414 



ELEMENTS OF ELECTRICAL ENGINEERING. 



series, provided the shunt field winding of each machine is con- 

 nected directly to the supply mains. If the shunt field windings 

 are designed for the voltage of the supply mains they should of 

 course be connected in parallel to the mains ; if they are designed 

 for half of the supply voltage they should be connected in series. 

 Of course the speed of the motors will be only half of what it 

 would be if the full supply voltage were to be applied to the arma- 

 ture terminals of each machine. 



Two rigidly coupled shunt motors in parallel. When two 

 shunt motors which are coupled together mechanically, are con- 

 nected in parallel to the supply mains (that is when the arma- 

 tures are connected in parallel, the field windings being connected 

 directly to the mains), a very slight dissimilarity between the two 

 machines causes a great inequality in the distribution of the total 

 load between them. This is due to the fact that a slight increase 

 of current through one armature causes a decrease of the counter 

 electromotive force in this armature by its demagnetizing action, 

 and this tends to further increase the current through this arma- 



Fig. 48. 



ture. At the same time the decrease of current through the 

 other armature causes its counter electromotive force to increase 

 thus tending to reduce the current still more. That is, any 



