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AC Demagnetisers are suitable for small, low field
demagnetisation, i.e. cathode ray tubes and certain steel applications.
They are the simplest form of demagnetiser. A coil or
demagnetisation fixture is connected to a mains supply that may or may
not be controlled by electronics to control the magnetic field’s
magnitude. For the smallest and simplest of cases a hand held AC
demagnetiser may be used.
To achieve the demagnetising effect, the applied
oscillating field needs to reduce over time. This can be achieved by
relative movement of the fixture and work piece or electrically
reducing the current in the fixture.
For relative movement, the fixture is moved away from
the work piece, or the work piece is moved away from the static
fixture. Either way the resultant demagnetisation is very dependent
upon the speed of relative movement. If this relative movement is made
by an operator, then the efficiency of the demagnetisation process can
be seriously impaired. It is therefore usual for the fixture to be
supplied with a conveyor belt system to give good smooth control of
this relative movement.
For the electrically controlled system, a control box is
fitted to the fixture. This electronically reduces the peak current
that flows into the coil on each successive oscillation of the mains,
thereby reducing the magnetic field experience by the work piece. The
time for the current to reduce to zero is adjustable of the order of a
few seconds dependent upon the components to be demagnetised.
A variation on a mains demagnetiser is to incorporate a
resonant technique to offer higher fields with low continuous input
current. Dependent on the application, this can achieve effective
demagnetisation without the need for component removal from the
demagnetising location.
This technique can be adopted for small and large
applications.
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