IEEE TRANSACTIONS ON MAGNETICS, VOL. 53, NO. 11, NOVEMBER 2017 8110706

A Brushless Dual-Mechanical-Port Dual-Electrical-Port MachineWith Spoke Array Magnets in Flux Modulator

Xiang Ren, Dawei Li, Ronghai Qu, and Tianjie Zou

State Key Laboratory of Advanced Electromagnetic Engineering and Technology, School of Electricaland Electronic Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

Brushless dual-mechanical-port dual-electrical-port (BLDD) permanent magnet (PM) machines have been gaining more andmore attentions in recent years, with the merits of two decoupled rotors and contactless torque transmission. However, existingBLDD machines tend to suffer from low torque density due to low working flux density. In this paper, a BLDD machine with spokearray PMs in flux modular is proposed, which improves the torque density significantly. The structure and operation principle of theproposed machine is introduced. Detailed performance comparison between three different BLDD machine topologies, i.e., surface-mounted PM BLDD machine, flux-bidirectional modulation BLDD machine, and the proposed BLDD machine, is presented throughfinite-element analysis. The analyzing results show that although the modulated magnetic field coupled with the modulation windingis slightly reduced, the torque transmission capability of the regular winding in the proposed BLDD machine is significantly enhancedwhen compared with that of its two counterparts.

Index Terms— Brushless dual-mechanical-port (DMP) dual-electrical-port (BLDD) machine, flux modulation effect, magnetic-gearedmachine (MGM), spoke array permanent magnet (PM).

I. INTRODUCTION

AS ONE type of electrical continuously variable trans-mission device, dual-mechanical-port (DMP) machine is

widely researched in recent years due to its high compactness,high power density, etc. Owing to these merits, it has greatpotential in hybrid electrical vehicle, wind power generation,etc. [1]–[3]. However, brushes and slip rings are used toguide injected current into rotational armature winding inDMP machines, which result in lower reliability and moremaintenance.

Based on the flux modulation principle [4]–[6], a newDMP machine type which is named as brushless DMP dual-electrical-port (BLDD) machine was proposed and analyzed,while brushes and slip rings are eliminated and the armaturewinding is moved to stator. There were different BLDDpermanent magnet (PM) machine topologies presented later.In [7] and [8], a BLDD machine which consists of onemagnetic-geared machine (MGM) and one additional torqueregulation machine was proposed, the two parts are separatedin axial direction. In [9] and [10], two more compact BLDDmachine topologies were proposed, where the whole machinesare integrated into one frame, while the control strategy wasinvestigated in [11].

Generally, BLDD machines should consist of one MGMpart and one PM synchronous machine (PMSM) part, andtwo torque components, which can be called MGM andPMSM torque component, are produced by two machine parts,respectively. The two machine parts can be placed separately inaxial direction as in [7] and [8], or integrated into one machineframe as in [9] and [10]. Obviously, the later placement is morecompact. However, the PMSM torque component is relatively

Manuscript received March 10, 2017; revised April 30, 2017; acceptedMay 15, 2017. Date of publication May 18, 2017; date of current versionOctober 24, 2017. Corresponding author: D. Li (e-mail: daweili@hust.edu.cn).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2017.2705912

Fig. 1. Cross section. (a) SPM BLDD structure. (b) ProposedBLDD structure.

low considering the magnetic circuit structure, which causeslow torque density of the whole BLDD machines.

In this paper, a BLDD machine topology with spoke arraymagnets in flux modulator is proposed. Due to the special mag-netic circuit design, the torque capability of the PMSM part inthe proposed machine is increased significantly. In Section II,the topology and operation principle of the proposed BLDDmachine is introduced. In Section III, operation features ofthe proposed BLDD machine is analyzed, and performancecomparison between three different BLDD machine topolo-gies is presented. In Section IV, some mechanical issues arediscussed. And finally the conclusion is drawn in Section V.

II. STRUCTURE AND OPERATION PRINCIPLE

A. Structure

The cross sections of surface-mounted PM (SPM) andthe proposed BLDD PM machine topologies are shownin Fig. 1(a) and (b). Two topologies both have one stator andtwo coaxial rotors. The special stator configuration is usedfor both two topologies, i.e., there are auxiliary teeth in thestator. There are two sets of windings, which can be called

0018-9464 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

8110706 IEEE TRANSACTIONS ON MAGNETICS, VOL. 53, NO. 11, NOVEMBER 2017

Fig. 2. MGM parts of BLDD machines. (a) SPM BLDD machine.(b) Proposed BLDD machine.

as modulation winding and regular winding, are placed intomain teeth and auxiliary teeth, respectively. This special statorstructure provides the two windings more freedom to selectslot/pole combinations, which makes it possible to employnon-overlapping winding configuration for both windings.To guarantee the machine performance, the two windingsshould be decoupled electrically.

