a new parallel double exitation synchronous machine

2252 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 9, SEPTEMBER 2011

A New Parallel Double Excitation Synchronous MachineB. Nedjar�, S. Hlioui�, Y. Amara�, L. Vido�, M. Gabsi�, and M. Lécrivain�

SATIE, ENS Cachan, CNRS, F-94230 Cachan, FranceGREAH, EA 3220, Université du Havre, 76063 Le Havre, France

This paper presents 3-D finite-element analysis of a new double excitation synchronous machine. It is shown that the machine has truefield regulation capability. The principle of operation and design aspects of this new machine are presented in the paper. Comparison of3-D FEA with an experimental study done on a prototype having a different rotor structure is also investigated.

Index Terms—Electromagnetic analysis, field weakening, hybrid excitation, permanent-magnet machines, synchronous machines, 3-Dfinite-element method.

I. INTRODUCTION

D OUBLE excitation (or hybrid excitation) consists ofcombining wound field and permanent-magnet excita-

tions in the same synchronous machine [1]–[17]. The goal ofdouble excitation principle is to combine advantages of per-manent-magnet machines, high power density and efficiency,with these of wound field synchronous machines, good fieldweakening capability. Double excitation machines can be di-vided into two categories: series double excitation and paralleldouble excitation machines [8], [11]. Machine presented in thispaper belongs to the second category (parallel double excitationmachines).

First, a nonexhaustive state of the art of double excitationstructures is presented. The principle of operation and designaspects of this new machine are then discussed. Finally, com-parison with a parallel double excitation prototype having a dif-ferent rotor structure is also investigated.

II. DOUBLE EXCITATION SYNCHRONOUS MACHINES

The double excitation principle allows a wide variety of struc-tures to be realized. Many criteria can then be chosen for theclassification of double excitation machines. Classical criteriaused for classification of other types of electric machines can beused; as an example:

1) radial field and axial field machines;2) 2-D and 3-D structure machines.Fig. 1(a) shows an example of an axial flux double excitation

machine [16] and Figs. 1(b) and (c) show examples of doubleexcitation machines with 2-D and 3-D structures, respectively[17], [11]. 3-D structures are in general more difficult to analyzeand manufacture.

Regarding the particular structure of double excitation ma-chines, the presence of two excitation flux sources, two criteriaseem more specific for classification of these machines:

1) By analogy with electric circuits, the first criterion con-cerns the way the two excitation flux sources are combined:series and parallel double excitation machines [8], [11].

Manuscript received October 06, 2010; revised March 04, 2011; acceptedMarch 15, 2011. Date of publication April 05, 2011; date of current versionAugust 24, 2011. Corresponding author: S. Hlioui (e-mail: [email protected]).

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

Digital Object Identifier 10.1109/TMAG.2011.2134864

Fig. 1. Different double excitation structures.

2) The second criterion concerns the localization of the ex-citation flux sources in the machine: both sources in thestator, both sources in the rotor and mixed localization. Bymixed localization it is meant that one source (excitationcoils or permanent magnets) is located in the rotor or the

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NEDJAR et al.: A NEW PARALLEL DOUBLE EXCITATION SYNCHRONOUS MACHINE 2253

Fig. 2. 3-D cut view of the double excitation machine.

stator and the other source in the stator or the rotor, respec-tively. Having excitation coils in the stator is favored toavoid sliding contacts.

The first criterion is more linked to flux control capability ofdouble excitation machines. Since excitation flux created by ex-citation coils should pass through the permanent magnets, whichhave a low relative permeability , in series double exci-tation machines, the flux control capability of the series doubleexcitation structures should be less efficient than that of the par-allel double excitation structures.

The second criterion is more linked to ease of manufactureand operation of double excitation machines. Indeed, havingboth excitation sources (excitation coils and permanent mag-nets) located in the stator presents some advantages from man-ufacturing and operating point of views because they are fixed:it is far more easier to evacuate losses from fixed parts thanmoving ones; the presence of excitation flux sources in the statorimplies a completely passive rotor which means no need ofa containment system and an improved high speed operatingcapability.

The studied double excitation machine has excitation coils lo-cated in the stator. Open circuit flux control principle of studiedmachine is described in following section.

