Generalized Approach of Stator Shifting in Interior Permanent-Magnet Machines Equipped With Fractional-Slot Concentrated Windings

Abstract

Electrical drive systems, which include electrical machines and power electronics, are a key enabling technology for advanced vehicle propulsion systems that reduce the petroleum dependence of the ground transportation sector. To have significant effect, electric drive technologies must be economical in terms of cost, weight, and size while meeting performance and reliability expectations. Interior permanent magnet machines with fractional-slot concentrated windings have been shown to be good candidates for hybrid traction applications. One of the key challenges is the additional stator magnetomotive force sub-and superharmonic components that lead to higher losses in the rotor as well as saturation effects. This paper tries to address this issue by looking into the concept of stator shifting. The generalized concept of stator shifting in the context of the harmonic components that are targeted for cancellation is presented; the focus is on single-layer and double-layer windings that have stator space subharmonics. It is shown that the stator shifting can reduce the loss-producing harmonics on the rotor as well as help the flux weakening performance of the fractional-slot concentrated winding designs. The cancellation of the loss harmonics is introduced as a method in which a particular harmonic can be targeted as well as reduce the phase inductance of the machine allowing for more room in terms of the operating voltage at higher speed. The concept of stator shifting will be explained, and the effect of varying the shift angle on the various harmonic components and winding factors will be investigated. Various designs, arising out of single-layer and double winding layer 10-pole, 12-slot configuration (targeting the FreedomCAR specifications) with varied shift angles are evaluated. The comparison between these designs in terms of their power density, efficiency, and torque ripple is presented.

Harmonic reduction methods for electrical generation: a review

“…Above all, fractional slot windings present the significant benefit of increasing the degrees of freedom in the choice of the number of slots and the pitching length as well as the opportunity to remove certain harmonics [21]. However, this particular winding configuration does produce high sub-harmonics, leading to a reduction of the overall efficiency [29]. More recently, multi-phase armature windings have been investigated for attenuating specific harmonics on the output of field-wound SGs [24].…”

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“…It is shown in [31] that the stator slot-opening influence the amplitude of harmonics in the MMF distribution. To cancel some undesirable harmonics (e.g., the additional stator MMF sub-and super-harmonic components that lead to higher losses in the rotor as well as saturation effects) in fractional-slot machines with concentrated windings, a concept called stator shifting is investigated by [32]. This technique has been applied to a six-phases permanent-magnet machine with low space harmonics for electric vehicle application [33].…”

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“…A number of techniques, reported in the literature, to mitigate the above detrimental effects, include segmentation of magnets both axially and circumferentially [15], using different number of turns per coil side [16], and shifting of stator by mechanical angle to target specific MMF space harmonic [17]. All these techniques either increase the manufacturing cost and material wastage or impose challenges to achieve (a) higher torque density, (b) higher winding factor, and (c) ease of manufacturing.…”

“…The lower and higher order space harmonics in the stator MMF rotate at different speeds relative to the rotor, and hence, cause many undesirable effects such as localized core saturation, eddy current loss in permanent magnets, acoustic noise and vibrations [12], [13]. These space harmonics also make it difficult to obtain high reluctance torque from interior permanent magnet (IPM) machines with fractional-slot windings [14].A number of techniques, reported in the literature, to mitigate the above detrimental effects, include segmentation of magnets both axially and circumferentially [15], using different number of turns per coil side [16], and shifting of stator by mechanical angle to target specific MMF space harmonic [17]. All these techniques either increase the manufacturing cost and material wastage or impose challenges to achieve (a) higher torque density, (b) higher winding factor, and (c) ease of manufacturing.…”