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 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. BS EN 60404-2:1998+A1 contains an updated version of IEC 60404-2.
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 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, Pc, is calculated as:
where: - number of turns of primary winding
- number of turns of secondary winding
- reading of the wattmeter in watts
- total resistance of the instruments in the secondary circuit in ohms
- average secondary voltage in volts
Specific power loss, Ps, are calculated as:
where: - length of the sample in metres
- average magnetic path length = 0.94 (constant value)
- 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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—Julius J. Epstein (1909–1952)
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—Henry David Thoreau (1817–1862)