Why short pitch winding is used?

Answer 1

In AC electrical machines, to produce a rotating magnetic field, distributed winding is used at stator pheripheri. These windings are of two types, single-layer and double-layer. In a double-layer diamond winding, the slot is divided into an upper and a lower part, and there is one coil side in each half slot. The coil side at the bottom of the slot belongs to the bottom layer of the slot, and the coil side adjacent to the air gap belongs to the upper layer.

A double-layer diamond winding is constructed like the single-layer winding. There are two zone rings, the outer illustrating the bottom layer and the inner the upper layer. The distribution of zones does not have to be identical in the upper and bottom layers. The zone distribution can be shifted by a multiple of the slot pitch. A single zone shift equals a single slot pitch. By comparing the width of the coil with the coil span of the winding, we can see that the coil is now one slot pitch narrower; the coil is said to be short pitched. Because of short pitching, the coil end has become shorter, and the copper consumption is thus reduced. On the other hand, the flux linking the coil decreases somewhat because of short pitching, and therefore the number of coil turns at the same voltage has to be higher than for a full-pitch winding. The short pitching of the coil end is of more significance than the increased number of coil turns, and as a result the consumption of coil material

decreases.

Short pitching also influences the harmonics content of the flux density of the air gap. A correctly short-pitched winding produces a more sinusoidal current linkage distribution than a full-pitch winding. In a salient-pole synchronous generator, where the flux density distribution is basically governed by the shape of pole shoes, a short-pitch winding produces a more sinusoidal pole voltage than a full-pitch winding.