It may be variously defined as
(i) The periphery of the amature divided by the number of poles of the generator i.e the distance between two adjacent poles .
(ii)It is equal to the nmber of armature conductors per pole
The length of a wire lying in the magnetic field and in which an e.m.f is induced,is called a conductor as,for example length AB or CD in fig.
Coil and Winding Element
With reference to right side fig, the two conductors AB and CD along wth their end connections constitute one coil of the armature winding.The coil may be single turn or multi-turn coil.A single turn coil wil have two conductors.But a multi-turn coil may have many conductors per coil side.
Coil-span or Coil-pitch (Ys)
It is the distance, measured in tems o armature slots between two sides of a coil.It is,in fact, the periphery of the armature spanned by the two sides of the coil.
If the coil-pitch is equal to the pole-pitch,then winding is called full-pitched.It means that coil span is 180 electrical degrees.In this case,the coil sides lie under opposite poles,hence the induced e.m.fs in them are additive.Therefore,maximum e.m.f is induced in he coil as a whole.it being the sum of the e.m.fs induced in the two coil sides.
If the coil-pitch is less than the pole-pitch,then the winding is fractional-pitched.In tis case,there is a phase difference between the e.m.fs in the two sides of the coil.
Pitch of a Winding (Y)
In general, it may be defined as the distance round the armature between two successive conductors which are directly connected together.Or it is the istance between the beginings of two consecutive turns.
Y = YB - YF -------- for lap winding
Y = YB + YF -------- for wave winding
Back Pitch (YB)
The distance, measured in terms of the armature conductors,whic a coil advances on the bak of the armature is called back pitch.
Front Pitch (YF)
The number of armature conductors or elements spanned by a coil on the front is called the front pitch.
Alternatively, the front pitch ma be defined as the distance between the second conductor of one coil and the first conductor of the next coil which are connected together at the front i.e commutator end of the armature. Both front and back pitches for lap winding are shown in fig.
Resultant Pitch (YR)
It is the distance between the begining of one coil and the begining of the next coilto which it is connected.It is also shown in fig.
Commutator Pitch (YC)
It is the distance between the segments to which the two ends of a coil are connected.From fig it is clear that for lap winding , Yc is the difference of YB and YF where as for wave winding it is the sum of the two.