Slot fill factor

Ratio between the cross-sectional area of all conductors in one slot and the entire slot area.

The slot fill factor is equal to the ratio of the conductor area over the total slot area, see Figure 1. For example, a slot fill factor of 0.5 would signify that half (50%) of the slot area is occupied by the conductors (i.e. pure copper or aluminum). The other half of the slot area is occupied by conductor insulation, slot insulation, and inevitable gaps in between the conductors and between the conductors and the slot sides.

Illustration of slot fill factor

Fig. 1 Illustration of slot fill factor.

The slot fill factor depends on the insulation thickness around the conductors and the slot, as well as the conductor shape. It is usually between 0.4 and 0.6. Higher slot fill factors can be achieved with rectangular conductors instead of round conductors. Achieving a high slot fill factor also requires suitable winding equipment and special attention during manufacturing.

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