J. T. Shi

Influence of Stator and Rotor Pole Arcs on Electromagnetic Torque of Variable Flux Reluctance Machines

The influence of stator and rotor pole arcs on electromagnetic torque of variable flux reluctance machines (VFRMs) having different stator and rotor pole combinations is investigated. Different from 6/4 stator/rotor pole switched reluctance machines, which usually employ equal stator pole arc and stator slot opening, unequal stator pole arc and slot opening can boost the torque density of the VFRM. Moreover, the optimal rotor pole arc to rotor pole pitch ratio in VFRMs is \(\sim ~1/3\) and the optimal stator pole arc is equal to or slightly smaller than the optimum rotor pole arc. For a 6-pole stator, the 4-, 5-, 7-, and 8-pole rotor VFRMs with the optimal stator and rotor pole arcs can increase the average torque by 1%, 4%, 30.7%, and 72.9% compared with those having equal stator pole arc and stator slot opening. In addition, the 6/7 stator/rotor pole VFRM exhibits the largest torque density. Prototype machines with optimal stator and rotor pole arcs are manufactured and measured to validate the analysis.

Novel Doubly Salient Permanent Magnet Machines With Partitioned Stator and Iron Pieces Rotor

In this paper, novel partitioned stator doubly salient permanent magnet (PS-DSPM) machines with separated PM excitation and armature windings are proposed. Compared with conventional DSPM machines with single stator in which PMs are inserted in the yoke and windings are arranged on the teeth, the proposed PS-DSPM machine has two stators with PMs and windings located separately. Two sets of PS-DSPM machines, i.e., PS-DSPM-I and PS-DSPM-II machines, are proposed based on the conventional DSPM-I and DSPM-II machines in which the PMs are located on the stator yoke with intervals of every number of phases and every stator pole, respectively. The electromagnetic performance including back electromotive force (EMF) and torque characteristics of the proposed PS-DSPM machines are analyzed and compared with those of the conventional DSPM machines based on the optimal designs for the highest average electromagnetic torque by finite-element (FE) analysis in this paper. FE results show that the proposed PS-DSPM-I and PS-DSPM-II machines exhibit 8.49% and 207% higher torque density than the conventional DSPM-I and DSPM-II machines with same copper loss, respectively. The influence of main design parameters on the average electromagnetic torque in the proposed PS-DSPM machines is investigated, together with the conventional DSPM machines. A prototype machine of PS-DSPM-II is manufactured and tested to verify the FE analysis.

Comparative Study of Partitioned Stator Machines With Different PM Excitation Stators

The partitioned stator (PS) machine adopts two stators to allocate windings and permanent magnets (PMs) separately, increasing the space for PMs as well as armature coils to boost the electromagnetic torque and improving the thermal condition of PMs. In this paper, the PS switched-flux PM (PS-SFPM) machine and the PS flux reversal PM (PS-FRPM) machine are proved to inherently share the same operating principle and similar machine topology but with interior PM (IPM) and surface-mounted PM stators, respectively. Furthermore, four globally optimized PS machines with different inner stator topologies are compared in terms of back EMF, cogging torque, electromagnetic torque, torque per PM volume, and flux-weakening capability. The results reveal that the spoke-shaped IPM (IPM-spoke) inner stator exhibits the highest back EMF and hence the highest average torque, while the I-shaped IPM (IPM-I) stator has the best flux-weakening capability, and the V-shaped IPM (IPM-V) produces the highest torque per PM volume. Furthermore, four machines are redesigned with the same PM usage volume, and the results show that the PS-SFPM machine still exhibits the highest back EMF as well as torque although with sacrificed advantages. The finite-element analyses and experiments are used to confirm the predictions.

Comparative study of novel biased flux permanent magnet machine with doubly salient permanent magnet machine

A comparative study between the novel biased flux permanent magnet machine (BFPMM) and the doubly salient permanent magnet machine (DSPMM) with the same machine size is presented in this paper. The choice of rotor pole number in BFPMM can be any integers except the phase number and its multiples, which is more flexible than that in DSPMM. Different from 6/4 stator/rotor pole DSPMM which has the unipolar phase flux-linkage and trapezoidal phase back-EMF, they become bipolar and essentially sinusoidal in 6/7 stator/rotor pole BFPMM. Hence, compared with DSPMM under the optimal current angle and rated copper loss, the torque ripple in BFPMM can be reduced significantly by ~70%-80% in both the original design (ORI) and the design with flux focusing technique (FC). Moreover, BFPMM exhibits larger average torque than that of DSPMM in both ORI- and FC-designs under the same copper loss and optimal current angle.