In the SPM BLDD machine shown in Fig. 1(a), the outerrotor is only formed by steel segments (iron pieces), whilespoke array PMs are introduced between them in the proposedtopology. Further, the pole-pair number of modulation windingPwr equals that of spoke PM array Pro, which also differs fromthat in the SPM BLDD machine, as shown in (1), where Priis the pole-pair number of inner rotor PMs

Pwr = Pro(for the proposed BLDD machine)

Pwr = Pri(for the SPM BLDD machine). (1)

The inner PM rotor is totally the same as that of SPM rotor,and the pole-pair number of modulation winding Pwm, SPMsin inner rotor Pri, and number of segment pieces in outer rotornro satisfy (2), which is same for the SPM BLDD machine

Pwm = nro − Pri. (2)

In the proposed BLDD machine, it is obvious that the pole-pair number of spoke array magnets in the outer rotor equalshalf of the number of steel segments, that is

Pro = nro

2. (3)

B. Operation Principle

Generally, BLDD machines consist of two machine parts.According to (2), the MGM part is formed by the modulationwinding, inner rotor, and steel segments in outer rotor, whichis same for both SPM and the proposed BLDD machines,as shown in Fig. 2. Operation of MGM is based on fluxmodulation principle, in which steel segments work as apole-pair transformer, and flux density in the airgap can becalculated as (4), where B , Fri, Aro, θ , t , �ro, and �ri representairgap flux density of MGM part, magnetomotive force (MMF)produced by inner surface PMs, airgap permeance function,spacial position, time, and speeds of outer and inner rotors,

Fig. 3. PMSM parts of BLDD machines. (a) SPM BLDD machine.(b) Proposed BLDD machine.

Fig. 4. No-load magnetic circuits of PMSM machine parts. (a) SPMBLDD machine. (b) Proposed BLDD machine.

respectively, and the airgap permeance function is assumedonly consists of constant term �0 and fundamental �1

B(θ, t) = Fri�ro

= Fri cos[Pri(θ − �rit)]{�0 + �1 cos[nro(θ − �rit)]}= Fri�0 cos[Pri(θ − �rit)]

+ 1

2Fri�1 cos[(nro ± Pri)θ − (nro�ro ± Pri�ri)t].

(4)

According to (2) and (4), a Pwm-pole-pair field exists in theairgap and it interacts with the modulation winding.

According to (1), PMSM parts of the proposed and SPMBLDD machines are technically different. The PMSM part inthe proposed BLDD machine is composed of regular windingand outer rotor, while that in the SPM BLDD machine iscomposed of the regular winding and inner rotor, as shownin Fig. 3.

As shown in Fig. 4, the magnetic circuit of PMSM machinepart in the proposed machine is quite different from that inthe SPM BLDD machine, where Fm , Rg , and Rm are MMFof magnet and reluctance of airgap and magnet, respectively.As well known, spoke PM arrangement has benefit of flux-focusing effect, and the main magnetic field only passesthrough one airgap in the proposed BLDD structure, whichmeans lower reluctance and less flux leakage, hence the pro-posed BLDD machine produces higher Pwr-pole-pair magnetic

REN et al.: BLDD WITH SPOKE ARRAY MAGNETS 8110706

Fig. 5. Initial design process of the proposed BLDD machine.

field in the outer airgap, and sequentially higher PMSM torquecomponent is produced.

Given the two machine parts mentioned above, torques andspeeds of two rotors can be controlled by currents in 2 sets ofwindings, as shown in

Tro = Trom + Tror (5)

Tri = Trim (6)

Trom = −nro

PriTrim (7)

Pro�ro = 60 fwr (8)

nro�ro − Pri�ri = 60 fwm (9)

where Tro, Trom , and Tror are total, MGM, and PMSM torquecomponents on the outer rotor, respectively, Tri and Trim arethe total and MGM torque components on the inner rotor,respectively, and fwr and fwm are current frequencies inregular and modulation windings, respectively.

III. FEA STUDY

In order to further investigate the principle and performanceof the proposed machine, several finite-element analysis (FEA)simulations have been performed. First, the machine opera-tion features, including flux distribution, back electromotiveforces (EMFs) and electromagnetic torque are calculated. Thenperformances comparison between SPM, flux-bidirectionalmodulation and the proposed BLDD machine topologies ismade.

