T.O. 33B-1-13-103.1.7.1 Alternating Current (AC).Alternating current, which is single phase when used directly for magnetizing purposes, is taken from commercialpower lines or portable power sources and is can be 50 or 60 Hertz. Magnetizing currents up to several thousandamperes are used, derived from step-down transformers connected to common line voltages, e.g., 115, 230, or 460volts.3.1.7.2 Direct Current (DC).Rectified alternating current is by far the most satisfactory source of direct current. By the use of rectifiers,commercially available single and three phase AC can be converted to a unidirectional current. Rectified three phaseAC is equivalent to straight DC, but exhibits a slight ripple.3.1.7.3 HalfWaveRectifiedSinglePhaseAlternatingCurrent(HWDC).Half-wave rectified single phase AC results in a pattern of unidirectional current flow made up of positive half cycles ofthe original AC waveform. The negative (reverse) half of each cycle is completely blocked out. The result is apulsating unidirectional current. That is, the current rises from zero to a maximum and drops back to zero (replicatingthe AC’s half cycle), is blocked during the reverse cycle (no current flows), and then repeats the first half cycle. Thistype of current is also called Half-Wave Direct Current (HWDC).3.1.7.4 Full Wave Rectified Single Phase Alternating Current (FWDC).This pulsating unidirectional current is sometimes used in MPT for certain special purpose applications. In general,however, it possesses no advantage over single-phase half-wave rectified waveforms. Because of its extreme “ripple,” itis not as satisfactory as rectified three phase current when DC is required. Further, it is more costly since it draws ahigher average current from the AC line than does rectified half-wave AC for a given magnetizing strength.3.1.7.5 InducedCurrentMagnetization.When direct current in a circuit is instantly cut off, the field surrounding the conductor collapses, or falls rapidly tozero. If an electrically conductive ferromagnetic material is present in such a field, the collapse of that field will inducea current in the material the same direction as present in the neighboring conductor before cut-off. This phenomenoncan be used to solve specific magnetizing problems that have no other practical solution. A useful application of thecollapsing field technique has been found in the inspection of ring-shaped parts, such as bearing races, without the needto make direct contact with the surface of the part. Regardless of the type of magnetizing current employed, whetherDC, AC, or half-wave, the induced current technique is usually faster and more satisfactory than the contact method.Only one operation is required, and the possibility of damaging the part due to arcing is completely eliminated since noexternal contacts are made on the part.3.1.8 FerromagneticMaterialCharacteristics.All ferromagnetic materials, after having been magnetized, will retain some residual magnetic field. The strength anddirection of the residual field depend upon all the magnetizing forces applied since the material was last demagnetizedand the retentivity of the material. The manner in which ferromagnetic materials respond to magnetizing forces ismost often portrayed in a plot of the flux density (B) as a function of the magnetizing force (H). The flux density (B) isthe number of magnetic lines of flux that are formed per cross-sectional area as a result of the magnetizing force (H).For an encircling coil, the magnetizing force is the accumulative effect of each turn of the coil and the current passingthrough it. Therefore, H equals the current passing through the coil, multiplied by the number of turns in the coil.Figure 3-16 shows a typical B-H curve for a ferromagnetic material starting in a demagnetized condition and thencycled to saturation in two opposite directions.
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