AC current

In contrast to a direct injection of dc or ac currents in the sample to be tested, the induction of eddy currents by an external excitation coil generates a locally limited current distribution. Since no electrical connection to the sample is required, eddy current NDE is easier to use from a practical point of view, however, the choice of the optimum measurement parameters, like e.g. the excitation frequency, is more critical. Furthermore, the calculation of the current flow in the sample from the measured field distribution tends to be more difficult than in case of a direct current injection. A homogenous field distribution produced by e.g. direct current injection or a sheet inducer [1] allows one to estimate more easily the defect geometry. However, for the detection of technically relevant cracks, these methods do not seem to be easily applicable and sensitive enough, especially in the case of deep lying and small cracks. 

Induction laws and experiments show that the impedance of a coil crossed by an AC current put near a conductive piece is modified by the creation of eddy currents. The presence of an anomaly in this material structure modifies the impedance of the generating coil. The impedance variation measure is at the root of non destructive testing by eddy currents. Any variation inside a piece (variation of conductivity or permeability) modifies the intensity and the course of the eddy currents and consequently the coil impedance. 

Historically, the first and most important capacitance method is the vibrating capacitor approach implemented by Lord Kelvin in 1897. In this technique (now called the Kelvin probe), the reference plate moves relative to the sample surface at some constant frequency and tlie capacitance changes as tlie interelectrode separation changes. An AC current thus flows in the external circuit. Upon reduction of the electric field to zero, the AC current is also reduced to zero. Originally, Kelvin detected the zero point manually using his quadrant electrometer. Nowadays, there are many elegant and sensitive versions of this technique. A piezoceramic foil can be used to vibrate the reference plate. To minimize noise and maximize sensitivity, a phase-locked... 

Since the potential and current are sinusoidal, the impedance has a magnitude and a phase, which can be represented as a vector. A sinusoidal potential or current can be pictured as a rotating vec tor. For standard AC current, the rotation is at a constant angular velocity of 60 Hz. 

From the data in Table 2-1 this results in a corrosion rate for Fe of 0.1 mm a for an effective ac current density of 20 A m . Thus only ac current densities above... 

The action of effects in the environment and cathodic current densities on ac corrosion requires even more careful investigation. It is important to recognize that ac current densities above 50 A m can lead to damage even when the dc potential is formally fulfilling the protection criterion [40]. 

With metals other than Fe, the percent of the ac current leading to corrosion can be considerably different. Cu and Pb behave similarly to Fe [36], whereas A1 [36] and Mg [39] corrode much more severely. This has to be watched with sacrificial anodes of these materials if they are subjected to ac. 

Figure 3-50 Methods of constant current limiting (a) discrete overcurrent limiting (constant-current limiting) (b) precision resistive current-sensing overcurrent protection (constant-current limiting) (c) use of a current transformer to sense ac current.

The power switch and rectifier ac-current loops contain very high trapezoidal current waveforms typical in PWM switching power supplies. These waveforms are rich in harmonics which extend far above the basic switching frequency. These ac currents can have peak amplitudes two to five times that of the... 

These ac current loops should be routed before any other traces in the power supply. The three major components that make up each loop the filter capacitor, the power switch or rectifier, and the inductor or transformer must be located adjacent to one another. The components must also be oriented such that the current path between them is as short as possible. A good example of a layout of the power section of a buck (or step-down) converter can be seen in Figure 3-60. 

The major concern of both output and input filter capacitors is the ripple current entering the capacitor. In this application, the ripple current is identical to the inductor ac current. The maximum limits of the inductor current is 2.8 A for I peak and about one-half the maximum output current or 1.0 A. So the ripple current is 1.8 A p-p or an estimated RMS value of 0.6 A (about one-third of the p-p value). 

Quasi-resonant converters are a separate class of switching power supplies that tune the ac power waveforms to reduce or eliminate the switching loss within the supply. This is done by placing resonant tank circuits within the ac current paths to create pseudo-sinusoidal voltage or current waveforms. Because the tank circuits have one resonant frequency, the method of control needs to be modified to a variable frequency control where the resonant period is fixed and the control varies the period of the non-resonant period. The quasi-resonant converters usually operate in the 300 kHz to 2 MHz frequency range. 

In the common-mode filter the windings of the transformer are in phase, but the ac currents flowing through the windings are out of phase. The result is that the common-mode ac flux within the core for those signals that are equal and opposing phases on the two power lines cancel out. 

Skill effect is the apparent increase in wire resistance when high-frequency ac currents are passed through them. A wire s real resistance when involving losses within a switching power supply is given in Equation El. 

