Analysis and comparison of magnetic sheet insulation tests

ELSEVIER

journalof magnetism ~ H and magnetic ~ i materials

Journal of Magnetism and Magnetic Materials 133 (1994) 396-398

Analysis and comparison of magnetic sheet insulation tests M.C. Marion-P6ra *, A. Kedous-Lebouc, B. Cornut, P. Brissonneau Laboratoire d'Electrotechnique de Grenoble (CNRS URA 355), ENSIEG, BP 46, 38402 St Martin d'H~res Cedex, France

Abstract

Magnetic circuits of electrical machines are divided into coated sheets in order to limit eddy currents. The surface insulation resistance of magnetic sheets is difficult to evaluate because it depends on parameters like pressure and covers a wide range of values. Two methods of measuring insulation resistance are analyzed: the standardized 'Franklin device' and a tester developed by British Steel Electrical. Their main drawback is poor local repeatability. The Franklin m e t h o d allows better quality control of industrial process because it measures only one insulating layer at a time. It also gives more accurate images of the distribution of possible defects. Nevertheless, both methods lead to similar classifications of insulation efficiency.

1. Introduction

Magnetic circuits in electrical machines are assembled using laminations insulated from each other in order to limit eddy currents. If defects appear in the insulation, intralaminar losses and local overheating can occur and damage the core. It is therefore of great interest to improve the insulation. It is with this aim in mind that we analyze and compare in this paper two devices used for the determination of surface resistance of magnetic sheets: the Franklin m e t h o d used by most standards [1], and a tester developed by British Steel Electrical (BSE) [2].

2. Description of m e a s u r e m e n t devices

2.1. Franklin device

of 2 N / m m 2. Electrical contact with the substrate of the test specimen is achieved by twist drills which pierce the insulating layer. A constant stabilized voltage of 500 mV is maintained between the electrodes and the drills. 5 ~ resistances in series with each electrode limit the current to 1 A in the event of a short circuit (Fig. 1). N measurements are performed at different points on the sample. The current flowing through the ten electrodes is collected (total current). The coefficient of surface insulation resistance is calculated as follows:

C=A

V/

~ j

It/

-

(•

mm2),

(1)

where R = resistance (5 ~ ) , N = number of measurements, A = area of the ten electrodes (645 mm2), V = voltage (500 mV), and Itj = total current. Our device also allows the currents flowing through each

The Franklin device allows the m e a s u r e m e n t of the surface resistance of one side of the sheet. Ten metallic contacts of fixed area (64.5 mm 2 each) are applied to the surface of the sample with a standard pressure

* Corresponding author. Tel: +33 (76) 82 64 61; fax: +33

(76) 82 63 00. 0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 0 1 6 3 - L

I

~

sheet

Fig. 1. Measurement principle of the Franklin device.

M. C Marion-P~ra et al. / Journal of Magnetism and Magnetic Materials 133 (1994) 396-398 Table 1 Measurements of total currents under standard conditions Sample Mineral coating, 3 p~m M1 M2 Mineral filled organic coating, 2 jxm O21 022 Organic coating, 4 I~m O41 042

Average I t (mA)

Standard deviation (%)

C (12 cm 2)

370 153

38 63

5 18

390 260

28 38

5 9

63 71

55 89

47 42

electrode to b e m e a s u r e d individually (individual currents). T h e data are t h e n r e p r e s e n t e d by a histogram.

2.1.1. Measurements under standard conditions S o m e of t h e tests carried out u n d e r s t a n d a r d conditions of voltage a n d p r e s s u r e are p r e s e n t e d in T a b l e 1 a n d Fig. 2. T h e m e a s u r e m e n t s reveal a wide r a n g e of values. H e n c e m e a n values c a n n o t b e c o n s i d e r e d as sufficient criteria for the assessment of insulation quality. 2.1.2. Repeatability T h e repeatability has b e e n studied by ten successive m e a s u r e m e n t s at the same place. T h e results are quite poor, a n d s t a n d a r d deviations can r e a c h high levels (even m o r e t h a n 100%). This p h e n o m e n o n has b e e n a t t r i b u t e d to m o r e or less r a n d o m e l e c t r o d e - c o a t i n g contacts. However, if silver-painted discs are applied to the c o a t e d s h e e t b e f o r e the m e a s u r e m e n t s , t h e results are

160

iiiiiiiiii!iiiiiiii

397

m u c h m o r e reproducible, a n d the s t a n d a r d deviations fall to a few percent. Nevertheless, n o n - r e p e a t a b i l i t y c a n n o t b e c o n s i d e r e d to be responsible for the scatter in the m e a s u r e m e n t s . In fact, the histograms of rep e a t e d m e a s u r e m e n t s at the same place a n d of meas u r e m e n t s over the whole sample are different: the r a n g e of t h e f o r m e r is smaller t h a n t h a t of the latter. Finally, even if local repeatability is not ensured, a statistical o n e has b e e n noticed. Two series of measurements, m a d e on the same sample, lead to similar distributions.

2.1.3. Influence of pressure A study has shown t h a t t h e coefficient of surface insulation resistance decreases w h e n t h e test p r e s s u r e increases. In fact, the real contact surface b e c o m e s larger. T h e choice of a suitable value for this p a r a m e ter is not so easy. O n the o n e h a n d , the p r e s s u r e must b e high e n o u g h to establish correct electrical contacts b e t w e e n the electrodes a n d the coating. O n the other, the test should b e as close as possible to the situation in electrical machines. This criterion is not well defined b e c a u s e in practice t h e sheets are not pressed h o m o g e n e o u s l y a n d strains are s t r o n g e r a r o u n d clamping elements. T h e p r e s s u r e for the F r a n k l i n test (2 N / m m 2) is certainly h i g h e r t h a n the m e a n value in machines, but can be justified by t h e fact t h a t the coatings are s u p p o s e d to b e a r the worst conditions. 2.2. BSE tester This device allows b o t h sides of the sheet to b e tested simultaneously, i.e. two insulating layers in series in the case of a d o u b l e - s i d e d c o a t e d sample (Fig. 3). T h e electrode area is 645 m m 2, the test pressure is 0.7 N / m m 2 a n d the applied dc stabilized voltage is 100 mV. M e a s u r e m e n t s are given directly in o h m s a n d the m a x i m u m d e t e c t a b l e value is 1000 II. T h e o b t a i n e d results are as s c a t t e r e d a n d as poorly r e p e a t a b l e as the F r a n k l i n total currents.

