Magnetic control of industrial motors /

Part 1. Alternating-current control devices and assemblies.--Part 2. Alternating-current controllers.--Part 3. Direct-current controllers.

Magnetic circuits and transformers; a first course for power and communication engineers,

"A publication of the Technology Press, Massachusetts Institute of Technology."

A distributed equivalent magnetic current based FDTD method for the calculation of E-fields induced by gradient coils

This paper evaluates a new, low-frequency finite-difference time-domain method applied to the problem of induced E-fields/eddy currents in the human body resulting from the pulsed magnetic field gradients in MRI. In this algorithm, a distributed equivalent magnetic current is proposed as the electromagnetic source and is obtained by quasistatic calculation of the empty coil's vector potential or measurements therein. This technique circumvents the discretization of complicated gradient coil geometries into a mesh of Yee cells, and thereby enables any type of gradient coil modelling or other complex low frequency sources. The proposed method has been verified against an example with an analytical solution. Results are presented showing the spatial distribution of gradient-induced electric fields in a multi-layered spherical phantom model and a complete body model. (C) 2004 Elsevier Inc. All rights reserved.

An equivalent distributed magnetic current based FDTD method for the calculation of e-fields induced by gradient coils in MRI

This paper evaluates a low-frequency FDTD method applied to the problem of induced E-fields/eddy currents in the human body resulting from the pulsed magnetic field gradients in MRI. In this algorithm, a distributed equivalent magnetic current (DEMC) is proposed as the electromagnetic source and is obtained by quasistatic calculation of the empty coil's vector potential or measurements therein. This technique circumvents the discretizing of complicated gradient coil geometries into a mesh of Yee cells, and thereby enables any type of gradient coil modeling or other complex low frequency sources. The proposed method has been verified against an example with an analytical solution. Results are presented showing the spatial distribution of gradient-induced electric fields in a multilayered spherical phantom model and a complete body model.

Nuclear magnetic resonance spectroscopy and ultrasound

The work described in this thesis is directed to the examination of the hypothesis that ultrasound may be used to perturb molecular motion in the liquid phase. These changes can then be detected by nuclear magnetic resonance (NMR) in spin-lattice and spin-spin relaxation times. The objective being to develop a method capable of reducing the pulsed NMR acquisition times of slowly relaxing nuclei. The thesis describes the theoretical principles underlying both NMR spectroscopy and ultrasonics with particular attention being paid to factors that impinge on testing the above hypothesis. Apparatus has been constructed to enable ultrasound at frequencies between 1 and 10 mega-hertz with a variable power up to 100W/cm-2 to be introduced in the NMR sample. A broadband high frequency generator is used to drive PZT piezo-electric transducer via various transducer to liquid coupling arrangements. A commercial instrument of 20 kilo-hertz has also been employed to test the above hypothesis and also to demonstrate the usefulness of ultrasound in sonochemistry. The latter objective being, detection of radical formation in monomer and polymer ultrasonic degradation. The principle features of the results obtained are: Ultrasonic perturbation of T1 is far smaller for pure liquids than is for mixtures. The effects appear to be greater on protons (1H) than on carbon-13 nuclei (13C) relaxation times. The observed effect of ultrasonics is not due to temperature changes in the sample. As the power applied to the transducer is progressively increased T1 decreases to a minimum and then increases. The T1's of the same nuclei in different functional groups are influenced to different extents by ultrasound. Studies of the 14N resonances from an equimolar mixture of N, N-dimethylformamide and deuterated chloroform with ultrasonic frequencies at 1.115, 6, 6.42 and 10 MHz show that as the frequency is increased the NMR signal to noise ratio decreases to zero at the Larmor frequency of 6.42 MHz and then again rises. This reveals the surprising indication that an effect corresponding to nuclear acoustic saturation in the liquid may be observable. Ultrasonic irradiation of acidified ammonium chloride solution at and around 6.42 MHz appears to cause distinctive changes in the proton-nitrogen J coupling resonance at 89.56 MHz. Ultrasonic irradiation of N, N-dimethylacetamide at 2 KHz using the lowest stable power revealed the onset of coalescence in the proton spectrum. The corresponding effect achieved by direct heating required a temperature rise of approximately 30oC. The effects of low frequency (20 KHz) on relaxation times appear to be nil. Detection of radical formation proved difficult but is still regarded as the principle route for monomer and polymer degradation. The initial hypothesis is considered proven with the results showing significant changes in the mega-hertz region and none at 20 KHz.

