Hysteresis loss subdivision

Assuming that different energy dissipation mechanisms are at work along hysteresis, a hysteresis loss subdivision procedure has been proposed, using the induction at maximum permeability (around 0.8 T, in electrical steels) as the boundary between the “low-induction” and the “high-induction” regions. This paper reviews the most important results obtained in 10 years of investigation of the effect of microstructure on these components of the hysteresis loss. As maximum induction increases, the “low-induction loss” increases linearly up to 1.2 T, while the “high-induction loss” is zero up to 0.7 T and then increases as a power law with n=5. Low-induction loss behavior is linearly related to H_c between 0.4 and 1.2 T. Grain size has a larger influence on low-induction losses than on high-induction losses. Texture has a much stronger influence on high loss than on low-induction loss, and it is related to the average magnetocrystalline energy. 6.5%Si steel shows smaller hysteresis loss at 1.5 T than 3.5%Si steel only because of its smaler high-induction component. The abrupt increase in hysteresis loss due to very small plastic deformation is strongly related to the high-induction loss component. These results are discussed in terms of energy dissipation mechanisms such as domain wall movement, irreversible rotation and domain wall energy dissipation at domain nucleation and annihilation.     

An intercomparison of rotational loss measurements in non-oriented Fe–Si alloys

We report a comparison of rotational energy loss measurements in the same non-oriented Fe–Si laminations carried out by two laboratories Istituto Nazionale di Ricerca Metrologica (INRiM) in Torino, Italy and Wolfson Centre for Magnetics (WCM) in Cardiff, United Kingdom. The measurements were performed on disk samples at magnetizing frequencies between 5 and 200 Hz with controlled circular flux density loci ranging between 0.2 and 1.9 T. Energy loss was measured applying both the fieldmetric and the rate-of-rise of temperature methods. The latter, exploiting the rate of rise of temperature under quasi-adiabatic conditions, is conveniently adopted on approaching magnetic saturation. Results from the two laboratories agree well up to 1.4 T, despite the different physical principles of the fieldmetric vs. rate-of-rise of temperature methods and the different size of sample and measuring areas. The rate-of-rise of temperature method seems to be the natural approach at high induction values.     

A three-phase single sheet tester with digital control of flux loci based on the contraction mapping principle

Tight control of flux loci in 2D testing of soft magnetic laminations is realized by a method based on the principle of contraction mapping. It is implemented through digital control of the currents supplying a three-phase yoke magnetizer and the use of circular samples. Faithful realization of the prescribed loci and good measuring accuracy are demonstrated in grain-oriented and non-oriented Fe–Si laminations.     

Magnetic losses evolution of a semi-processed steel during forced aging treatments

An ultra-low carbon steel (30 ppm after decarburization) containing Al and Si was aged for distinct soaking times at 210 °C. The core loss increased continuously until around 24 h. After that, only slight changes were verified. It was found that only the hysteresis loss component changed during the aging treatment. By internal friction test and transmission electron microscopy it was seen that carbon precipitation caused the magnetic aging. By scanning electron microscopy it could be concluded that the increase of aging index was attributed to the high number of carbides larger than 0.1 μm.     

Recent development of non-oriented electrical steel sheet for automobile electrical devices

This paper describes non-oriented electrical steel sheet for automobile motors and reactors. Electrical steel sheets for energy efficient motors show high magnetic flux density and low iron loss. They are suitable for HEV traction motors and EPS motors. A thin-gauge electrical steel sheet and a gradient Si steel sheet show low iron loss in the high-frequency range. Therefore, the efficiency of high-frequency devices can be greatly improved. Since a 6.5% Si steel sheet possesses low iron loss and zero magnetostriction, it contributes to reduce the core loss and audible noise of high-frequency reactors.     

Magnetic losses at high flux densities in nonoriented Fe–Si alloys

We present and discuss power loss measurements performed in Fe–(3.5 wt%)Si nonoriented laminations up to very high flux densities. The results are obtained on disk samples using a 1D/2D single-sheet tester, where the fieldmetric and the thermometric methods are applied upon overlapping polarization ranges. The power loss in the highest polarization regimes (e.g. J_p>1.8 T) is measured, in particular, by the rate of rise of temperature method, both under controlled and uncontrolled flux density waveform, the latter case emulating the conditions met in practical unsophisticated experiments. Lack of control at such extreme J_p levels is conducive to strong flux distortion, but the correspondingly measured loss figure can eventually be converted to the one pertaining to sinusoidal induction at the same J_p values. This is demonstrated as a specific application of the statistical theory of magnetic losses, where the usual formulation for the energy losses in magnetic sheets under distorted induction is exploited in reverse fashion.     

