Equivalence of Measurements on Soft Magnetic Materials in the U.K. and Measurements for Operational Conditions
Measurements on soft magnetic materials were made to determine the measurement capabilities available to the partners of a government-funded collaborative research project, with 16 partners from industry and academia. One of the objectives of this project was the development of advanced magnetic materials for novel electrical machines for use in harsh environments such as gas turbines. To establish the measurement confidence necessary to deliver this project objective, an equivalence study was undertaken. A reference core of soft magnetic material that has been measured over a period of many years at the National Physical Laboratory (NPL) was distributed and the dc and ac properties were measured. The measurement systems of the three laboratories are independent and this ensured that no correlations that could skew the results existed. In the analyses, the measurements at NPL were used as the baseline. With the core capabilities established, it was possible to develop the new measurements required to deliver the overall project objectives. At NPL, a standard permeameter method was modified for the measurement of strips. This was incorporated into a stress machine to measure the dc properties of the soft magnetic materials used by the partners, at stress levels of up to 700 MPa. The results for seven materials are presented and the changes in the properties with applied stress compared to establish which materials exhibit favorable properties.
Effect and compensation of ADC nonlinearity on magnetic accommodation measurements
Abstract In this paper we analyze how static errors of Analog-to-Digital converters (ADCs) can influence the measurement of magnetic accommodation in soft ferrite core. In a first analysis, the discussion is carried out through a series of simulated ADC INLs applied to simulated signals and experimental data. Possible impacts of the integral nonlinearity (INL) on the reconstruction of the B  −  H trajectory are shown. In a second phase, a real ADC INL is considered to better quantify the impact of real ADC errors on the measurements. An easy to implement procedure based on physical symmetry property of the material is presented and adopted to reduce the INL effect on magnetic accommodation measurements. Finally, a comparison between the proposed approach and a mid-point linearization compensation technique of INL is reported showing the validity of the presented procedure.
Comments on "Improving accuracy of intrinsic coercivity measurement for magnetically soft materials"
The authors of the paper by Hilton et al. (see ibid., vol. 41. no. 8, P.2322-7, Aug. 2005) conclude among other things that "the commercial imperative is to obtain accurate coercivity results. How slow does the magnetizing field decay have to be in order to achieve this? There is no universal answer; it depends on the material and on the dimensions. For routine coercivity measurement of soft iron bars, a 60-s linear ramp time is advised. This protocol allows accurate coercivity measurement within an acceptable amount of laboratory time." The present author would like to mention that years ago a number of experiments were carried out indicating that coercivity of magnetically soft materials depends strongly on the rate of rise of the driving field. In the case of a linear ramp driving held, the coercivity, depends linearly on the square root of this rate of rise, k/sub i/. It was shown experimentally that the above dependence holds also for soft ferrites and Permalloy cores as well as for magnetic thin films.