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| [[File:Epstein frame.jpg|thumb|right|Epstein frame]]
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| An '''Epstein frame''' or '''Epstein square''' is a standardised measurement device for measuring the [[magnetic properties]] of soft magnetic materials, especially used for testing of [[electrical steel]]s.
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| In [[Europe]], the measurement configuration and conditions are defined by a standard: ''IEC 60404-2:1996 Magnetic materials - Part 2: Methods of measurement of the magnetic properties of electrical steel sheet and strip by means of an Epstein frame''.<ref>[http://webstore.iec.ch/webstore/webstore.nsf/mysearchajax?Openform&key=60404-2&sorting=&start=1&onglet=1 IEC 60404-2]</ref> ''BS EN 60404-2:1998+A1''<ref>[http://shop.bsigroup.com/en/SearchResults/?q=60404-2%26&no=0&d=&c=10&f=&t=d BS EN 60404-2:1998+A1]</ref> contains an updated version of IEC 60404-2.
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| An Epstein frame comprises a [[primary winding|primary]] and a [[secondary winding]]. The sample under test should be prepared as a set of a number of strips (always a multiple of four) cut from electrical steel sheet or ribbon. Each layer of the sample is double-lapped in corners and weighted down with a force of 1 N{{Clarify|reason=explain abbreviation|date=October 2013}} (see photo).
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| The power losses are measured by means of a [[wattmeter]] method in which the primary current and secondary voltage are used. During the measurement, the Epstein frame behaves as an unloaded [[transformer]].
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| Power loss, ''P<sub>c</sub>'', is calculated as:
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| <math>P_c = \frac {N_1}{N_2} \cdot P_m - \frac {\left( 1,111 \cdot |\bar{U_2}| \right)^2}{R_i} </math>
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| where:<br>
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| <math>N_1~</math> is the number of turns of primary winding<br>
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| <math>N_2~</math> is the number of turns of secondary winding<br>
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| <math>P_m~</math> is the reading of the wattmeter in [[watt]]s<br>
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| <math>R_i~</math> is the total resistance of the instruments in the secondary circuit in [[ohm]]s and<br>
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| <math>|\bar{U_2}|</math> is the average secondary voltage in [[volt]]s.
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| Specific power loss, P<sub>s</sub>, is calculated as:
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| <math>P_s = \frac {P_c \cdot 4 \cdot l}{m \cdot l_m}</math>
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| where:<br>
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| <math>l~</math> is the length of the sample in metres<br>
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| <math>l_m~</math> is the average magnetic path length = 0.94 (constant value)<br>
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| <math>m~</math> is the mass of the sample in kilograms
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| If all conditions are as defined in the standard, the [[standard deviation]] of the reproducibility of the values is not greater than 1.5% up to 1.5 T{{Clarify|reason=explain abbreviation|date=October 2013}} for [[non-oriented electrical steel]] and up to 1.7 T for [[grain-oriented electrical steel]].
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| == See also ==
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| * [[Soken tester]]
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| ==References==
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| {{reflist}}
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| [[Category:Magnetism]]
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