Types of Armature winding of an AC machine

In general armature winding is classified as,

1. Single-layer and double-layer winding
2. Full pitch and short pitch winding
3. Concentrated and distributed winding

1. Single-layer and Double-layer winding- If a slot consists of only one coil side, the winding is said to be a single layer. This is shown in figure-

While there are two coil sides per slot, one at the bottom and one at the top the winding is called Double-layer winding.

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 degrees electrical. the value of 'n', slots per pole indicates how many slots are contributing 180-degree 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 the first slot, the winding is said to be full pitch winding and coil is called full pitch coil.

  For example in 2 poles, 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 degrees electrical apart, the coil is called full pitch coil.

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

3. 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 the number of slots or degrees electrical. So if the coil span is 'n' slots or 180 degrees electrical the coil is called full pitch coil. This is shown in figure-

     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-degree electrical, the coils are called, Short pitched coils or fractional pitched coils. 

So in 18 slots, 2 pole alternator instead of connecting a coil side in slot No. 1 to slot No. 10, it is connected to a coil side in slot No. 9 or slot No. 8, the coil is said to be short pitched coil and winding is called short pitch winding. This is shown in figure-

Advantages of Short Pitch Coils-

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

1. 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.
2. Short pitching eliminates high-frequency harmonics which distort the sinusoidal nature of emf Hence waveform of an induced emf is more sinusoidal due to short pitching.
3. As high-frequency harmonics get eliminated, eddy current and hysteresis losses which depend on frequency also get minimized. This increases efficiency.

To be continued.....