New Models to Analyse the Performance of Magnetic Cores in Electromagnetic Machines for Renewable Energy Systems

Abstract

The magnetic core made, from electrical steels, is an essential part of an electromagnetic machine to link primary and secondary windings with a low reluctance path of magnetic flux to transfer electrical energy between windings. Despite this, the inevitable core loss remains a long-standing troublesome problem hindering the design of efficient and reliable machines. Therefore, it is indispensable to fully understand the magnetic characteristics of electrical steels under magnetic excitation to predict the magnetic core performance when an electromagnetic machine is in service within a power system.
The best way to investigate the magnetic characteristics of electrical steels is to model the magnetic hysteresis and calculate the energy losses under sinusoidal and non-sinusoidal excitation for a wide range of frequencies and peak flux densities. There are numerous excellent models to trace hysteresis loops at low frequencies. However, no models have been developed for simulating the hysteresis loops at high frequencies due to the distortion and irregularity of the curves caused by the complexity of the physical mechanism at high frequencies. To investigate the performance of electrical steels as a magnetic core, this study proposes a novel single equation model derived from the analysis of magnetic domain patterns in ferromagnetic materials to trace hysteresis loops and calculate the energy losses for both low and high frequencies, including both major and minor hysteresis loops.
The magnetic domain patterns in electrical steels consist of two categories, anisotropic domain and isotropic domain; these domain patterns exist in both grain-oriented electrical steels (GOESs) and non-oriented electrical steels (NOESs), but the domain shape and size are somewhat different. The magnetic properties are dominated by the proportion of domain pattern components in the electrical steels. The GOESs are characterised mainly by the anisotropic domain component, and the NOESs are determined mainly by the isotropic domain component. So, it is reasonable for the proposed single equation model containing two items representing anisotropic and isotropic domains to be applied for both GOESs and NOESs.
The energy losses of the magnetic core are the primary concern of power system companies considering the operation cost and climate protection. Typically, the prediction of energy losses is made to evaluate the area of the hysteresis loops. It is difficult for some models to calculate the hysteresis loop area, so finite element computational methods are used to calculate the losses. The calculation of energy losses using the single equation model is advantageous for predicting energy losses because it is made to integrate the equation over the excitation field range.
The physical mechanism of energy loss separation is analysed to propose a novel theory of ferromagnetism to provide the proposed model with the necessary physical grounding. The new separation principle of energy loss is investigated according to the microstructure variation of the ferromagnetic material under an external field. Accordingly, the energy loss of the magnetic core includes three components, hysteresis loss, eddy current loss, and magnetisation loss. The components are calculated to fit the relevant measurement data.