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.
Improving accuracy of intrinsic coercivity measurement for magnetically soft materials
This paper considers problems associated with an established coercivity measurement procedure, the "pull out" method, used at the Magnet Centre for many years. Such measurements are an important quality control tool for magnetically soft materials. The main emphasis of this paper is on low carbon relay steels, but other materials have been investigated. Relays and actuators play an increasingly important role in automotive and aerospace technology. Here we review the procedure in detail, but our findings are relevant to other "open circuit" methods of intrinsic coercivity measurement. We have found that errors are caused by rapid changes in applied magnetic field strength at a specific part of the hysteresis cycle, between saturation and the open circuit remanent flux density. We have devoted much attention to the geometry of the samples, with particular emphasis on thickness and dimension ratio, as these are major factors in intrinsic coercivity determination. We propose a modification of the relevant international standard for intrinsic coercivity measurement.
Optimization of single-yoke magnetic testing by surface fields measurement
The main aim of the work is to discuss ways of optimizing a single-yoke magnetization set-up with respect to fluctuations of the measurement results due to imperfections of the yoke–sample contact. The nondestructive testing method is proposed to be optimized by simultaneous multi-point determination of surface magnetic fields of the investigated sample. The presented measurements of the surface fields are accompanied by computational modelling of the magnetic system by the finite element method.