Proceedings of the 2008 International Conference on Electrical Machines
Paper ID 909
Soft Magnetic Composite with Lamellar Particles Application to the Clawpole Transverse-Flux Machine with Hybrid Stator Patrick Lemieux1, O. Jude Delma2, Maxime R. Dubois3, Roderick Guthrie4 1,4 Mcgill Metal Processing Center, 2,3 Laboratoire d’Électrotechnique, d’Électronique de Puissance et de Commande Industrielle (LEEPCI), 1
[email protected] [email protected] [email protected] 4
[email protected]
Abstract
magnetic parts. The magnetic isotropy and production method of SMC material with iron powder [2] have opened new perspectives in the design of electrical rotating machines. New machine topologies putting emphasis on the maximization of power density were developed. In [3] for example, the notches of a stator of a permanent magnet machine were extended up to the end winding, allowing it to collect more magnetic flux. However, this type of material has less attractive magnetic properties than electrical steels. Because SMC is made from a mixture of ferromagnetic material and electrically insulating material, they have low saturation flux densities and permeability due to fact that there are air gaps between the iron particles. Moreover the shaping process for SMC’s induces strain and stress in the particles. These can only be partially removed by a stress annealing process, since the electrically insulating material cannot withstand the temperatures needed for complete annealing. That limitation increases hysteresis losses considerably. The SL-SMC presented in this article combines the shaping advantages of powder metallurgy with the excellent magnetic properties of laminated electrical steels by using a composite of lamellar particles that are partially insulated, yet fully sintered [4]. This magnetic material is anisotropic but as it is moldable, some types of parts hardly possible with laminated steel can be pressed with this material. As an example of its application, we compare the performances of two Transverse-Flux Machines with a Hybrid Stator [5] where the concentrators made with SMC are replaced by concentrators made with SL-SMC.
This paper proposes a Sintered Lamellar Soft Magnetic Composite Material (SL-SMC) which combines the shaping advantage of soft magnetic composite material (SMC) using iron particles with the excellent magnetic properties of laminated electrical steels wherein the particles are lamellar and partially insulated/sintered. In the paper, the manufacturing process for this new material and its physical attributes are described. The effects of the latter on its magnetic properties are also discussed. A comparison is made between two Clawpole Transverse-Flux Machines (TFM) with a hybrid stator using concentrators fabricated with SMC for one of them and a SL-SMC for the other, in order to determine the improvements that this new material can bring to this type of machine
I.
INTRODUCTION
The excellent ferromagnetic properties of iron have lead to iron being used for decades as a core material for magnetic circuits. However, as iron is a very good conductor, the use of massive parts of the pure metal leads to high power losses in the form of Eddy Currents which are induced in the metal in the presence of time-varying magnetic fields. Several magnetic materials have been developed using iron in order to obtain magnetic circuits with optimal magnetic performances and low losses. These materials can be classified according to the range of electric frequencies associated with their use. Two types of magnetic materials are mostly used in design of electromagnetic devices working at low frequencies. They are the Fe-Si electrical steels [1] and soft magnetic composites (SMC), [2] With electrical steel, magnetic circuits are established by stacking magnetic sheets. In these magnetic circuits, the magnetic flux has to flow in directions parallel to sheet steel’s surfaces. This kind of material has very good magnetic proprieties but the anisotropy of laminated electrical steel and constraints due to their manufacturing impose limits on the geometries of the magnetic circuits possible. SMC`s are also used in the design of electromagnetic devices, largely operating in the kHz range and, more recently, in some new designs of rotating machinery. They find their application in topology of machine where the magnetic flux has to flow in the three dimensions in the 978-1-4244-1736-0/08/$25.00 ©2008 IEEE
II. DESCRIPTION OF THE SOFT MAGNETIC COMPOSITE WITH LAMELLAE PARTICLES
The SL-SMC is made of lamellar metallic particles. These particles are cut from different rolled sheets whose top and bottom surfaces are coated with a dielectric coating capable of withstanding a temperature well above 1000°C. The rolled sheets can be made of metallic material containing at least one of Fe, Ni and Co. Sheet thicknesses must be less than 150µm in order to limit Eddy Currents in the final part. The dielectric is a thin alumina layer produced by a new high productivity Sol-Gel spray technology. The sol gel solution contains an organic additive [6] which improves the coating’s adhesion on rolled sheet and decreases its tendency 1
Proceedings of the 2008 International Conference on Electrical Machines a)
b)
Fig. 3. a) Texture of forged part; b) Texture of sintered part.
