Eddy current inspection methods are used extensively in industry for the nondestructive testing of a wide variety of materials and product applications. The general theory of eddy current inspection is described. Equations defining the depth of penetration are derived from Maxwell's equations. Signal analysis methods are described using coil impedance diagrams. The impedance diagrams for a number of inspection applications are presented. A discussion of the techniques for theoretical calculation of the impedance of a coil is described. Typical coil configurations and instrumentation techniques are discussed.

Thesis. 1975. M.S.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Bibliography: leaves 36-37. / M.S.

MnZn ferrites are widely used in power electronics applications where the switching frequency is in the range of several tens of kilohertz to a megahertz. In this range of frequencies the combination of relatively high permeability and relatively low conductivity found in MnZn ferrite helps to minimize the size of magnetic devices while maintaining high efficiency. The continuing improvement in semiconductor switches and circuit topologies has led to use of high-frequency switching circuits at ever increasing power levels. The magnetic devices for these high-power, high-frequency circuits require magnetic CORES that are significantly larger than standard ferrite-core devices used at lower power levels. Often such large ferrite cores must be custom designed, and at present this custom design is based on available material information without regard for the physical size of the structure. This thesis examines the issues encountered in the use of larger MnZn ferrite cores for high-frequency, high-power applications. The two main issues of concern are the increased power dissipation due to induced currents in the structure and the change in inductance that results as the flux within the core is redistributed at higher frequencies. In order to model these problems using either numerical or analytical methods requires a reliable and complete set of material information. A significant portion of this work is devoted to methods for acquiring such material information since such information is not generally available from the manufacturers. Once the material constants required for the analysis are determined, they are used in both closed-form and numerical model to illustrate that large ferrite cores suffer significant increases in loss and significant decreases in inductance for frequencies as low as several hundred kilohertz. The separate impacts of the electrical and magnetic losses in the core are illustrated through the use of linear finite element analyses of several example core structures. The device impedances calculated using the FEA tools show fair agreement with measurement. An analysis of gapped structures and segmented cross-sections shows that these design options can reduce the severity of the dimensional problems for some designs. / Ph. D.

In a grounded direct current system, motor faults may occur which will not be indicated by ordinary protective devices, but which will affect motor operation or create unsafe conditions. All such faults are manifested by the fact that the current in one lead supplying the motor will be greater than that in the other. This current differential can be made to operate a direct current differential relay. About the two supplying leads, when such a fault occurs, will exist a magnetomotive force. This mmf will set up a flux in the core and armature of a relay which forms a magnetic circuit about the supplying conductors. Since the armature of the relay is balanced on a knife edge, the force of attraction between core and armature which is caused by the flux passing from one to the other will move the armature, and in so doing close a pair of contacts. A model relay was built which operates on this principle. Testing demonstrated that it would operate on a differential of approximately one and one-half amperes and that the relay would release when the differential was reduced to approximately four-tenths amperes. The test also showed that these differentials did not vary when the total currents involved changed. These results indicate that a relay built with a slightly different design, (so as to overcome the shortcomings of test model, notably the insecure armature mounting, and so as to provide certain other features, such as control of the current differential for which the relay will operate), would be responsive to current differentials of less than one-half ampere. Such a relay would be a practical device and would be capable of performing, in conjunction with a second, conventional, relay and a circuit breaker, an important control function. / M.S.

A method of measuring the negative-sequence current in an unbalanced three-phase circuit. A four-winding current transformer, in conjunction with a phase shift network and an alternating current ammeter, was used for obtaining a current proportional to, and in phase with, the negative-sequence current. After calibration, readings were taken of the three line currents in a three-phase three-wire circuit and of the current in the negative-sequence ammeter. The negative-sequence current. was calculated from the line currents and compared with the ammeter reading. The difference between the two values, in most cases, was less than five per cent of the ammeter reading. It was found that. the circuit had to be calibrated for each type of ammeter used. It was concluded that direct readings of negative-sequence currents can be made with accuracy consistent with commercial requirements. / Master of Science

Master of Science

The original circuit diagram was drawn previous to any reference work on the subject; however, in the construction of the system it was found necessary to add to and subtract from the original circuit. For instance, in order to isolate the oscillator from the modulator and to amplify the carrier voltage it was necessary to add buffer amplifier stages; since only one system was built and since the system worked on the tuned detector principle, no filters were deemed necessary. Other circuit changes of minor importance are; the use of individual plate voltage supplies to minimize feedback between stages; the use of regenerative detection; and a variation in the obtaining of the oscillator output. The system works satisfactorily, but is very critical; this is largely due to the fact that regenerative detection is used. Once the system is adjusted for quiet distortionless operation, it is found that an increase in modulating potential sufficient to over modulate the carrier introduces distortion. This is probably due to the fact that the tube is not operating on a parabolic portion of the grid voltage plate current characteristic curve during a large portion of each cycle, thus introducing third order, fourth order, fifth order, and nth order distortion terms.* * It is to be remembered that the plate current can be expressed as a power series of n terms of the grid voltage, i.e. --- [see equation] The number of terms depends upon the portion of the curve over which the tube is operating; thus, for "square-law" or second order modulation, which occurs over the parabolic portion of the characteristic curve, n = 2; for third order modulation, n = 3; etc. / M.S.

