IEC TR 62981:2017
(Main)Studies and comparisons of magnetic measurements on grain-oriented electrical steelsheet determined by the single sheet test method and Epstein test method
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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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
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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
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.030 ISBN 978-2-8322-4332-9
– 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
Y
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
Figure 13 – Overall dispersion (all labs, J values, and samples) of the laboratories'
p
best estimates P of the power loss at 50 Hz around their reference values . 23
i
Figure 14 – Overall dispersion (all labs, J values, and samples) of the laboratories'
p
best estimates S of the apparent power at 50 Hz around their reference values, with
i
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
p
and samples) of the laboratories' best estimates P of the power loss at 50 Hz around
i
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
P
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
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
– 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
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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.
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
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• reconfirmed,
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IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
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– 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:
• 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)
m
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.
– 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
S
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:
– 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 this purpose. This will become more evident by the results shown in
Clauses 5 and 6.
5 Preliminary comparisons and experiments
5.1 General
In the first phase of this IEC project, a comparison of the relative difference
δP = (P – P )/P measured by steel manufacturers on their own products using their
SE SST Eps Eps
own set-ups was performed. It turned out that the information was not sufficient for specifying
reference values for SST sheet samples (see 5.2 and 5.4).
In order to assess the influence of the yokes on the SST measurement results, further
preliminary comparisons and experiments were subsequently made in China. Four
laboratories and six SST fixtures with yokes having stacked lamination were involved. These
experiments were to improve, besides the knowledge about the dispersion, the knowledge of
the systematic error performance of the SST which becomes more significant when SST
results would be upgraded to independent reference values (see 5.3 and [18]).
– 10 – IEC TR 62981:2017 © IEC 2017
5.2 Comparison of the relative difference δP = (P – P )/P measured by steel
SE SST eps Eps
manufacturers on their own products using own set-ups
In 2012, seven manufacturers took part in this exercise and made their data measured on
related pairs of Epstein and SST samples available for comparison. Two of them contributed
data measured on non-oriented materials of grades 270-50A, 400-50A, 470-65A, 600-50A and
700-50A (5 sample pairs each). These δP results turned out to be between +14 % and
SE
–9 %. They were inconsistent and partly contrary to results published earlier. Therefore, and
because the number of two contributors was too low for any statistical evaluation, further
consideration of these findings related to non-grain-oriented products was abandoned.
However, in the case of grain-oriented material, the simpler sample preparation, wider
applicability and a measurement result that is closer to an imagined true value are the
impetus for the great interest in the Epstein-SST relationship, or, very recently, in the
intention of introducing SST reference values for the grading of grain-oriented materials.
Correspondingly, six manufacturers have contributed δP results measured on 5 or more
SE
samples for some of the following grades of their grain-oriented products: M90-23P, M100-
27P, M103-27P, M105-30P (2x), M130-27P, M110-23S, M120-23S, M120-27S, M130-27S,
M130-30S, M140-30S (2x), M150-35S, M155-35S. Figure 2 shows the resulting relative
difference δP = 100∙(P – P ) / P , averaged for each manufacturer and grade,
SE SST EP EP
determined by the 6 contributors, as circles [6]. The different colours of the fillings are
assigned to the different contributors. The continuous curve represents the least square fit to
the measurement results achieved for 240 of the related grain-oriented Epstein-SST sample
pairs (almost all of S-type, a few of P-type material) [3] which is quoted as the informative
conversion factor in Annex C (informative) of
IEC 60404-3:1992/IEC 60404-3:1992/AMD1:2002 [2].
IEC
NOTE The circles are the data from 6 industry laboratories on 13 g.-o. grades (colours assigned to
manufacturers). The blue continuous curve is δP representing the least square fit to the older PTB measurements
[3] quoted as the informative conversion factor in IEC 60404-3 [2] (the uncertainty of the curve is characterized by
a relative standard deviation of about σ = 2 %[3].
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
The discussion of these findings within IEC TC 68 considered these results as unsatisfactory
with regard to the purpose of introducing SST reference values for the grading of grain-
oriented material. In the course of this discussion, experts from steel manufacturing industry
[5] opened a new view on the Epstein-SST problem by pointing to the deceptive role of the
assessment of Epstein results as seemingly absolute reference values, based on arguments
given in 4.2. Moreover, whilst in general the dispersion of Epstein and SST loss values are
similar, the Epstein method shows a larger dispersion than the SST method when applied to
high-permeability material at the key magnetic polarization 1,7 T (see σ-values in
Figure 15 a) and b)). As a consequence, the realization of a thorough comparison of
measurements on grain-oriented SST sheet samples including high permeability and domain
refined material according to IEC 60404-3 and its evaluation independent of Epstein
measurements was proposed. Epstein measurements were to be executed in parallel in order
to achieve a parallel assessment of the two dispersion characteristics.
