IEC 60404-2:2008 pdf download – Magnetic materials – Part 2: Methods of measurement of the magnetic properties of electrical steel strip and sheet by means of an Epstein frame
3.4 Air flux compensation The mutual inductor for air flux compensation shall be located in the centre of the space enclosed by the four coils, its axis being directed normal to the plane of the axes of these coils. The primary winding of the mutual inductor shall be connected in series with the primary winding of the Epstein frame, and the secondary winding of the mutual inductor shall be connected to the secondary winding of the Epstein frame in series opposition (see figure 3). The adjustment of the value of the mutual inductance shall be made so that, when passing an alternating current through the primary windings in the absence of the specimen in the apparatus, the voltage measured between the non-common terminals of the secondary windings shall be no more than 0,1 % of the voltage appearing across the secondary winding of the test apparatus alone. Thus the average value of the rectified voltage induced in the combined secondary windings is proportional to the peak value of the magnetic polarization in the test specimen.
3.5 Power supply The power supply shall have a low impedance and a high stability of voltage and frequency. During measurements, the voltage and frequency variations shall not exceed ±0,2 % of the required value. For the determination of the specific total loss, the specific apparent power and the r.m.s. value of the magnetic field strength, the form factor of the secondary voltage shall be 1 ,1 1 1 ± 1 %. NOTE This is possible in several ways: for example by using an electronically controlled power supply or a negative feedback power amplifier. The form factor of the secondary voltage is the ratio of its r.m.s. value to its average rectified value. Two voltmeters, one responsive to r.m.s. values and the other responsive to average rectified values shall be used to determine the form factor. NOTE The waveform of the secondary induced voltage should be checked with an oscilloscope to ensure that only the fundamental component is present.
3.6 Voltage measurement The secondary voltage of the Epstein frame shall be measured by means of appropriate voltmeters having an input impedance greater than or equal to 1 000 Ω /V.
NOTE For the application of digital sampling methods, see Annex A.
3.6.1 Average type voltmeter
A voltmeter responsive to average rectified values having an accuracy of ±0,2 % or better shall be used.
NOTE The preferred instrument is a digital voltmeter.
3.6.2 RMS voltmeter
A voltmeter responsive to r.m.s. values having an accuracy of ±0,2 % or better shall be used.
NOTE The preferred instrument is a digital voltmeter.
3.6.3 Peak voltmeter
A voltmeter responsive to peak values having an accuracy of ±0,5 % or better shall be used.
3.7 Frequency measurement
A frequency meter having an accuracy of ±0,1 % or better shall be used.
NOTE For the application of digital sampling methods, see Annex A.
3.8 Power measurement
The power shall be measured by a wattmeter having an accuracy of ±0,5 % or better at the actual power factor and crest factor.
NOTE For the application of digital sampling methods, see Annex A.
The resistance of the voltage circuit of the wattmeter shall be at least 5 000 times its reactance, unless the wattmeter is compensated for its reactance.
If a current measuring device is included in the circuit it shall be short-circuited when the secondary voltage has been adjusted and the loss is being measured.
4 Procedure for the measurement of the specific total loss
NOTE For the application of digital sampling methods, see Annex A.
4.1 Preparation for measurement
The Epstein frame and measuring equipment shall be connected as shown in figure 3.
The test specimen shall be weighed and its mass determined to within ±0,1 %. After weighing, the strips shall be stacked into the coils of the Epstein frame with double lapped joints at the corners and with the same number of strips in each branch of the frame such that the length of the internal side of the square so formed is 220 − + 0 1 mm. Where strips are cut half parallel and half perpendicular to the direction of rolling, the strips cut in the direction of rolling shall be inserted in two opposite branches of the frame and those cut perpendicular to the direction of rolling inserted in the other two branches. Care shall be taken to ensure that the air gap between the strips in the overlapping portions is as small as possible. It is permissible to apply a force of about 1 N to each corner, normal to the plane of the overlapping strips. The test specimen shall then be demagnetized in a decreasing alternating magnetic field of an initial level higher than used in previous measurements.