Dynamical Resonances and SSF Singularities for a Magnetic Schrödinger Operator

We consider the Hamiltonian H of a 3D spinless non-relativistic quantum particle subject to parallel constant magnetic and non-constant electric field. The operator H has infinitely many eigenvalues of infinite multiplicity embedded in its continuous spectrum. We perturb H by appropriate scalar potentials V and investigate the transformation of these embedded eigenvalues into resonances. First, we assume that the electric potentials are dilation-analytic with respect to the variable along the magnetic field, and obtain an asymptotic expansion of the resonances as the coupling constant ϰ of the perturbation tends to zero. Further, under the assumption that the Fermi Golden Rule holds true, we deduce estimates for the time evolution of the resonance states with and without analyticity assumptions; in the second case we obtain these results as a corollary of suitable Mourre estimates and a recent article of Cattaneo, Graf and Hunziker [11]. Next, we describe sets of perturbations V for which the Fermi Golden Rule is valid at each embedded eigenvalue of H; these sets turn out to be dense in various suitable topologies. Finally, we assume that V decays fast enough at infinity and is of definite sign, introduce the Krein spectral shift function for the operator pair (H+V, H), and study its singularities at the energies which coincide with eigenvalues of infinite multiplicity of the unperturbed operator H.

New SR drive with integrated charging capacity for plug-in hybrid electric vehicles (PHEVs)

Plug-in hybrid electric vehicles (PHEVs) provide much promise in reducing greenhouse gas emissions and, thus, are a focal point of research and development. Existing on-board charging capacity is effective but requires the use of several power conversion devices and power converters, which reduce reliability and cost efficiency. This paper presents a novel three-phase switched reluctance (SR) motor drive with integrated charging functions (including internal combustion engine and grid charging). The electrical energy flow within the drivetrain is controlled by a power electronic converter with less power switching devices and magnetic devices. It allows the desired energy conversion between the engine generator, the battery, and the SR motor under different operation modes. Battery-charging techniques are developed to operate under both motor-driving mode and standstill-charging mode. During the magnetization mode, the machine's phase windings are energized by the dc-link voltage. The power converter and the machine phase windings are controlled with a three-phase relay to enable the use of the ac-dc rectifier. The power converter can work as a buck-boost-type or a buck-type dc-dc converter for charging the battery. Simulation results in MATLAB/Simulink and experiments on a 3-kW SR motor validate the effectiveness of the proposed technologies, which may have significant economic implications and improve the PHEVs' market acceptance.

Magneto-Electric Nano-Particles for Non-Invasive Brain Stimulation

This paper for the first time discusses a computational study of using magneto-electric (ME) nanoparticles to artificially stimulate the neural activity deep in the brain. The new technology provides a unique way to couple electric signals in the neural network to the magnetic dipoles in the nanoparticles with the purpose to enable a non-invasive approach. Simulations of the effect of ME nanoparticles for non-invasively stimulating the brain of a patient with Parkinson’s Disease to bring the pulsed sequences of the electric field to the levels comparable to those of healthy people show that the optimized values for the concentration of the 20-nm nanoparticles (with the magneto-electric (ME) coefficient of 100 V cm21 Oe21 in the aqueous solution) is 36106 particles/cc, and the frequency of the externally applied 300-Oe magnetic field is 80 Hz.

Yield of metastable atoms from a rare gas discharge in a longitudinal magnetic field

Atomic beam experiments are limited by intensity. Intensity limitations are specially critical in the measurements of metastable atoms, since their relative population is several order of magnitude smaller than the beam population. This thesis provides a method for increasing the intensity of metastable argon and neon beams effusing from a hot cathode, glow discharge by use of a longitudinal magnetic field. The argon and neon metastable atom intensities have been measured for a range of discharge pressure, voltage, and current for a magnetic field strengths from 0 to 31 mT. For both argon and neon, the metastable atom beam intensity rises to a maximum value about one order of magnitude above the zero field case. A qualitative discussion of the theory of this phenomenon is also presented.

