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AC measurements of soft magnetic materials

Soft magnetic materials testing

Soft magnetic materials, in the form of electrical steel strips and sheets, are used widely in transformers and motors. AC measurements are made on these materials which typically have intrinsic coercivity less than 1000 A/m. They are used primarily to enhance and/or channel the flux produced by an electric current. 

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 and IEC 60404 Part 3 (Epstein strips and single sheets respectively). 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 ratio of outer to inner diameter of less than 1.2 are required. Measurements can be made at various magnetic flux density levels, with actual levels achievable dependant on material and geometry.

The measurement of the magnetic properties of electrical steels for sinusoidal conditions are useful for the purposes of purchase and sale. In electric motors, the waveform conditions are often far from sinusoidal and an estimation of the loss properties requires measurements made for the same conditions.

NPL has experience in making measurements on electrical steel strips being subjected to tensile stress, which is of use to simulation and modelling tools where the correct data for specific applications is required. 

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