Epstein frame

 ( Magnetism ) 

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 .

The International Standard for the measurement configuration and conditions are defined by the standard IEC 60404-2:2008 Magnetic materials - Part 2: Methods of measurement of the magnetic properties of electrical steel sheet and strip by means of an Epstein frame IEC 60404-2:2008 published by International Electrotechnical Commission.

An Epstein frame comprises a primary winding 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 (see photo).

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.

Power loss, P_c, is calculated as:

$P\_c\; =\; \backslash frac\; \{N\_1\}\{N\_2\}\; \backslash cdot\; P\_m\; -\; \backslash frac\; \{\backslash left(\; 1,111\; \backslash cdot\; |\backslash bar\{U\_2\}|\; \backslash right)^2\}\{R\_i\}$

where:
$N\_1~$ is the number of turns of primary winding
$N\_2~$ is the number of turns of secondary winding
$P\_m~$ is the reading of the wattmeter in
$R\_i~$ is the total resistance of the instruments in the secondary circuit in and
$|\backslash bar\{U\_2\}|$ is the average secondary voltage in .

Specific power loss, P_s, is calculated as:

$P\_s\; =\; \backslash frac\; \{P\_c\; \backslash cdot\; 4\; \backslash cdot\; l\}\{m\; \backslash cdot\; l\_m\}$

where:
$l~$ is the length of the sample in metres
$l\_m~$ is the average magnetic path length = 0.94 (constant value)
$m~$ is the mass of the sample in kilograms

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 for non-oriented electrical steel and up to 1.7 T for grain-oriented electrical steel.

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