III. PRINCIPLE OF OPERATION AND DESIGN ASPECTS

A. Principle of Operation

Fig. 2 shows a cut view of studied machine. It combines apermanent-magnet excitation with a wound field excitation.

Excitation coils are located in the stator, on top of armatureend windings, thereby avoiding sliding contacts. Radially mag-netized rare earth permanent magnets are located in the rotor.This machine has 12 magnetic poles .

The stator is composed of a laminated core, solid iron yokeand end-shields, conventional ac three-phase windings and twoexcitation annular coils. Solid iron components (external yokeand end-shields) provide a low reluctance path for wound fieldexcitation flux.

The rotor is, amongst other things, composed of two solidiron collectors and 12 rare earth permanent magnets. Parts ontop and below magnets can either be laminated or massive. Ef-fect of laminating these parts or not on machines performanceis investigated. Rotor back iron is magnetically insulated fromother ferromagnetic parts of the rotor.

Fig. 3. PM excitation flux trajectories: (a) active flux lines, (b) nonactive fluxlines.

Fig. 3(a) shows principal flux trajectory of PM excitation flux.This flux circulates from one pole to another as for classical sur-face PM machines. These flux lines participate to power conver-sion contrary to these shown in Fig. 3(b). Flux lines shown inFig. 3(b) are designated as nonactive because they do not passthrough armature windings and therefore do not participate topower conversion. Design procedure should increase reluctancepath for these lines. Fig. 4 shows wound field excitation fluxtrajectories. The machine has two annular excitation coils. Eachcoil is acting in one kind of magnetic poles.

The flux created by an excitation coil passes one time throughactive part’s air gap (homoplar path). Depending on DC exci-tation current direction, excitation coils can either be used toenhance or decrease excitation flux passing through armaturewindings.

Finite-element calculations, shown in the following section,will assess the effectiveness of excitation flux control in thismachine. Advantage related to the structure of this machine,no sliding contacts and good excitation flux control, should be,however, counterbalanced by a significant increase in materialrequirement. Stator and rotor core require tangential and axialflux conduction capacity; reasons which make the machineheavier than classical PM machines.

B. Design Aspects

The first element which has been look at is the spacing be-tween claws on top of permanent magnets (Fig. 5). Fig. 5(a)shows leakage flux lines, which increase if spacing betweenclaws is reduced. This leakage flux path reduces air gap fluxand then machines performance. Fig. 5(b) shows the same rotorbut with an increased spacing between claws. Increasing thisspacing will impact on wound field excitation flux cross-section.


Fig. 4. Wound field excitation flux trajectories.

Fig. 5. Leakage flux between rotoric claws.

Fig. 6. Rotor with empty spaces filled with permanent magnets. (a) 3-D view.(b) Cut view.

Effect of this parameter (spacing between claws) is investigatedin the next section.

Something else which can be done to reduce this leakage fluxand in the same time increase air gap flux is to fill empty spacesby permanent magnets (Fig. 6). Fig. 6 shows a 3-D view of therotor with all spared spaces filled with permanent magnets. Forsome configurations the area available for magnets is wide andtherefore inexpensive ferrite magnets can be utilized without theperformance penalty usually associated with their low residualflux density. At least ferrite permanent magnets can be used tofill empty spaces.

Since wound field excitation flux has an important axial com-ponent (Fig. 4), air gap flux control can be improved by usingsolid iron for claws instead of laminated sheets. Using solid ironimplies, however, higher iron loss and it also offers a low reluc-tance path for nonactive flux lines [Fig. 3(b)]. For ease of man-ufacturing rotor back iron can also be realized using solid iron.Another parameter which has been studied is the air gap thick-ness between rotoric flux collectors and statoric end-shields.

Fig. 7. Stator/rotor air gaps.

Fig. 8. 3-D finite-element mesh.

These components of the machine are perfect cylinders madeof solid iron and as a consequence the air gap between themcan be reduced as regards to air gap thickness in active part ofthe machine (Fig. 7). Finite-element calculations with an air gapthickness of half that of active part have been done. Results ofthis analysis are reported in the next section.

