Electrical eddy current

The nondestructive electrical eddy current test is a method in which eddy current flow is induced in the test object. Changes in the flow by variations in the test specimen... 

Damping torque loss DoLMotor = OLMotor- (by electric eddy current), depending on the rotational speed N, with... 

The intensity of the magnetic field produced by eddy current is depended on electrical conductivity and magnetic permeability of the studied area. In case of a uniform structure, when the conductivity of the material is high, the intensity of the induced magnetic field is big and signal received by probe Hp is small. 

Attention should be given in the fact, that penetration of eddy currents in residual austenite will be slightly deeper than in the martensite structure of steel, as austenite shows low electrical conductivity. The signal originatimg from the austenite structure will be amplified in effect of the influence of the structure found at greater depth. There will be no error as the method of measurement is compartable and the samples made for reference purposes will have the same structure as the studied part. 

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. 

Eddy Currents ttike their name from the swirls (eddies) observed in turbulent water flow. The Greek mythology tells us about Charybdis. A monster eddy current between Italy and Sicily whose attractive field pulled unwary sailors to their destruction. Our kind of eddy currents are usually not so dangerous, they flow in electrical conductors and are a branch of Electromagnetics. Where does that spring from Could it make eddy currents the very oldest NDT technique ... 

Exciting developments based on electromagnetic induction raced along from that time, giving us the sophisticated products our everyday lives depend on. During most of the period productive uses for eddy current technology were few and few people believed in it as a usefiil tool eddy currents caused power loss in electrical circuits and, due to the skin effect, currents flowed only in the outer surfaces of conductors when the user had paid for all the copper in the cable. The speedometer and the familiar household power meter are examples of everyday uses that we may tend to forget about. The brakes on some models of exercise bicycle are based on the same principle. 

Maximum gap between the eddy-current converter and the expected surface- 10mm Inspected surface electrical conductivity- 0.5-r60MS/m. 

Spinel ferrites, isostmctural with the mineral spinel [1302-67-6] MgAl204, combine interesting soft magnetic properties with a relatively high electrical resistivity. The latter permits low eddy current losses in a-c appHcations, and based on this feature spinel ferrites have largely replaced the iron-based core materials in the r-f range. The main representatives are MnZn-ferrites (frequencies up to about 1 MH2) and NiZn-ferrites (frequencies 1 MHz). 

High purity 50% ferrosihcon containing <0.1% Al and C is used for production of stainless steel and corded wire for tires, where residual aluminum can cause harm fill alumina-type inclusions. These are also useflil in continuous cast heats, where control of aluminum is necessary. High purity grades of 50 and 75% ferrosihcon containing low levels of aluminum, calcium, and titanium are used for sihcon additions to grain-oriented electrical steels, where low residual aluminum content contributes to the attainment of desired electrical properties, eg, significant reduction of eddy currents. 

These deratings are meant only for non-magnetic enclostires. where the heal generated is only through induced electric currents (hR) and hence low. There are no hysteresis or eddy current lo.sses. 

These balanced enclosure currents also induce electric fields into nearby structures, RCC beams and columns in the same way as the main conductors, and hence nullify most of the space magnetic fields. These space fields (fields outside the enclosure) are otherwise responsible for causing eddy current and hysteresis losses in the metallic (magnetic) structures, RCC beams and columns in the vicinity. The electrical bonding of enclosures thus... 

The philosophy of any EMI shield is to encourage eddy currents to flow within the surfaces, thus dissipating the noise energy. Also, the assembled enclosure should act as a gaussian enclosure where there is good electrical conduction totally around the enclosure. So removable hatches and enclosure members need very good electrical connections around their peripheries. RF gasketing is sometimes used in particularly troublesome cases. 

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