THE SINGLE-PHASE SERIES MOTOR. 



313 



the commutator leads pass. The two coils tt f are wound on a 

 small iron core, and they are so connected that their magnetizing 

 actions on the iron core are equal and opposite when equal cur- 

 rents flow into (or out of) the armature winding through both 

 coils ; whereas their magnetizing actions work together when 

 current tends to flow out of the armature winding through one 

 coil and into the armature winding through the other coil. The 

 consequence of this arrangement is that current can flow into or 

 out of the armature through both of the coils tt' without being 

 choked, whereas opposite currents in the coils t and /' (the 

 short-circuit current in the section) are greatly choked. 



The complete arrangement of the balanced choke coils is 

 shown in Fig. 270 ; each commutator lead is split into two 



Fig. 270. 



branches, and each branch contains a choke coil which is bal- 

 anced against a choke coil in an adjacent commutator lead. 

 With this arrangement a brush must never touch less than two 

 commutator bars, because of the excessive choking of useful cur- 

 rent when it has to enter the armature through the two choke 

 coils in one lead ; but if the brush never touches less than two 

 bars, it must at certain instants touch three bars, and when this 

 occurs, a short circuit is established which is unchoked except 



