Types of Armature Windings

Types of Armature Windings

In general armature winding is classified as,

i) Single layer and double layer winding

ii) Full pitch and short pitch winding

iii) Concentrated and distributed winding.

Let us see the details of each classification.

1. Single Layer and Double Layer Winding

If a slot consists of only one coil side, winding is said to be single layer. This is shown in the Fig. 1.11.1 (a).

While there are two coil sides per slot, one at the bottom and one at the top the winding is called double layer as shown in the Fig. 1.11.1 (b).

A lot of space gets wasted in single layer hence in practice generally double layer winding is preferred.

2. Full Pitch and Short Pitch Winding

As seen earlier, one pole pitch is 180° electrical. The value of 'n', slots per pole indicates how many slots are contributing 180° electrical phase difference. So if coil side in one slot is connected to a coil side in another slot which is one pole pitch distance away from first slot, the winding is said to be full pitch winding and coil is called full pitch coil.

For example in 2 pole, 18 slots alternator, the pole pitch is n =  18/2 = 9 slots. So if coil side in slot No. 1 is connected to coil side in slot No. 10 such that two slots No. 1 and No. 10 are one pole pitch or n slots or 180° electrical apart, the coil is called full pitch coil.

Here we can define one more term related to a coil called coil span. 

a. Coil Span

It is the distance on the periphery of the armature between two coil sides of a coil. It is usually expressed in terms of number of slots or degrees electrical. So if coil span is 'n' slots or 180° electrical the coil is called full pitch coil. This is shown in the Fig. 1.11.2.

As against this if coils are used in such a way that coil span is slightly less than a pole pitch i.e. less than 180° electrical, the coils are called, short pitched coils or fractional pitched coils. Generally coils are shorted by one or two slots.

So in 18 slots, 2 pole alternator instead of connecting a coil side in slot No.l to slot No.10, it is connected to a coil side in slot No.9 or slot No. 8, coil is said to be short pitched coil and winding is called short pitch winding. This is shown in Fig. 1.11.3.

b. Advantages of Short Pitch Coils

In actual practice, short pitch coils are used as it has following advantages :

a) The length required for the end connections of coils is less i.e. inactive length of winding is less. So less copper is required. Hence economical.

b) Short pitching eliminates high frequency harmonics which distort the sinusoidal nature of e.m.f. Hence waveform of an induced e.m.f. is more sinusoidal due to short pitching.

c) As high frequency harmonics get eliminated, eddy current and hysteresis losses which depend on frequency also get minimised. This increases the efficiency.

3. Concentrated and Distributed Winding

In three phase alternators, we have seen that there are three different sets of windings, each for a phase. So depending upon the total number of slots and number of poles, we have certain slots per phase available under each pole. This is denoted as'm'.

m = Slots per pole per phase = n/number of phases

= n/3 (generally number of phases is 3) 

For example in 18 slots, 2 pole alternator we have

n = 18/2 = 9

and m = 9/3 = 3

So we have 3 slots per pole per phase available. Now let 'x' number of conductors per phase are to be placed under one pole. And we have 3 slots per pole per phase available. But if all 'x' conductors per phase are placed in one slot keeping remaining 2 slots per pole per phase empty then the winding is called concentrated winding.

Key Point : So in concentrated winding all conductors or coils belonging to a phase are placed in one slot under every pole.

But in practice, an attempt is always made to use all the'm' slots per pole per phase available for distribution of the winding. So if 'x' conductors per phase are distributed amongst the 3 slots per phase available under every pole, the winding is called distributed winding. So in distributed type of winding all the coils belonging to a phase are well distributed over the 'm' slots per phase, under every pole. Distributed winding makes the waveform of the induced e.m.f. more sinusoidal in nature. Also in concentrated winding due to large number of conductors per slot, heat dissipation is poor.

Key Point : So in practice, double layer, short pitched and distributed type of armature winding is preferred for the alternators.

Review Question

1. Discuss the comparison between the following in an alternator :

i) Single layer and double layer windings

ii) Full-pitch and short pitch coils

iii) Concentrated and distributed winding.