Effect of slot configuration and airgap and magnet thicknesses on rotor electromagnetic loss in surface PM synchronous machines

This paper presents a systematic study of the effect of slotting, i.e. number of slots and slot opening; and airgap and magnet thicknesses on rotor electromagnetic losses in surface magnet synchronous generators. All machines have the same overall diameter, stator bore diameter and active length. They were all designed to have approximately the same emf; the magnet thickness was adjusted to achieve this. All machines were also assumed to output the same sinusoidal current, at the same power factor. Rotor losses were calculated using both analytical and transient rotating mesh finite element analysis methods. The results are presented in the paper in a normalised fashion showing losses per unit surface area of the rotor surface versus s /λ and g /λ, where s, g, λ denote slot-opening, total gap between hub and stator bore, and slot pitch. In addition to providing an insight into the effect of slotting and gap geometry on losses, such graphs may be used to make quick estimates of rotor losses. The validity of this approach is investigated by comparison of estimates of losses from the normalised data with those calculated using FEA for machines of different sizes and number of slots.