National Physical Laboratory

AC measurements of magnetically soft materials: specific total loss, specific apparent power and AC permeability

Epstein frame 

The specific total loss, the specific apparent power and AC permeability of sheets, strips and ring cores of electrical steels and high permeability nickel-iron alloys can be measured in accordance with IEC 60404 Part 2: 1996 and IEC 60404 Part 3: 2002, (electrical steels). Test specimens in the form of strips 305 mm x 30 mm are tested in the Epstein frame. Sheet specimens of electrical steel 500 mm x 500 mm can be tested in a single sheet tester built to the IEC recommended design.

The measurement of specific total loss, specific apparent power and AC permeability for ring cores can be made up to a frequency of 100 kHz in accordance with IEC 60404 part 6. To determine the material properties cores with a ration of outer to inner diameter of less than 1.2 are required. Measurements can be made at various magnetic flux density levels. Typical levels include 50, 100 150 and 200 mT. The actual levels achievable will depend on the loss of the core and the mass of the core. It may be necessary to make measurements on a smaller core at higher frequencies to achieve the required magnetic flux density. At these frequencies and for reasonable loss figure the core temperature will increase. Depending on the material, the required properties could have significant temperature coefficients and the temperature at the surface of the core will be reported. As required by the written standard IEC 60404 part 6, these measurements are made with either the magnetic field strength waveform of the magnetic flux density waveform sinusoidal.

AC measurements 

The measurement of the magnetic properties of soft magnetic materials for sinusoidal conditions are useful for the purposes of purchase and sale. In machines, the waveform conditions are often far from sinusoidal and estimation of the loss properties requires measurements made for the same conditions. Work on measuring these quantities for non-sinusoidal waveforms is under way.

For both Hard and Soft magnetic material measurements, a number of instruments are required to measure the B and the H. For magnetic flux density, B, measurements, detection coils and integrators. The effective area of search coils can be calibrated at NPL using a mutual inductance bridge. The calibration of integrators across the full working range is possible using calibrated fixed value mutual inductors. To determine the magnetic field strength, H, Hall probes are typically used. Using a number of coil systems and an electromagnet these can be calibrated up to 3 T in most cases directly against a Nuclear Magnetic resonance magnetometer.


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Last Updated: 18 Aug 2015
Created: 11 Jun 2009


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