Epstein frame

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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.

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