Journal of Magnetism and Magnetic Materials 160 (1996) 171-173 Journal of magnetism and magnetic materials

ELSEVIER

Experimental analysis of the field distribution in a large RSST

N. Nencib, A. Kedous-Lebouc *, B. Comut Laboratoire d'Electrotechnique de Grenoble, CNRS-URA 355, ENSIEG, BP 46. 38402 Saint Martin d'H~res Cedex, France

Abstract In this paper, the distribution of the magnetic field in a 300 mm RSST (rotational single sheet tester) is determined.

Measurements have been carried out on GO and NO samples, using Hall probes, for three excitation axes: rolling, transverse and 45 °. The influence of the material anisotropy and the yoke-sample air gap is reported. Variation of the field perpendicular to the sample plane justifies using the double H-coil method.

Kewvords: RSST: H distribution; Hall probe; H coil

1. Introduction 2. RSST and the field analysis system

A rotational single sheet tester (RSST) is the main device which allows rotational in-plane characterization of magnetic material sheets. Many problems related to this device exist and have not yet been overcome [1]. To achieve uniform magnetization of the whole sample is the main problem. Several efforts have been made to widen the sheet area in which the magnetic field remains homo- geneous. It is very effective to size the search coils cor- rectly for measurement of the field H and the flux density B. The distribution of these two variables in the plane of the sheet has to be determined. Numerical simulations based on finite element software can help the optimization and the design of an RSST [2], but are not sufficient to analyse the H homogeneity in the sample precisely. In fact, many assumptions are needed and the macroscopic anisotropy of the test specimen is not taken into account. Experimental analysis is then required in order to deter- mine the homogeneous area of the field corresponding to the measurement area. Such an investigation is presented in this paper. The H distribution in a large RSST is analysed and the influence of the anisotropy of the mate- rial on flux leakages and the role of the air gap between the sample and the yoke are highlighted. The H variation perpendicular to the sample is also studied.

Corresponding author. Fax: +33-76826300; email: afif.lebouc @leg.ensieg.fr.

The study was carried out on a large RSST designed for a 300 mm side or larger square sample [3]. It consists of two double vertical yokes which hold the exciting coils. They are obtained from two GO SiFe wound cores. The air gap between the yoke and the specimen is adjusted pre- cisely.

To obtain a detailed field distribution map, pin-point measurements have been performed using Hall effect probes. A two-perpendicular-probe system with its signal treatment circuits is set precisely on the sample thanks to a suitable non-magnetic support. The whole plane of the sheet can then be analysed by displacement of the system (2 mm above the sample surface) in different directions with a 20 mm step. The two H components are measured with a two-channel synchronous voltmeter. The repeatabil- ity of the sample setting and the field measurement are first tested. The standard deviation of H does not exceed 0.8%, which is satisfactory.

3. Field distribution with zero air gap

The field homogeneity was analysed on grain-oriented and non-oriented SiFe material. The exciting field was sinusoidal and was applied along three different axes: the rolling (RD), transverse (TD) and 45 ° directions. In this case, the air gap between the sample and the yoke is limited to the residual gap. The main results are summa- rized in Figs. 1 and 2. The curves describe the standard deviation of the field obtained in variable squared areas which are chosen in the centre of the sample and which simulate the H coil. These results show that the best homogeneity of the field is obtained in the 45 ° excitation

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172 N. Nencib et al. / Journal of Magnetism and Magnetic Materials 160 (1996) 171 173

80 r ~ 15oo RD F ~

70 ~ He d~ 2000 TD ~ / / " -- (Nm) ,-~ 55o //z~ /~ q

g 60 f " - "0- -55oo 45 ¢ / / x~ " ~ q L o ,~oo / / / -

40 1 . . . . . . . . . . . . . . . . . . . . . . . . . .

"~ 30 .................................................