During pressing, the lamellar particles deformed such that their insulated top and bottom faces, rotate perpendicular to the pressing direction. Therefore, during sintering or hotforging, the welds between the particles are principally in the directions perpendicular to pressing direction. The resulting part has structural similarities with those made by laminated steel stacking. This molded part consequently has good mechanical properties and good permeability perpendicular to the direction of pressing, with almost no air gaps. Furthermore, given the high thermal resistance of the dielectric coating, thermal treatments can be done to remove the stress in particles and promote grain growth of metal. Hysteresis losses can accordingly be reduced considerably. SL-SMC brings the possibility to form magnetic parts with new varied geometry due to its molding process while keeping magnetic properties similar to laminated steels. Therefore parts are anisotropic and have directions of favorable magnetization (easy directions) and directions of non-favorable magnetization (hard direction).
2 mm
Fig. 1. Lamellae particles making process.
to crack at high thickness. The final calcined dielectric thickness is under 1µm. Fig.1 shows the process of cutting the metallic lamellar particle from rolled-coated sheets. As a result, the two major surfaces of each metallic micro-lamina are coated with the dielectric while its cut edges are not. The preferred size of a square lamella is approximately 2 x 2 mm. Parts are obtained by pressing the lamellae in a mould. Given that the edges of the lamella are not covered with insulation, some metallic contact can be established between neighbouring lamella. The part is then sintered or forged at high temperature. Sintering promotes “necks” to form between adjacent particle edges, while complete stress relief, and even recrystallization of the lamellae, can be achieved, giving excellent mechanical and magnetic properties. Preheating and forging has the same effect while increasing importantly the density of the material. Pictures a) and b) in fig. 3 show the texture of parts respectively forged and sintered, in a direction parallel to the direction of compression. The black zones correspond to non-ferromagnetic zones like air. We can see that a forged part has a higher density of magnetic components compared to a sintered part.
III. EVALUATION OF MAGNETIC PERFORMANCES OF SOFT MAGNETIC COMPOSITE WITH LAMELLAR PARTICLES
In order to evaluate the magnetic performance of soft magnetic composite with lamellar particles, samples with annular and parallelepiped shapes were fabricated. See fig.2 and 4. The ring test identified the material’s magnetic properties in the easy directions. The parallelepiped samples allowed the material’s magnetic properties in the hard direction to be measured. A magnetic circuit of amorphous
Samples
Fig. 2. Parts made with SL-SMC; at top right, part in forging matrix
Fig. 4. Methods for the evaluation of magnetic properties (at top left rings for properties perpendicular to the pressing direction, at top right parallelepiped samples for the properties parallel to the pressing direction).
