Armature Winding Pole Pitch Coil Pitch Commutator Pitch

Before going to the actual discussion, we need to know some related terms of the armature winding.

Pole Pitch

It is the distance on the armature periphery between two adjacent poles. We usually measure it in terms of armature slots per pole. That means, if a DC generator has four magnetic poles and it has 48 armature slots, then slots per pole of the generator will be 12. Hence, this 12 is nothing but pole pitch of the generator.

pole pitch

Conductor

The conductor is the portion of the coil wire which comes under the direct effect of the magnetic field. As we know that some wire loops make a coil. The entire circuit cannot come under the influence of the magnetic field. Only the side portion of the loop comes under the direct effect of the magnetic field. We call this portion of the loop as the conductor. For example, in the figure below, ABCD is a loop. So, the AB and CD portion of the loop are conductors.

coil conductor

Coil

A coil may constitute multiple turns of copper wire. Again each turn in the coil forms a loop. On the other hand, each loop constitutes two conductors. A coil may be a single turn or multi-turn. Considering single turn or single loop coil, there are only two conductors in the coil. But in the multi-turn coil, there is a number of loops, and hence there are numbers of conductors per coil side. All insulated conductors of a coil side are wrapped together and placed in a single armature slot.

armature coil

Coil Side

Likewise, the conductor, the portion of the coil comes under the direct effect of the magnetic field is the coil side.

coil side

Coil Span or Coil Pitch

Coil span or coil pitch is the distance between two coil sides of a single coil. Similar to the pole pitch, we measure coil pitch in terms of armature slots. After placing one coil side in an armature slot, the other coil side is placed in another armature slot depending upon the design criteria of the machine. The number of armature slots in between these two coil sides is the pitch of the coil.

coil span or coil pitch

Full Pitch Coil

If the coil pitch is precisely equal to the pole pitch in terms of armature slots, then the coil is said to be a full pitched coil. In this case, if one side of the coil comes in front of the north pole, then the other side comes in the front of the adjacent south pole. Hence, the induced EMF is maximum in both of the coil sides at the same time. That means EMFs are in phase. So, the resultant EMF per coil will be a simple sum of the EMF of both coil sides.

Short Pitch Coil

When the coil pitch is smaller than the pole pitch, we refer the coil as a short pitch coil.  Consequently, there will be a phase difference between the EMF of both coil sides. Resultant EMF of the coil would be the vector sum of the EMF of both coil sides. We also call a short pitch coil as a fractional pitch coil. The resultant EMF of a fractional pitch coil is always less than that of a full pitch coil. To improve commutation in the machine we intentionally use fractional pitch coil.

Pitch of a Winding

Pitch of a winding is the distance between two successive conductors which are directly connected.

back pitch front pitch resultant pitch

Back Pitch

Back Pitch is measured in the term of armature conductors. The coil advances on the back of the armature. We call this advancement as back pitch and denote it with YB. In other words, this is the distance between two sides of a coil.

Front Pitch

The front pitch is the distance between the right coil side of one coil to the left coil side of the adjacent coil. Usually, we denote the front pitch with YF.

Resultant pitch

The resultant pitch is the distance between the right coil side of two adjacent coils or the left coil side of two adjacent coils. We denote the resultant pitch with YR.

Commutator Pitch

The commutator pitch is the distance between two commutator segments connected to two sides of a single coil. We measure it in terms of commutator segments. We denote the commutator pitch with YC.

Single Layer Armature Winding

This type of winning is one of the simplest forms of the armature winding. In this type of armature winding, we place each coil side in a single armature slot.

Double Layer Armature Winding

In double layer armature winding, each armature slot contains two coil sides one above other. Here, we place one side of a coil in the lower half of the slot, and the other side of this coil in the upper half of another slot.

Types of Armature Winding

There are two types of armature winding. One is wave winding, and another is lap winding.  We shall discuss wave winding and lap winding in different articles in details.

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