General Electric

About: General Electric is a company organization based out in Boston, Massachusetts, United States. It is known for research contribution in the topics: Turbine & Signal. The organization has 76365 authors who have published 110557 publications receiving 1885108 citations. The organization is also known as: General Electric Company & GE.

Introduction to computer aided tomography

Abstract: In recent years, Computer Aided Tomography (CAT) has had a major impact on the medical fields of radiology and neurology and nuclear medicine. This paper introduces the basic physics and mathematics underlying the production of reconstructed tomographic images. It discusses the evolution and philosophies of different possible data collection and reconstruction schemes.

Overview of PI-Based Solutions for the Control of DC Buses of a Single-Phase H-Bridge Multilevel Active Rectifier

Abstract: The H-bridge-based multilevel active rectifier is an attractive topology that allows feeding of multiple dc loads. The main control problem is to manage state variables (one current plus dc voltages) with only switching functions. In this paper, three PI-based solutions are discussed, and the tuning of the best solution is addressed in case resonant controllers are used instead of classical integrators. Moreover, practical problems such as the implementation and the influence of phase-locked-loop systems and of analog filters (first order and Butterworth) on the measurements are discussed.

Propagation of Large Barkhausen Discontinuities. II

Abstract: Large Barkhausen discontinuities have previously been observed by Forrer and by Preisach in nickel wires and hard-drawn wires of the nickel-iron series respectively under stress. A prediction that these discontinuities occur in form of a propagation along the wire, starting at a nucleus, has now been substantiated. In the experiments an additional local field was used to start the propagation at a definite point on the wire, which was in a uniform magnetic field, and the velocity was determined by measuring the short time interval elapsing between the passage through two search coils around the wire. With a fixed value of tension on the wire, the velocity $v$ was found to vary approximately linearly with the applied uniform field $H$, so that $v=A(H\ensuremath{-}{H}_{0})$. $A$ is the slope of the velocity-field characteristic and ${H}_{0}$ is called the critical field. Measured velocities range from 500 to 40,000 cm ${\mathrm{sec}}^{\ensuremath{-}1}$. ${H}_{0}$ varies with composition, amount of cold working, and with the stress applied to the wire. Increasing tension reduces the critical field over the greater part of the Ni-Fe alloy composition range. The behavior of ${H}_{0}$ with increasing and decreasing tension shows the presence of elastic hysteresis. $A$ is nearly constant for changes in tension, in diameter of wire, for composition of wire, and is the same for a strip. Its value is approximately 25,000 cm ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{gauss}}^{\ensuremath{-}1}$.The existence of eddy currents limits the speed with which magnetism can penetrate the wire. A rough calculation of this time gives values in the neighborhood of ${10}^{\ensuremath{-}2}$ sec. Thus a discontinuity travelling at ${10}^{4}$ cm per sec. occupies a length of some 100 cm on the wire. This was substantiated both by measurements of the peak voltage induced in a search coil and by oscillograms taken of the induced voltage. The observed passage times agree well with the theoretical penetration times.The constancy of the $v\ensuremath{-}H$ slope for wires of different diameters is believed toindicate that the velocity depends upon surface phenomena rather than volume phenomena. The velocity would thus be determined by conditions existing near the front edge of the discontinuity where the penetration is still slight. The critical field is believed to represent a threshold value of magnetic field which must be exceeded at all points of the wire before reversal of magnetism can occur. The excess of the impressed field over the critical is nullified during propagation by the fields arising from the eddy currents. A possible picture of the discontinuity is one in which the reversal occurs within a minute distance of an approximately conical surface in the wire, the edge of the base of the cone forming the front of the wave.The explanation advanced by Preisach for the asymmetric hysteresis loops found if one limit of the magnetization cycle was reduced has been extended. In this case magnetic inhomogeneities act as nuclei. Mechanical distortion introduces inhomogeneities of another type which also lead to the very easy formation of a nucleus. The phenomena found with torsion are more complicated than for tension. In some cases the slope of the $v\ensuremath{-}H$ lines shows appreciable variation with direction of twist. The results of tests with various compositions of the nickel-iron series are described using both tension and torsion, but no newrelations to other properties of these alloys can be given so far. Identifying ${H}_{0}$ with coercive field, R. Becker's theory has been compared with our results. It appears that in most cases increased elastic tension and increased cold working stresses shift ${H}_{0}$ in opposite directions.