The effects of stress, temperature and cutting on the magnetic properties of electrical steel

Mallett, Paul 2024. The effects of stress, temperature and cutting on the magnetic properties of electrical steel. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Non-oriented electrical steel is a key component in electric motors, used as a medium through which torque is generated via magnetic induction. The degree of magnetisation for a given field and its subsequent power losses are dependent on the microstructure of the material, which is heavily engineered to engender a specific grain structure in order to achieve desired magnetic permeability and loss properties. Changes to microstructure affect those engendered magnetic properties and thus performance in situ can vary as a function of manufacturing and operational processes. This thesis presents two novel experimental systems designed and assembled to investigate these effects by applying heat and stress to non-oriented electrical steel laminations variant in grain size and conducting magnetic measurements. The other system investigated local magnetic properties through the use of a needle probe to illustrate magnetic degradation from different cut edges. These systems were verified to have a specific total loss uncertainty of ±2.05% and local flux density uncertainty of ±3.3%, respectively. It was found that a degradation from cutting highly depends on cutting optimisation, with optimised laser cutting yielding comparable results to those of EDM-cut material, and unoptimised guillotine cutting yielding over 10 mm magnetic degradation with highly asymmetric flux density across the lamination. A new model to describe the local variation in properties as a function of distance from cut edge is presented and validated with experimental data from five different cutting techniques. Bulk and average local permeability reduction was investigated as a function of cut-edge-to-surface-area ratio and found to reduce by at least 30% for ratios above 0.2 regardless of cutting technique. The effects on magnetic properties due to heat and stress were found to depend on the grain size, with smaller grains exhibiting a stronger loss response to temperature and weaker response to stress. The magnetic sensitivity to applied stress and temperature was related to the non-linear grain-boundary density as a function of grain size, indicating materials of grain size ≤ 60 µm will respond more favourably to operational motor temperatures and stresses. The shared dependence of these effects on grain-boundary density affords speculation as to a composite stress-, temperature- and spatially-dependent local loss model.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Engineering
Uncontrolled Keywords:	1) electrical steel 2) magnetic materials 3) heat 4) stress 5) cutting 6) hysteresis
Date of First Compliant Deposit:	23 July 2024
Last Modified:	23 Jul 2024 13:56
URI:	https://orca.cardiff.ac.uk/id/eprint/170855

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