2
Proceedings of the 2008 International Conference on Electrical Machines TABLE 1 COMPARATIVE TABLE OF MAGNETIC PROPERTIE FOR SL-SMC (LAMELLAR), SMC AND M19 0.35mm DC magnetic properties
SL-SMC / easy direction SL-SMC / hard direction Magnetic steel M19 0.35mm SMC2 1 2
Eddy currents losses 1T 100
AC losses 1T (W/kg)
Bmax 1 (T)
HC (A/m)
µmax
60Hz
1.59
60
2300
1.95
26.4
-
390
900
40
1118
1.7
77
10000
1.36
18.9
1.4
150
290
11
77
60
400Hz
40 20 0 0
PCore _ SL −SMC ( f , B)
600 800 Frequencies (Hz)
1000
1200
(1) m Where PCore/m represents the specific core losses. In fig.5 , we can see that the curve generated by eq. 1 represents the measured core losses for 1T induction very well. The two terms of eq. (1) represent hysteresis and Eddy current losses respectively. Fig. 6 shows the contribution of each of these losses according to the frequency of magnetic field. At frequencies lower than 100 Hz, core losses are mainly due to hysteresis losses. As expected, we see that the hysteresis losses in the SLSMC are much less important than in the SMC. Sintering or forging of parts allow for the removal of air gaps, plus complete stress relief in particles, plus metal grain growth. The small quantity of Eddy current losses confirms the excellent thermal resistance of the coating and the layered structure of the part so molded. The graphs in fig.7 were drawn from eq. (1), (2) and (3) which model, respectively, core losses in magnetic circuits made with SL-SMC, SMC [5] and M19 rolled sheet steel [5]
PCore _ SMC ( f , B) m PCore _ M 19 ( f , B) m
= 0.026 B 2.07 f + 9.9 ⋅ 10 −5 B 2 f 2
= 0.18B1.7 f + 5 ⋅ 10−5 B 2 f 2
(2)
= 0.024B1,7 f + 6 ⋅10 −5 B 2 f 2
(3)
We can notice that the SL-SMC has a magnetic behavior comparable to that of rolled sheet steels. The scale of Eddy Current will be influenced by the thickness of
140 Cores losses (W/kg)
400
of favourable magnetization. The obtained model is given by eq. (1)
material was used to channel the magnetic flux to the parallelepiped samples. Amorphous material losses are negligible and its permeability is very high compared to that of the lamellar material. The results presented in this paper are for parts made with an iron-3% silicon alloy. These parts were forged, annealed and sintered at 1212°C under hydrogen. It offers both low iron losses and high flux density. In table 1, we observe that the SL-SMC has a high permeability and saturation flux density close to that of laminated electrical steel. As explained previously, this high permeability is due to the metallic links existing between particles in the direction of favorable magnetization due to the sintering process. Such is not the case with SMC. Furthermore, the lamellar structure allows more compact layouts which substantially increase the part density and its saturation flux density compared to SMC materials. It also minimizes the thickness of air between the layers of magnetic materials, providing rather good permeability in the direction of unfavorable magnetization (that is, in the direction parallel to pressing direction). In the direction of favorable magnetization, core losses are also much lower than SMC. Fig. 5 shows the iron losses measured on a ring sample. A mathematical model of the iron losses was derived from the experimental data. This model was determined for various frequencies of sinusoidal excitement field and various levels of induction in the sample. This modelling has been done firstly in the direction
Model 1T
200
Fig. 6 Hysteresis and Eddy current losses in SL-SMC. (easy direction)
:@ H = 8000A/m : @ ref: [7,8]
120
Hysteresis losses 1T
80
Core losses (W/kg)
Materials
120
Measure 1T
100
250
SMC ATOMET EM-1 1T M19 0.35mm 1T SL-SMC Fe3%Si 1T
80
200 core losses (W/kg)
60 40
150
20
100
0 0
200
400
600
800
1000
1200
Fréquencies (Hz)
Fig: 5. Real and modeling core losses as function of frequency of an applied field for SL-SMC (easy direction).
50 0 0
200
400
600
800
1000
1200
Frequency (Hz)
Fig: 7. Losses as function of frequency of an applied field for SMC, rolled steel sheet and SL-SMC (easy direction).
3
Proceedings of the 2008 International Conference on Electrical Machines particles and the electrical resistivity of the alloy as in rolled sheet steels. The thickness of the coating also influences Eddy Currents between particles, which affect losses. Due to its molding properties, it constitutes an advantageous option compared to SMC and to rolled sheet steels in certain machine topologies. Similar measurements were carried out on a parallelepiped sample of the lamellar material for the hard direction. The results are reported in fig. 8. The core losses this time were mathematically modelled for the unfavourable direction (see eq. 4).
P( f , B) = 0.30 B 2.87 f + 6.23 ⋅10−3 B 2 f 2 m
TABLE II MECHANICAL PROPERTIES OF STUDING SL-SMC PARTS AND SMC PARTS Identification
SL-SMC SMC 1
(4)
Measure 1T
Cores losses (W/kg)
600 400 200 0 50
100
150
200
250 300 Fréquencies (Hz)
350
400
450
Fig: 8. Real and modeled core losses as a function of frequency of an applied field for SL-SMC (hard direction).
