Plastic Moulded Soft Magnetic Materials for Electrical Machines M. Sarasa1), M. Bosse2), D. Gerling3), G. Ziegmann2), G. Kastinger1) Energy and Body Systems, Engineering-Design-Testing (EB/EDT4), Robert Bosch GmbH, Postfach 1163, 77813 Buehl [email protected], [email protected], 2) Fachbereichs Physik, Metallurgie und Werkstoffwissenschaften, Institut für Polymerwerkstoffe und Kunststofftechnik, Technische Universität Clausthal Agricolastraße 6 38678 Clausthal-Zellerfeld [email protected], [email protected] 3) Fakultät für Elektrotechnik und Informationstechnik (EIT), Institut für Elektrische Antriebstechnik, Universität der Bundeswehr München Werner-Heisenberg-Weg 39 D - 85577 Neubiberg [email protected] 1)

Keywords Moulding Manufacturing Technology, Electrical Machines, Soft Magnetic Modified Plastic Abstract This paper presents the implementation of a new soft magnetic filled and injection moulded polymer in electrical machines. The first part reviews the current moulding manufacturing technologies and describes the advantages of high-filled plastics and the limits of the polymer process. Then, the properties of the new material are discussed by taking into account their maximum permeability µrmax, saturation flux density Bs (saturation polarisation Js), and resistivity R´. Material property limits imposed by the moulding process are also mentioned and considerations relative to the use of moulded compounds in electrical machines are examined. These include complex shaping and integration of different motor components. Simulations of such motors are compared with equivalent production motors. Finally, future plans including the manufacturing and testing of a small-sized electrical machine with the most favourable injection moulded material are detailed. 1 Introduction To manufacture more integrated and complex motor topologies, new soft magnetic materials, combining integration and complex shaping properties as well as suitable soft magnetic characteristics and leakage power, can be used. Soft magnetic modified plastics could be ideally suited as they offer a higher potential in manufacturing complex shapes than laminated sheets or sintered materials. Higher integration of different motor components (e.g. injection-mould around inserts) is also an advantage over conventional materials. Reduced eddy-current and hysteresis losses could provide an additional benefit, too. On the other hand, these plastics have lower saturation flux density, Bs, and relative permeability, µr, in comparison with conventional soft magnetic materials such as laminated sheets.

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Ultra high filled thermoplastics

2.1. Compound extrusion The supply of a well dispersed ultra high filled mixture of a thermoplastic and a functionalising filler material is the main task of the chosen plastics machining process. A concurrent flow twin screw extruder is one of the appropriate processes for that. In the present case 71vol.% of a spherical iron silicon powder material containing 6.8mass% silicon (FeSi6.8) was mixed into a thermoplastic Polypropylene (PP) matrix. To support the process a package of additives containing antioxidant, metal deactivator and slip agent was used. Fig. 2-1 shows the installed twin screw extrusion machine and points out the main process highlights. compound discharge

preheated filler material Degassing

pressure zone

Fig. 2-1

plastics and additives Degassing

working zone

melting zone

Twin screw extruder used for production of ultra high filled thermoplastics.

After pouring the thermoplastics – additives mixture into the hopper the melting zone prepares a homogenous melt. The material degasses for the first time. By using a side feeder the preheated metal powder is being added. The extrusion working zone creates a good dispersion of the filler inside the melt. After that the material degasses for another time before it is being delivered to the pressure zone. It is then discharged through the extruder nozzle. To retain the ability of being injection moulded after the extrusion process it must be possible to discharge the compound through the extruder’s nozzle without transcending the given pressures and torques of the machine. Under these conditions the maximum filling degree of iron silicon powder in PP was determined to 95mass% (= 71vol.%). The extrusion parameters used are shown in Table 2-1. Table 2-1

Extrusion parameters.

Rotational speed Temperatures Pressure Torque

400 min-1 feeding zone: 25°C, melting zone: 240°C, working zone: 240°C, pressure zone: 230°C ≤ 100 bar ≤ 70 Nm

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2.2. Injection Moulding For producing functionalised parts out of the high filled thermoplastic compound the injection moulding process was chosen. This is the most common process for the rational production of components with complex geometry. The high content of filler brings about some process specialities. Due to the fact that a dispersion with non molten particles must be processed some important properties of the melt changes: A big quantity of visco-elastic polymer is replaced with rigid undeformed filler material, so the shear thinning effect of a polymer material will partially disappear; the absolute value of the viscosity will increase. The filler material is a much better thermal conductor than the polymer. Inside a mould the injected melt will cool down faster. Therefore the maximum flow length will decrease. The absolute shrinkage of the compound is mainly conditioned by the remaining thermoplastics. Because of it’s small amount the shrinkage decreases down to nearly zero percent between the mould and the component geometry. The tough plastics behaviour decreases to refractory material behaviour. The mould geometry must be suitable for that. The rheological behaviour of a high filled thermoplastic material extremely differs to an unfilled one. Due to measurements the optimal flow can be achieved at shear rates ≤ 200 s-1. Large sprue bush diameters are necessary for that. In consideration of those features it was possible to produce different geometries out of the highly filled compound. Fig. 2-2 shows the designed mould cavity for a toroid geometry and the produced toroid itself in Fig. 3-1 (left side).

