Winding Pitches

Winding Pitches

• Now we will consider the definitions of different types of winding pitches, required in the design of the armature winding.

1. Back Pitch

• The distance which is measured interms of armature conductors i.e. between top and bottom coil sides of a coil measured around the back of the armature i.e. away from the commutator is called back pitch and is denoted by y_ь. It is measured interms of coil sides.

Key Point: Since it is difference between odd and even number, it is always odd number.

• The size of the coil is decided by the back pitch. The back pitch is normally equal to coilsides per pole. It is shown for lap and wave type of winding in the Fig. 3.5.1.

• The back pitch for the winding shown in the Fig. 3.5.1 is y_b 14 – 1 = 13 as coilsides 1 and 14 are of coil number I.

2. Front Pitch

• It is defined as the distance measured between two coilsides which are connected to same commutator segment. It denoted by Y_. The front pitch for the winding shown in the Fig. 3.5.2 is Y_f = 14 – 3 = 11.

This is the front pitch for lap winding while for wave winding y_f = 27-14= 13. The front pitch for both lap and wave type winding is odd. If for a lap winding the connections are traced then the movement is backwards (coilside 14 to coilside 3). Hence the type of winding can be determined from front pitch and it does not affect the size of coils.

3. Winding Pitch

• It is defined as the distance between the starts of two consecutive coils measured in terms of coilsides. It is denoted by Y.

For Lap winding, Y = Y_b-Y_f

For Wave winding, Y = Y_b +Y_f

• For the lap winding shown in the Fig. 3.5.1 the winding pitch is Y = 13 - 11 = 2 while for wave winding Y = 13+ 13 = 26. It is also the distance round the armature between two consecutive conductors. Practically, coil pitches or winding pitches as low as eight tenths of a pole pitch are employed without much reduction in e.m.f.

Key Point: Fractional pitched windings are intentionally used so as to achieve saving in copper of the end connections and for improvement in commutation.

4. Commutator Pitch

• It is defined as the distance between the two commutator segments to which the two ends i.e. starts and finish of a coil are connected. It is measured in terms of commutator segments and it is denoted by yc. For the windings shown in the Fig. 3.5.1.

У_с = 2 – 1 = 1 for simplex lap winding

Y_c = 12 – 1 = 11 for simplex wave winding

• For lap winding y_c, is the difference between Y_b and y_f while for wave winding y_c, is the sum of Y_b and y_f. The number of bars between the coil leads is commutator pitch. In general y_c, is the 'plex' of lap wound armature. It is 1, 2, 3 for simplex, duplex, triplex lap windings respectively.

• On tracing through simplex lap winding, a segment adjacent to the first one is reached after traversing through one coil. For simplex wave winding, the adjacent segment is reached after traversing through P/2 coils.

Review Question

1. Define and explain back pitch, front pitch, commutator pitch winding pitcg for armature winding.