Studies and comparisons of magnetic measurements on grain-oriented electrical steelsheet determined by the single sheet test method and Epstein test method

IEC TR 62981:2017(E), which is a Technical Report, provides the results of international exercises and comparisons focusing on achieving the knowledge of the statistical performance of single sheet tester (SST) measurements made on grain-oriented electrical steel. These experiments aim at specifying obligatory reference values, measured by the single sheet test method, for the grading of high permeability (P grades) grain-oriented (g.-o.) materials, independently from the Epstein classification as it is practiced today. Besides this, Epstein test measurements have been made in order to gain more up-to-date statistical performance for comparison with the SST statistical characteristics. A few experiments were carried out aiming at improved knowledge on the systematic error performance of the SST, i.e. they were to determine the correlation between the quality of insulation separating laminations in the SST yokes and the measured loss.

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Published
Publication Date
21-May-2017
Current Stage
PPUB - Publication issued
Start Date
22-May-2017
Completion Date
22-May-2017
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IEC TR 62981
Edition 1.0 2017-05
TECHNICAL
REPORT
colour
inside
Studies and comparisons of magnetic measurements on grain-oriented
electrical steelsheet determined by the single sheet test method and Epstein
test method
IEC TR 62981:2017-05(en)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC TR 62981
Edition 1.0 2017-05
TECHNICAL
REPORT
colour
inside
Studies and comparisons of magnetic measurements on grain-oriented
electrical steelsheet determined by the single sheet test method and Epstein
test method
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.030 ISBN 978-2-8322-4332-9

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TR 62981:2017 © IEC 2017
CONTENTS

FOREWORD ........................................................................................................................... 4

1 Scope .............................................................................................................................. 6

2 Normative references ...................................................................................................... 6

3 Terms and definitions ...................................................................................................... 6

4 Background ..................................................................................................................... 7

4.1 Historical background and former concepts of the SST-Epstein relationship ............ 7

4.2 Establishing reference values for grain-oriented electrical steels determined

by independent SSTs – A new approach to the purpose .......................................... 8

5 Preliminary comparisons and experiments ....................................................................... 9

5.1 General ................................................................................................................... 9

5.2 Comparison of the relative difference δP = (P – P )/P measured
SE SST eps Eps

by steel manufacturers on their own products using own set-ups .......................... 10

5.3 Preliminary comparisons and experiments made by four Chinese

laboratories using six SSTs with stacked yokes .................................................... 11

5.4 Necessity of comparing independent SST results .................................................. 13

6 International comparison of SST measurements on grain-oriented electrical steel

and accompanying Epstein measurements .................................................................... 15

6.1 General conditions, samples, participants ............................................................. 15

6.2 Circulation of the samples and measurement procedure ....................................... 16

6.3 Results and analysis of the measured quantities ................................................... 17

6.4 Conclusions of the international comparison ......................................................... 30

7 Summary and conclusions ............................................................................................. 31

Bibliography .......................................................................................................................... 32

Figure 1 – Epstein frame and single sheet tester, schematic view, windings partly

omitted ................................................................................................................................... 7

Figure 2 – Relative difference δP = 100 (P – P ) / P versus peak magnetic
SE SST EP EP

polarization J measured by six contributors on samples of their own products ....................... 10

Figure 3 – Contact pattern for the measurement of lamination resistance in the air gap

of SST yokes ........................................................................................................................ 11

Figure 4 – Ratio of the power loss P to that of the SST with the best yokes,
SST

P , versus lamination conductivity factor C of the yokes .............................................. 12

SSTopt Y
Figure 5 – Ratio of the power loss at 100 Hz to that at 40 Hz, P /P , at 1,7 T,
100 40

versus lamination conductivity factor C of the yokes ............................................................ 13

Figure 6 – Relative difference δP = 100(P – P ) / P versus magnetic
SE SST EP EP

polarization ........................................................................................................................... 14

Figure 7 – Relative difference δP = 100(P – P ) / P at 1,7 T determined by
SE SST EP EP

three standard laboratories, IEN, NPL and PTB, on S- and P-type g.-o. sample pairs ........... 14

Figure 8 – Dispersion of manufacturer’s grain-oriented material production in form of

Epstein samples (PTB 1999) ................................................................................................. 15

Figure 9 – Example of scattering of the laboratories’ best estimates around the

reference value (CGO sample No. 2, unweighted average, dash-dotted line) ........................ 18

Figure 10 – Example of scattering of the laboratories’ best estimates around the

reference value (HGO sample No. 4, unweighted average, dash-dotted line) ........................ 19

Figure 11 – Example of scattering of the laboratories’ best estimates around the

reference value (HGO sample No. 5, unweighted average, dash-dotted line) ........................ 19

