Three-dimensional PDESMs. (a) Radial flux topology. (b) Axial flux topology. 

Context in source publication

Context 1

... of a double excitation machine with a 2-D structure [12]. It should be noticed that the SDESM and PDESM studied in this paper respectively have 2-D and 3-D structures. Three- dimensional structures are in general more difficult to analyze and manufacture. Regarding the particular structure of double excitation machines, i.e., the presence of two excitation flux sources, the following two criteria seem more specific for the classification of these machines: 1) by analogy with electric circuits, the first criterion concerns the way the two excitation flux sources are combined: SDESM and PDESM [1], [3]; 2) the second criterion concerns the localization of excitation flux sources in the machine: both sources in the stator, both sources in the rotor and mixed localization. By mixed localization, it is meant that one field source (wound field excitation coils or PMs) is located in the rotor or stator and the other field source in the stator or rotor, respectively. Having wound field excitation coils in the stator is favored to avoid sliding contacts. The first criterion is more linked to the flux control capability of double excitation machines. For reasons explained earlier, the flux control capability of series double excitation structures should be less efficient than that of parallel double excitation structures. The second criterion is more linked to the ease of manufacture and operation of double excitation machines. Indeed, having both excitation sources (excitation coils and PMs) located in the stator presents some advantages from manufacturing and operating point of views because they are fixed: it is far more easier to evacuate losses from fixed parts than moving ones, and the presence of excitation flux sources in the stator implies a completely passive rotor, which means no need for a containment system and an improved high-speed operating capability. The studied PDESM has excitation coils located in the stator and PMs located in the rotor, whereas they are both located in the rotor for the series double excitation structure (SDESM). An updated overview of the series and parallel double excitation topologies is described in following sections. For SDESMs, excitation field sources, i.e., PMs and excitation coils, are placed in series (Fig. 2). Fig. 2 shows the SDESM principle schematic and the corresponding magnetic equivalent circuit (MEC) (Fig. 2(a) and (b), respectively). The iron core relative permeability is considered to be infinite. P AG and P PM represent, respectively, the air gap and PM permeances. PM excitation field enhancing or weakening (Fig. 3(a) and (b), respectively) is done by feeding excitation coils with a current in a given direction or the opposite one. Fig. 4 shows two examples of SDESMs. For the machine shown in Fig. 4(a) [2], both excitation flux sources are located in the rotor, whereas for the machine shown in Fig. 4(b) [13], excitation flux sources are both located in the stator avoiding the sliding contacts. For SDESMs, since the excitation flux created by excitation coils should pass through the PMs (Fig. 3), which have a low relative permeability ( μ r ≈ 1) , i.e., low permeance (Fig. 2(b), P ), the flux control capability should be less efficient than that of parallel double excitation structures. Furthermore, a risk of PM demagnetization can be feared. However, the fact that excitation fluxes created by both sources take the same way, iron loss reduces when the PM’s excitation flux is weakened, which is not the case of all PDESMs [1]. For PDESMs, the excitation fluxes created by PMs and excitation coils have different trajectories. The flux created by excitation coils does not pass through PMs. Compared with the first group, i.e., SDESMs, parallel double excitation allows a wide variety of structures to be realized. Fig. 5 shows the PDESM principle schematic and corresponding MEC (Fig. 5(a) and (b), respectively). As previously, the iron core relative permeability is considered to be infinite. Since the excitation flux created by excitation coils does not pass through the PMs, the demagnetization risk is avoided. Figs. 6–9 show different structures based on the principle of parallel doubled excitation. They illustrate the diversity of structures based on this principle. Fig. 6 shows two machines based on the principle of parallel double excitation [14], [15]. These machines have a 2-D structure. Both excitation flux sources are located in the rotor for these machines. Fig. 7 shows two PDESMs having a 3-D structure [16]–[18]. The machine shown in Fig. 7(a) [16], [17] consists of two stator and rotor portions. The rotor consists of a PM and a wound field rotor portions put adjacent to each other in the axial direction under a single armature winding, which span the two rotor portions. Both excitation flux sources are located in the rotor. Fig. 7(b) shows a PDESM with two annular excitation coils located