Epstein frame

Epstein frame or Epstein square - standardised measurement device for measuring the magnetic properties of soft magnetic materials, especially used for testing of electrical steels.

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." [ [http://webstore.iec.ch/webstore/webstore.nsf/artnum/020774 IEC Web Store | Publication Detail > IEC 60404-2 Ed. 3.0 Bilingual ] ]

Epstein frame comprises a primary and a secondary winding. The sample under test should be prepared as a set of a number of strips (always multiplicity of 4) cut from electrical steel sheet or ribbon. Each layer of the sample is double-lapped in corners and weighed down with a force of 10 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:

where:$N\_1~$ - number of turns of primary winding
$N\_2~$ - number of turns of secondary winding
$P\_m~$ - reading of the wattmeter in watts
$R\_i~$ - total resistance of the secondary winding in ohms
- average secondary voltage in volts

Specific power loss, P_s, are calculated as:

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

where:$l~$ - length of the sample in metres
$l\_m~$ - average magnetic path length = 0,94 (constant value)
$m~$ - 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.

References