Analysis of Novel Multi-Tooth Variable Flux Reluctance Machines With Different Stator and Rotor Pole Combinations

Novel multi-tooth variable flux reluctance machines (VFRMs), which adopt doubly salient stator and rotor structure with nonoverlapping stator ac armature and dc field windings, are investigated in this paper. The rotor pole number can be any integer except the phase number and its multiples. Meanwhile, to obtain symmetrical bipolar phase flux-linkage and back electromotive force waveforms, the ratio of stator pole number to the greatest common divisor of stator- and rotor-pole numbers should be even integers. Furthermore, under the same rated copper loss and stator outer radius as well as 6-pole stator, 4-tooth per stator pole VFRMs in which the stator and rotor pole number determined by Nr = nNs ± 1 (n is the number of small teeth per stator pole) exhibit the highest average torque. Moreover, compared with the optimal 6/7 stator/rotor pole single-tooth VFRM, the 6/25 stator/rotor pole 4-tooth VFRM has more sinusoidal and larger back EMF, negligible cogging torque, lower torque ripple, and higher torque capability at relatively low copper loss. Similar to single-tooth VFRMs, 4-tooth VFRMs with Nr = nNs ± 1 exhibit more sinusoidal back EMF and larger torque than that of VFRMs with Nr = nNs ± 2. The analyses are experimentally validated by several prototype machines.

Comparative study of novel synchronous machines having permanent magnets in stator poles

Novel stator permanent magnet synchronous machines (SPMSMs) which adopt doubly salient stator and rotor structure and non-overlapping winding are investigated in this paper. Several new SPMSMs topologies are developed, including magnets in the stator poles and on the stator pole surface, respectively. It shows that amongst various SPMSMs, the surface-mounted SPMSM exhibits higher torque density, lower torque ripple and more sinusoidal back-EMF than the pole-mounted SPMSM under the condition of same copper loss.

Comparison of Partitioned Stator Switched Flux Permanent Magnet Machines Having Single- or Double-Layer Windings

A novel type of partitioned stator switched flux permanent magnet (PS-SFPM) machine with either single-layer or double-layer windings is developed in this paper. The proposed PS-SFPM machines have two stators, which separately accommodate the armature windings and the PMs, and between which is the rotor made of iron pieces, while the number of stator poles with PMs may be equal or half of that with armature windings. All the machines are optimized under fixed copper loss for maximum average torque by genetic algorithm. Their electromagnetic performances are compared, such as open-circuit flux-linkages and back-electromotive forces (EMFs), cogging torque, static torque waveforms, average torque against current, PM utilization ratio, and flux-weakening performances. The results show that due to more PM usage and higher open-circuit back-EMF, the PS-SFPM machines having the number of stator poles with PMs the same as that with armature windings exhibit higher average torque, irrespective of the winding topologies, either single-layer or double-layer windings. However, the single-layer winding PS-SFPM machines having the number of stator poles with PMs half of that with armature windings have the best PM usage and the highest ratio of average torque to PM volume, as well as good flux-weakening capability. A prototype machine is manufactured and tested to validate the analyses.

Comparison of partitioned stator machines with different PM excitation stator topologies

The partitioned stator (PS) machine adopts two stators to allocate coils and permanent magnets (PMs) separately, which can increase the space for PMs and coils to boost the electromagnetic torque and to improve the thermal condition of PMs which are on the stator. In this paper, the PS switched flux PM machine and PS flux reversal PM machine are proved to inherently share the same operating principle and the same machine topology but with interior PM (IPM) and surface-mounted PM (SPM) stators, respectively. Based on this concept, four PS machines with different PM excitation stator topologies are introduced and compared in terms of back-EMF, cogging torque, electromagnetic torque, torque per PM volume and flux-weakening capability. The results show that the spoke-IPM inner stator exhibits the maximum back-EMF and hence the largest average torque, while the IPM inner stator has the best flux-weakening capability and the V-shaped IPM (VIPM) produces the highest torque per PM volume.

Comparison of partitioned stator switched flux permanent magnet machines having single- and double-layer windings

In this paper, the electromagnetic performance of partitioned stator switched flux permanent magnet (PM) (PS-SFPM) machines with single- and double-layer windings are compared when the inner stator is equipped with the same number or half the number of PMs as that of the outer stator teeth. Electromagnetic performance of these PS-SFPM machines is investigated by 2-dimensional finite element analysis (FEA), based on the globally optimized designs for the maximum average electromagnetic torque. FEA results show that for 12-outer-stator-tooth and 12-inner-stator-PM PS-SFPM machines the double-layer windings produce higher torque than the single-layer windings, but for 12-outer-statortooth and 6-inner-stator-PM PS-SFPM machines it is opposite, i.e. the double-layer windings produce less torque than the single-layer windings. Overall, the 12-outer-stator-tooth and 12-inner-stator-PM PS-SFPM machines produce higher torque and less torque ripple than the 12-outer stator-tooth and 6-inner stator-PM PS-SFPM machines.

Influence of flux focusing on electromagnetic torque of novel biased flux PM machines

A novel biased flux permanent magnet (PM) machine (BFPMM) with doubly salient structure and PMs located in the stator back-iron is investigated in this paper. The stator is constituted by T-shaped laminated segments between which are placed circumferentially magnetized magnets of alternate polarity, and a concentrated winding is wound around each stator pole. The rotor has salient structure which is similar to that in a switched reluctance machine (SRM), whilst the choice of its pole number is flexible and can be chosen to be any integer except equal to the phase number and its multiples. When the number of stator and rotor poles differed by one, 6-stator pole BFPMM exhibits bipolar phase flux-linkage and sinusoidal phase back-EMF. Three types of flux focusing are proposed to increase the slot area and improve the torque performance. The results show that they can produce higher phase flux-linkage and back-EMF as well as larger torque capability at rated copper loss.