General Information About Power Electronics

There are many possibilities of implementing power stages for unconventional actuators. In the following, we will introduce their fundamental operating principles and some of their combinations. 

The current and voltage at the output of a power amplifier can be depicted as functions of time or in a voltage-current coordinate system, see Fig. 6.141. Depending on whether the user would like to operate an ohmic, an inductive or a capacitive load, and depending on whether this operation should be unipolar or bipolar, he will require an amplifier with one, two or four-quadrant operation. 

If an amplifier used for ohmic loads generates positive current only, one quadrant will suffice. If the load is to be operated with negative voltage as well, two quadrants are required, see Fig. 6.141. 

These three circuit concepts differ significantly in terms of their output signal quality, and the efficiency of their energy use. 

In the lefthand circuit diagram, a piezo actuator is connected as a load. The coil (choke) protects it from rectangular switching voltages, but not from force impulses due to high peaks that can occur in the triangular current- 

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The combination of the cluster model approach and modem powerful quantum chemistry techniques can provide useful information about the electronic structure of local phenomena in metal oxides. The theoretical description of the electronic states involved in local optical transitions and magnetic phenomena, for example, in these oxides needs very accurate computational schemes, because of the generally very large differential electron correlation effects. Recently, two very promising methods have become available, that allow to study optical and magnetic phenomena with a high degree of precision. The first one, the Differ-... 

Examination of powdered materials with an electron microscope can generally disclose the presence of surface imperfections and pores. However, those imperfections or irregularities smaller than the microscope s resolving power will remain hidden. Also hidden is the internal structure of the pores, their inner shape and dimensions, their volume and volume distribution as well as their contribution to the surface area. However, by enveloping each particle of a powder sample in an adsorbed film, the method of gas adsorption can probe the surface irregularities and pore interiors even at the atomic level. In this manner a very powerful method is available which can generate detailed information about the morphology of surfaces. 

Electron microscopy is a powerful direct experimental technique. Using electron microscopy information can be obtained on the presence of inhomogeneities, and on their shapes, sizes, size dispersion and number density. The experimental and theoretical aspects of this technique have been reviewed by Hirsh a/. (1965). There are two methods of observation. In the first, the topography of the sample surface is replicated and it is this replica, and not the sample which is then examined in the electron microscope. Generally carbon is used as the replicating material and shadowing at an angle with heavy elements (Pt) is used to accentuate the surface reUef. The resolution limit is about 50 A due to a microstructure in the rephca. As the sample itself is not examined, a diffraction pattern is not obtained. The sample surface can be either etched or unetched. An unetched surface wiU reveal cracks, voids, and polyphase microstructures if the various phases... 

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