Main Parameters of Armature Winding

Pole Pitch Definition

Pole pitch is defined as the peripheral distance between the center of two adjacent poles in a DC machine. This distance is measured in terms of armature slots or armature conductors that come between two adjacent pole centers.

Pole Pitch is equal to the total number of armature slots divided by the total number of poles in the machine.

For example, if there are 96 slots on the armature periphery and 4 poles, the number of armature slots that come between two adjacent pole centers would be 96/4 = 24. Hence, the pole pitch of that DC machine would be 24.

Hence pole pitch is equal to total numbers of armature slots divided by total numbers of poles, we alternatively refer to it as armature slots per pole.

Coil Span Definition

Coil span (also known as coil pitch) is defined as the peripheral distance between the two sides of a coil, measured by the number of armature slots between them. It indicates how many slots apart the two sides of the coil are placed on the armature.

If the coil span is equal to the pole pitch, then the armature winding is said to be full – pitched. In this situation, two opposite sides of the coil lie under two opposite poles.

Hence emf induced in one side of the coil will be in a 180o phase shift with emf induced in the other side of the coil. Thus, the total terminal voltage of the coil will be nothing but the direct arithmetic sum of these two EMFs.

If the coil span is less than the pole pitch, then the winding is referred to as fractional pitched. In this coil, there will be a phase difference between induced EMFs on two sides, less than 180o. Hence resultant terminal voltage of the coil is the vector sum of these two EMFs and it is less than that of the full-pitched coil.

In practice, coil span as low as eight-tenths of a pole pitch is used without significantly reducing EMF. Fractional-pitched windings are used to save copper in end connections and improving commutation.

Full-Pitched Winding

A full-pitched winding has a coil span equal to the pole pitch, resulting in induced EMFs that are 180 degrees out of phase, summing directly.

Fractional-Pitched Winding

A fractional-pitched winding has a coil span less than the pole pitch, leading to a phase difference of less than 180 degrees and a vector sum of EMFs.

Commutator Pitch Definition

Commutator pitch is defined as the distance between two commutator segments connected to the same armature coil, measured in commutator bars or segments.

Single Layer Armature Winding

We place armature coil sides in the armature slots differently. In some arrangement, each one side of an armature coil occupies a single slot.

In other words, we place one coil side in each armature slot. We refer to this arrangement as single-layer winding.

Two Layer Armature Winding

In other types of armature winding, arrangement two coil sides occupy every armature slot; one occupies the upper half, and another one occupies the lower half of the slot.We so place the coils in two layers winding that if one side occupies upper half, then another side occupies the lower half of some other slot at a distance of one coil pitch away.