Excitation sources direct current

Direct-current motors are classified as separately excited motors, series motors, shunt motors, and compound motors. The field winding of a separately excited motor is in a circuit that is energized by a separate dc source the field winding is not physically connected to the armature circuit (containing the armature winding). 

In synchronous motors, the excitation is supplied by a separate direct current source, either as a separate motor-generator (M-G) set or as an exciter mounted directly on the motor shaft. The current can be made to lead to various degrees by varying the magnitude of the field strength. 

Straight Shunt-Wound Motor. A straight shunt-wound motor is a direct-current motor in which.the field circuit is connected either in parallel with the armature circuit or to a separate source of excitation voltage. The shunt field is the only winding supplying f ield excitation. 

Flames and plasmas can be used as atomisation/excitation sources in OES. Electrically generated plasmas produce flame-like atomisers with significantly higher temperatures and less reactive chemical environments compared with flames. The plasmas are energised with high-frequency electromagnetic fields (radiofrequency or microwave energy) or with direct current. By far the most common plasma used in combination with OES for analytical purposes is the inductively coupled plasma (ICP). 

The examples we saw are for L C circuits supplied from a direct current source. What happens when an L C circuit is excited by an alternating current source Once again, oscillatory response will be present. The oscillatory waveform superimposes on the fundamental waveform until the damping forces sufficiently attenuate the oscillations. At this point, the system returns to normal operation. In a power system characterized by low resistance and high values of L and C, the effects would be more damaging than if the system were to have high resistance and low L and C because the natural frequencies are high when the values of L and C are low. The... 

For special applications direct current plasma (DCP) (Leis et al., 1989) and micro-wave-induced plasma (MIP) may be used. The MIP first became widely used as a spectroscopic radiation source after a stable discharge at atmospheric pressure had been obtained (Beenakker, 1977 Beenakker et al., 1978). The MIP is not capable of taking up wet aerosols, but is useful for the excitation of dry aerosols, produced by electrothermal evaporation from a graphite furnace (Aziz et a ., 1982). Direct sample insertion has been discussed recently by Blain and Savin (1992). 

A plant for this purpose comprises a vacuum chamber with ancillary equipment. The articles to be plated are mounted on a water-cooled cathode and, following evacuation of the chamber and filling with argon to a pressure of 0.002 Torr, direct current from a supply in the range 2000 to 5000 V is applied—to give a glow discharge in the area of the cathode. The article or articles mounted there thus are cleaned by ionic bombardment. Next, the evaporation source is excited, and the atoms of source material are deposited on the work pieces. 

ICP-OES is an analytical system that can do simultaneous or sequential determination of up to 50 elements at all concentration levels with a high degree of accuracy and precision. Excellent vaporization-atomization-excitation-ionization is obtained with an argon-supported ICP operated at atmospheric pressure. The emitted spectra is observed with a polychromator or a scanning spectrometer may be used depending on whether simultaneous or sequential determinations are desired. This atomization-excitation process does not exhibit interelenent effects often seen in AAS, and ppb range detection is routine. Effective nebulization of samples needs to be improved on however, ICP and direct-current (DC) plasmas are extremely effective atomization sources that provide the most effective instrumental technique for simultaneous elemental analysis. 

PACVD method is another typical way of making diamond films. Precursor gas molecules can be decomposed into radicals under the effect of plasma. There are three plasma sources commercially available (Davis 1993). Microwave plasma typically uses excitation frequencies of 2.45 GHz. Radio frequency (RF) plasma excitation typically employs frequencies of 13.56MHz (or less commonly 450kHz). Direct current plasmas can be run at low electric powers, named as cold plasma, or at high electric powers, which create an arc, named as thermal plasma. Microwave PACVD method is the most common one among the three methods. 

Emission spectrography includes an excitation source (in this instance a direct-current arc), an optical unit using a dispersion system to provide monochromatic images of the input slit on its focal surface, and a detection system (in this instance, a photographic emulsion). 

The major advantage of this method lies in its multi-element capability and high sensitivity (detectivity) (Hill etal. 1993). On-line combinations with separation techniques are easily set up. The excitation source is an inductively coupled plasma or - less commonly - a direct current- (DCP) or microwave- plasma (MIP) plasma temperatures are around 5000-9000°K. Chemical interferences, such as molecular emissions are rarely observed, but background compensation should be applied in any case. Sample introduction is performed via a nebulizer and spray chamber (see earlier in this chapter for details of nebulizer types and related problems and solutions). Sequential... 

During the 1980s, a rapidly increasing number of methods have been published for mercury determination by AES (often called OES = optical emission spectrometry) after excitation/ionization in a gas plasma, usually argon. The plasma source most frequently used is an ICP, but also other kinds of plasma sources are used, e.g. alternating current plasma (ACP), direct current plasma (DCP), and microwave-induced plasma (MIP). AES has a wide multi-element capability the linear range extends over 4-6 orders of magnitude. 

A number of electrical excitation-sources are available for emission spectroscopy. In most commercial spectrochemical instruments, more than one excitation source is contained in a single power-supply cabinet a typical combination may include a spark, a direct-current arc, and an alternating-current arc. A list of the various electrical excitation-sources, some of their characteristics, their approximate cost and the types of samples generally required is given in Table 11.1. Because of the actual or potential widespread use in emission spectroscopy, only the arc, spark, and inductively coupled plasma discharges will be described here in detail. 

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