434 ELEMENTS OF ELECTRICAL ENGINEERING. 



This may be shown clearly by making a diagram of a chord winding and indicating 

 by arrows the direction of the current in each inductor. The neutral zone on the 

 commutator includes those commutator bars in which terminate all of the winding 

 elements which do not include inductors under the pole faces ; between such commu- 

 tator bars the electromotive force is nearly zero. In a winding of which front and 

 back pitches are very nearly equal to Zip the neutral zone is nearly as wide as the 

 angular space between adjacent pole tips, because, if the pitch were ZJp, every 

 inductor which, for a given position of the armature, lies between pole tips would be 

 connected front and back to other inductors which also lie between pole tips. On the 

 other hand, when the pitch, front or back, differs considerably from Z//, there is 

 only a narrow band of inductors, between a given pair of pole tips, which connect to 

 other inductors not under the pole faces, for a given position of the armature. 



For a given number of inductors the front and back pitches 

 must be chosen to comply with the following requirements : 



(a) All the winding elements must be similar mechanically 

 and electrically and they must be placed symmetrically on the 

 armature core.* 



(b) In tracing through a simplex winding every inductor must 

 be passed over once and once only, and the winding must close 

 on itself or be reentrant. 



(c) In a multiplex winding which consists of a combination of 

 complete and independent simplex windings, each simplex wind- 

 ing must satisfy condition (). 



(d) A multiplex winding which does not consist of complete 

 and independent simplex windings must as a whole satisfy condi- 

 tion (&). Such a multiplex winding is called a singly reentrant 

 multiplex winding. 



(e) In a two layer winding (in slotted armatures, two inductors 

 per slot) the inductors in the lower layer should be considered 

 even numbered, and those in the upper layer odd numbered. 



Lap windings. (a) Front and back pitches are opposite in 

 sign. 



(b) Front and back pitches cannot be equal. Equality of 



* In some cases armature core stampings already in stock are used for the armature 

 of a newly designed machine, even though a suitable arrangement of the windings 

 necessitates the leaving out of two half-coils in two of the slots. In such a case the 

 two slots which aie half filled by the winding, are filled up with the two halves of a 

 " dummy" coil. 