δP (%)
SE
5.3 Preliminary comparisons and experiments made by four Chinese laboratories
using six SSTs with stacked yokes
Two laboratories in China measured the magnetic loss of one grain-oriented SST sample and
found a difference of 7 % between their results determined under equivalent conditions. The
search for the reason revealed a considerable difference in the inter-lamination resistance of
the yokes of the two SSTs. This encouraged the initiation of a comparison of measurements
among 4 laboratories: China Jiliang University Hangzhou, Bao Steel Shanghai, Wuhan Iron
and Steel Company (WISCO) and National Institute of Metrology Beijing (NIM) using 6 SST
fixtures with stacked lamination yokes. Specific power loss P and apparent power S at
S S
f = 50 Hz were measured at peak magnetic polarisation levels of J = 1,5 T, 1,7 T, 1,8 T on
four related SST-Epstein sample pairs, each cut adjacently from the same coil, of the grain-
oriented grades: M130-30S, M105-30P (a and b) and M095-23P. Besides, the inter-lamination
resistance in the air gaps of the SST yokes was measured over 5-cm-sections.
For the determination of the conductivity factor C of the yokes' lamination, the following
Y
Formula (1), derived from the classical power loss definition [9], was used:
N
S
C = ⋅ d
Y s
∑
(1)
R
Si
i=1
where
N is the number of sections;
s
d is the length;
s
R is the resistance of section i.
si
The contact paperboard strip shown in Figure 3 was turned over for the measurement of
upper and lower yokes weighted together in parallel circuitry.
IEC
NOTE Paperboard, 0,3 mm thick, copper contacts arranged to 50 mm wide sections; hatched part outside the air
gap, with reinforced contacts.
Figure 3 – Contact pattern for the measurement of
lamination resistance in the air gap of SST yokes
In almost all cases, only the front side allowed proper access for the measurements so these
values were considered as representative for the whole. Since, intentionally, the SSTs were
selected for having yokes of widely different quality, the C covered a very wide range
Y
-1 2 -1 2
extending from 8 Ω cm to 8 000 Ω cm .
– 12 – IEC TR 62981:2017 © IEC 2017
Figure 4 shows the ratio of the power loss P measured on the 4 sheet samples using the
SST
six SSTs, to the power loss value measured by the "best" SST, P (showing the lowest
SSTopt
conductivity factor), plotted versus the lamination conductivity factor of the SSTs, C . It
Y
appears an evident correlation between the yokes’ quality, characterized by the inter-
lamination conductivity, and the excess loss exceeding the value measured by the best SST.
From these results, recommended limits for the yokes’ inter-lamination resistance could be
derived as 10 Ω per section and 100 Ω over the whole length of the parallel upper and lower
yokes’ resistance.
The participants of the China studies also measured the air gap widths profile of their SSTs.
The profile was averaged over the air gap length (45 cm), and a correlation between the
average value and an excess value, corresponding to the resistance procedure, of their
apparent power ratio S /S was searched for – however, a correlation was not found.
SST SSTopt
-1 2
(Ω cm )
IEC
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
SSTopt Y
Encouraged by the positive result regarding the correlation of lamination resistance to excess
loss, the participants of the IEC RRT (2013-14) (see Clause 6) were asked to carry out the
same resistance and air gap widths measurements. The results of the China studies could not
be reconfirmed by the IEC RRT results. The diagrams showed stochastic distributions and no
trend. F. Fiorillo has proposed a possible explanation of this phenomenon based on the
hypothesis that the heterogeneity of the system leads to incorrect results in the resistance
measurements. Similar findings on soft magnetic composite material were presented by C.
Cyr [20]. However, this does not explain why the effect is found with the China experiment,
and with the IEC RRT it does not appear.
On the other hand, this negative result was confirmed by another experiment, i.e. the
frequency dependence of the magnetic loss in the light of the lamination conductivity factor
C . The participants of the IEC RRT (2013/14, see Clause 6) have measured the magnetic
Y
quantities also in the frequency range from 40 Hz to 100 Hz (some started from 20 Hz).
Figure 5 shows the ratio of power loss measured at 100 Hz to that at 40 Hz plotted against
the yokes’ lamination conductivity factor C in logarithmic plotting. It was expected that there
Y
would be a significantly increasing trend in the curves with the increase of the measured
yokes’ lamination conductivity factor C . Apparently, this did not appear, i.e. according to this
Y
finding the lower resistance of the lamination of the yokes seems to have no observable
influence on the loss measurement result. This contradicts earlier experiences.