in the stator, whereas the PMs are located in the rotor [18]. Fig. 8 shows two other PDESMs having a 3-D structure [19], [20]. For both machines, the excitation coils are placed in the stator and PMs in the rotor. These two machines are based on the same parallel double excitation principle. In recent years, a particular effort has been focused on the study of flux switching double excitation machines [Figs. 1(c) and 9] [12], [14], [21]–[25]. This effort is being made due to the interesting characteristics of flux switching structures, i.e., a completely passive rotor allowing an easier high-speed operation, the presence of both excitation flux sources in the stator that allows an easier cooling. Fig. 9 shows three different flux switching double excited synchronous machines. These machines can be classified within parallel double excitation structures. III. PDESM Fig. 10(a) and (b) show, respectively, the stator and rotor of a 3-kW prototype of PDESM used in the comparison study. Fig. 10(c) shows a lamination sheet used to build the proto- type’s rotor. Table I gives the PDESM main data. Fig. 11 shows a 3-D cut view of the PDESM. The stator is composed of a laminated core, solid iron yoke and end-shields, conventional ac three-phase windings, and two excitation annular coils. Solid iron components (external yoke and end-shields) provide a low reluctance path for the wound field excitation flux. The rotor is, among other things, composed of two solid iron collectors and 12 ferrite PMs. This prototype has been designed using a simple magnetic circuit model; it has not been fully optimized [3]. The operating principle and flux control capability of this machine are presented in following sections. The operating principle of the studied PDESM was described in [3]. Some of the features necessary to the comparison study will be however recalled in this section. This machine has a 3-D parallel double excitation structure. Excitation coils are located in the stator, on top of the armature end windings, as shown in Fig. 10(a), thereby avoiding sliding contacts. Azimuthally magnetized ferrite PMs are located in the rotor. The flux focusing principle is used to obtain reasonable values of air-gap flux density. Fig. 12 shows the main trajectory of flux created by PMs. This flux circulates from one pole to another as for classical PM machines using the flux focusing principle. The presence of end-plates used as flux path for excitation flux created by excitation coils induces a leakage flux path for the PMs (about 10% flux loss when the end plates are present compared with flux measurements when the end plates are removed). Fig. 13 shows the wound field excitation flux trajectories. The machine has two annular excitation coils. Each coil is acting in one kind of magnetic pole. The flux created by an excitation coil passes one time through the active part’s air- gap (homopolar path). Depending on dc excitation current direction, excitation coils can be used to either enhance or decrease (weaken) the excitation flux passing through armature windings. More details about the operation of this structure can be found in [3]. As can be seen, the flux created by excitation coils does not pass through PMs, which means that the PM demagnetization risk due to excitation coils is completely cancelled as compared with SDESMs. The open circuit flux control capability of this machine was studied experimentally and using a 3-D finite-element analysis (FEA) (Fig. 14). Fig. 14 shows the 3-D finite-element mesh of the studied machine. The FEA study was used to help establish an analytical model based on MEC; a model that is more convenient to use in the process of design optimization [26]. Fig. 15(a) shows the variation of the maximum air-gap flux versus field MMF obtained by measurements and 3-D FEA. It can be seen that a wide range of air-gap flux control can be achieved. The field MMF is given for one excitation coil. The air-gap flux changes with a variation of + 95% when the air-gap flux is enhanced and − 70% when it is weakened with respect to the no-field excitation flux. It can also be noticed that results from the 3-D FEA agree fairly well with the experimental ones. Fig. 15(b) shows the EMF for different field ampere-turns measured for a machine speed of 170 r/min. IV. SDESM The series double excitation structure (SDESM) used for the comparison study is shown in Fig. 4(a). Both excitation sources, i.e., PMs and excitation coils, are located in the rotor. In the following sections, the operating principle of this structure is first presented. Then, a design study is conducted to optimize the open circuit flux control capability. An analytical model based on formal solution of Maxwell’s equations in low permeability regions is used for that purpose. To have a comparison framework, the stator inner radius of the studied SDESMs is set equal to that of the PDESM used for the comparison study (Fig. 10 and Table I). Fig. 16 illustrates the open circuit air-gap flux control principle for ...