Magnetic ordering in Er₂Ti₂O₇ and elastic softening in Tb₂Ti₂O₇

Geometric frustration occurs in the rare earth pyrochlores due to magnetic rare earth ions occupying the vertices of the network of corner-sharing tetrahedra. In this research, we have two parts. In the first one we study the phase transition to the magnetically ordered state at low temperature in the pyrochlore Er₂Ti₂O₇. The molecular field method was used to solve this problem. In the second part, we analyse the crystal electric field Hamiltonian for the rare earth sites. The rather large degeneracy of the angular momentum J of the rare earth ion is lifted by the crystal electric field due to the neighboring ions in the crystal. By rewriting the Stevens operators in the crystal electric field Hamiltonian ᴴCEF in terms of charge quadruple operators, we can identify unstable order parameters in ᴴCEF . These may be related to lattice instabilities in Tb₂Ti₂O₇.

Fibrillar Elastomeric Micropatterns Create Tunable Adhesion Even to Rough Surfaces

Acknowledgements V.B., N.K.G., and E.A. contributed with conception and experimental design. V.B. performed the experiments. V.B., R.H., A.G., and R.M.M. carried out analysis and interpretation of data. V.B., R.H., A.G., and E.A. wrote the manuscript. V.B. and R.H. contributed equally to this work. V.B. acknowledges funding by SPP 1420 of the German Science Foundation DFG. E.A., N.K.G., and R.H. acknowledge funding from the European Research Council under the European Union/ERC Advanced Grant “Switch2Stick,” Agreement No. 340929.

Crystallographic and magnetic identification of secondary phase in orientated Bi 5Fe 0.5Co 0.5Ti 3O 15 ceramics

The fabrication of highly-oriented polycrystalline ceramics of Bi 5Fe 0.5Co 0.5Ti 3O 15, prepared via molten salt synthesis and uniaxial pressing of high aspect ratio platelets is reported. Electron backscatter images show a secondary phase within the ceramic which is rich in cobalt and iron. The concentration of the secondary phase obtained from scanning electron microscopy is estimated at less than 2% by volume, below the detection limit of x-ray diffraction (XRD). The samples were characterized by x-ray diffraction, polarization-electric field measurements, superconducting quantum interference device as a function of sample orientation and vibrating sample magnetometry as a function of temperature. It is inferred from the data that the observed ferromagnetic response is dominated by the secondary phase. This work highlights the importance of rigorous materials characterisation in the study of multiferroics as small amounts of secondary phase, below the limit of XRD, can lead to false conclusions.

Strain-Tunable Spin Moment in Ni-Doped Graphene

Graphene, due to its exceptional properties, is a promising material for nanotechnology applications. In this context, the ability to tune the properties of graphene-based materials and devices with the incorporation of defects and impurities can be of extraordinary importance. Here we investigate the effect of uniaxial tensile strain on the electronic and magnetic properties of graphene doped with substitutional Ni impurities (Ni_sub). We have found that, although Ni_sub defects are non-magnetic in the relaxed layer, uniaxial strain induces a spin moment in the system. The spin moment increases with the applied strain up to values of 0.3-0.4 \mu_B per Ni_sub, until a critical strain of ~6.5% is reached. At this point, a sharp transition to a high-spin state (~1.9 \mu_B) is observed. This magnetoelastic effect could be utilized to design strain-tunable spin devices based on Ni-doped graphene.

Electric Field Effects on Graphene Materials

Understanding the effect of electric fields on the physical and chemical properties of two-dimensional (2D) nanostructures is instrumental in the design of novel electronic and optoelectronic devices. Several of those properties are characterized in terms of the dielectric constant which play an important role on capacitance, conductivity, screening, dielectric losses and refractive index. Here we review our recent theoretical studies using density functional calculations including van der Waals interactions on two types of layered materials of similar two-dimensional molecular geometry but remarkably different electronic structures, that is, graphene and molybdenum disulphide (MoS2). We focus on such two-dimensional crystals because of they complementary physical and chemical properties, and the appealing interest to incorporate them in the next generation of electronic and optoelectronic devices. We predict that the effective dielectric constant (ε) of few-layer graphene and MoS2 is tunable by external electric fields (E ext). We show that at low fields (E ext < 0.01 V/Å) ε assumes a nearly constant value ∼4 for both materials, but increases at higher fields to values that depend on the layer thickness. The thicker the structure the stronger is the modulation of ε with the electric field. Increasing of the external field perpendicular to the layer surface above a critical value can drive the systems to an unstable state where the layers are weakly coupled and can be easily separated. The observed dependence of ε on the external field is due to charge polarization driven by the bias, which show several similar characteristics despite of the layer considered. All these results provide key information about control and understanding of the screening properties in two-dimensional crystals beyond graphene and MoS2