-1 2
(Ω cm )
IEC
NOTE 085-23P is laser-scribed material.
Figure 5 – Ratio of the power loss at 100 Hz to that at 40 Hz, P /P ,
100 40
at 1,7 T, versus lamination conductivity factor C of the yokes
Y
5.4 Necessity of comparing independent SST results
In order to achieve a comparative view of several comparison studies and to assess their
value for establishing a list of reference loss values for high grade grain-oriented material, the
quantity δP = 100 (P – P )/P was plotted versus the applied magnetic polarization,
SE SST eps Eps
combined in Figure 6, for the conversion curve in
IEC 60404-3:1992/IEC 60404-3:1992/AMD1:2002, Annex C, for the comparison of
manufacturer results (see 5.2), for the China studies (2012) and for the IEC RRT comparison
(2013-14) (anticipated from Clause 6.). The two last studies are presented only by their
values at 1,7 T, the most important normative value. At this polarization, the δP values
SE
spread over a range of about 10 %, clustering partly within the different studies’ results. Thus,
these results suggest that it is inappropriate to base the establishment of high grade SST
reference values on their relation to Epstein results.
Similar conclusions can be drawn from the results shown in Figure 7. The smaller symbols
(the connecting lines have no physical meaning) represent the related power loss difference,
δP , measured, within an euromet comparison project, by the three Standard Laboratories,
SE
IEN (Italy), NPL (UK) and PTB (Germany), on 15 related grain-oriented SST and Epstein
samples. Their grades are indicated in the line "euromet grades". The S-type samples show,
with the exception of the outlying black curve, a relatively good agreement. An explanation
could be that the small grains form a statistically averaging situation in the legs and corners of
the Epstein frame, compared with the more chaotic situation with large grains, and secondly,
the magnetization is closer to technical saturation compared with the P-type samples which is
again in the more chaotic Barkhausen stage at this magnetic polarization.
– 14 – IEC TR 62981:2017 © IEC 2017
J (T)
IEC
NOTE Light grey: copy of Figure 2. Coloured symbols: squares (China 2012) and triangles (IEC RRT 2013/14):
average of participants at 1,7 T. 085-23P is laser-scribed material.
Figure 6 – Relative difference δP = 100(P – P ) / P
SE SST EP EP
versus magnetic polarization
P S
IEC
NOTE Small symbols: euromet comparisons (1999, [19] eval.in [6]), on 15 related Epstein-SST-sample pairs of
2 different P-grades and 2 different S-grades ("euromet grades"). Large circles: IEC RRT (2013/14) [8], measured
on 5 sample pairs ("IEC grades"), for details see Clause 5.
Figure 7 – Relative difference δP = 100(P – P ) / P at 1,7 T determined by three
SE SST EP EP
standard laboratories, IEN, NPL and PTB, on S- and P-type g.-o. sample pairs
δP (%)
SE
Supplier 9
Supplier 8
Supplier 7
Supplier 6
Supplier 5
Supplier 4
Supplier 3
Supplier 2
Supplier 1
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5 8,0 8,5 9,0
σ
r
IEC
NOTE Histogram of the relative standard deviations σ within the 53 groups of 4 or 5 samples each; one group =
r
one manufacturer, one grade.
Figure 8 – Dispersion of manufacturer’s grain-oriented
material production in form of Epstein samples (PTB 1999)
Another relevant aspect regarding the scope of 5.4 is the dispersion performance of the
Epstein values as it appears with different Epstein samples prepared from different coils but
for the same nominal grade. This performance was investigated by PTB in 1998 in connection
with the SST-Epstein-relationship experiments leading to the curve of
IEC 60404-3: 1992/IEC 60404-3:1992/AMD1:2002, Annex C [2]. The 240 grain-oriented
Epstein samples were divided in 53 groups consisting of 4 or 5 Epstein samples of the same
grade, each produced by one manufacturer. The relative standard deviations of the 4 or 5
samples of these groups, σ , are arranged in the histogram (see Figure 8). The combined
r
dispersion of the scattering of the material production and of the Epstein preparation within
one producer is presented and shows a focus at 2,5 % to 3,0 %. The scatter of the Epstein
sample preparation assigned to a group of various producers is still not included. A crucial
experiment could be thought of to clarify this: a batch of P-type 50 cm-sheet samples, as
homogeneous as possible, should be supplied by one manufacturer and circulated to other
manufacturers, one sample to each, for preparing an Epstein sample. Those samples should
then be me
...
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