Residual magnetism is the condition where a relay is energized and then when
the current is removed, the relay sticks, appearing to remain energized
momentarily. You can verify that residual magnetism is occurring by getting
the condition to occur and then turning the machine off. If the relay still
sticks after the power is off, then you know that you either have residual
magnetism or something mechanically binding. Closing and releasing the
relay by hand will tell you if a mechanical problem exists.
Residual magnetism can be countered by applying tape between the relay
solenoid coil and the actuator plate. The small space taken up by the tape
and the tape's insulating properties is usually just enough to break the
magnetic field and allow the relay to function properly.
Residual magnetism can be caused by a relay which is attached to its metal
base by a non-brass screw. Check that your problem relay has a brass
screw. A steel screw will carry the magnetic field into the metal base and
affect the relay is screwy ways.
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