ELECTRICAL ENGINEERING. 



the neutral axis, the net counter-electromotive force between the 

 brushes is decreased, speed remaining unchanged. As a result, 

 the impressed voltage, E x , forces an increased current through 

 the armature, thus producing an increased torque which causes 

 the motor speed to increase until the counter-electromotive force 

 reaches a value sufficiently large to reduce the current to the 

 value required to supply the necessary driving torque. 



57. Speed regulation of the shunt motor. A motor which is 

 designed for supplying mechanical power at constant speed is 

 called a " constant speed " motor. Thus a shunt motor is called 

 a " constant speed " motor, although the maintenance of a strictly 

 constant speed by such a motor can be accomplished only by the 

 manipulation of a rheostat. The range of variation of speed of a 

 " constant speed" motor with change of load, when the rheostat 

 is left wholly unchanged, is an important practical matter. The 

 change of speed from full load to no load expressed as a percentage 

 of full load speed, is called the "speed regulation " of the motor, 

 voltage of supply and resistance of armature circuit and of field 

 circuit being kept constant. Thus, if the speed of a motor rises 

 from 1,000 revolutions per minute at full load to 1,050 revolutions 

 per minute at no load, its speed regulation is 5 per cent. 



It is to be noted that the word 



supply main . . . i j - i r 



regulation is applied to changes of 

 o speed inherent in the machine itself, 

 'f while the word control is applied to 



changes of speed due to deliberate 





O 

 O 



Resistance > O 



O 

 O 



2 adjustments by an attendant. 



Arm&ture fj o | Influence of armature resistance 

 supply main { U P^ the speed regulation of a shunt 



motor. When the resistance of 

 the armature of a shunt motor is 



large, or when a considerable resistance is connected in series 

 with the armature as shown in Fig. 80, then the speed of the 

 motor falls off greatly with increase of load. The cause of 

 this great drop in speed under load is evident from equations 



