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| The magnetizer
(magnetizing pulse generator) consists of a power supply operating off line
AC current which produces DC current at a very high voltage, at moderate
current levels. The current is then stored in a bank or banks of capacitors.
In order to insure proper operation, both the value of the capacitance and
charge voltage must be matched to the fixture which is in use. A switch
capable of carrying very high currents is then closed to allow the magnetizing
pulse to flow. In Oersted Technology products these are solid-state SCR
(silicon controlled rectifier) devices, which are fast-acting and have very
little voltage drop or lost power. The load fixture stores energy in its
magnetic field, and behaves electrically like a nonlinear inductance. If
the inductance is high enough, the current would surge forward, then slow
up, stop, and reverse direction, which is called ringing. Reversed current
in the fixture would be disastrous, because it would destroy the capacitors
and also partially demagnetize the part. Reversed current into the capacitors
is blocked by separate diodes (which only conduct electrical current in
one direction), one for each bank. If the current were to be abruptly cut
off in the fixture, however, the voltage across the fixture would rise to
a very high value, until it forced a path to dissipate the stored energy.
That would destroy the electrical insulation of the fixture, burn out the
blocking diodes, and possibly cause a fire or endanger the operator. Instead,
a large diode (the freewheeling, or "flyback" diode) is provided
to allow the energy to continue out one side of the fixture winding and
back into the other, until it is dissipated safely in the electrical resistance
of the fixture.
Many magnetizer designs use line-frequency transformers to obtain the high charging voltage. Oersted Technology uses a high-frequency design instead which saves weight and space. It also results in even charging, so that the capacitor banks are charged faster for the same maximum line current, with less stress on the capacitors. |
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