Variable Flux Reluctance Machines Research Articles

A Partitioned Stator Variable Flux Reluctance Machine

This paper proposes a partitioned stator variable flux reluctance machine (PS-VFRM) in which field and armature windings are placed on two separate stators. The influence of different stator/rotor-pole combinations on the electromagnetic performance is examined using finite-element analysis. Candidate designs maximize the average electromagnetic torque. Compared against the conventional single stator VFRM, the proposed PS-VFRM is shown to provide 24.6% and 12.7% increases in torque density and efficiency, respectively. The static electromagnetic performance is validated using experimental prototypes.

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 N r = nN s ± 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 N r = nN s ± 1 exhibit more sinusoidal back EMF and larger torque than that of VFRMs with N r = nN s ± 2. The analyses are experimentally validated by several prototype machines.

Integrated Field and Armature Current Control Strategy for Variable Flux Reluctance Machine using Open Winding

This paper proposes an integrated field and armature current control strategy for a variable flux reluctance machine (VFRM) with an open-winding topology. By using an open-winding inverter, the field and armature currents can be injected into a single coil as a sinusoidal current biased by the dc offset rather than the separated field and armature windings. The integrated current control can reduce the copper loss to half and extend the operating speed range owing to the reduction in the winding resistance. In order to utilize the zero-sequence current as a field current, a zero-vector modification technique is proposed, in which the switching-on time of the zero vectors is modified to generate the constant zero-sequence voltage between two inverters. The proposed scheme is implemented in a synchronous dq0-axis frame with space vector modulation. For the validation of the proposed method, a machine model of the VFRM is developed and implemented in MATLAB/Simulink. The simulation and experimental results verify that the proposed strategy can effectively reduce the copper loss and extend the operating speed range.

Stator/Rotor Pole Combinations and Winding Configurations of Variable Flux Reluctance Machines

In this paper, stator/rotor pole combinations, winding configurations, and electromagnetic performance of novel variable flux reluctance machines (VFRMs), which employ a doubly salient structure similar to switched reluctance machines (SRMs) but with stator-located dc field windings, are investigated. VFRMs with 12 stator poles are taken as examples to illustrate the method for determining the winding connections and winding factors. The back-electromotive force (EMF), self- and mutual inductances, cogging torque, static torque, torque ripple, and unbalanced magnetic force (UMF) are investigated by finite-element analyses. It is found that many stator/rotor pole combinations, i.e., 12/8 (which may be derived from the conventional three-phase SRM), 12/10, 12/11, 12/13, and 12/14, are feasible for the 12-stator-pole VFRMs. Among these pole number combinations, the 10- and 14-rotor-pole VFRMs can eliminate the inherent UMF in 6/5 and 6/7 VFRMs and exhibit more sinusoidal back-EMF waveforms and have higher torque density than an 8-rotor-pole VFRM, whereas the 11- and 13-rotor-pole VFRMs exhibit similar torque density as the 10- and 14-rotor-pole VFRMs, but with negligible cogging torque and torque ripple, albeit with UMF. Five prototype VFRMs with 12 stator poles and different rotor poles have been designed, manufactured, and tested to verify the analyses.

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.

Electromagnetic Performance of Novel Synchronous Machines With Permanent Magnets in Stator Yoke

Novel synchronous machines with doubly salient structure and permanent magnets (PMs) in stator yoke have been developed in this paper. The stator is constituted by T-shaped lamination segments sandwiched with circumferentially magnetized PMs with alternate polarity, while the rotor is identical to that of switched reluctance machines (SRMs). The stator pole number is multiples of six, which is the number of stator poles in a unit machine. Similar to variable flux reluctance machines (VFRMs), the rotor pole numbers in the novel machines are not restricted to those in SRMs. When the stator and rotor pole numbers differ by one (or the number of multiples), the novel synchronous machines show sinusoidal bipolar phase flux linkage and back electromotive force (EMF), which make the machines suitable for brushless ac operation. Moreover, two prototype machines with six-pole stator and five-pole, seven-pole rotors are designed and optimized by 2-D finite element analysis. It shows that, compared with VFRMs, the novel machines can produce ~70 % higher torque density with the same copper loss and machine size. Meanwhile, the proposed machines have negligible reluctance torque due to very low saliency ratio. Experimental results of back EFM, cogging torque, and average torque on the prototypes are provided to validate the analysis.

Comparative Study of Novel Variable Flux Reluctance Machines With Doubly Fed Doubly Salient Machines

This paper presents a comparative study between novel variable flux reluctance machines (VFRMs) and doubly-fed doubly salient machines (DFDSMs), having the same stator outer diameter and axial length. For comparison, a 6-pole stator is used in VFRMs and DFDSMs, while a 5-pole rotor is optimized for VFRMs and a 4-pole rotor for DFDSMs. Compared with DFDSMs in which the fluxlinkage and back-EMF are asymmetric particularly under heavy magnetic saturation, they become symmetrical and essentially sinusoidal in the VFRMs. Hence, the torque ripple in VFRMs can be significantly reduced. Further, due to the shorter flux path in the VFRMs, their average torque under the same copper loss can also be increased by 20% ~ 30% for the optimally designed VFRMs and DFDSMs within the same space envelop.

Electromagnetic Performance of Novel Variable Flux Reluctance Machines With DC-Field Coil in Stator

In this paper, novel variable flux reluctance machines (VFRMs) with different rotor pole numbers, which employ a doubly salient structure together with field windings identically located in the stator for each phase, are investigated and compared. Different from other doubly salient machines, such as switched reluctance machines (SRM) and doubly fed doubly salient machines (DFDSM), the equation for determining the stator/rotor pole numbers, i.e., $N_{\rm r}=N_{\rm s}\pm 2k$ , is no longer limited in VFRMs. More feasible selections of rotor pole numbers can be used if the stator/rotor pole numbers can satisfy $N_{\rm r}\ne kN_{\rm ph}$ . Thus, for a 6-stator pole machine not only the rotor pole number can be selected as even numbers, such as 4- and 8-rotor poles, which are commonly used for SRM and DFDSM, but odd numbers, such as 5- and 7- rotor poles, are also feasible. The benefits of using 5- and 7-rotor poles are the cancellation of even-order harmonics together with the third-order harmonic in the flux-linkage and back-EMF. Hence, more sinusoidal back-EMF can be obtained to produce a lower torque ripple. The cancellation in 5- and 7-rotor pole machines is because the flux generated by field and armature windings can pass through the adjacent stator poles to form a shorter flux path, which can further increase their average torque. To validate the analysis, four prototype machines with different rotor pole numbers are manufactured and tested.