...~.-..'...~..~...~...[...&..~...~...~-..'...~...i..~...~...;...:....L..:

""~'"'""~...i...~.-..[...~...~...4...5...'...4...&..~...i...~...[....i...:

120

3. Comparison of the Franklin device and the BSE tester

"'~'-.'...~..'~-..~....~..~...~...4...5...'...,~...~...~...~...~...'....5....: "..~'...'...~...~...~....~...&.°&...,~...~°..'...&..~,..&..~...~...L...3..."

80

3.1. Test conditions ...~...:..~...~...&~.~.~.&..~....'s.~5......~.5~..&..~...5..~'~..-:..~.5..~.

...~..........~...~...~....~...&..~...~...~....:...~...~..~...~..~...:.....~...i

40

0

0.02

0.04

0.06

0.08

0.1

I (A) Fig. 2. Histogram of individual currents of a mineral coating.

E x p e r i m e n t s have b e e n m a d e o n t h e same samples with the t h r e e m e t h o d s : individual currents, total currents, a n d the B S E tester. In o r d e r to have test conditions as close as possible, the coating has b e e n rem o v e d on o n e side of the sample to m e a s u r e only o n e insulating layer. B S E pressure has b e e n c h o s e n for all m e a s u r e m e n t s . F r a n k l i n c u r r e n t s have b e e n t r a n s l a t e d into equivalent resistances a n d r e l a t e d to the same surface of m e a s u r e m e n t .

398

M.C. Marion-Pdra et al. /Journal of Magnetism and Magnetic Materials 133 (1994) 396-398 Table 2 Comparison of the Franklin device and the BSE tester

I

[ electrode J :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

Sample 02, I t 02, I i

[

l

1

Fig. 3. Measurement principle of the BSE tester. Comparison tools include histograms, the most probable distribution value, extreme values, the arithmetic average and the harmonic mean representing the equivalent resistance of N resistances, detected on the sample, in parallel, related to the measurement surface of one electrode.

3.2. Results and discussion Three kinds of coatings are tested: mineral coating (M), thick organic coating (O4), and thin minerals filled organic coating ( 0 2 ) (Table 2).

02, BSE 04, I t O4, I i 04, BSE M, I t M, I i M, BSE

Rmi n

Rma x

Rarithm

Rharm

(fD

(~)

(a)

(~)

0.3 8×10 3 0.28 5.1 8×10 2 0.4 1 10 -2 0.2

2.48 177.5 1 416 35×103 1000 416 35x103 1000

0.9 14.5 0.5 273 17×103 848 141 9101 630

0.72 0.3 0.44 50.4 12.7 6.8 6 0.8 1.3

harmonic values are of about the same order, particularly for individual currents and the BSE tester.

3.2.3. Evaluation of insulation quality The insulation quality of the three coatings are compared using four criteria: maximum detected resistances: Rmax(02) < R m a x ( M ) < Rmax(04); minimum detected resistances:

3.2.1. Resistance distribution The three ways of measuring lead to the same shape of distributions. The resistance histogram of the thin coating 0 2 , obtained by the three methods, is rather symmetric around the mean value, but histograms of the two others are asymmetric with mainly high resistances (the highest detectable by the devices used). The most probable distribution values are in good agreement, but the range of the distribution obtained by the individual currents is always wider. In fact, since the individual Franklin electrodes have the smallest areas, they are able to detect higher and lower resistances than the other methods. The resistance distribution is then more accurate and gives more information for the same number of measurements. The maximum values detectable by the three methods differ because the sensibilities of the devices are different: 5 × 104 f~ (10 6 A) for individual currents, 500 f~ (10 -3 A) for total currents, and 1000 fl for the BSE tester. This fact explains the large difference between the maximum values measured with the BSE tester and the Franklin total currents for the mineral and the organic coating 04. 3.2.2. Means The resistance distribution ranges are wide, so that the arithmetic and harmonic means are very different except for measurements of the thin coating with the total currents and the BSE tester. Because the maxim u m detectable resistances are so different for the three methods and are preponderant in the calculus, arithmetic means cannot be in good agreement. But

Rmin(O2 ) < Rmin(M) < Rmin(O4); harmonic and arithmetic means: R . . . . ( O 2 ) < R ..... ( M )

< R ....

(04);

most probable value: Rp(O2) < Rp(M) < Rp(O4). These four criteria lead to the same classification whatever the chosen method: 0 2 provides the worst insulation, the mineral coating a much better one, and the thick organic coating the best.

4. Conclusions

The surface insulation resistance of magnetic sheets is difficult to evaluate because it depends on parameters like pressure and presents a wide range of values. N o n e of the measurement methods considered is completely satisfactory. In particular, they present poor local repeatability. Nevertheless, both methods lead to similar classifications of the insulation quality of the sheets. The Franklin individual currents method allows better quality control of the industrial process and gives a more accurate image of the distribution of possible defects.

References

[1] IEC 404-11, part 11 (1991). [2] P. Beckley, J. Mater. Eng. 12 (1990) 47.