Updated: Feb 19, 2019
THE ADVANTAGES & DISADVANTAGES
AC (Alternating Current)
Very much a “ skin effect”.
Ideal for all surface breaking defects.
(e.g. in-service related defects caused by stress, fatigue and heat, which all start at the surface ).
Provides the best movement and vibration of particles, allowing migration in and around the defects for clear defined indications.
Leaves the least residual magnetism in the part and therefore is the easiest to de-magnetise.
Does not indicate even slightly sub-surface defects very well.
AC is a very satisfactory medium for diminishing de-magnetisation.
HWDC (Half Wave Direct Current)
Developed by rectifying AC and removing one polarity.
Much better than AC for indicating slightly sub-surface defects.
Still provides excellent movement and vibration of particles, allowing migration in and on top of defects.
The residual magnetism left in the part is still not a problem for conventional diminishing AC current to demagnetise.
Lectromax technical dep’t firmly believes this wave-form (HWDC) to be the best for Magnetic Particle Applications in general.
FWDC (Full Wave Direct Current)
The most penetrative magnetic field produced from mains electricity.
Can find (slightly) deeper defects than AC or HWDC. It must be noted that for “serious” sub-surface defects, Magnetic Particle Testing is not the best method. (Ultrasonics and/or Radiography are processes much better suited for these defects).
The only way normally to demagnetise is with FWDC reversing diminishing current. This means electronically reversing the polarity at the same time as diminishing the current. A very expensive feature to add to the equipment.
The magnetising is also a significantly more expensive process requiring 3 main magnetising transformers
The most satisfactory method of Magnetic Particle Testing generally is to magnetise in HWDC, demagnetise with diminishing AC.
This happens to be the most cost effective method also.