3 2 DIRECT CURRENT MOTORS. [Exp. 



weakened by the demagnetizing effect of armature reactions 

 (8a, Exp. i-A). This causes the flux to decrease with load, 

 so that the speed does not decrease as much as it would with the 

 brushes in the neutral position. On account of armature reac- 

 tions, therefore, the speed regulation of a motor is better; the 

 voltage regulation of a generator is worse ( 16, Exp. i-B). 



The proper brush position for best commutation is the posi- 

 tion which gives minimum speed. 



9. If the backward lead of the brushes is increased, the speed 

 of the motor under load can be increased until it equals or ex- 

 ceeds the speed at no load. Such a control of speed by brush 

 adjustment is not practicable, however, on account of bad com- 

 mutation and destructive sparking; the brushes should be given 

 the position of best commutation. A small variation of speed can 

 be made, if desired, by shifting the brushes, provided it is not 

 enough to cause much sparking. 



10. Speed Control. From equation (5) it is seen that the 

 speed of a motor can be varied : by changing the impressed volt- 

 age, E; by varying resistance, R (series controller) ; or, by vary- 

 ing flux, <. Each of these methods is in use for operating 

 variable speed motors. 



(a) Varying line voltage. Several line voltages can be ob- 

 tained by using a number of line wires. Such a system is called 

 a multiple-voltage system. 



(b) Varying resistance. The series controller is in common 

 use with series motors ; it is used occasionally with shunt motors 

 of small size. 



(c) Varying flux. This can be accomplished either by a 

 change in excitation (magnetomotive force) or a change in 

 reluctance; for flux magnetomotive force -7- reluctance. 



( i ) Speed control by varying excitation is obtained in a shunt 

 motor by a rheostat in series with the field (7) ; in a series 

 motor, by an adjustable resistance in parallel with the field. 



