Difference in Unbalanced Magnetic Force of Fractional-Slot PM Machines between Internal and External Rotor Topologie

This paper presents a comparative investigation into unbalanced magnetic force(UMF)of asymmetric permanent magnet machines without rotor eccentricities,particularly focusing on the difference between internal-and external-rotor topologies.The asymmetric field distribution results in radial and tangential asymmetric force waves.Although the radial and tangential stresses are in different direction,the UMF components they produce are nearly aligned.The UMF from asymmetric radial force wave can be additive or subtractive to that from asymmetric tangential force wave.Investigation shows that for the same pole slot number combination,if the UMFs due to radial and tangential force waves are additive in internal rotor machine,they are subtractive in the external rotor counterpart,and vice versa.Investigation reveals a general rule determining whether additive or cancelling:for a UMF produced by any two field harmonics,they are additive if the higher order is produced by the outer part outside the airgap,but cancelling if the higher order is produced by the inner part.Therefore,for a machine with pole number 2p=3k+1,they are additive if it is an external-rotor machine,but otherwise subtractive.On the other hand,for a machine with pole number 2p=3k-1,they are subtractive if it is an external-rotor machine,but otherwise additive.For the UMF due to armature reaction only,they are subtractive for external-rotor machines,but otherwise additive.The investigation is carried out by an analytical model and validated by finite element analysis.

Magnetomotive Force Harmonic Reduction Techniques for Fractional-Slot Non-Overlapping Winding Configurations in Permanent-Magnet Synchronous Machines

Compared to conventional distributed winding configurations,the fractional-slot non-overlapping(concentrated)windings exhibit advantages such as short end-winding length,high copper packing factor(particularly with segmented stator structure),low cogging torque,good field weakening capability owing to relatively large d-axis inductance,and better fault tolerant capability due to low mutual inductance.However,one of the key problems of employing concentrated windings in Permanent-magnet Synchronous Machines(PMSMs)is the high eddy-current losses in rotor magnets and/or rotor iron due to the presence of a large number of lower and higher order space harmonics in the stator Magneto-Motive Force(MMF).These MMF harmonics also result in other undesirable effects,such as acoustic noise and vibrations,and localized core saturation which tend to reduce reluctance torque.This paper reviews the current state-of-the-art of the MMF harmonic reduction techniques for concentrated winding configurations in PMSMs,including winding split and shift,delta-star connected windings,multiple 3-phase windings,multilayer windings,uneven turn numbers,and stator flux barriers.Their concepts,advantages and disadvantages are presented and assessed.

Investigation of Eccentric PM on High-frequency Vibration in FSCW PM Machine Considering Force Modulation Effect

This study investigates the negative influence of an eccentric permanent-magnet(PM)design on high-frequency electromagnetic vibration in fractional-slot concentrated-winding(FSCW)PM machines.First,an analytical expression for the sideband current harmonics was derived using the double Fourier series expansion method.Then,the characteristics of the flux-density harmonics are studied from the perspective of the space-time distribution and initial phase relationship.The influence of the eccentric PM design on high-frequency electromagnetic and concentrated forces was studied based on the electromagnetic force modulation effect.Consequently,an eccentric PM design is not conducive to reducing the 2pth-order high-frequency electromagnetic forces.Finally,two FSCW PM machines with conventional and eccentric PM designs are manufactured and tested to verify the theoretical analysis.The results show that the eccentric PM design worsens high-frequency vibrations.