Section IV presents finite-element analysis of the studied ma-chine. 3-D FEA is used to determine flux distribution and theflux control capability of this machine. It is also used to study ef-fect of some design parameters on machine performance. Mainmachine data are identical to that of a previous prototype. Acomparison study between studied machine and the prototypeis presented in this paper.

IV. FINITE-ELEMENT ANALYSIS

The structure of studied machine requires the use of 3-Dfinite-element analysis. Fig. 8 shows the 3-D finite-elementmesh of studied machine. The nonlinearity of B-H curves ofmachine’s different parts is considered in this finite-elementanalysis.

Fig. 9 shows two cross sections of the initial machine design.Fig. 9(a) shows an axial cross section (perpendicular to rotationaxis) in the active part of the machine and Fig. 9(b) shows a crosssection parallel to the axis of the machine [AA plane, Fig. 9(a)].These figures also shows the main dimensions of the machine.Table I gives the machine’s main data.

The initial design parameters of this machine have been de-rived from a simple analytical model based on a simple reluc-tance network. For the double excitation circuit’s design, theprinciple of equalization of flux cross-sections has been used[ , Fig. 9(b)]. This machine will be referred next as“Machine 1”.


Fig. 9. Initial design main dimensions (Machine 1).

The stator structure and dimensions of this machine is iden-tical to a previously built prototype [11]. The smallest woundfield excitation flux cross-section in the stator is the sectionof the cylinders located at the inner radius of end shields [ ,Fig. 9(b)]. The design of the rotor claws was based on the equal-ization of contact sections between claws and rotoric flux col-lector with the section [Fig. 9(b)].

Fig. 8 shows the 3-D mesh of 1/12 of the machine (only onepole pair is considered (1/6 of the machine) and half of the ma-chine’s axial length). Finite-element calculations are done on1/6 of the machine. The machine’s model should consider theentire axial length of the machine. The mesh of only 1/12 of themachine is shown to highlight the smoothness of the mesh.

The finite-element calculations are done considering two airvolumes at axial ends of the machine (Fig. 10). Fig. 10 showsthe different boundary conditions imposed to nodes belongingto outer bounding surfaces of the finite-element model. Dirichlet

TABLE IDOUBLE EXCITATION SYNCHRONOUS MACHINE DATA

boundary condition is applied to bounding surface in both axiallimits of the finite-element model and for radii equal to 0 m and

(machine’s external radius), respectively.The 3-D mesh of the structure is obtained by extruding a 2-D

mesh. In the 2-D geometry, it is necessary, from the very start,to envisage all surfaces which make it possible by extrusion tocreate volumes of the structure in 3-D.

The developed model takes into account the rotor movement.The air-gap is divided into two parts (Fig. 11); a part is linked tothe rotor and the other part to the stator. When creating the 2-Dgeometry, used for the extrusion, at the border of the two partsof air-gap, two lines are confused. One line belongs to the halfair-gap linked to the stator and the other one to that linked to therotor. The same number of elements is imposed on these lines.The number of elements imposed on these lines is equal to 120.These elements are uniformly distributed. As mentioned earlier,due to symmetry considerations, finite-element calculations arecarried out on a sixth of the machine, that is to say 60 , whichmeans that there is a node every 0.5 on the lines at the borderbetween the two air-gap parts.

The simulation of the movement of the rotor, in this case, canbe realized only between two angles which are multiple of 0.5 ;the intermediate positions must be also multiple of 0.5 . For theposition where, the lines at the border are completely confused,the nodes on these lines, having same location, are merged. Forother positions, where the lines at the border are not any morecompletely confused (Fig. 12), the steps to be followed are:

1) on the air-gap, the nodes in parts where these lines are inter-sected, it is sufficient to merge nodes having same location;

2) remaining nodes should be coupled as shown in Fig. 12(b).Fig. 13 shows different electromotive force (EMF) wave-

forms per turn, at 1000 rpm, for different excitation currentvalues. It can be seen that the double excitation is very effective.The EMF maximum value has been nearly doubled when en-hancing air gap excitation flux compared to the case where nofield current is applied. It can be noticed that air gap excitation


Fig. 10. 3-D finite-element model. (a) 3-D view. (b) Side view. (c) Front view.

Fig. 11. Air-gap division in two parts.

flux can be completely cancelled by applying an adequate fieldcurrent value.