1200 Model 1T
Cores losses (W/kg)
1000
(kg/m )
(MPa)
(MPa)
7400
210
1
95
7100
124
-
: @ ref: [8]
Studies of Transverse-Flux Permanent Magnet Machines (TFPM) have shown a good potential for very high force densities leading to very compact electrical machines. In [9], a theoretical study based on Double-rotor, double sided TFPM geometry with flux-concentration, as shown in fig. 10 a) indicates that force densities up to 200 kN/m2 are possible. Among the various geometries proposed for TFPM machines [10], one finds the Clawpole TFM proposed by [11]. In fig.10 b). the Clawpole TFM has the advantage of singlesside stator and a single row of permanent magnets. These characteristics greatly facilitate the machine’s construction and manufacture. With the Clawpole TFM, the flux density vector has high components in 3 dimensions, mainly in the stator core. It thus requires the use of SMC for stator core. The force density of TFPM machines is strongly dependent on pole pitch and air gap thickness [12][13]. The TFM’s generally have a high number of poles and thus, short pole pitches. The combination of short pole pitches and SMC material leads to high core losses in the stator. In [5], a solution is proposed for the reduction of iron losses in the Clawpole TFM by using a combination of Fe-Si rolled sheet steel and SMC in the stator; see fig.11. Laminations are used to carry the flux around the coil while SMC is used near the air gap to deviate the flux coming from the air gap towards the laminated C-core. The use of SMC material contiguous to an air gap on the rotor and stator provides easy machining of rotor and stator thus leading to very thin air gaps as little as 0.55 mm. In the rotor, flux-concentrators are also made of SMC for this reason. The available flux in the air-gap is limited by the permeability (reluctance) of the concentrators as well as by their maximal flux density. In certain TFPM machines, concentrators are made by rolled sheet steel [10]. But in this case, the air gap cannot be kept low and regular. In fact, a
800
0
UTS
IV. INTEREST OF SMC IN THE DESIGN OF CLAWPOLE TRANSVERSE-FLUX MACHINE WITH HYBRID STATOR
1200 Model 1T
TRS
3
Fig.9 shows the contribution of each type of loss. In the hard direction, the eddy currents are more important because they circulate on the particle’s plane. Effective electrical insulation thickness is thus limited to something above 2 mm taking into account inter-particular contacts. Regarding hysteresis losses, in this direction, the thickness of metallic grain is limited by the thickness of particles (125 µm). As with regular SMC, each particle interface can be considered as an air gap, limiting permeability by arming the movement of Bloch walls and producing demagnetizing fields. Table 2 presents the mechanical properties of hot forged iron-silicon based SL-SMC compared to a SMC using 1% resin as an insulating and binding coating. 1000
Density
Measure 1T
800 600 400 200 0 0
50
100
150
200
250 300 Fréquencies (Hz)
350
400
45
Fig. 9 Hysteresis and Eddy current losses in SL-SMC. (hard direction) a) Fig. 10 a) Double-rotor, double sided TFPM geometry with fluxconcentration. b) Clawpole TFM concept, with full SMC stator.
4
b)
Proceedings of the 2008 International Conference on Electrical Machines 2,5
B SMC (T) SL-SMC (T) B M19 (T)
B (T)
2 1,5 1 Fig. 11 Clawpole TFM with Hybrid Stator
0,5
stack of steel sheets cannot be machined easily, and it would be intricate to use sheets with different width to assemble concentrators. Thus the global reluctance in such kind of machine is high due to thicker air gaps, limiting the flux in the stator. Parts made with SL-SMC are molded. Bow forms can be obtained. Moreover, as parts are highly dense and sintered, they can eventually be machined. Thus, SL-SMC is an interesting material for making flux-concentrators with low reluctance, high flux density and low losses, without increasing the machine air gap.