On the Steinmetz hysteresis law

The behavior of the Steinmetz coefficient has been described for several different materials: steels with 3.2% Si and 6.5% Si, MnZn ferrite and Ni–Fe alloys. It is shown that, for steels, the Steinmetz law achieves R²>0.999 only between 0.3 and 1.2 T, which is the interval where domain wall movement dominates. The anisotropy of Steinmetz coefficient for non-oriented (NO) steel is also discussed. It is shown that for a NO 3.2% Si steel with a strong Goss component in texture, the power law coefficient and remanence decreases monotonically with the direction of measurement going from rolling direction (RD) to transverse direction (TD), although coercive field increased. The remanence behavior can be related to the minimization of demagnetizing field at the surface grains. The data appear to indicate that the Steinmetz coefficient increases as magnetocrystalline anisotropy constant decreases.     

Magnetic properties and power losses in Fe–Co-based bulk metallic glasses

Magnetic properties of Fe–Co-based bulk metallic glasses have been experimentally investigated. Samples were prepared by water-cooled Cu-mold injection casting technique. The samples have cylindrical shapes with 0.8 mm diameter and 30 mm length. Amorphous structures were confirmed by the presence of a main halo in X-ray diffraction patterns and by the detection of crystallization signal around 650 °C using differential scanning calorimetry.     

Lean non-oriented electrical steel grades

A “Lean Grade Policy” has been adopted involving reduced losses of a given steel composition and supplied grades with typical losses closer to the guarantees. Increased final annealing temperatures and modified hot rolling have resulted in reduced losses and improved permeability. A “lean grade” will have higher heat conductivity and permeability compared to a “standard grade”. The policy could result in less variability and better quality control.     

Comparison of engineering methods of loss prediction in thin ferromagnetic laminations

The loss predictive methods based on the static and dynamic components of power loss are compared with the methods where the total loss is subdivided into hysteresis, classical and excess components. It is explained why the simplest two-component methods can be preferable. An approach to the characterization of a given steel is outlined.     

Loss analysis of non-oriented soft-magnetic materials associating results from hexagonal single sheet testers and Epstein frame testers

The main subject of this work was to analyze magnetic losses of non-oriented soft magnetic materials, which are used in rotating machines like motors or generators, under the conditions of alternating and rotating magnetic flux. For this purpose a hexagonal rotational single sheet tester (HRSST) and a standardized 25 cm Epstein frame tester (EFT) were used, respectively. Since each HRSST sample represents only one random sample, whereas a stack of EFT samples represents a basic population of a production lot, a simple procedure is introduced to derive characteristic rotational loss data from EFT measurements and previously acquired loss factors.     

The effect of compound addition Dy2O3 and Sn on the structure and properties of NdFeNbB magnets

NdFeNbB with the additions of Dy₂O₃ and Sn permanent magnets have been attained by means of powder-blending technique, and their magnetic properties, temperature performance and microstructure were studied in this paper. The addition of just 2.0 wt% Dy₂O₃ or 0.3 wt% Sn proved to be very effective in improving the permanent magnetic properties of NdFeNbB magnets. Dy₂O₃ additions result in the increase in the H_ci and temperature dependence due to the increase of T_c, formation of (NdDy)-rich phase and grain refinement of Φ phase. This improvement of the coercivity stability of the magnets from the addition of Sn is attributed to the smoothing effect of the Sn addition at the grain boundaries. The magnetic properties, the temperature dependence and Curie temperature of NdFeNbB with Dy₂O₃ and Sn combined addition were found to be considerably improved. From the X-ray diffraction, SEM-EDAX studies and the thermo-magnetic study, the improved properties due to the solution of Dy and Sn to the Φ phase, the reduced N_eff and the smaller Φ phase.     

Dependence of the power losses of a non-oriented 3% Si-steel on frequency and gauge

The power losses of a non-oriented 3% Si-steel rolled to gauges between 0.05 and 2 mm and heat-treated thereafter have been measured under sinusoidal polarizations at frequencies between 15 Hz and 10 kHz. The losses were analysed using a loss separation model based on statistical theory. For the thick samples the skin effect caused the model to fail above a certain frequency, while for the very thin samples the model seems to describe the losses well at all frequencies studied.     

A universal DC characterisation system for hard and soft magnetic materials

A fully automatic system has been designed for the accurate measurement of the DC magnetic properties of soft and hard ferromagnetic materials utilising discrete calibrated instruments in order to provide a traceable calibration route separate from the transfer of standard magnetic test samples. Custom written software is used to operate the system in one of three modes, constant dH/dt, variable dH/dt and a second quadrant demagnetisation curve mode. The first two of these modes are utilised for soft magnetic materials with the second mode varying dH/dt in order to keep dB/dt relatively constant. Both modes use cycle times of between 60 and 300 s and may utilise a variety of test configurations including a bar permeameter, electromagnet, ring samples or Epstein frame. The minimum cycle time and the most appropriate mode is dependent on the particular sample and the effect of this on materials with a large dB/dH is significant. Measurements on soft materials include major BH loop, minor BH loops, first-order reversal curves, remanence, coercivity, normal magnetisation curve, peak permeability and loop area. The third mode is used with an electromagnet to measure the demagnetisation curve of hard magnetic materials up to a maximum demagnetisation field of 1.6 MA/m. The measurement algorithm modulates dH/dt depending on dB/dt and incorporates holdback in order to accommodate rare earth materials which exhibit high viscosity.     

