T.O. 33B-1-1
3-10
3.1.7.1
Alternating Current (AC).
Alternating current, which is single phase when used directly for magnetizing purposes, is taken from commercial
power lines or portable power sources and is can be 50 or 60 Hertz. Magnetizing currents up to several thousand
amperes are used, derived from step-down transformers connected to common line voltages, e.g., 115, 230, or 460
volts.
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 phase
AC is equivalent to straight DC, but exhibits a slight ripple.
3.1.7.3
Half Wave Rectified Single Phase Alternating Current (HWDC).
Half-wave rectified single phase AC results in a pattern of unidirectional current flow made up of positive half cycles of
the original AC waveform. The negative (reverse) half of each cycle is completely blocked out. The result is a
pulsating unidirectional current. That is, the current rises from zero to a maximum and drops back to zero (replicating
the AC’s half cycle), is blocked during the reverse cycle (no current flows), and then repeats the first half cycle. This
type 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,” it
is not as satisfactory as rectified three phase current when DC is required. Further, it is more costly since it draws a
higher average current from the AC line than does rectified half-wave AC for a given magnetizing strength.
3.1.7.5
Induced Current Magnetization.
When direct current in a circuit is instantly cut off, the field surrounding the conductor collapses, or falls rapidly to
zero. If an electrically conductive ferromagnetic material is present in such a field, the collapse of that field will induce
a current in the material the same direction as present in the neighboring conductor before cut-off. This phenomenon
can be used to solve specific magnetizing problems that have no other practical solution. A useful application of the
collapsing field technique has been found in the inspection of ring-shaped parts, such as bearing races, without the need
to make direct contact with the surface of the part. Regardless of the type of magnetizing current employed, whether
DC, 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 no
external contacts are made on the part.
3.1.8
Ferromagnetic Material Characteristics.
All ferromagnetic materials, after having been magnetized, will retain some residual magnetic field. The strength and
direction of the residual field depend upon all the magnetizing forces applied since the material was last demagnetized
and the retentivity of the material. The manner in which ferromagnetic materials respond to magnetizing forces is
most often portrayed in a plot of the flux density (B) as a function of the magnetizing force (H). The flux density (B) is
the 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 passing
through 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 then
cycled to saturation in two opposite directions.
