Integral slot winding & fractional slot winding of AC machine

1. Integral slot winding-The value of slots per pole per phase decides the class of the winding.

m= Slots/Pole/Phase

Note-  When the value of m is an integer, then the winding is called integral slot winding.

Consider 2 pole, 12 slots alternator hence,

n= Slots/Pole =12/2 =6

Pole pitch= 180 degree = 6 slots

m= n/2 = 6//2 =3

As m is an integer, the type of winding is integral slot winding. This winding can be full pitch winding or short pitch winding.

Let, the winding is full pitch winding. For integral slot winding, coils of one coil group lying under one pole pair are connected in series. Thus the end of the first coil is connected to start of the next coil lying to the right of the first coil. The alternate coil groups must be reverse connected such that emf induced in them is addictive in nature.  Any slot contains the coil sides which belong to the same phase. Such a winding is shown in figure-

If the short pitch coils are used for integral slot winding then in each group of the slots per pole phase, the coil sides of different phases exist.

2. fractional slot winding- This is another class of winding which depends on the value of m.

Note-  The winding in which slots per pole per phase (m) is a fractional number is called fractional slot winding.

    In such a winding, the number of slots (S) must be divisible by 3. Thus slots per phase is an integer which is necessary to obtain symmetrical three-phase winding. But slots per pole (n) and slots per pole per phase (m) both are fractional. As n is a fraction, the coils cannot be the full pitch. Thus if there are 54 slots and 8 poles then the slots per pole n= 54/8 =6.75 hence coil span can be 7 or 6. Generally, short pitch coils are used. Such a fractional slot winding can be easily achieved with double layer winding.

    In a balanced three-phase winding, a basic unit under a pole pair (N and S) is repeated for remaining pole pairs where m is an integer. In fractional slot winding, they are reduced to an irreducible fraction by taking out the highest common factor in the number of slots and poles.

Let  S= Number of Slots
       P= Number of Poles

then for a 3 phase winding,

 The number k indicates the number of repeatable units and the number of possible parallel paths. The characteristics ratio indicates that there are Sk coils per phase distributed among Pk poles.

Thus the winding is to be considered only of Pk poles out of P poles and for other poles it is repeated.

Similarly, the winding arrangement is to be considered for Sk slots out of total S slots and for other slots it is repeated.

In a double layer winding, only the arrangement of the top layer is to be considered. This gets repeated in the bottom layer in which the corresponding coil sides are located one coil span away.

Advantages of Fractional slot winding-

The various advantages of fractional slot winding are-

1. Though appear to be complicated, easy to manufacture.
2. The number of armature slots (S) need not be an integral multiple of the number of poles (P)
3. The number of slots can be selected for which notching gear is available, which is economical
4. There is saving in machine tools.
5. High-frequency harmonics are considerably reduced.
6. The voltage waveform available is sinusoidal in nature.