Figure 12 – Samples No. 1 to No. 5: ratio of SST to Epstein power loss reference

values δP (J ) = (

) /

at 50 Hz versus peak polarization ................ 20

SE p SST Epst Epst
---------------------- Page: 4 ----------------------
IEC TR 62981:2017 © IEC 2017 – 3 –

Figure 13 – Overall dispersion (all labs, J values, and samples) of the laboratories'

best estimates P of the power loss at 50 Hz around their reference values ........................... 23

Figure 14 – Overall dispersion (all labs, J values, and samples) of the laboratories'

best estimates S of the apparent power at 50 Hz around their reference values, with

and without outliers ............................................................................................................... 24

Figure 15 – Dispersion around the reference value of the laboratories' best values of

the power loss P measured at 50 Hz by the Epstein and the SST methods at 1,7 T .............. 25

Figure 16 – Dispersion around the reference value of the laboratories' best values of

the apparent power S measured at 50 Hz by the Epstein and the SST methods at 1,7 T ....... 26

Figure 17 – Overall dispersion (European metrological laboratories only, all J values

and samples) of the laboratories' best estimates P of the power loss at 50 Hz around

their reference values, with and without outliers .................................................................... 27

Figure 18 – Dispersion of the laboratories’ best estimates of SST (a) and Epstein (b)

power loss at 50 Hz .............................................................................................................. 28

Figure 19 – Dispersion of the laboratories’ best estimates of SST (a) and Epstein (b)

power loss at 50 Hz .............................................................................................................. 29

Figure 20 – Dispersion of the laboratories’ best estimates, represented by the standard

deviation σ of SST (red) and Epstein (blue) power loss (a) and apparent power (b) at

50 Hz, versus the peak value of the polarization, J , summarizing Figures 18 and 19 ........... 30

Table 1 – Participating laboratories ....................................................................................... 16

Table 2 – Circulated grain-oriented electrical steel test samples ........................................... 17

Table 3 – Reference values at 50 Hz for the power loss P and the apparent power S ............ 21

Table 4 – Standard deviations associated with the reference values at 50 Hz for the

power loss P and the apparent power S (Table 3) .................................................................. 22

Table 5 – Reference values at 50 Hz of the polarization at H = 800 A/m J and
800

standard deviation of the distribution of the laboratories’ best estimates ............................... 22

Table 6 – Relative standard deviations of 50 Hz power loss P and apparent power S

distributions around their reference values ............................................................................ 27

---------------------- Page: 5 ----------------------
– 4 – IEC TR 62981:2017 © IEC 2017
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
STUDIES AND COMPARISONS OF MAGNETIC MEASUREMENTS
ON GRAIN-ORIENTED ELECTRICAL STEELSHEET DETERMINED BY
THE SINGLE SHEET TEST METHOD AND EPSTEIN TEST METHOD
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

all national electrotechnical committees (IEC National Committees). The object of IEC is to promote

international co-operation on all questions concerning standardization in the electrical and electronic fields. To

this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,

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indispensable for the correct application of this publication.

9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of

patent rights. IEC shall not be held responsible for identifying any or all such patent rights.

The main task of IEC technical committees is to prepare International Standards. However, a

technical committee may propose the publication of a technical report when it has collected

data of a different kind from that which is normally published as an International Standard, for

example "state of the art".

IEC TR 62981, which is a technical report, has been prepared by IEC technical committee 68:

Magnetic alloys and steels.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
68/535/DTR 68/543/RVC

Full information on the voting for the approval of this technical report can be found in the

report on voting indicated in the above table.
---------------------- Page: 6 ----------------------
IEC TR 62981:2017 © IEC 2017 – 5 –

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

The committee has decided that the contents of this publication will remain unchanged until

the stability date indicated on the IEC website under "http://webstore.iec.ch" in the data

related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates

that it contains colours which are considered to be useful for the correct

understanding of its contents. Users should therefore print this document using a

colour printer.
---------------------- Page: 7 ----------------------
– 6 – IEC TR 62981:2017 © IEC 2017
STUDIES AND COMPARISONS OF MAGNETIC MEASUREMENTS
ON GRAIN-ORIENTED ELECTRICAL STEELSHEET DETERMINED BY
THE SINGLE SHEET TEST METHOD AND EPSTEIN TEST METHOD
1 Scope

This document, which is a Technical Report, provides the results of international exercises

and comparisons focusing on achieving the knowledge of the statistical performance of single

sheet tester (SST) measurements made on grain-oriented electrical steel. These experiments

aim at specifying obligatory reference values, measured by the single sheet test method, for

the grading of high permeability (P grades) grain-oriented (g.-o.) materials, independently

from the Epstein classification as it is practiced today. Besides this, Epstein test

measurements have been made in order to gain more up-to-date statistical performance for

comparison with the SST statistical characteristics. A few experiments were carried out

aiming at improved knowledge on the systematic error performance of the SST, i.e. they were

to determine the correlation between the quality of insulation separating laminations in the

SST yokes and the measured loss.