A. Operation Features

A 2-D FEA model is built to calculate the operation featuresof the proposed machine and verify the principle introducedin Section II. The initial design process is illustrated in Fig. 5,

TABLE I

MAIN PARAMETERS OF SPM AND PROPOSED BLDD MODELS

Fig. 6. Magnetic field distributions excited by PMs at different partsseparately. (a) Surface PMs in the inner rotor. (b) Spoke PMs in the outer rotor.

where Do and Di represent outer and inner diameters ofthe stator, TMG and TSM stand for MGM and PMSM torquecomponents, Vwm, Vwr, Awm, and Awr are rated voltage andelectrical loading of modulation and regular windings, andBMG and BSM denote magnetic loading of the MGM and thePMSM machine parts, respectively. Prior to specific designprocess, some design indexes, including torque capability,rated voltages, and machine outer radius are given. First,rotor pole-pair numbers, together with slot/pole combinationof two windings are carefully selected. Second, the electricaland magnetic loadings are practically chosen. The stator innerdiameter is then roughly calculated based on the followingsizing equations:

TMG = α

(√2π

4

)(D2

i Ls

)kwm AwmBMG

TSM =(√

2π

4

) (D2

i Ls

)kwr AwrBSM (10)

where α, kwm, and kwr represent pole ratio, winding factors ofmodulation, and regular winding, respectively.

More specific stator and rotor dimensions are designedaccording to winding cooling condition and flux densityrestriction After the FEA-based design process, some mainparameters of the proposed machine are given in Table I.

Distributions of magnetic fields excited by inner surface andouter spoke PMs separately are shown in Fig. 6. In Fig. 6(a),it can be seen that magnetic fields in stator and inner rotorhave different pole pairs, which verifies flux modulation effect

8110706 IEEE TRANSACTIONS ON MAGNETICS, VOL. 53, NO. 11, NOVEMBER 2017

Fig. 7. No-load flux density distributions of BLDD machines.(a) SPM topology. (b) Proposed topology.

Fig. 8. Back EMF waveform in different operation conditions.(a) �ro = 500 rpm, �ri = 0. (b) �ro = 0, �ri = 500 rpm.(c) �ro = 500 rpm, �ri = 250 rpm.

in the MGM part. Fig. 6(b) shows the field distribution ofPMSM machine part, an 11-pole-pair field is produced inthe stator. No-load flux density distributions of SPM and theproposed BLDD machines are shown in Fig. 7(a) and (b),respectively. Due to excitation of the outer spoke PMs, theproposed BLDD machine produces higher flux density in thestator core, especially in auxiliary teeth.

No-load back EMF waveforms of two sets of windings atdifferent rotor speeds are shown in Fig. 8. When the outerrotor rotates at 500 rpm and the inner rotor is static, period ofno-load back EMF in the regular winding is 10.91 ms, whichequals double of that in the modulation winding. When theouter rotor is static and inner rotor rotates, almost no EMFinduced in the regular winding. When outer and inner rotorsboth rotate, periods of EMF in two windings differs from thatshown in Fig. 8(a). It is noticed that EMF frequencies shownin Fig. 7 match well with (8) and (9).

Output torque characteristics of the proposed machine atdifferent operation conditions are presented in Fig. 9. Whenthe regular winding is at open circuit, only MGM torque com-ponent acts on two rotors. As shown in Fig. 9(a), Tro and Triare proportional to Iwm, which matches well with (7). Whenthe modulation winding is at open circuit, only the PMSMtorque component works on the outer rotor, which means

Fig. 9. Electromagnetic torques on two rotors at different operationconditions. (a) Iwr = 0. (b) Iwm = 0. (c) Iwm = 4.05 A.

Fig. 10. Magnetic fields and spectrums in airgaps. (a) Inner gap.(b) Outer gap.

Tri equals zero and Tro is proportional to Iwr, as shownin Fig. 9(b). In Fig. 9(c), Iwm is constant and Iwr varies, whichmeans Tri and Trom are constant and Tror varies linearly withthe increase of Iwr.

B. Performance Comparison

An SPM BLDD machine as shown in Fig. 1(a) is designedfor performance comparison. Its main parameters are givenin Table I, which have been optimized for higher torqueproduction. Compared with the proposed machine, it has samestator but different rotor topology and sizes.

Flux density waveforms and spectrums of SPM and theproposed BLDD machines in inner and outer airgaps areshown in Fig. 10(a) and (b), respectively. The 11-pole-pairfield of the SPM BLDD machine reaches 0.85 T in the innergap, however, due to flux modulation effect and flux leakage,only 0.27 T remains in the outer gap, which equals about 1/4 ofthat in the proposed BLDD machine, and this result verifiesthe analysis of magnetic circuits in Section II.

REN et al.: BLDD WITH SPOKE ARRAY MAGNETS 8110706

Fig. 11. No-load back EMF comparison of the SPM and proposedBLDD machines. (a) Modulation winding. (b) Regular winding.

Fig. 12. Torque components comparison between SPM and proposedBLDD machines. (a) MGM torque component. (b) PMSM torque component.

