Alternators:Short-pitch Winding : Pitch factor/chording factor

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Short-pitch Winding : Pitch factor/chording factor

So far we have discussed full-pitched coils i.e. coils having span which is equal to one pole-pitch i.e. spanning over 180° (electrical).

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As shown in Fig. 37.16, if the coil sides are placed in slots 1 and 7, then it is full-pitched. If the coil sides are placed in slots 1 and 6, then it is short-pitched or fractional-pitched because coil span is equal to 5/6 of a pole-pitch. It falls short by 1/6 pole-pitch or by 180°/6 = 30°. Short-pitched coils are deliberately used because of the following advantages:

1. They save copper of end connections.

2. They improve the wave-form of the generated e.m.f. i.e. the generated e.m.f. can be made to approximate to a sine wave more easily and the distorting harmonics can be reduced or totally eliminated.

3. Due to elimination of high frequency harmonics, eddy current and hysteresis losses are reduced thereby increasing the efficiency.

But the disadvantage of using short-pitched coils is that the total voltage around the coils is somewhat reduced. Because the voltages

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induced in the two sides of the short-pitched coil are slightly out of phase, their resultant vectorial sum is less than their arithmetical sum.

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Example 37.1. Calculate the pitch factor for the under-given windings : (a) 36 stator slots, 4-poles, coil-span, 1 to 8 (b) 72 stator slots, 6 poles, coils span 1 to 10 and (c) 96 stator slots, 6 poles, coil span 1 to 12. Sketch the three coil spans.

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