Fig. 12. Movement consideration in the finite-element model.

Fig. 13. EMF waveforms for different excitation currents.

Fig. 14 (Machine 1) shows variation of maximum air gap fluxversus field magnetomotive force (MMF). It can be seen that awide range of flux control can be achieved.

Fig. 14 compares flux control capability of different ma-chines. All these machines have same basic structure. Machine2 has the same structure as machine 1 but uses solid iron claws.It can be seen that flux control is greatly improved by usingsolid iron claws. However, an increase of rotor’s iron loss canbe feared. Machine 3 has different claws shapes [Fig. 5(b)]compared to machine 1 [Fig. 5(a)].

The leakage flux between claws has been reduced by nearly10% compared to machine 1, but it hasn’t affected the open cir-cuit air gap flux when no field current is applied. Furthermore,


Fig. 14. Flux control capability for different machines.

Fig. 15. Sensitivity analysis for the spacing between claws. (a) Angle � defi-nition. (b) Flux control capability.

effectiveness of flux control, as easily expected, has been re-duced compared to machine 1.

Fig. 6 shows rotor of machine 4. All empty spaces have beenfilled with ferrite permanent magnets ( T). Com-pared to other machines the open circuit air gap flux has beenincreased, but the field weakening capability has been reduced.For a field MMF of ATs the flux is reduced in same pro-portions as machine 1, but since flux value has been increased,when no field current is applied, cancellation of open circuit airgap is no more possible.

Fig. 16. Air-gap flux density distribution for different field ATs (Machine 1).(a) Field �� AT. (b) Field �� AT. (c) Field ��

�� AT.

A sensitivity analysis concerning the spacing between clawswas carried out using the 3-D finite-element model. Fig. 15 il-lustrates results of this study. Claws shape corresponds to theone shown in Fig. 5(a) for an angle equal to 3 (Machine 1)and to the one shown in Fig. 5(b) for an angle equal to 16 (Ma-chine 3), respectively. This study confirms previous conclusionsconcerning the comparison between machines 1 and 3.

Calculations done with an air gap thickness of 0.25 mm be-tween rotoric flux collectors and statoric end-shields has shownpractically no difference with previous case ( mm).Double excitation flux passes through laminated claws which


Fig. 17. Air-gap flux density distribution for different field ATs (Machine 2).(a) Field �� AT. (b) Field �� AT. (c) Field ��

�� AT.

offer a high reluctance path. Equivalent reluctance of this pathshould be higher than that offered by air gap between rotoricflux collectors and statoric end-shields and as consequence re-duces the effect of this last one.

Fig. 16 shows radial component distribution, of air gap fluxdensity, over a magnetic pole (North Pole) in the active part ofmachine 1, for three different values of excitation current. Fora null excitation current air gap flux density distribution is onlydue to permanent magnets [Fig. 16(b)]. It can be noticed that

Fig. 18. Lamination effect modeling.

Fig. 19. Stator of double excitation prototype: Details of (a) armature and ex-citation windings, and (b) end shields.

axial penetration of excitation flux created by field coil is limitedby laminated nature of claws [Figs. 16(a) and (c)].

Fig. 17 shows the same distribution for machine 2. Sinceclaws are made of solid iron, excitation flux created by fieldcoil will act over the entire length of active part and air gap fluxcontrol is more effective. Flux density is more uniformly dis-tributed, over axial length, compared to machine 1. Machineslaminated parts have been modeled with anisotropic magneticproperties. The equivalent value of laminated parts permeabilityin axial direction has been derived based on experience acquiredwith this kind of machines [9].

Fig. 18 illustrates how the value of relative permeability in zdirection is estimated. Laminated parts are considered as succes-sion of ferromagnetic and nonmagnetic (lamination insulationand parasitic air gaps) materials. A packing factor , definedas the total length of ferromagnetic steel parts divided by totallaminated pack length (active length), is set to 97%. Equation(1) gives then the value of the equivalent relative permeabilityin axial direction

(1)

where is the relative permeability of ferromagnetic parts.With a value of (corresponding to linear part ofthe B(H) curve) and , a value of is obtained.

