Slot Effect on the Reluctance of Air-Gap, Carter like Calculation well suited for Machines with Large Air-Gap or Surface Mounted Permanent Magnets

Slots causes an increase of the air-gap reluctance which is traditionally taken into account using the Carter’s formula. However, that formula was established under the hypothesis of a thin air-gap with respect to the tooth width. This assumption is not verified in the case of surface mounted permanent magnets (SMPM) machines because the thickness of these magnets must be considered as being a part of the air-gap. We establish in this communication a new expression under the assumption of an infinite air-gap. We compare then the errors of the two formulae for any air-gap thickness.