Armature Winding

Armature Winding Points : Armature Winding, Pole-pitch, Conductor, Coil and Winding Element, Coil-span or Coil pitch, Pitch of a Winding (Y), Pitch of a Winding (Y), Front Pitch, Resultant Pitch Now, we will discuss the winding of an actual armature. But before doing this, the meaning of the following terms used in connection with armature winding should be clearly kept in mind. 1. Pole-pitch It may be variously defined as:
(i) The periphery of the armature divided by the number of poles of the generator
i.e. the distance between two adjacent poles.
(ii) It is equal to the number of armature conductors (or armature slots) per pole.
If there are 48 conductors and 4 poles, the pole pitch is 48/4 = 12. 2. Conductor The length of a wire lying in the magnetic field and in which an e,m.f. is induced, is called a conductor (or inductor) as, for example, length AB or CD in. 3. Coil and Winding Element The two conductors AB and CD along with their end connections constitute one coil of the armature winding. The coil may be single-turn coil or multi-turn coil. A single-turn coil will have two conductors. But a multi-turn coil may have many conductors per coil side. for example, each coil side has 3 conductors. The group of wires or conductors constituting a coil side of a multi-turn coil is wrapped with a tape as a unit and is placed in the armature slot. It may be noted that since the beginning and the end of each coil must be connected to a commutator bar, there are as many commutator bars as coils for both the lap and wave windings.

4. Coil-span or Coil pitch (Y_s) It is the distance, measured in terms of armature slots (or armature conductors) 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 pole span or coil pitch is equal to the pole pitch (as in the case of coil A where pole-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.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 example, if there are 36 slots and 4 poles, then coil span is 36/4 = 9 slots. If number of slots is 35, then Y_s = 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 314th of the pole pitch), then the winding is fractional pitched.
In this case, there is a phase difference between the e.m.f.s in the two sides of the coil. Hence, the total e.m.f. round the coil which is the vector sum of e.m.f.s in the two coil sides, is less in this case as compared to that in the first case.

5. 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 distance between the beginnings of two consecutive turns.
Y = Y_B - Y_f for lap winding
Y = Y_B - Y_f for wave winding
In practical, coil-pitches as low as eight-tenths of a pole pitch are employed without much serious reduction in the e.m.f. Fractional-pitched windings are purposely used to effect substantial saving in the copper of the end connections and for improving commutation. 6 Back Pitch (Y_b) This 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 Y_B.
As element 1 is connected on the back of the armature to element 8. Hence, Y_B = (8 - 1) = 7 7. Front Pitch (Y_F) The number 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 designated by Y_F. Again in element 8 is connected to element 3 on the front of the armature, the connections being made at the commutator segment. Hence, Y_F =8 – 3 = 5.
Alternatively, the front pitch may be defined as the distance (in terms of armature conductors) 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 and wave winding are.

8. Resultant Pitch (Y_R) It is the distance between the beginning of one coil and the beginning of the next coil to which it is connected , As a matter of precaution, it should be kept in mind that all these pitches, though normally stated in terms of armature conductors, are also sometimes given in terms of armature slots or commutator bars because commutator is, after all, an image of the winding. 9. Commutator Pitch (Y_G) It is the distance (measured in commutator bars or segments) between the segments to which the two ends of a coil are connected. it is clear that for lap winding, Y is the difference of Y_B and Y_F whereas for wave- winding it is the sum of Y_b and Y_F. Obviously, commutator pitch is equal to the number of bars between coil leads. In general, Y_C 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-windings.