# Reduction of coil span

This value expresses the reduction of the coil span compared to full pitch. The higher the reduction of coil span, the shorter the end-turns of your winding.

In the Emetor winding calculator, the reduction of coil span compared to full pitch is expressed in number of slot pitches.

The reduction of coil span can be expressed as: $$\frac{Q_s}{p} - \mbox{coil span,}$$ where $\frac{Q_s}{p}$ is the coil span for full pitch.

Single-layer integer-slot windings have always a coil span that is equal to full pitch, ie. the reduction of coil span gets zero. For example, the 2-pole 6-slot winding in Fig. 1 has a coil span of 3, a full pitch of $\frac{Q_s}{p} = \frac{6}{2} = 3$, and consequently a reduction of coil span of 0:

 A c B a C b

Fig. 1 Single-layer 2-pole 6-slot integer-slot winding with a winding factor of 1.0.

In order to reduce the coil span, which has the advantage of reducing the length of the end-turns in your winding, a two-layer winding becomes necessary, see Fig. 2. In this case, the 2-pole 6-slot winding has a coil span of only 2, an unchanged full pitch of $\frac{Q_s}{p} = \frac{6}{2} = 3$, and consequently a reduction of coil span of 1 slot pitch:

 A c B a C b b A c B a C

Fig. 2 Double-layer 2-pole 6-slot integer-slot winding with reduced coil span and a winding factor of 0.866.

Fractional-slot windings and concentrated windings have full pitches that are not integers, as opposed to integer-slot windings. In addition, concentrated windings have always a coil span of one, due to their nature of being wound around a single tooth.
Examples:
Full pitch of 2-pole 9-slot fractional-slot winding: $\frac{Q_s}{p} = \frac{9}{2} = 4.5$
Full pitch of 4-pole 9-slot concentrated winding: $\frac{Q_s}{p} = \frac{9}{4} = 2.25$