As seen in the previous pages,
electric current is often used to establish the magnetic field
in components during magnetic particle inspection. Alternating
current and direct current are the two basic types of current
commonly used. Current from single phase 110 volts, to three phase
440 volts, are used when generating an electric field in a component.
Current flow is often modified to provide the appropriate field
within the part. The type of current used can have an effect on
the inspection results, so the types of currents commonly used
will be briefly reviewed.
Direct current (DC)
flows continuously in one direction at a constant voltage. A battery
is the most common source of direct current. As previously mentioned,
current is said to flow from the positive to the negative terminal. In actuality, the electrons flow in the opposite direction.
DC is very desirable when inspecting for subsurface defects because DC generates a magnetic
field that penetrates deeper into the material. In ferromagnetic
materials, the magnetic field produced by DC generally penetrates
the entire cross-section of the component. Conversely, the field
produced using alternating current is concentrated in a thin layer
at the surface of the component.
(AC) reverses in direction at a rate of 50 or 60 cycles per second.
In the United States, 60 cycle current is the commercial norm
but 50 cycle current is common in many countries. Since AC is
readily available in most facilities, it is convenient to make
use of it for magnetic particle inspection. However, when AC is
used to induce a magnetic field in ferromagnetic materials, the
magnetic field will be limited to narrow region at the surface
of the component. This phenomenon is known as the "skin effect"
and occurs because the changing magnetic field generates eddy currents in the test object. The eddy currents produce a magnetic field that opposes the primary field, thus reducing the net magnetic flux below the surface. Therefore, it is recommended that
AC be used only when the inspection is limited to surface defects.
View a short video
on the AC versus DC. (195
Rectified Alternating Current
Clearly, the skin effect limits the use of AC since many inspection
applications call for the detection of subsurface defects. However,
the convenient access to AC, drives its use beyond surface flaw
inspections. Luckily, AC can be converted to current that is very
much like DC through the process of rectification. With the use
of rectifiers, the reversing AC can be converted to a one directional
current. The three commonly used types of rectified current are
Half Wave Rectified Alternating
When single phase alternating
current is passed through a rectifier, current is allowed to flow
in only one direction. The reverse half of each cycle is blocked
out so that a one directional, pulsating current is produced.
The current rises from zero to a maximum and then returns to zero.
No current flows during the time when the reverse cycle is blocked
out. The HWAC repeats at same rate as the unrectified current (60 hertz typical). Since half of the current is blocked
out, the amperage is half of the unaltered AC.
This type of current is often
referred to as half wave DC or pulsating DC. The pulsation of
the HWAC helps magnetic particle indications form by vibrating
the particles and giving them added mobility. This added mobility
is especially important when using dry particles. The pulsation
is reported to significantly improve inspection sensitivity. HWAC
is most often used to power electromagnetic yokes.
Full Wave Rectified Alternating
Current (FWAC) (Single Phase)
Full wave rectification
inverts the negative current to positive current rather than blocking
it out. This produces a pulsating DC with no interval between
the pulses. Filtering is usually performed to soften the sharp
polarity switching in the rectified current. While particle mobility
is not as good as half-wave AC due to the reduction in pulsation,
the depth of the subsurface magnetic field is improved.
Three Phase Full
Wave Rectified Alternating Current
Three phase current
is often used to power industrial equipment because it has more
favorable power transmission and line loading characteristics.
This type of electrical current is also highly desirable for magnetic
particle testing because when it is rectified and filtered, the
resulting current very closely resembles direct current. Stationary
magnetic particle equipment wired with three phase AC will usually
have the ability to magnetize with AC or DC (three phase full
wave rectified), providing the inspector with the advantages of
each current form.