Types of Armature Winding

Types of Armature Winding

• We have seen that there are number of armature conductors, which are connected in specific manner as per the requirement, which is called armature winding. According to the way of connecting the conductors, armature winding has basically two types namely, a) Lap winding, b) Wave winding.

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 of part of the armature winding in lap fashion is shown in the Fig. 3.4.1.

•As seen from the Fig. 3.4.1, there is overlapping of coils while proceeding.

Key Point: Due to such connection, the total number 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. To get an idea of wave winding a part of armature winding in wave fashion is shown in the Fig. 3.4.2.

• Both coils starting from slot 1 and slot 2 are br progressing in wave fashion.

Key Point: Due to this type of connection, the total number of conductors 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.

3. Comparison of Lap and Wave Type Windings

4. Winding Terminologies

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

b) 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

c) 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 with more than one turn is called multiturn coil.

Hence if number of coils, alongwith number of turns per coil are specified, it is possible to determine the total number of turns and hence total number of armature conductors 'Z' required to calculate generated e.m.f.

d) Pole-pitch: The distance between the two adjacent poles is called a pole pitch. It is measured interms of number of slots. Thus total slots along the periphery of armature divided by the total number of poles is called a pole pitch.

          Pole pitch = Number of armature slots / P

If armature bore diameter is given then the pole pitch can be obtained as,

Pole pitch = π × Вore diameter / P

Thus expressing bore diameter in metres, the pole pitch in metres can be obtained.

e) Coil side: Two coil sides form a coil. The group of conductors on one side of a coil forms one coil side while the group of conductors on other side of a coil forms second coil side. A coil is formed by joining such two coil sides placed in two different slots.

f) Coil span: The distance between the two coil sides which are joined together to form a coil is called coil span. It is generally measured in terms of number of slots. If the coil span is equal to one pole pitch then the coil is called full pitch coil. If coil span is less than one pole pitch then the coil is called short pitched or short chorded coil.

5. Single Layer and Double Layer Winding

• Basically there are two physical types of the windings. These are i) Single layer winding ii) Double layer winding. The sequential arrangement of coils around the armature is different for both these types of windings.

a. Single Layer Winding

• In this type of winding, the complete slot is containing only one coil side of a coil. This type of winding is not normally used for machines having commutators. It is shown in the Fig. 3.4.5.

• In single layer windings permit the use of semi enclosed and closed types of slots. Also the coils can be pushed through the slots from one end of the core and are connected during the process of windings at the other end. Here the insulation can be properly applied and consolidated which is advantageous in large output machines with high voltage.

• The single layer windings used in high voltage machines use small groups of concentrically placed coils. The interlinking between these coils is in such a way so as to minimize the space taken up outside the slot and in the overhang connections.

b. Double Layer Winding

• It is shown in the Fig. 3.4.6. It consists of identical coils with one coilside of each coil in top half of the slot and the other coilside in bottom half of another slot which is nearly one pole pitch away.

• In the Fig. 3.4.6 (a) there are two coilsides per slot while in Fig. 3.4.6 (b) there are eight coilsides per slot. Each layer may contain more than one coil side if large number of coils are required. For placing double layer windings, usually open slots are used.

Advantages of Double Layer Winding

The double layer winding has following advantages,

1. It provides neat arrangement as all coils are identical.

2. Greater flexibility can be achieved with double layer winding as coil span can be easily selected.

6. Closed and Open Windings

• Armature windings are classifed into two different types namely i) Closed type winding ii) Open type winding.

a. Closed Type Winding

• In this type of winding, a closed path is formed around the armature. The starting point of the winding is reached again after passing through all the turns. The current passing through closed type winding is through brushes placed on commutator. The commutator segments are connected to various armature coils.

.• The armature current gets divided into different parallel paths. The current flowing through the coil changes continuously but from brush side the winding view remains same and polarity is maintained which is in effect due to use of commutator segments.

• The closed type of winding is normally used in a.c. and d.c. commutator machines. This type of winding is usually double layer.

b. Open Type Winding

• In case of a.c. machines, commutator is not used and hence closed winding is not required to be used. In such cases open type winding is used. The armature is left open at one or more points.

. • The ends of each section of the winding can be brought at the terminals to do the required type of interconnection externally. The open type of winding is preferred over closed type as it gives better flexibility in design and freedom of connections.

• These type of windings are either single layer type or double layer type and are mainly used in induction machines and synchronous machines.

Review Questions

1. What are the ordinary types of armature winding for d.c. machine? Explain the essential difference between them and give relative merits and the applications of the two types windings.

2. Distinguish between lap and wave windings.

3. Define the following terminology of armature winding of a d.c. machine: i) Conductor ii) Turn iii) Coil and coil side iv) Coil span.