20 40 60 80 100 120 140 160 180

Size of the central square of the sheet (mm)

Fig. 1. Standard deviation of H, cr H, measured near the sample surface for RD, TD and 45 ° excitations and different fields Hc in the center of the sheet, NO SiFe.

o=

7O

5O

L

2o i

10

0 , 20

2400 RD [] 7.~ /a

H c .Ik 1500 TD / {#drn da I If~) ~ "

O 11oo

. . /

& ~ j . J ~:r:'>"

40 60 80 100 120 140 160 180

Size of the central square of the sheet (mm)

Fig. 2. As for Fig. 1, but for the GO SiFe sample.

direction. The TD remains the most unfavourable and an increase of the applied field leads to an improvement in homogeneity. This behaviour can be explained by the anisotropy of the material. For GO SiFe, it is very difficult to magnetize the transverse direction, and leakage fluxes are important along the RD; they lead to poor homogeneity of the field in the sheet plane. When H is high, almost all the magnetic domains are oriented along the 90 ° axis and

the effect of the rolling direction decreases. In the case of NO material, similar phenomena apply, but the difference between RD and TD is smaller because the anisotropy of the material is weaker. For 45 ° excitation, the flux leak-

ages generated by the two main directions are balanced and consequently the field homogeneity is very good.

4. Influence of the air gap between the sample and the yoke

The preceding tests were repeated for different air gaps. No air gap effect is noticed for the 45 ° direction. The observed H homogeneity is reported in Table 1 for NO material and for RD and TD. In the case of GO material, the air gap influence is negligible along RD because this direction remains strongly preferred. The TD behaviour is

similar to that of the NO sheet. The air gap plays an important role in the field distribu-

tion. It increases the independence between the two per- pendicular axes by increasing the core reluctances. This

makes the flux cross in the middle of the sheet and leads to a more uniform variation of the field in the surface.

For different air gaps, the measurements were carried out with the same exciting current. Table 1 shows that the insertion of an air gap also increases the exciting field in the middle of the sample. Additional experiments confirm that H increases with up to a 2 or 3 mm air gap and then

decreases. This result is due to the joint action of the flux leakage and the increase of the yoke reluctance.

Table 1 Influence of the air gap (H c = field in the center of the sample, H m = mean field in the considered area)

Air gap H c Measurementarea, centralsquare side(mm)

(mm) (A/m) 40 80 120 160

NO SiFe, RD excitation 0.0 1450 H m (A/m) 1600 1800 2060

~n (%) 15 32 46 0.2 1820 H m (A/m) 1920 2100 2350

~# (%) 9 26 40 1.8 2040 Hm(A/m) 2040 2220 2390

~H (%) 4 15 29 NO SiFe, TD excitation

0.0 1960 Hm(A/m) 2260 2520 2730 ~H (%) 21 34 43

0.2 2030 Hm(A/m) 2280 2550 2880 ~H (%) 18 31 46

1.8 2070 H m (A/m) 2360 2480 2950 ~H (%) 14 21 42

2360 59

2650 53

2810 47

2980 53

3290 56

3290 53

N. Nencib et al. / Journal c~f Magnetism and Magnetic' Materials I60 (1996) 171-173 173

5. Field distribution perpendicular to the sheet plane

The H variations along different axes perpendicular to the sheet surface were determined in order to optimize the H-coil setting. The results show that the field amplitude varies rapidly when we move away from the sample surface. Indeed, at 5 mm height, the H variation is about 20%. In addition, this variation is roughly linear. This study confirms the need to investigate double H coils to achieve precise measurements, and validates the linear extrapolation of the two H-coil signals for field calculation [4].

6. Conclusion

In this study, the H distribution in a 300 mm RSST was determined. The homogeneity is strongly affected by

the quality of the material and the exciting directions. A 1.5 mm air gap between the yoke and the sample was chosen. This is optimum for both a good homogeneity and high level of the field. The RSST was then equipped with double H coils of 80 mm size. This large area corresponds to a good field homogeneity. It represents the material correctly and offers measurable search-coil signals.

References

[1] J. Sievert, J. Magn. Magn. Mater. 112 (1992) 50. [2] N. Nencib et al., J. Magn. Magn. Mater. 133 (1994) 553. [3] N. Nencib et al., J. Magn. Magn. Mater. 160 (1996) 174 (these

Proceedings). [4] T. Nakata et al., IEEE Trans. Magn. 23 (1987) 2596.