As one can see, the larger-diameter wires suffer a much more rapid degradation in ac resistance with increasing frequency than do smaller-diameter wires. So it is advantageous to use multiple strands of smaller wires instead of one large diameter wire. The ac current density of the smaller wires (>30 AWG) can actually be pushed to two to three times the assumed current density used in the charts because their surface area to cross-sectional area ratio is much greater. 

In 1874 Heaviside established the ordinary symbolic method of analyzing alternating current circuits in common use today. It was a technique developed about fifteen years before AC came into commercial use. He emphasized the role of metallic circuits as guides, rather than conductors of AC currents. He discussed in detail causes of line distortion and suggested ways of alleviating it. 

The power dissipated in an AC circuit with current of maximum amplitude flowing through a resistance is less than the power produced by a constant DC current of magnitude flow ing through the same resistance. For a sinusoidal AC current, the root mean square (rms) value of current I is the magnitude of the DC current producing the same power as the AC current with maximum amplitude I. The rms value I is given by... 

The device of Figure 2-70 can also operate as a motor if a DC current is applied to the rotor windings as in the alternator and an AC current is imposed on the stator windings. As the current to the stator flows in one direction, the torque developed on the rotor causes it to turn until the rotor and stator fields are aligned (5 = 0°). If, at that instant, the stator current switches direction, then mechanical momentum will carry the rotor past the point of field alignment, and the opposite direction of the stator field will cause a torque in the same direction and continue the rotation. 

Circuits that carry AC current employing two, three, or more sinusoidal potentials are C2 ed polyphase circuits. Polyphase circuits provide for more efficient generation and transmission of power than single-phase circuits. Power in a three- (or more) phase circuit is constant rather than pulsating like the single-phase circuit. As a result, three-phase motors operate more efficiently than single-phase motors. 

Electric power is almost always transmitted as three-phase AC current. In domestic use, current is often distributed from a substation at 13,200, 6,600, or 2,300 V, which is stepped down by a transformer close to the point of use to 600, 480, and 240 V for three-phase current for commercial power and 240 and 120 V for single-phase, three-wire current for household power and lights. If DC current is required, synchronous converters or rectifiers are used to convert the AC supply to DC. 

Electrical Stability of Emuisions. The electrical stability test indicates the stability of emulsions of water in oil. The emulsion tester consists of a reliable circuit using a source of variable AC current (or DC current in portable units) connected to strip electrodes. The voltage imposed across the electrodes can be increased until a predetermined amount of current flows through the mud emulsion-breakdown point. Relative stability is indicated as the voltage at the breakdown point. 

When a pure sinusoidal AC current passes across the electrode/solution interface, the cell voltage (a two electrode arrangement is used) shows a sinusoidal perturbation. It contains multiples of the fundamental frequency of the modulation, the first harmonie dominates. The magnitude of the effect is comparable to Faradaie rectification, but experiments may be easier to perform. Measurement and evaluation have been described in detail [60Old, 72Hil2]. (Data obtained with this method are labelled FD.)... 

Nonfaradaic components associated with the uncompensated resistance between reference electrodes (7 ) and the double layer capacitance (Qi) can be accurately determined by AC impedance measurements. In this technique, a small AC potential perturbation is superimposed to the DC bias, and the resulting AC current is measured as a function of the frequency of modulation. The simplest circuit considered for a polarizable... 

FIG. 9 Real component of the AC current (a) and imaginary part of the normalized potential-modulated reflectance (b) for the TCNQ reduction by ferrocyanide at the water-DCE interface. Experimental conditions as in Fig. 5. The potential modulation was 30 mV rms at 3.2 EIz. (c) Optical Randles plot obtained from the frequency-dependent analysis of the PMR responses. (Reprinted from Ref 43 with permission from Elsevier Science.)... 

As indicated above, the Model 611 does not require a separate temperature probe and so it has no temperature knob to be operated its circuits instead perform the following functions (abbreviated as in the Orion specification) (1) induce ac signal across pH probe (2) measure average dc potential of probe (3) convert amplitude of ac signal to dc potential (V) (4) calculate log V (5) measure in-phase ac current through probe (6) convert current to dc potential proportional to current (/) (7) calculate log / (8) calculate log R (resistance of probe) = log V - log I (9) convert log R into signal proportional to temperature (displayed) (10) use temperature signal to correct pH, to be read. 

Further, if within the electrical circuit the ohmic resistance R can be neglected, the ic wave leads to the potential by 90°, as is known, which means that shows a positive 7t/2 phase angle shift ( between tt/2 and zero. Our main objective in AC polarography, however, is the faradaic current, so a separating condenser is placed between the amplifier and normal resistor in order to filter out the d.c. current and to evaluate the ac current component. As we want to understand the relationship between idc(i ) and iac(i ) as a function of Edc and Eac applied, we may consider Fig. 3.41(a) and (b). 

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