There are various designations for "non-oriented electrical sheet steel" and for "grain-oriented

electrical sheet steel" in use, for example in the IEC 60404 classification and specification

standards, and there are also abbreviations like CGOS (for conventional grain-oriented steel)

often used in industry. In this report, the following designations and abbreviations are used:

– electrical steel as generic term;

– n.-o- electrical steel and g.-o. electrical steel as generic terms for these two types;

– S-type electrical streel or c. g.-o. electrical steel for "conventional grain-oriented electrical

steel";
– P-type g.-o. electrical steel or high-permeability g.-o. electrical steel;

– DR g.-o. electrical steel for "domain refined grain-oriented electrical steel";

– where two terms are used, it can depend on the context;
– "electrical steel" can be replaced with "material", depending on the context.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their

content constitutes requirements of this document. For dated references, only the edition

cited applies. For undated references, the latest edition of the referenced document (including

any amendments) applies.

IEC 60050-121, International Electrotechnical Vocabulary – Part 121: Electromagnetism

(available at http://www.electropedia.org)

IEC 60050-221, International Electrotechnical Vocabulary – Chapter 221: Magnetic materials

and components (available at http://www.electropedia.org)
3 Terms and definitions

For the purposes of this document, the terms and definitions given in IEC 60050-221 and

IEC 60050-121 apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
---------------------- Page: 8 ----------------------
IEC TR 62981:2017 © IEC 2017 – 7 –
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
4 Background
4.1 Historical background and former concepts of the SST-Epstein relationship

The magnetic characteristics of electrical steel are significant in two regards. Firstly, they are

decisive for the possible applications of the material. Secondly, the magnetic loss

performance is essential for the material grading and for the efficiency of the energy

transformation, i.e. for the energy costs and the economic and environmental aspects.

The Epstein method [1] and the single sheet tester (SST) method [2] are the two

standardized methods for measuring the magnetic properties of electrical steel. Whilst the

Epstein method, based on the 25-cm-frame, was designed about 60 years ago, the first

edition of the single sheet tester standard was published in 1982 after intense discussions at

IEC meetings (see Figure 1). This SST(82) standard comprised 500 mm x 500 mm sheet

samples forming the closed magnetic circuit together with two symmetrical flux closure yokes

made of grain-oriented electrical steel or nickel iron alloy. This first 1982-version was

characterized by reference to the Epstein test method, i. e. it had to be calibrated using

Epstein strips, 50 cm long and 30 mm wide, measured in the Epstein square and then,

inserted side by side, in the SST. This method turned out to be considerably dispersive for

reasons which are mentioned in 4.2 and 5.4.

Therefore, 10 years later, IEC published the independent single sheet test method in the

IEC SST(92) standard [2] that includes the use of a conventional effective magnetic path

length of l = 45 cm. However, due to the different designs of their magnetic circuits, SST(92)

and Epstein methods show, in particular with high grade GOES materials, significant

differences of their results when applied to the same material (for details, see 4.2).

IEC
Key
N magnetizing winding
N secondary winding
Figure 1 – Epstein frame and single sheet tester,
schematic view, windings partly omitted

The Epstein method has been in use continuously, from its beginning to the present time,

defined as the only reference method determining the quality reference in the specification

standard. Correspondingly, the grade designations are directly related to the Epstein loss

values, for instance the designation M150-35S5 designates a conventional (S-type) grain-

oriented steel of 0,35 mm thickness with a maximum specific loss value of 1,50 W/kg

measured by the Epstein method at 1,7 T and at 50 Hz. Thus, Epstein loss values have been

the reference values for trade and application purposes, laid down in the lists of the

__________
Numbers in square brackets refer to the Bibliography.
---------------------- Page: 9 ----------------------
– 8 – IEC TR 62981:2017 © IEC 2017

specification standards, for about 60 years. For this reason, the Epstein to SST relationship

was the subject of intense studies during the last two decades [3] [4] [6]. These studies are

described in detail in Clause 5.

It is not easy to change this situation although the SST method is superior when applied to

grain-oriented electrical steel because of its practical simplicity (no stress-relief annealing of

sample needed) and also its suitability to the highest grade materials (e.g. domain refined

grades which do not withstand stress relief annealing without deterioration of their properties).

Therefore, an increasing part of the industry involved requests that SST reference values be

included in the specification standards for these material grades [5].

