THE CONTINUOUS CURRENT MOTOR 167 



Armature Reaction and Resistance Act to Neutralize One 

 Another. This effect of field weakening may be about 

 sufficient to overcome the effect of armature resistance 

 drop, in which case the speed load curve will be nearly flat, 

 giving no speed decrease with increase of load. If the 

 brushes are shifted too far back, the weakening of the 

 field caused by the armature reaction may be sufficient 

 to make the motor speed up with increase of load. 



If the brushes are shifted forward, the armature reaction 

 tends to magnetize the main field and so the speed-load 

 curve of the shunt motor with brushes having a forward 

 shift, is nearly the same as that of a compound motor. 

 This condition is never met in practice as the brushes of a 

 motor spark viciously if shifted forward to any extent. 

 35. Effect of Line Voltage on Speed. If the voltage 

 impressed on a motor varies, the speed of the motor may be 

 expected to vary correspondingly. In the case of a shunt 

 motor the field flux decreases with a decrease in the impressed 



TJ _ Tr> 



voltage so that, in the formula for speed, N = 



both E and < vary and, if the field of the motor is not oper- 

 ated near saturation, the change in speed for a small change 

 in line voltage is not marked. And if E increases, < in- 

 creases in nearly the same ratio so that, as E increases, the 

 speed does not increase very much. 



In the case of a series motor there is no shunt field 

 and thus for a given current in the armature circuit, the 

 speed varies in nearly the same ratio as the impressed 

 voltage. 



The compound motor, having both shunt and series 

 coils, has a change of speed, with change in line voltage, 

 greater than that of the shunt motor and less than that 

 of the series motor. 



36. Motors with Commutating Poles. The problem of 

 commutation is just as difficult to solve for the motor as 

 for the generator. If a motor is not equipped with com- 



