This is another class of winding which depends on the value of m.
Note : When the value of m is non-integer i.e. fraction then the winding is called fractional slot winding.
If S is the number of slots then the value of m is selected as,
m = S / 3P .......... for fractional slot winding
where S must be divisible by 3. Thus slots per phase is integral.
While the pole pitch i.e. S/P is also fractional. Thus coil span can
not be full pitch.
When m is integer then the basic unit under a pole pair N and S
keeps on repeating n the winding for any number of pole pairs. In
fractional slot winding, in order to construct a basic unit, the ratio
S/3P is further reduced to irreducible fraction by cancelling out the
highest common factor from S and P. Thus,
m = S/3P = S'/3P'
The coil sides under P' poles from a basic unit which repeats in the winding for P/P' times.
In double layer winding, phase grouping of coil sides for bottom
layer is same as the phase grouping of coil sides for top layer where
corresponding coil sides are located one coil span away.
The steps for designing fractional slot winding are,
1. Calculate the slot angle which is also fractional.
2. Starting with 0o, calculate the angle for serially arranged slots. If angle exceeds 180o, subtract 180o.
3. The phase group R is located for 0 angle ∟60o
The phase group B is located for 60o angle ∟120o
The phase group Y is located for 120o angle ∟180o
The ordering of phase group is RB' YR' BY'.
4. The starting points of the phases are located at 0o, 60o, and 120o.
1.1 Advantages of Fractional Slot Winding
The various advantages of fractional slot winding are,
1. Thought appear to be complicated, easy to manufacture.
2. The number of armature slots need not be integral multiple of number of poles.
3. The number of slots can be selected for which notching gear is available.
4. There is saving in machine tools.
5. High frequency harmonics are considerably reduced.
6. The voltage waveform available is sinusoidal in nature.
