Types of Armature Windings

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.

1.1) 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(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(b).

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

 1.2) Full Pitch and Short Pitch Winding

       As seen earlier, one pole pitch is electrical. The value of 'n', slots per pole indicates how many slots are contributing 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 No.10 such that two slots No.1 and No.10 are one pole pitch or n slots or 180^o electrical apart, the coil is called full pitch coil. 

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

1.2.1 Coil Span

       It is the distance on the periphery of the armature between two coil sides of a coil. It is usually expressed interms of number of slots or degrees electrical. So if coil span is 'n' slots or 180^o electrical the coil is called full pitch coil. This is shown in the Fig. 2.

Fig. 2  Full Pitch Coil

       As against this if coils are 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. 1 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. 3.

Fig. 3 Short Pitch Coil

1.2.2 Advantages of Short Pitch Coils

       In actual practice, short pitch coils are used as it has 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 e.m.f. Hence waveform of an induced e.m.f. 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 minimised. This increases the efficiency.

1.3 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 no. 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 slot per pole per phase empty then the winding is called concentrated winding.

Note : 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.

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

Example 1. : Draw the developed diagram for full pitch armature winding of a three phase, 4 pole, 24 slots alternator. Assume single layer winding and of distributed type. 

Note : This example will explain all the winding terminologies discussed earlier.

 Solution : P = 4, 24 slots , 3 phase

.^..              n = Slots per pole = 24/4 = 6

                m = Slots per pole per phase = n/3 = 6/3 = 2

                β  = Slot angle = 180^o /n = 180^o /6 = 30^o

       Full pitch coils are to be used so if phase 1 say R is started in slot 1, it is to be connected to a coil in slot 7 so that coil span will be 6 slots i.e. 'n' slots i.e. 1 pole pitch.

       As distributed winding is to be used, both the slots per pole per phase (m = 2) available are to be used to place the coils. And all coils for one phase are to be connected in series. So from slot No.7 we have to connect it to coil slot No.2 and slot No.2 second end to slot No.8 and so on. After finishing all slots per phase available under first pair of pole, we will connect coil to slot No.13 under next pole ans winding will be repeated in similar fashion. The starting end R_s and final end R_f winding for R-phase are taken out finally. Connections for R-phase only are shown in the Fig. 3.

Fig. 3  Developed winding diagram for 'R' phases

       Now, we want to have a phase difference of 120^o between 'R' and 'Y'. Each slot contributes 30^o as β = 30^o . So start of 'Y' phase should be 120^o apart from start of 'R' i.e. 4 slots away from start of R. So start of 'Y' will be in slot 5 and will get connected to slot No.11 to have full pitch coil. Similarly start of 'B' will be further 120^oapart from 'Y' i.e. 4 slots apart start of 'Y' i.e. will be in slot No.9 and will continue similar to 'R'. Finally all six terminals of three sets will be brought out which are connected either in star or delta to three ends R,Y and B outside to get three phase supply. The entire winding diagram with star connected windings is shown in the Fig. 4.

Fig. 4  Winding diagram for all three phases