Magnetic characterization of HSLA steel by power-law decay exponents of Barkhausen emission signal

The general trend of magnetic behaviour of materials is that the mechanically hard materials are also magnetically hard. However for the high strength low alloy (HSLA) steel tempered at various aging temperatures, the correlation was reported as negative. The anomaly could not be explained by the magnetic parameters like RMS voltage calculated from the Barkhausen emission signal and the coercivity from the magnetic hysteresis loop. This paper reports another magnetic parameter known as power-law decay exponent which shows excellent correlation with the mechanical properties and thus explains the progressive evolution of the microstructural constituents in HSLA steel.     

Magnetic circuit of a contactless torque sensor for electric power steering

Modern passenger cars are increasingly equipped with electromechanical steering assist rather than hydraulic systems known for many decades. Major benefits are reduced fuel consumption (up to 0.2l/100 km) and increased functionality. As such a system reacts to the drivers input in terms of steering torque or steering effort, a sensor is required that accurately measures the steering torque. Valeo has adopted a magnetic technology and has improved the performance by adding specially designed flux concentration devices. The magnetic circuit consists of a multi-pole ring magnet and a pair of ring-shaped soft magnetic parts rotating together with the steering shaft and an additional pair of soft magnetic flux concentration devices which are fixed stationary inside the housing. The steering torque causes a relative twist between magnet and the soft magnetic rings, therefore implementing a proportional magnetisation of the latter. A U-shape was chosen for the flux concentration devices in order to compensate mechanical tolerances of the system. The main focus of this paper will be on the tolerance behaviour of the sensor system and the impact of the flux concentration devices. Because of the nonlinear nature of the magnetisation curve of the NiFe alloy used extensive 3D FEM simulation was necessary. Simulation enables us to have a look inside the soft magnetic material and predict the spatial magnetisation distribution with the benefit of avoiding saturation. The result is an optimised sensor, which meets both the harsh environmental conditions inside the motor compartment as well as the cost pressure in the automotive business.     

On the correlation between microstructure and magnetic losses in electrical steel

The magnetic properties of electrical steel such as magnetization curves, magnetization behavior and specific magnetic losses are related to the microstructure and the texture of the steel. A quantitative model, which describes the effect of microstructure and texture and their interplay on the magnetic losses P, is still missing. Based on experimental data for nonoriented electrical steels and FeSi-samples with high (Si, Al)-content, a more general formula is proposed for the dependence of P, at a given value of magnetic induction B, as a function of the mean value of the grain size d of the material and of the intensities of the relevant magnetic texture components.     

A novel technique for measurement of core loss in low permeability materials

A survey of existing techniques for measuring core loss in magnetic materials is presented. It is shown that the most challenging combination is a low permeability material at low measurement frequency. Such materials have application in components of power converters, and thus are of considerable technological significance. Measurement difficulties stem from losses in the windings employed, which are difficult to disentangle from loss in the core, as well as the stringent requirements on phase accuracy of measurement instruments. Another practical problem is the large power capacity required of the drive source to achieve significant induction levels. A new resonant measurement setup, which we call the “virtual impedance” technique, is proposed as a solution to these problems. Its utility is demonstrated with measurements of a commercial composite core with permeability 14μ₀ whose low-core loss defeats conventional methods of measurement.     

Similarity rules of magnetic minor hysteresis loops in Fe and Ni metals

We have studied similarity rules of quasistatic minor hysteresis loops for Fe and Ni single crystals in the wide temperature range from 10 to 600 K. Two similarity rules of M_R*/M_a*∼3/4 and W_R*/W_F*∼1/6, were found in a medium field range where irreversible movement of Bloch walls plays a crucial role for magnetization; M_a*, M_R*, W_F*, and W_R* are magnetization, remanence, hysteresis loss, and remanence work of a minor hysteresis loop. The similarity rules hold true, being almost independent of kinds of ferromagnets, applied stress, and temperature. The origin was discussed from the viewpoint of pinning effects due to dislocations as well as eddy current effects which become predominant at low temperatures for samples with low dislocation density.     

Real time dynamic domain observation in bulk materials

The paper briefly reviews the development of domain observation techniques used to study basic properties of electrical steels. A new real-time, dynamic, high magnification domain observation technique is presented as an example of a state-of-the-art system, capable of imaging non-repetitive domain wall motion at power frequencies. Advantages of real-time domain observation over stroboscopic magneto-optic studies are presented and the need for real-time observations in developing future loss theories is proposed. Possible relationships between Barkhausen noise, hysteresis and non-repetitive wall motion in electrical steels are suggested and finally examples of non-repetitive wall motion in other magnetic materials are illustrated.