Active front

Water Aggregation. An interesting question arises at the outset as to what constitutes an aqueous phase. How many water molecules are required before an electrochemical process can be activated Conversations with many well-known electrochemists have led us to use a IM solution as a reference. Another basis for using IM is the observation that the pH at the active front under a cathodically delaminating coating approaches a value of slightly under 14, i.e., approximately IM in hydroxyl ions. A IM solution is 55M with respect... 

Fig. 36. (A) Self-sustained oscillations during the dissolution of a 3 cm long iron wire in 1 M sulfuric acid at different electrode potentials (a) 0.290 V, (b) 0.285 V, and (c) 0.280 V vs. SCE. (B) Position of the activation front (i.e. during the rising part of the oscillations) versus time. Electrode potentials (a) 0.38 V, (b) 0.34 V, (c) 0.30 V, (d) 0.28 V vs. SCE. (Reproduced from R. Baba, Y. Shiomi, S. Nakabayashi, Chem. Eng. Science 55 (2000) 217 - 222 with permission from Elsevier Science, 2000).

Thollander M, Svensson TH, Hellstrom PM. Stimulation of P-adrenoceptors with isoprenaline inhibits small intestinal activity fronts and induces a postprandial-like motility pattern in humans. Gut 1997 40 376-380. 

Regarding the results discussed above, the interesting aspect of these experiments is that the front velocities took on a constant value. Some data can be seen in Fig. 59. The first three examples show activation fronts in the bistable regime of Fe, Au, and Zn dissolution, respectively the last two curves display examples of pulses in an excitable regime, again for metal dissolution reactions, hi all examples, two stationary electrodes were used to probe the local potential. The velocity of the fronts or pulses were extracted from the time difference at which the transitions were measured at the two probes. In all five examples, the readings of the two probes seem to be just time-shifted versions of each other. This indicates that the structures propagate with constant shape and velocity. 

AFS (active front rear steering). SbW (steer-by-wire). 

The circuits in Fig. 32 are suited for impedances not exceeding 1 MQ. If very small electrodes are used, e.g., for measuring of single cells, their impedance values can exceed 1,(XX) MQ. Although shielding can prevent excessive noise, care should be taken of parasitic elements, especially of stray capacitances. Moreover, if active front ends in the immediate vicinity of the electrodes are used, they may heat up the chamber. 

Connection to the supply/line can be by a Diode Front End (DFE) arrangement for two quadrant operations. Addition of a chopper circuit with dumping resistor extends the operation of a DFE drive for accommodation of short term regenerative loads. More commonly for the four quadrant control required by a hoist a fully controlled bridge is used. This is referred to as an Active Front End (AFE). 

The presence of the DC Link with an Active Front End provides a number of advantages ... 

For the purpose of the present document, the area of interest here is the Incoming Power Conversion section of the Variable Frequency Drive (VFD), the Front End of the drive, this is what the AC feeder network sees and from which the effects in concern originate from. All other considerations downstream such as Inverter frequency, motor harmonics, motor power factor etc. are totally independent of the AC front end side, effectively isolated by the DC intermediate circuit. In the above diagram, the term AC to DC converter is used as a generic term to describe Passive and Active Front End types. 

Active Front End can be further classified in two categories based on their principle of operation, switching frequency and power range. For the purpose of the present description those will be classified as Inverter Supply Unit (ISU) and Active Rectifier Unit (ARU). 

Mine hoist drive systems have evolved from DC-based systems to AC systems in the majority of new apphcations. Early AC apphcations involved the use of cycloconverter drive systems, however the trend in modern hoisting systems is to utihze cage induction motors on many new installations coupled with Pulse Width Modulated (PWM) style Vcuiable frequency drives (VFDs) complete with Active Front End (AFE) technology. 

L Moran, J. Espinoza, M. Ortiz, J. Rodrique, J. Dixon, Practical Problems Associated with the Operation of ASDs Based on Active Front End Converters in Power Distribution Systems, Industrial Applications Conference, 2004, Vol. 4, 3-7 Oct. 2004, pp 2568-2572... 

AC Voltage source inverter drives (VSI) with pulse width modulation control (PWM) and active front end (APE) have the following advantages ... 

Many mining operations in remote areas have emergency power supplies required for mine evacuation in the event of a power utility failure. The advantage of the PWM AC drive with active front end (AFE) is that the kVA demand is proportional to the real power produced at the hoist motor. Operating at reduced speed reduces the kVA demand proportionally. When considering kVA demand only, it would appear that an emergency generator rated at some fraction of the hoist motor would be able to power the hoist at reduced speed. 

As can be seen from Figure 17, using the three-level active front end AFE, enables the grid side input current to be nearly sinusoidal, and no matter the energy is fed in or feed out the system can work with unity power factor. 

Figure 17. Grid side voitage and current waveforms of three-level active front end AFE operation...

Rectifier Unit Fuiiy regenerative Active Front End (AFE) type, based on FIV-iGBT, 3-ievei... 

The hoist drive solution presented in this work, which is based on the Ingedrive MVlOO medium voltage AG frequency converters, contains two completely independent Active Front End Rectifier and Inverter sets under the well-known AC-DC-AC topology. The entire drive system solution allows the operation at full speed and full load (then full hoist performance, with no limitation) even in the case that one frequency converter (AG-DC-AC set) is unavailable. The solution presents some extra benefits comparing to the original hoist system solution that was based on two slip ring rotor wound induction motors in which the speed control... 

The three discrete potential maxima (50 ms) reflect activation fronts in three regions of the heart. From left to right right ventricular outflow tract anterior wall of left ventricle posterior wall of left ventricle. 

Figure 13 shows similar results but for a different source configuration. There are two activation fronts, one extending from 20° to 30°, and the other from -20° to 30°. Figure 13 (top) shows the surface data and the inverse-reconstructed epicardial potentials. Figure 13 (bottom) shows the actual forward-computed epicardial and body surface potentials for comparison. The surface distribution exhibits only a single potential maximum and does not reflect the two discrete activation fronts. The inverse-reconstructed epicardial potentials, on the other hand, contain two separate maxima which clearly reflect the true nature of the source. The location of the activation fronts and their extent are accurately reflected in the location and extent of the reconstructed maxima. This result demonstrates the capability of the inverse procedure to accurately reconstruct multiple local cardiac events from the low resolution, smooth surface potential data. As already mentioned, this property of the inverse procedure permits detailed examination of regional electrical events within the heart in a fashion that is not possible directly from the body surface potential distribution. 

ANS These are artifacts resulting from the truncation of an infinite series expansion to a finite number of terms. If we add 5-8 terms, the artifact disappears. Note that the deviations in the epicardial solutions are present only in the more difficult situation of two activation fronts on the posterior of the heart. The posterior of the heart is farther from the torso surface than is the anterior of the heart, and the surface potentials are correspondingly lower. The inverse solution is more difficult in the case where the information on the torso consists of low potentials with very smooth gradients. Nevertheless, accurate solutions are obtained with a higher number of multipoles. 

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