Fig. 2-2

Injection moulding cavity for a toroid.

The injection moulding parameters used are shown in Table 2-2. Table 2-2

Used injection moulding parameters.

Temperatures: Injection rate: Holding pressure: Tool temperature:

Hopper: 30°C, zone 1: 180°C, zone 2: 190°C, zone 3: 200°C, zone 4: 210°C, zone 5: 220°C, nozzle: 220°C 10 ccm/s at maximum 1000 bar 1 s at 800 bar, 6 s at 750 bar 30°C

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3 New Soft Magnetic Modified Plastic PP FeSi 6.8 This new moulded, soft magnetic modified plastic has promising properties, allowing its use in electrical machines. Its most interesting feature is its complex shaping properties, enabled by its magnetic isotropic structure. Additionally, it offers the ability to manufacture different components in one moulding process and to integrate different components. Detrimental properties of these plastics are reduced saturation flux density Bs (0.71T) and reduced permeability µr (17.5). When using modified plastics it is important to exploit the advantages which the material has to offer, because a direct replacement of laminations degrades the machine properties [2]. 3.1. Properties of the Filler Material The filler material of the compound is an iron-silicon powder (PP FeSi 6.8) with 6.8wt.% of silicon. The powder consists of spheres with a maximum particle diameter of 106µm. The particle diameter distribution of the powder was determined by a sieve-analysis and standardised to 101 particles. The average values of the different particle sizes are listed in Table 3-1. Knowledge of the particle size distribution is necessary to enable calculations of eddy-current losses in the compound. Eddycurrent losses have a direct influence on the heating of the material; and if used in electrical machines on the machine-heating and machine-efficiency. Thus information about these currents are important for an optimised motor-design. Table 3-1

Particle diameter distribution determined by a sieve-analysis and standardised to 101 particles [1].

Number of Particles 3 7 21 17 29 24

Particle Diameter [µm] 98,0 82,5 64,0 49,0 40,5 28,0

The estimated relative permeability of the filler material is µr ≈ 9600, the saturation flux density Bs is 1,6T. 3.2. Properties of the Compound As opposed to the first moulded soft magnetic compound (cylindrical geometry, see Fig. 3-1 right) with FeSi 6.8 as filler material, the new compound (toroidal geometry, see Fig. 3-1 left) also with FeSi 6.8 offers suitable mechanical properties for use in electrical machines. It is believed that the compounds are strong enough to sustain the strains of mass production, e.g. automated winding.

Fig. 3-1

A new moulded PP FeSi 6.8-compound (left side) and an older one also with PP FeSi 6.8 as filler material is shown.

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In comparison with conventional laminated steel (St37-100 eddy-current loss: PLEC = 4,67 W/kg, hysteresis loss PLHys = 3,97 W/kg), the new material offers a substantial reduction in eddy-current losses as well as isotropic magnetic characteristics. On the other hand this compound suffers from reduced magnetic properties such as µr and Bs. The BH- and the µH-characteristic of the compound are displayed in Fig. 3-2. BH- Characteristic PPFe6,8Si

µH- Characteristic PPFe6,8Si 20

0,8

18

0,7

16

0,6

14 12

0,4

µr

B[T]

0,5

10

0,3

8

0,2

6 4

0,1

2 0 0

10

20

30

40

50

0 0

H [ kA/m ]

Fig. 3-2

5

10

15

20

25

30

35

40

45

50

H [ kA/m ]

BH- and µH- characteristic of the compound with FeSi 6.8 as filler material.

The complete power losses PL (with differentiation between hysteresis losses PH and eddy current losses PE) at 50 and 100Hz, the saturation flux density Bs, the maximum relative permeability µrmax and the resistivity ρ are listed in Table 3-2. Table 3-2

Magnetic and electrical Properties of PP FeSi 6.8

PL [W] PP FeSi 6,8

PH [W]

PE [W]

50Hz

100Hz

50Hz

100Hz

50Hz

100Hz

*

*

*

*

*

*

Bs [T]

µrmax

ρ [µΩm]

0.71

17.5

9.9 · 105

* Measurement is not possible because of to small power losses.

The toroid features a mass of m = 111,50g and a density of ρ = 5,36g/cm³. The described material will be used to mould the armature geometry of a small-sized dc machine designed for an automotive application. 4 The Use of New Plastic Moulded Materials in selected Motor Designs The first step of checking the suitability of the PP FeSi 6.8-compound was to find a suitable electrical machine where this material could be used. An appropriate motor must fulfil many requirements. One of these being a maximum armature thickness of 4mm due to moulding technology requirements. At higher thickness the generation of vacuoles can not be excluded [3]. The vacuoles inside a component reduce the component filling factor and therefore the component permeability. Another requirement is the use of a permanent magnet excitation. In most cases permanent magnet machines feature a relatively large air gap (the relative permeability of hard magnetic materials is similar to the vacuum permeability) and thus the lower permeability of the soft magnetic modified compound is less important than in electrically excited machine [2]. A further requirement was the choice of a mass production motor, because of existing experiences with this machine. As a result of the listed requirements a small sized, permanent excited dc-motor was chosen. The armature of the chosen motor is seen in Fig. 4-1.

