Q. Define coil pitch, Back pitch, front pitch, pole pitch, commutator pitch, resultant pitch, Multiplex winding and degree of Re-entrant of a winding, with respect to a D.C. machine.
Sol. Coil - span or coil pitch (Ys) : It is the distance, measured in terms of armature slots (or armature conductors) between two sides of coil. It is, in fact, the periphery of the armature spanned by the two sides of the coil.
If the pole span or coil pitch is equal to the pole pitch (as in the case of coil A, coil wher coil - pitch of 4 has been assumed), 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 the coil as a whole, it being the sum of the e.m.f.s induced in the two coil sides. For ex, if there are 36 slots and 4 poles, then coil span is 36/4 = 9 slots. If no. of slots is 35, then Ys = 35/4 = 8 because it is customary to drop fractions.
If the coil span is less than the pole pitch (as in coil B where coil pitch is 3/4th of the pole pitch), then the winding is fractional - pitched coil. In this case, there is a phase difference b/w the e.m.fs. in the two sides of the coil. Hence, the total e.m.f. round the coil which is the vector sum of e.m.fs. in the two coil sides, is less in this case as compared to that in the first case.
Back Pitch (YB)
The distance, measured in terms of the armature conductors, which a coil advances on the back of the armature is called back pitch and is denoted by YB.
Front Pitch (YF)
The no. of armature conductors or elements spanned by a coil on the front (or commutator end of an armature) is called the front pitch and is denoted by YF.
Alternatively, the front pitch may be defined as the distance (in terms of armature conductors) b/w 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 and wave - winding.
Resultant Pitch (YR) : It is the distance b/w the beginning of one coil and the beginning of the next coil to which it is connected.
Pole Pitch : It may be variously defined as ;
(i) The periphery of the armature divided by the no. of poles of the generator i.e. the distance b/w two adjacent poles.
(ii) It is equal the no. of armature conductors (or armature slots) per pole. If there are 48 conductor and 4 poles, the pole pitch is 48/4 = 12
Commutator Pitch (YC)
It is the distance (measured in terms of commutator bars or segments) b/w the segment to which the two ends of a coil are connected. It is clear that for lap winding, YC is the difference of YB and YF whereas for wave winding it is the sum of YB and YF.Obviously, commutator pitch is equal to the no. of bars b/w coil leads. In general, YC equals the 'plex' of the lap - wound armature. Hence, it is equal to 1, 2, 3, 4 etc. for simplex, duplex, triplex and quadruplex etc. lap - winding.
Degree of Re - entrancy of an armature winding
A winding is said to be single re - entrant, if on tracing through it once, all armature conductors are included on returing to the starting point. It is double re - entrant if only half the conductor are included in tracing through the winding once and so on.
In such windings, there are several sets of completely closed and independent windings. If there is only one set of closed winding, it is called simplex wave windings. If there are two such windings on the same armature, it is called duplex winding and so on. The multiplicity affects a no .of parallel paths in the armature. For a given no. of armature slots and coils, as the multiplicity increases, the no. of parallel path in the armature increases thereby increasing the current rating but decreasing the voltage rating.