2-A] SPEED CHARACTERISTICS. 3 1 



or, 



E-RI 



(5) 



This is the speed equation for a motor. It is seen that if < 

 is reduced the speed will increase. The equation shows the defi- 

 nite numerical relations of the quantities involved. How an 

 increase in speed is brought about by a decrease in flux is made 

 more clear in 7. 



6. Speed of a Shunt Motor. A shunt motor with constant 

 supply voltage has a constant field current and therefore a con- 

 stant flux. It accordingly follows that the speed is nearly con- 

 stant. The RI drop causes it to decrease with load (compare 

 equation 5) ; this is partially offset, however, by the effect of 

 armature reactions, as seen later (8). 



7. It is seen from equation (5) that the speed may be in- 

 creased or decreased by weakening or strengthening the field. 

 The process is explained as follows: 



When the field is weakened the counter-electromotive force is 

 reduced ; this permits more current to flow in the armature, thus 

 giving greater torque* and speed. The speed accordingly in- 

 creases until E' has increased so as to limit the current (and 

 hence the torque) to a value which will give no further accel- 

 eration. 



The cause for increase of speed is surplus torque. 



8. Armature Reactions and Brush Position. If the brushes 

 are given a backward lead (which is usual in motors running in 

 one direction, in order to obtain better commutation) the field is 



* (7a). As an example, suppose the field is weakened so that the flux 

 is reduced 2 per cent, and E' the same amount; and suppose the armature 

 current increases 50 per cent. Torque is proportional to flux and armature 

 current and in this assumed case is increased 47 per cent. ; for .98 x 1.50 

 = 147. This increase is only temporary, for the armature current and the 

 torque decrease as higher speeds are reached. 



