T.O. 33B-1-13-243.3.4.2.2The prevailing approach for obtaining direct current for magnetic particle inspection is through rectification ofalternating current using solid state rectifiers. A rectifier (diode) is a device that allows electric current to flow throughit in only one direction. By proper connection of rectifiers, the back and forth flow of alternating current is convertedto a current flow in only one direction. This is a form of direct current. A rectifier circuit, which converts bothalternations (back and forth flow) of the alternating current to one direction of current flow, is called a full-waverectifier.3.3.4.2.3Single-phase alternating current can be rectified using a full-wave rectifier circuit to obtain direct current for magneticparticle inspection. Single-phase rectification, however, is seldom used to obtain direct current, except in the case ofsmall hand-held magnetizing devices. Since three-phase power is so readily available in industry, direct current formagnetic particle inspection units is usually obtained using three-phase full-wave rectifiers.3.3.4.3 ComparisonOfResultsUsingDifferentTypesOfCurrent.Figure 3-21 is a comparison of indications of the same set of fine surface cracks on a ground and polished piston pin,obtained by using 60 cycle AC, DC from storage batteries (straight DC), and DC from rectified three-phase 60 cycleAC respectively. Four values of current were used in each case with a central conductor to magnetize the hollow pin.The indications produced with AC are heavier than the DC indications at each current level, although the difference ismost pronounced at the lower current values. Straight DC and rectified AC are comparable in all cases. The ACcurrents are meter (R.M.S. or Root Mean Square) values, so that peak of cycle currents, and therefore magnetizingforces, are 1.41 times the meter reading shown.Figure 3-21. Comparison of Indications of Surface Cracks on a Part Magnetized with AC, DC and Three PhaseRectified AC3.3.4.3.1Another comparison can be made using the Ketos ring specimen, the drawing for which is shown in Figure 3-22. Thespecimen, made of unhardened (annealed) tool steel (0.40 percent carbon), is 7/8 inch thick. Holes, 0.07 inch indiameter and parallel to the cylindrical surface, are located at increasing depths below the surface.
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