Types of DC Generator Armature Winding

TYPES OF ARMATURE WINDING

There are two methods of placing the armature conductors in the slots namely:

(i) Single layer winding

(ii) Double layer winding

In single layer winding, only one conductor or coil side is placed in each armature slot. Such a winding is not much used.

In double layer winding, there are two conductors or coil sides placed in each slot.

According to the way of connecting the conductors, armature winding has basically two types namely:

1. Lap winding,

2. Wave winding

Both the winding techniques are explained below.

1. Lap Winding

In this case, if connection is started from conductor in slot 1 then connections overlap each other as winding proceeds till starting point is reached again.

Developed view part of the armature winding in lap fashion is shown in the Fig. 2.5

As seen from the Fig. 2.5 there is overlapping of coils. While proceeding, due to such connection, the total numbers of conductors get divided into P number of parallel paths, where P= number of poles in the machine. Large number of parallel paths indicate high current capacity of machine, hence lap winding is preferred for high current rating generators.

2. Wave Winding

In this type of connection, winding always travels ahead avoiding overlapping. It travels like a progressive wave hence called wave winding.

A part of armature winding in wave fashion is shown in Fig. 2.6.

Both coils starting from slot 1 and slot 2 are progressive in wave fashion.

Due to this type of connection, the total number of conductor get divided into two number of parallel paths always, irrespective of number of poles of the machine. As number of parallel paths are less, it is preferable for low current, high voltage capacity generators.

The number of parallel paths in which armature conductors are divided due to lap or wave fashion of connection is denoted as A. So A=P for lap connection and A=2 for wave connection.

Winding Terminologies

1. Conductor

It is the actual armature conductor which is under the influence of the magnetic field, placed in the armature slot.

2. Turn

The two conductors placed in different slots when connected together, forms a turn, while describing armature winding the number of turns may be specified from which, the number of conductors can be decided.

 Z=2×Number of turns.

3. Coil

For simplicity of connections the turns are grouped together to form a coil. If coil contains only one turn it is called single turn coil while coil contains more than one turn is called multi turn coil.

4. Commutator Pitch (Yc)

The commutator pitch is the number of commutator segments spanned by each coil of the winding. It is denoted by Yc.

In Fig. 2.9 (a), one side of the coil is connected to the commutator segment 1 and the other side connected to commutator segment 2. Therefore the number of commutator segments spanned by the coil is 1 i.e., Yc = 1.

In Fig. 2.9 (b), one side of the coil is connected to commutator segment 1 and the other side to commutator segment 8. Therefore, the number of commutator segments spanned by the coil =8‒1=7 segments. i.e., Yc=7. The commutator pitch of winding is always a whole number, since each coil has two ends and as two coil connections are joined at each commutator segments,

Number of coils = Number of commutator segments.

For example, if an armature has 30 conductors, the number of coils will be 30/2 = 15. Therefore, number of commutator segments is also an important factor in determining the type of DC armature winding.

5. Pole pitch

The distance between the centers of two adjacent poles is called pole pitch. One pole pitch equals to 180 electrical degrees. It is also defined as the number of slots per pole.

Thus if a 4‒pole generator has 16 coils, then number of slots = 16

.. Pole pitch = 16/4= 4 slots.

6. Coil span

The distance between the two coil sides of a coil is called coil span. It is expressed in electrical degrees or in number of slots.

7. Full Pitched winding

If the coil span is equal to pole pitch, the winding is called as full pitched winding.

8. Short Pitched Winding

If the coil span is less than the pole pitch, the winding is called as short pitched or short chorded winding.

For example, the pole pitch for a 4‒pole, 24 slot machine is 24/46 slots. If the coil sides of one coil are placed in slots 1 and 7 as shown in Fig. 2.10, then it is full pitched winding. If they are placed in 1 and 6, then it is short pitched winding. (short chorded by one slot i.e., 180/6 = 30 electrical degrees).

9. Back pitch (Y_B)

It is the distance measured in terms of armature conductors between the two sides of coil at the back of the armature (see Fig. 2.11). It is denoted by Y_B. For example if a coil is formed by connecting conductor 1 (Upper conductor in a slot) to conductor 12 (bottom conductor in another slot) at the back of the armature, then back pitch is Y_B = 12−1 = 11 conductors.

10. Front Pitch (Y_F)

It is the distance measured in terms of armature conductor between the coil sides attached to any one commutator segment (see Fig. 2.11). It is denoted by Y_F. For example, if coil side 12 and coil side 3 are connected to the same commutator segment, then front Side pitch is Y_F = 12‒3 = 9 conductors.

11. 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. (See Fig. 2.11) It is dented by Y_R. Therefore the resultant pitch is the algebraic sum of the back and front pitches.