64 TMK DIRECT-CURRENT MOTOR (Ml IV 



.-i iti 

 the speed increases with the induction factor; ob = 2 / ; 



1 ?] E"* 

 at b the speed is a maximum and 1><I= t ' . If the 



J' J.LV 



induction factor is increased beyond ofc, the speed will 

 diminish until, when M is infinite, the speed is nothing. 



Tjl 



The current curve is also plotted in the figure; ";/ = 



B 



and/6 =^p 



From considerations of efficiency motors are usually 

 worked on the speed curve to the right of d, where an 

 increase of M involves a decrease of the speed. But it is 

 easy to arrange an experiment in which the strength of the 

 magnets can be varied and the motor brought to rest by 

 increasing M. Care must be taken that the current when 

 the speed is nothing shall not be great enough to injure 

 the motor. 



The experiment should be arranged so that the load, 

 when unbalanced, tends to turn the armature in the reverse 

 direction. If the induction factor is reduced below the 

 value oa, the direction of rotation will be changed and the 

 speed will increase in the reverse direction as the value 

 of M is reduced. When M is nothing, the speed will be 

 infinite, the current will also be infinite. 



In practice the load on the motor is generally incapable 

 of reversing the motion, so that if the induction factor be 

 reduced to nothing, the speed would tend first to increase, 

 and then to decrease to nothing, the current meanwhile 



increasing to its final value - ; the safety fuses would of 



course prevent so large a current from passing. 



The values of the speeds for different induction factors 



