# Epstein frame

Epstein frame

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 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.[1] BS EN 60404-2:1998+A1[2] contains an updated version of IEC 60404-2.

An 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 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[clarification needed] (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:

$P_c = \frac {N_1}{N_2} \cdot P_m - \frac {\left( 1,111 \cdot |\bar{U_2}| \right)^2}{R_i}$

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 watts
$R_i~$ is the total resistance of the instruments in the secondary circuit in ohms and
$|\bar{U_2}|$ is the average secondary voltage in volts.

Specific power loss, Ps, is calculated as:

$P_s = \frac {P_c \cdot 4 \cdot l}{m \cdot l_m}$

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[clarification needed] for non-oriented electrical steel and up to 1.7 T for grain-oriented electrical steel.