0 0
5000
10000 15000 20000 25000 30000 H (A/m)
Fig. 12. B-H curves of SL-SMC, SMC and laminated steel used in the FEA calculations.
V. EVALUATION OF THE PERFORMANCES OF CLAWPOLE TFM WITH CONCENTRATOR MADE WITH SL-SMC By simulation, we evaluated the impact of SL-SMC by incorporating it into the design of the Clawpole TFM with a hybrid stator of fig. 11. We compared two machines, one with concentrators made with SMC and the other with SLSMC. Both are simulated using the 3D Finite Element Analysis (FEA) magnetostatic package MagnetVI8 from Infolytica. The fig. 12 presents the B-H curve of the materials used in simulation. The magnetization curve of the SL-SMC for its easy direction is determined from the setting described in fig. 4. It was not possible to determine exactly this curve in the hard direction because the amorphous material used to channel the flux through parallelepiped samples saturates before the parallelepipeds themselves. In table 1, we can see that SL-SMC also has good permeability in its hard direction. We then assumed that the B-H curve is isotropic for the simulation. Cores losses are modelled using eq. 1 and 4 for SL-SMC and eq.2 for SMC. Knowing the variation of magnetic induction in each direction in the parts and knowing parts density (see table 2), core losses can be determined. The geometry used for the comparison is shown in fig. 13. In this application, the concentrator’s easy directions are directed according to machine radius and its rotation axis. Symmetrical boundary conditions are imposed in the calculations, as a matter, a single pole pair of the machine needs to be considered. Flux and torque are computed for 24 different positions in one cycle; that is for every 15 electrical degrees. In calculation A, the concentrator’s material chosen is SMC, while in calculation B, the concentrator’s material is the SL-SMC.
Fig. 13. One pole pair of the clawpole TFM used in the FEA computation.
Considering the machine in fig. 13, with 30 poles (p = 15) used as a generator at 240 RPM, the No-load induction in the C-core is plotted, for each type of concentrator in fig 14 as a function of time for one electric period. We can see a substantial increase in the maximum induction when the concentrators are made with Fe 3%Si SL-SMC, compared 1,5 SMC EM1 SL-SMC
1
B (T)
0,5 0
0,008333 -0,5
0,013333
0,018333
0,023333
-1 -1,5
time (s)
Fig. 14. Induction in the C-core when no charge is connected to the machines
5
Proceedings of the 2008 International Conference on Electrical Machines VI. CONCLUSIONS
TABLE 3 INFLUENCE OF SL-SMC ON THE PERFORMANCE OF A TRANSVERSE-FLUX MACHINE WITH HYBRID STATOR (SIMULATION) No-load RMS induction in C-core (T)
No-load core losses in Concentrators due to fundamental frequency W
SL-SMC
0.9
33.9
96.13
Powder SMC
0.78
38.5
94.14
Concentrator material
The paper presents the use of a new type of SMC material based on lamellar particles. These can be moulded into complex 3D parts as with more conventional SMC (powdered or water atomised, irregularly shaped particles). Experiments with this new composite material indicate that its magnetic properties are very close to those of laminated electrical steels. These are significantly better than conventional powdered SMC’s. Nonetheless, both types of particles share the advantage of being mouldable versus conventional large laminates. Simulations obtained by using the properties of the new composite in its different directions in a clawpole transverse-flux machine with an hybrid stator demonstrates that the lamellar composite is highly advantageous, even if the material is not always exposed to a field only oriented in its easy direction.
Total no-load core losses in the machine (W)
to an SMC concentrator. The 13.6% increase is due to the higher permeability and the higher magnetization of the SLSMC. It reduces the reluctance of the machine and increases the magnetic induction available in the machine air gap. This increase of the induction will be directly reflected in the apparent power of the machine. A slight modification in the design of the concentrators, the permanent magnets and/or the stator foot in order to optimise it for the use of the SLSMC material, could have given slightly better results. For quantitative comparison, however, the designs were kept identical, in order to validate all the results with a real bench test. As shown in table 3, the no-load core losses in SL-SMC concentrators are 12% less important than those in the SMC concentrators. The simulation shows that the alternative component of the magnetic field in the concentrators has three dimensions. The decreasing of the core losses indicates that this alternative component varies mainly in the concentrator’s “easy” directions. The increase of the magnetic flux in the air gap and accordingly in the stator, leads naturally to an increase in the stator core losses, particularly in the stator foot. However, this is still produced using SMC material in the mathematical model. Because of the important increase in the stator core’s losses, the machine’s global core losses are increased by 2% as shown in table 3. However, this increase in losses is much lower than the increase in the apparent power of the machine (13.6%) , giving a more efficient machine, in toto.