4.2 Establishing reference values for grain-oriented electrical steels determined by

independent SSTs – A new approach to the purpose

Earlier studies always based their considerations of the Epstein to SST relationship on the

following formula:
δP = (P – P ) / P (or on the equivalent ratio P / P ).
SE SST Ep Ep SST Ep

The different systematic error characteristics of the Epstein and SST methods with grain-

oriented materials can result, for instance, in differences of 4 % to 10 % between the specific

total loss values, P , measured by them at a peak magnetic polarisation of 1,7 T. The

systematic errors were found to be caused by the different magnetic circuit designs of the two

methods, i.e. the inhomogeneity of the Epstein circuit formed by the double-overlapping joints

of the strips (decrease of value), and, on the other hand, by the loss contribution through the

SST yokes (increase of value).

Above, the main sources of systematic errors of both, Epstein and SST, are mentioned. Whilst

systematic errors might be partly explainable, the statistical errors (dispersion), which are

almost of the same magnitude for Epstein and SST, can only partly be assigned to specific

phenomena. However, the Epstein to SST ratio, showing pretty good agreement between

laboratories when identical samples are circulated, shows significant higher dispersion when

the comparison refers to varieties of samples of the same grade (see for example 5.1). The

intrinsic properties of those sample individuals are supposed to vary to an extent which is

determined by the complexity of the process of sample preparation. Thus, it is probable that

there is a significantly larger dispersion with Epstein samples rather than with SST samples

(see also Figure 8 and [11]).

Recently, initiated through experts from industry closely involved in practical metrology [5],

the awareness has grown that the Epstein to SST relationship, comprising the systematic and

statistical error performance of both, Epstein and SST method, is an improper quantity for

upgrading the SST to a reference method for high grade g.-o. electrical steel. The main

reason is a phenomenon which was ignored with the studies published earlier, including the

empirical SST-Epstein relation curve shown in Annex C of [2] which was obtained

predominantly for conventional grain-oriented material. This phenomenon is the uncertainty

that has to be assigned to the preparation of the Epstein strip samples which necessitates a

stress relief annealing operation. This suppresses eventual internal stress due to the

production process and, thus, has a misleading impact on the Epstein to SST relationship.

This effect is more pronounced with high permeability g.o. material. This uncertainty accounts

for a dispersion component of the properties of individual Epstein strip samples caused by the

difference in the preparation procedures between laboratories and the randomly arranged

strips in the sample stack. Items causing this dispersion component are the following.

Firstly, cutting the plate into strips creates basically a specimen with different properties: the

flux is constricted to the strips. High permeability grades partly have grain sizes larger than

the Epstein strip width. Flux paths in legs and corners of the strip’s stack then undergo drastic

changes compared with the entire sheet, and they depend on the random stacking. Internal

stress is introduced through the cutting which shall then be removed through suitable

annealing. Variations in this procedure create further dispersion:
---------------------- Page: 10 ----------------------
IEC TR 62981:2017 © IEC 2017 – 9 –
– the method of cutting and sharpness of the cutting tools;

– the shape of the annealed samples – single strip or stack, with or without weight;

– the annealing procedure – duration, temperature, atmospheres, type of furnace;
– the handling of the samples.

This dispersion is not reflected by comparisons based on circulation of identical Epstein

samples to the participating laboratories as it was practiced in the past.

However, this consideration does not include the still more complicated situation with domain

refined grades which do not withstand stress relief annealing without deterioration of

properties (see below).

In the case of non-domain-refined grades, the cutting to Epstein strips and the process of

annealing the strips can, as mentioned above, change the intrinsic properties of the original

product; in particular, it can make an inferior quality product which includes severe internal

stresses seemingly better by releasing the stresses. This might be tolerated where the

building process of the transformer core involves an annealing stage (e.g. wound cores). For

manufacturers of stacked transformer cores, this is unacceptable [4].
Whilst companies having stable production processes and applying constant sample

preparation may achieve a reasonable in-house-reproducibility of the Epstein method, this is

not sufficient for the grading metrology worldwide. Generally, it can be stated that the higher

the grade, the stronger is the influence on the dispersion from the Epstein sample

preparation.

Finally, with laser domain refined materials, the Epstein test is even not applicable without an

expensive wire cutting of the strips to avoid stress. Also, in this case, a certain dispersion

caused by the different variations of the process of the Epstein sample preparation may be

assumed, however there is no information which allows to quantify this. What remains is the

random flux path fluctuation when large-grain material is cut to strips as was mentioned

above.

Thus, if single identical Epstein sample stacks are passed through various laboratories, a

small dispersion of the measured specific total loss does not tell us the full story. This might

also hold for SST samples, however to a smaller extent, because they are prepared in only

one step, the cutting. The items listed above suggest that the sample preparation procedure

makes the Epstein method results inappropriate as a reference for the conversion into

nominal SST values to be listed as specification of grain-oriented material of higher grades.

Thus, the independent SST method according to IEC 60404-3 [2] is needed as the more

appropriate method for t
...

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