The next section presents a comparison of flux control capa-bility of studied machine with a previously built double excita-tion prototype.

V. COMPARISON WITH ANOTHER PROTOTYPE

Fig. 19 shows the stator of a 3 kW machine prototype towhich studied machine has been compared. Both machines


Fig. 20. Rotor of double excitation prototype. (a) Lamination sheet. (b) Finalassembly.

TABLE IIDOUBLE EXCITATION SYNCHRONOUS PROTOTYPE DATA

Fig. 21. Excitation flux control characteristic for prototype machine (3-D FEanalysis and experimental results).

(studied machine and prototype) have same stator (same statorand overall dimensions) but different rotor structures.

Fig. 20 shows details of prototype’s rotor. Rotors of both ma-chines have same overall dimensions but different structures.Figs. 20(a) and (b) show respectively lamination sheet used tobuild prototype’s rotor and assembled rotor. The prototype has12 poles as studied machine. This prototype uses ferrite perma-nent magnets. The principle of flux concentration helps to reachhigh values of air gap flux density. Table II gives some comple-mentary data concerning this prototype.

Before comparing flux control capability of both machines, afinite-element analysis of prototype machine (Figs. 19 and 20)has been conducted. This study helps to estimate to which extentthe FEA is effective and accurate.

Fig. 21 shows flux variations versus field ampere turns forprototype machine. This figure also compares results from 3-DFEA and experimental results. It can be noticed that experi-mental and FEA results agree well. The air-gap flux changeswith a variation of %, when air gap flux is enhanced, and

Fig. 22. EMF waveforms for different field MMF for prototype machine (3-DFE analysis and experimental results). (a) Field �� AT. (b) Field�� AT. (c) Field �� AT.

%, when it is weakened, with respect to the no-field excita-tion flux.

3-D FEA has also been used to calculate EMF waveformsfor different excitation currents ( AT [Fig. 22(a)], 0 AT[Fig. 22(b)] and AT [Fig. 22(c)]. Figs. 22(a), (b), and (c)compare EMF waveforms obtained with the 3-D FEA and ex-perimental measurements for different field MMF.

The 3-D FEA is used to compute flux variations over an elec-trical period; the EMF waveforms are then obtained by meansof numerical derivation of flux waveforms. EMF measurements


Fig. 23. Flux control capability comparison.

are made at a rotational speed of 170 rpm. As seen an excellentagreement between FE results and measurements is obtained.This study confirms the fact that the finite-element analysis iseffective and accurate.

Fig. 23 compares flux variations versus field ampere turnsfor studied machine (3-D finite-element analysis) and proto-type machine (experimental measurements). The goal here is tocompare performance of both machines in terms of flux controlcapability.

Although, performance of studied machine seems to be lowerthan that of prototype machine no definitive conclusions canbe drawn. It should be noticed that both machines have notbeen optimized and that more investigations are needed. How-ever, the fact that air gap excitation flux can be completely can-celed for the new double excitation machine constitutes an in-teresting characteristic. A characteristic which can be advanta-geous in case of a fault accruing during operation, as a phaseshort-circuit.

Comparing different machines is not an easy task. Machinesshould be first optimized for different applications implying dif-ferent power levels and volume constraints before establishingadvantages and drawbacks of each structure.

VI. CONCLUSION

This paper presents a new double excitation machine withgood field weakening capability. This structure belongs toparallel double excitation machines. Excitation coils located inthe stator allow an effective air gap flux control while avoidingpermanent-magnet demagnetization risk. Comparison with adouble excitation prototype having same stator structure andoverall dimensions, but different rotor structure, is also pre-sented. This machine can be either used as a motor or generator.

Some design parameters of this structure have been investi-gated using 3-D FEA. In order to assess the effectiveness and ac-curacy of the finite-element method, a 3-D finite-element anal-ysis has also been applied to the double excitation prototype(having the same stator, but a different rotor structure). Compar-ison of FE and experimental results tends to confirm this fact.

This study will be used to help establish an analytical model,based on reluctance network, of the new structure [18], [19]. An-

alytical models are more convenient to use in design optimiza-tion process, especially for machines with 3-D structure (mag-netic flux flowing in the three dimensions).

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a new parallel double exitation synchronous machine

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