T.O. 33B-1-13-413.3.12.6.3.5Table 3-3 compares the differences in the current required to longitudinally magnetize the solid and hollow parts in theexamples of paragraphs 3.3.12.6.3.1 and 3.3.12.6.3.4 above. The only difference in the two parts is that one washollow and the other was solid. If the effective diameter Deff had not been considered, the current for the hollow partwould have been over-estimated by 927 amperes. This additional amperage would certainly result in excessivebackground and possibly false indications from over-magnetizing the part.Table 3-3. Comparison of Coil Amperages for Solid vs. Hollow PartsSolid Part Hollow PartPart Length 10 inches 10 inchesPart Diameter 2 inches 2 inchesCoil Description 5-turn,12-inch diameter5-turn,12-inch diameterAmps Required 1800 8733.3.12.6.3.6The inspector should be cautioned that when using the above rule-of-thumb formulas, the part length used in the L/Dratio is the part dimension measured in the direction of the coil axis and the diameter is the dimension measured in theplane of the coil. For example, a 2-inch diameter steel bar 10 inches long will have an L/D ratio of 5 when the bar isplaced in the coil with its axis parallel with that of the coil. If the bar is placed in the coil so that the bar and coil axisare at right angles to each other, the L/D ratio will be only 0.2, a figure which, if used, would indicate the need forimpracticably high amperages.3.3.12.6.3.7If the need arises to inspect parts having L/D ratios of 3 or less, the effective L/D ratio must be increased by placing thepart between two pole pieces while it is being magnetized. The length dimension for the L/D ratio then becomes thelength of the two pole pieces plus the part length. Such pole pieces must make good contact on eachside of the partand must be made of ferromagnetic material. Solid steel pole pieces may be used when directcurrent is used in the coiland the continuous method of inspection is used. If the continuous method is used with either AC or half-wave DCcurrent in the coil, the pole pieces must be made from laminated magnetic material similar to the silicon steel legs of ahand probe with articulated legs. This is also true for residual inspection. Pole pieces must be made from the propermaterial if residual inspection, or the wet continuous method of inspection with AC or half-wave DC, is to be used.3.3.12.6.4CableWrapCoil.Cables used are commonly 2/0 or 4/0 AWG (American Wire Gage), flexible stranded, insulated copper cable. Thenumber of turns used is kept low, from 3 to 5 turns to minimize cable resistance in the case of DC and coil impedancewhen AC is used.3.3.12.6.4.1Multiple inspections spaced approximately 15 to 18 inches along the length of a long part are preferable to oneinspection using one long coil of many turns. Cable lead lengths between the power source and coil wraps must be keptas short as practical so that maximum amperages are produced in the coil. When AC is being used, and to some extentwith half-wave DC, available amperages can be increased by twisting or taping together the cable lengths between thecoil and the power supply. This reduces the coil-circuit impedance the same way that reducing turns on the coil doesand makes it possible for more AC current to flow in the coil circuit. The total length of the cable together with theresistance of its connections determines the DC amperage obtainable in the coil. The longer the cable and the poorerthe electrical connections, the less will be the DC and the half-wave DC amperages that can be obtained. Increasedcable resistance also lowers available AC current, but in the case of AC, the impedance of the coil and coil lengthcircuit has a much greater effect than does resistance in lowering and limiting available AC current.
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