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Fig. 4-1

Armature of the chosen small sized dc- motor for building up a prototype. The conventional laminated armature will be replaced with PP FeSi 6.8 compound.

The second step was to compare the performances of the laminated motor with the simulated results obtained for the motor with a plastic moulded armature. In the first simulation, the laminations of the armature were directly replaced by the PP FeSi 6.8-compound. The results show that the stall torque of the dc-motor with the plastic moulded armature is 53% of the conventional motor. In Fig. 4-2, the torque-speed characteristics are presented for both motors. Eq. 4-1 confirms the simulated characteristics in Fig. 4-2. On the left the speed-torque- and current-torquecharacteristic is shown, on the right the speed-current-characteristic. Both simulated motors have got the motor constant c and the same current I.

T=

c ⋅Φ ⋅ I 2 ⋅π

Eq. 4-1 [4]

Only the magnetic flux Φ is reduced due to the use of the PP FeSi 6.8 which has a higher reluctance. Torque-Speed Characteristic

Current-Speed Characteristic 1

1

0,8

0,8

0,6

0,6

0,6

0,4

0,4

0,2

0,2

0

0 0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

T / Tmax n Laminated

Fig. 4-2

n FeSi 6.8

I Laminated

0,9

1

n / nmax

0,8

I / Imax

n / nmax

1

0,4

0,2

0 0

0,2

0,4

0,6

0,8

1

I / Imax I FeSi 6.8

Laminated

FeSi 6.8

Comparison of the revolution-torque-, current-torque- and revolutions-current-characteristic of the conventional dc- motor vs. the Motor with the plastic moulded armature.

The PP FeSi 6.8-motor cannot reach the required operating points. The results could be improved if the isotropic magnetic properties and the geometrical freedom of the material were better exploited. In this case, it would be possible to include rounded edges and to increase axially the tooth tip length, allowing an axial length reduction of the whole motor. With rounded edges the operation of winding the armature becomes easier and external insulation required when winding on laminations is no longer necessary. When exploiting these advantages the torque-to-weight-ratio could approximate the one of the conventional machine. 6

5 Future Developments For further developments and construction of a prototype, it is necessary to find a compound with higher permeabilities. One possibility is to use filler materials combining a very much higher permeability than 9600 and a suitable saturation flux density over 1.6T. Another possibility would be to increase the volume of the filler material, in order to achieve a filling factor of more than 71vol.%. Obviously, this should be done without deterioration of the mechanical properties. In all cases, the generation of vacuoles inside the moulded components should be avoided. For the prototype, it is probably not important because the sizes of the different parts of the armature are smaller than 4mm perpendicular to the moulding direction. For bigger sized motors or components with a maximum size over 4mm, the reduction of vacuoles is extremely important. The next part of this work is to build a prototype with a plastic moulded armature to check the operation of the motor and to verify the results of the simulation. In Fig. 5-1 a view to the chosen armature is shown. This armature model will be the base to manufacture the required injection moulding cavity. Unlike the laminated armature, this prototype will have rounded tooth-edges and longer tooth tips in the axial direction.

Fig. 5-1

Armature model for moulded prototype. Note the longer tooth tips in the axial direction and the rounded edges.

At the same time, the development of a suitable eddy-current calculation for soft magnetic modified plastics is important to have predictions about motor-characteristics. After construction and testing of the prototype, the technical feasibility of this production method for small sized dc-machines will be assessed with respect to mass production criteria. 6 Conclusion At present it is difficult for new soft-magnetic materials to replace conventional soft magnetic materials such as electric sheet or sintered materials. Due to lack of material knowledge, commercial use of these new plastic-moulded material is not yet realistic. At present the achievable permeability of the plastic-moulded material of µcomp = 13.8 is still too small [2]. However, by increasing the filling factor to over 71vol.% and selecting a soft-magnetic filler with a high permeability, substantially higher compound permeabilities can be achieved, which may make the commercial use of soft magnetic modified plastics in electrical machines possible.

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References: [1] [2] [3] [4]

M. Drache: „Simulation der maximalen Packungsdichte aus Siebfraktionen des Materials FeSi 6,8 – 106µm“, Information from Institut für Technische Chemie, TU- Clausthal, Dec. 2003, in German. M. Sarasa, D. Gerling, A. Schumacher, G. Kastinger: „Soft Magnetic Materials for Electrical Machines“, Joint Czech-Polish Conference on Low Voltage Electrical Machines, Brno-Šlapanice, 11.12.Nov.2003. M. Bosse, G. Ziegmann: „FeSi- Magnetkunststoff“, Report about the Manufacturing of a SoftMagnetic Modified Polymer, Mar. 2004, in German. D. Gerling: „Elektrische Maschinen und Antriebe I“, Lecture at Universität der Bundeswehr München, 2002, in German.

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