Fig. 11(a) shows no-load back EMFs in modulation windingof two models when their outer rotors both rotate at 500 rpm.Although the 2-pole-pair field in the outer gap of the proposedBLDD machine is lower than that of the SPM BLDD machine,the proposed machine has a higher gear ratio, hence a higherback EMF value is obtained. Fig. 11(b) shows back EMFcomparison in regular windings, EMF of the proposed BLDDmachine is about four times that of the SPM BLDD machine,which matches well the analysis of magnetic fields.

Output torque comparison between SPM and the proposedBLDD machine is shown in Fig. 12, Iwr and Iwm are 4.05and 5.67 A, respectively, for both models. The MGM torquecomponent increases by about 7%, and the PMSM torque com-ponent increases to about four times in the proposed BLDDmachine. Considering magnet material usage, the MGM torquecomponent per unit magnet usage in the proposed machine isabout 57.2% of the SPM BLDD machine, while this value is224.7% for the PMSM torque component.

In order to further illustrate features of the proposed BLDDmachine, another comparison with BFM BLDD machine [10]is made. Parameters and sizes of two models are shownin Table II. For a convictive comparison, the outer and innerradius, stack length, turns in series per phase, and pole ratioare kept the same for both machine models.

Table III summarizes the results, and shows that althoughthe proposed BLDD machine produces ∼3% lower funda-mental back EMF in the modulation winding and 10% lowerMGM torque than that of the BFM BLDD machine, the backEMF in the regular winding and the PMSM torque componentare ∼48% and ∼45% higher. Moreover, the magnet materialconsumption of the proposed machine is only 62% that of theBFM BLDD machine, and a more competitive value of torqueper unit magnet usage is achieved accordingly.

Furthermore, since the slot number for the regular andmodulator windings can be independent selected in the

TABLE II

PARAMETERS OF BFM AND PROPOSED BLDD MACHINES

TABLE III

PERFORMANCES OF BFM AND PROPOSED BLDD MACHINES

Fig. 13. 3-D structure of the outer rotor.

proposed machine due to its special stator configuration,the non-overlapping winding configuration can be used forboth the two windings to reduce the end winding length. Thismeans that the proposed machine can operate at a higherelectric loading to enlarge the output torque.

IV. MECHANICAL ISSUES

In this section, the mechanical structure of the proposedBLDD machine is presented. The mechanical stress and itsinfluence on electromagnetic performances are evaluated.

The 3-D structure of the outer rotor is shown in Fig. 13.A 0.5 mm magnetic bridge is added at the bottom of steelsegments, trapezoid magnets are adopted to mounted betweensegments, and mounting holes are punched at the center ofeach segment. The assembly graph of the whole machine isshown in Fig. 14, in which both the two rotors are with robustmechanical structure, i.e., two-side support.

The mechanical stress of the outer rotor at 500 rpm ischecked, as shown in Fig. 15. The maximum mechanical stressof the rotor support is 0.32 MPa, which is much smaller

8110706 IEEE TRANSACTIONS ON MAGNETICS, VOL. 53, NO. 11, NOVEMBER 2017

Fig. 14. Assembly graph of the proposed machine.

Fig. 15. Mechanical stress analysis of the outer rotor.

TABLE IV

ELECTROMAGNETIC PERFORMANCE COMPARISON BETWEEN BLDD

MACHINES WITH AND WITHOUT MECHANICAL STRUCTURES

than the yield strength of the carbon steel. The influenceof the magnetic bridge and mounting holes on electromag-netic performance of the proposed machine is summarizedin Table IV. The MGM and PMSM torque components are∼9% and ∼10% reduced, respectively, considering thesemechanical structures.

V. CONCLUSION

In this paper, a BLDD machine with spoke array magnetsin flux modular has been proposed, while spoke array magnetrotor structure is employed to work as flux source of thePMSM part and flux modulator of the MGM part at thesame time. With flux-focusing effect utilized and magneticresistance of the PMSM part decreased, the working fluxdensity and corresponding PMSM torque component are muchhigher.

Furthermore, decoupled control of speeds and torques ofthe proposed BLDD machine has been verified through FEA,Then, the proposed BLDD machine has been compared withexisting SPM and BFM BLDD machine topologies on elec-tromagnetic performance. The analysis results show that thePMSM torque component of the proposed machine are ∼300%and ∼45% higher compared with those of the SPM and BFMBLDD machine, respectively. Finally, an outer rotor structurewith magnetic bridge and mounting holes is further pro-posed for the consideration of practical fabrication. Reliablemechanical support of the outer rotor has been verified thoughmechanical stress evaluation.

ACKNOWLEDGMENT

This work was supported by the National Natural Sci-ence Foundation of China under Grant 51337004 and Grant51607079.

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