Designing smart devices based on magneto-rheological elastomers (MREs) is a quite challenging task due to the inherent nonlinear behavior of MREs in the presence of an applied magnetic field. In the early stage of the design process, the magnetic circuit parameters need to be optimized via either analytical approaches or finite element (FE) simulations. These require a precise nonlinear magnetic permeability model (<inline-formula> <tex-math notation="LaTeX">$B$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula> curve formulation) of MREs. The model should consider the effect of essential design factors [particle volume fraction (PVF) and anisotropy] and loading conditions (magnetic field and pre-strain). The main contribution of the present study is to formulate a field-dependent novel and reliable nonlinear <inline-formula> <tex-math notation="LaTeX">$B$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula> relationship for MREs as functions of the basic design and loading factors. Three batches of isotropic and anisotropic MRE samples were fabricated with different PVFs (15%, 30%, and 45%). Subsequently, a simple methodology was presented to determine the magnetic permeability and then the nonlinear <inline-formula> <tex-math notation="LaTeX">$B$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula> curves of MREs considering wide variations in pre-strain (0%, 12.5%, 25%, 37.5%, and 50%) and coil current, ranging from 0.2 to 8 A. Experimental results showed that the relative magnetic permeability of both types of MREs increased with increasing pre-strain. Phenomenological-based models were consequently developed to estimate the nonlinear <inline-formula> <tex-math notation="LaTeX">$B$ </tex-math></inline-formula>–<inline-formula> <tex-math notation="LaTeX">$H$ </tex-math></inline-formula> curves of the isotropic and anisotropic MREs accurately and efficiently over the entire design and loading conditions considered. The proposed model can be effectively used for modeling and design optimization of MRE-based smart devices.

Development of New Phenomenological Models for Predicting Magnetic Permeability of Isotropic and Anisotropic Magneto-Rheological Elastomers

B-H characteristics of magnetic materials are an essential input to the design of electrical machines and magnetics in power converters. Wide frequency range operation of machines and converters increasingly requires experimental characterization of the magnetic core material at low as well as high frequencies. Therefore, power amplifiers having wide bandwidth, high voltage and current gains, along with low output offset are needed for characterization. Linear closed-loop multistage amplifiers are often used. Direct coupling of amplifier stages is desirable to characterize the magnetic materials at very low frequencies. The middle stage of such linear multi-stage amplifiers provides the voltage gain. A conventional common-emitter based middle voltage gain stage is reviewed first. A folded cascode amplifier with buffered output is proposed as an alternative. This is capable of direct-coupling with the input (differential), and output (current amplification) stages. The proposed amplifier is shown experimentally to yield a high gain-bandwidth product. Experimental results of the proposed voltage amplifier and the closed loop power amplifier are presented. 

Improved Direct-Coupled High-Bandwidth Voltage Amplifier for B-H Characterization of Magnetic Materials

Aircraft and many other on-board applications require compact electric motor drives having high power densities. A typical motor drive includes an electrical machine controlled by a power-electronic converter. Ferromagnetic materials form an important part of such machines and integrated magnetics in power converters. Hence the knowledge of such materials at operating conditions of frequency and flux density is crucial in the electromagnetic and thermal design of such systems. The information in manufacturers’ datasheets is limited and typically available at conventional frequencies like 50 Hz. However, operating frequencies in such motor drives can extend upto hundreds and thousands of Hz. Hence, in-house characterization of magnetic materials becomes necessary over the frequency range of interest. In this paper, two SiFe steels- 50C600 and 50C800 are characterized over a frequency range of 333 Hz to 1000 Hz, and upto flux densities of around 1.5 T. Experimental B-H loops are obtained at different frequencies and compared. Detailed procedure for evaluation of magnetization and core-loss curves from multiple B-H loops is provided. Core-losses at working levels of flux-densities are observed to be much higher at higher frequencies. The experimental data obtained, can be used in the design and analysis of magnetic structures fabricated using such materials. 

Experimental Evaluation of High-Frequency Magnetization Characteristics of Core Materials

Transformer excitation magnetizing curves (TEMC) reflect the dynamic operation characteristics of iron-core materials. Using numerical analysis and the waveform recording function of digital oscilloscopes, we developed a cost-effective method for determining the TEMC. This approach eliminates the need for conventional analog integrator circuits. To address the potential obstacles to the digital generation of TEMC—namely, curve offsets and curve transients—we proposed solutions involving Fourier filtering and determining the initial point of integration. The results indicate that the proposed approach yields results consistent with those of conventional analog integrator circuits and highlight its promise for applications in data processing.

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A Study of Digital Measurement and Analysis Technology for Transformer Excitation Magnetizing Curve

B-H characteristics of magnetic materials are an essential input to the design of electrical machines and magnetics in power converters. Wide frequency range operation of machines and converters increasingly requires experimental characterization of the magnetic core material at low as well as high frequencies. Therefore, power amplifiers having wide bandwidth, high voltage and current gains, along with low output offset are needed for characterization. Linear closed-loop multistage amplifiers are often used. Direct coupling of amplifier stages is desirable to characterize the magnetic materials at very low frequencies. The middle stage of such linear multi-stage amplifiers provides the voltage gain. A conventional common-emitter based middle voltage gain stage is reviewed first. A folded cascode amplifier with buffered output is proposed as an alternative. This is capable of direct-coupling with the input (differential), and output (current amplification) stages. The proposed amplifier is shown experimentally to yield a high gain-bandwidth product. Experimental results of the proposed voltage amplifier and the closed loop power amplifier are presented.

Energy losses during the magnetization process of soft magnetic laminations depend on the excitation waveform. Manufacturers of magnetic materials may not provide losses for different excitation waveforms. This may affect the accuracy of the predicted losses which are used to estimate the performance of electrical equipment during the predesign stage. Thus, a characterization tool which is capable of measuring hysteresis loops under controlled magnetization is required. Digital characterization systems available in literature use expensive commercial power amplifiers. In this work, a cost-effective power amplifier circuit is developed and interfaced with a LabVIEW program to characterize soft magnetic materials. The measurements for a sinusoidal excitation from the proposed setup are validated with those from a standard commercial measuring setup. The system is also used to measure hysteresis loops for arbitrary magnetization conditions (magnetic flux density ( $ {B}$ ) with harmonics). The proposed setup can operate over a frequency range 35–500 Hz. A small Epstein frame is also designed for measurements at higher frequencies (>200 and <500 Hz). A provision is also made to predict three components of dynamic hysteresis losses at any frequency in the above range using the loss separation approach. This capability of the setup enables the user to predict the losses at frequencies beyond the capability of the tool. The predicted losses using this algorithm are validated with measured losses. The proposed setup is cost-effective and can be developed easily using basic electronic components.

Greeshma Mohan Unniachanparambil

Papers

A Novel Cost-Effective Magnetic Characterization Tool for Soft Magnetic Materials Used in Electrical Machines