REFERENCES [1] [2] [3] [4] [5] [6]
[7] [8] [9]
If the scenario is to run the machine at the same power output, it is important to note that the simulation of the total losses of the machine with SL-SMC does not also take into account the resistive losses (joules losses) in the electrical copper winding of the machine. Less copper winding would be needed for the machine, for the same power output, since it runs at a higher total induction with the SL-SMC. For future work, in addition of doing a bench testing validation of the simulated values, the replacement of the stator feet by SL-SMC could give different results. We know that this intricate geometry component, still fabricated as an SMC, contains important three dimensional components of its magnetic field, and that the path of the magnetic field lines are complex. The simulation is difficult to do at this stage because we have, first of all, to determine the best way to press it, in one or many sub-parts, so as to maximize the orientations of the lamellae in the main direction of the field path. Other machine designs must also be considered with this new material.
[10] [11] [12] [13]
6
A.J. Moses “Electrical steels: past present and future developments, ” Science, Measurement and Technology, IEE Proceedings A, Sep 1990 Page(s):233 - 245 Person M. ,Jansson P., Jack A.G. and Mecrow B.C. “Soft Magnetic Composite Materials – Use for Electrical Machines” Proc 1995 IEE Elec Mach. And Drives Conf. Alan G. Jack, Barrie C. Mecrow “Permanen-Magnet machine with Powdered Iron Cores and Preperssed Windings” Industry Applications, IEEE Transactions on Volume 36 July-Aug. 2000 Page(s):1077 - 1084 P. Lemieux, 2004, Patent PCT-CA04-00147, EP 1595267 “High performancemagnetic composite for AC applications and a process for manufacturing thesame”, 2004 Maxime R. Dubois, Nicolas Dehlinger, Henk Polinder and Daniel Massicotte. “Clawpole Transverse-Flux Machine with Hybrid Stator” ICEM 2006, Chania, Greece, paper 412 Chengbin Jing, Jinxia Hou, “Sol-Gel Derived Alumina/Polyvinylpyrrolidone Hybrid nanocomposite Film on Metal for Corrosion Resistance”, Journal Of applied Polymer Science, Vol. 105, 697-705 (2007) Transverse Rupture Strength, Standard #41 MPIF, Princeton NJ, (ASTM B 528). Registered mark of Quebec Metal Powders ltd for its water atomized irregular particles SMC. Weh H., “Permanentmagneterregte Synchronmaschinen hoher Kraftdichte nach dem Transversalflusskonzept,” etzArchiv, vol.10, no.5, pp. 143 - 149, 1988. [In German]. Maxime R. Dubois , Henk Polinder “Study of TFPM machines with toothed rotor applied to direct-drive generators for wind turbines” NORPIE 2004, Trondheim, Norvège, 14-16 juin 2004 Maddison C.P., Mecrow B.C. and Jack A.G., “Claw Pole Geometries For High Performance Transverse Flux Machines”, ICEM 98, Vigo, Spain, p. 340-345. Kang D.H., Chun Y.H., and Weh H., “Analysis and optimal designof transverse flux linear motor with PM excitation for railway traction”, IEE Proc. Electr. Power Appl., vol. 150, no. 4, July 2003. Hasubek B.E., Nowicki E.P., “Design Limitations of Reduced Magnet Material Passive Rotor Transverse Flux Motors Investigated using 3D Finite Element Analysis”, Canadian Conf. on Electr. & Computer Eng. Vol. 1, 7-10 March 2000, pp. 365 – 369.
