Types of armature winding of an AC machine

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 = 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.

Note:-  So in the 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' 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 a distributed type of winding all the coils belonging to a phase are well distributed over the' slots per phase, under every pole. Distributed winding makes the waveform of the induced emf more sinusoidal in nature. Also in concentrated winding due to a 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- Write the scheme of connections for a 3 phase, 1 layer stator lap winding of a synchronous machine having 6 poles and 36 slots.


Solution- P=6, 36 slots, n= Slots/pole =6

m= slots/pole/phase = n/3 =2, β= 180 degree/n = 30 degree

For full pitch coils if phase 1 say R starts in slot 1 then it must be connected in slot 7 so that coil span is 6 slots i.e. 6β degree i.e. 180 degrees. Thus the coils are full pitch coils.

  For distributed winding, both slots per pole per phase available are to be used. And all coils of one phase are to be in series. So from slot 7, connect it to coil in slot 2 for lap winding and the second end of slot 2 to coil in slot 8 and so on. After finishing all slots per pole per phase available under the first pair of poles connect the coil to slot 13 under next pole and the winding will be repeated thereon in a similar fashion. The starting end Rs and final end Rf for R phase is taken out finally. The connections for the R phase are shown in figure-

There must be a phase difference of 120 degrees between R and Y. Each slot contributes 30 degrees so the start of Y phase should be 120 degrees apart from Rs i.e. 4 slots away from Rs i.e. in slot 5. Similarly, the start of the B phase is further 120 degrees away from the Y phase i.e. 4 slots away from Ys i.e. in slot 9. Finally, all six ends Rs, Rf, Ys, Yf and Bs, Bf is brought out which are connected in star or delta to complete the winding.