Consider a AC machine of 18 slots,2 poles and 3-phase.
So the value of n=18/2=9 and m=9/3=3.b=180/9=20 degrees.
So we have 9 slots per pole and 3 slots per pole per phase.
R phase will occupy 3 slots of each pole. So there will be 6 coil sides-3 coils of R phase. Similarly 3 coils of Y phase and B phase.
Now coil span factor is a factor by which the coil is shorted.Ideally
But when we short the coil by one slot, we have
1--9,2--10,3--11. So we have shorted by 20 degress. Alpha = 20 degrees.
This is what is short pitch coil.