No / Voltage clamp was used to investigate the effects of N-[2-p-bromo-cinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89), a potent inhibitor of PKA, on transient outward K+ current (Ito) and inward rectifying K+ current (IK1) in rat cardiac muscle. Initial experiments, performed using descending voltage ramps, showed that H-89 inhibited both the outward and inward ramp currents in a concentration-dependent manner at concentrations between 5 and 60 ¿mol l¿1. A similar degree of inhibition was observed when Ito and IK1 were recorded using square wave depolarising and hyperpolarising voltage steps, respectively. The IC50 was 35.8 ¿mol l¿1 for Ito and 27.8 ¿mol l¿1 for IK1 compared to 5.4 ¿mol l¿1 for L-type Ca2+ current (ICa). The Hill coefficients for Ito, IK1 and ICa were ¿1.97, ¿1.60 and ¿1.21, respectively. In addition to inhibiting Ito amplitude, H-89 also accelerated the time to peak and the rate of voltage-dependent inactivation so that the time course of Ito was abbreviated. Paired-pulse protocols were performed to study the effects of H-89 on steady-state activation and inactivation as well as recovery from voltage-dependent inactivation. H-89 produced a concentration-dependent rightward shift in voltage-dependent activation but had no significant effect on steady-state inactivation. Recovery from voltage-dependent inactivation was delayed, although this was only visible at the highest concentration (60 ¿mol l¿1) used. In experiments investigating the effects of elevated cyclic AMP, the ß-adrenergic agonist isoprenaline and the phosphatase inhibitor calyculin A had no major effects on Ito or IK1. Data suggest that the effects of H-89 on K+ currents are more complex than simple inhibition of PKA-mediated phosphorylation.

Sand dune erosion is highly episodic occurring only when storm waves coincide with high tides generating swash that impacts the toe of the dune. Owing to the episodic nature of sand dune erosion, it is difficult to observe in nature. The removal of a structure and rip-rap sea-wall from the Stilwell Hall site located in southern Monterey Bay provided a unique opportunity to study erosion processes at an accelerated rate. A 1-D wave impact line erosion model (Larson et al., 2004) was tested against data acquired at this site between April, 2004 and April 2005. The model was optimally tuned to the data by a dimensionless coefficient that relates the impact force to the rate of recession. The coefficient values ranged from 0.7-1.3x10-3, for this field data, compared with values of 1.0-2.5x10-3 previously obtained for lab and field data. Migrating rip currents create a system of mega-cusps, which are nominally 10m in width and 200m in alongshore wavelength (Thornton, 2005). The presence of megacusps is hypothesized to accelerate sand dune erosion at their embayments where the beach is steeper and narrowest (Short, 1979;Shih and Komar, 1984;Revell, et al., 2002). It was determined that the highest recession occurred at the location of the rip current/mega-cusp embayment. Changes in the surf climate are of great interest to Naval Special Warfare (NSW) and U.S. Marine Corps (USMC) forces tasked with planning and executing operations in littoral areas. Naval history is replete with operations highlighting the importance of understanding and accurate prediction of nearshore dynamics. Without the ability to predict nearshore morphologic processes, providing such support is impossible.

Approved for public release, distribution is unlimited / Rip current spacings are compared with wave energetics and directional spreading in the Southern Monterey Bay. Southern Monterey Bay affords a unique environment to study rip currents owing to their prevalence created by near-normally incident waves on a sandy shoreline. It is hypothesized that rip current spacing is a function of wave directional spreading and energy flux, based on the morphodynamic modeling by Reniers et al. 2003. A gradient of wave energy flux exists due to headlands and refraction over Monterey Canyon. Rip currents are shown to occur between cusps in the shoreline, allowing cusp spacing to be a surrogate for rip spacing. Rip current spacing was inferred from beach morphology surveys, LIDAR imagery, and Argus cameras, and found to be O(150m) at Sand City and O(300m) at Marina, separated by 6km . Measured waves during a two month period using wave-rider buoys, show a gradient of across-shore energy flux between Sand City, 2 28000( / ) F Jm x . , and Marina, 2 33000( / ) F Jm x . . The two sites have the same peak directional spreading of energy value, 14 peak ̤= o , and slightly different bulk values for Sand City, 18 bulk ̤= o , and Marina, 20 bulk ̤= o . Therefore, the variations in rip current spacing could not be attributed to directional spreading but appear related to variations in energy flux. / Ensign, United States Naval Reserve