Armature winding | Pole Pitch – coil pitch – front pitch – back pitch

Armature winding is one of the most important parts of the DC Generator. In armature winding, you have to be clear with several terms like coil sides, coil pitch, pole pitch, front pitch, back pitch, commutator pitch, etc. Without a proper understanding of these terms, it is not possible to efficiently design the winding.

So, in this section, we will be discussing the terms used in the armature winding of a DC Generator. Take a look at the armature winding in an alternator.

Terms used in the armature winding

The following terms are used in connection with the armature winding.

Conductor

The conductor is an individual wire lying in the magnetic field. By relative motion of the conductor, the field induces an emf in it. It may have one or two or more parallel strands. In the following figure, AB and CD are the conductors.

Turn

A turn has two conductors in series lying in the magnetic field. In other words, a conductor in one slot is connected to a conductor in another slot, it forms a turn.

Coil

The above said two conductors AB and CD along with their end connections constitute a coil or winding. It may be a single turn coil or multi-turn coil. Single turn coils have only two conductors. Whereas the multi-turn coil will have many conductors. The figure below (b) shows 3 turn coil, which has 3 conductors on each coil side.

Coil Side

Each coil, either single turn or multi-turn, has two sides called the coil sides. A conductor can also be called a coil side. AB and CD are said to be the two coil side of the coil. A coil group may have one or more single coils. The number of coils are arranged in coil groups called the winding.

Front end and Back end connector

A wire that connects the end of a coil to a commutator segment is called a front end connector. The back end connector is a wire that connects one coil side to the other side of the coil. Back connections are opposite to that of the commutator.

Pole Pitch

Pole pitch is nothing but the peripheral distance between two adjacent poles. It is measured in terms of armature slots or armature conductors. It can also be defined as the number of armature slots(or armature conductors) per pole. Let there be 48 conductors and 4 poles in a dc generator, then the pole pitch is 48/4 = 12.

Coil Pitch or Coil span

It is the angular distance between two sides of a coil, measured in terms of armature slots. For example, if there are 36 slots and 4 poles, then the coil pitch is 36/4 = 9 slots.

If the coil pitch is equal to the pole pitch, then it is called a full pitch coil. For a full pitch coil, each coil side lies under the opposite pole as shown in the figure below. It means the coil span is 180 electrical degrees. If the coil pitch is less than the pole pitch, it is said to be short pitch coil or fractional pitch coil.

Pitch of a winding or winding pitch

Winding pitch is defined as the distance between two successive conductors which are directly connected together around the armature. It is the beginning of two successive coil sides and is denoted by Y. For lap winding, Y = Y_b – Y_f and for wave winding, Y = Y_b + Y_f.

Back Pitch

The distance at which a coil advances on the back of the armature is called back pitch, denoted by Y_b. It can also be defined as the distance between the first and the last conductors of a coil. It is the same as coil span and is shown in the below figure(a) and (b).

Front Pitch

It is the distance between the second conductor of one coil and the first conductor of the next coil. Both the coils should be connected to the same commutator segments on the front, as shown in the figure (a) and (b) below. It is denoted by Y_f.

Resultant Pitch

It is the distance between the beginning of one coil and the beginning of the next coil to which it is connected. It is illustrated in the above figure (a) and (b), denoted by Y_r.

Commutator Pitch

It is the distance between the Commutator segments to which the two ends of a coil are connected. From the figure, you can observe, for lap winding, Commutator pitch (Y_c) is the difference of back pitch(Y_b) and front pitch(Y_f). For wave winding, it is the sum of the back pitch and front pitch.

Calculation of pitch values

Let us find the different pitch values for a two layer armature winding shown below.

In the above diagram, the coil sides or conductors are numbered as 1, 2, 3, 4, etc for our calculations. The top layer is numbered as 1, 3, 5, 7, 9, 11 and the bottom layer is numbered as 2, 4, 6, 8, 10, 12.

To calculate the back pitch of an armature winding, consider the coil side 3 on the upper layer, which connects to the other side of the coil 10 on the bottom layer.

Back pitch, Y_b = 10 – 3 = 7

To calculate the front pitch, consider the conductor 8, which is the second conductor of the first coil and conductor 3, which is the first conductor of the next coil.

Front pitch, Y_f = 8 – 3 = 5

winding pitch for lap winding, Y = Y_b – Y_f = 7 – 5 = 2

winding pitch for wave winding, Y = Y_b + Y_f = 7 + 5 = 12

To find the resultant pitch, consider the beginning of conductors 5 and 7, Y_r = 7 – 5 = 2

The value of commutator pitch is similar to the winding pitch values.

Commutator pitch for lap winding, Y = Y_b – Y_f = 7 – 5 = 2

For wave winding, Y = Y_b + Y_f = 7 + 5 = 12