# 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: $\frac{Q_s}{p} = \frac{9}{4} = 2.25$

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