I have a conveyor belt under which there are electromagnets to hold a piece of steel during movement. At the end of the belt the pieces must be unloaded with suction cups. Over a certain speed the electromagnets do not discharge sufficiently quickly, making resistance to lifting. How can I speed up the electromagnet discharge process?
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\$\begingroup\$ Do you 'just' switch off or are you generating a deguassing current profile? See wikipedia \$\endgroup\$– OldfartCommented Nov 23, 2019 at 18:53
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\$\begingroup\$ @Oldfart The electromagnets are only turned off \$\endgroup\$– FraMasCommented Nov 23, 2019 at 18:56
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\$\begingroup\$ Are there diodes across the magnet coils to reduce arcing at the switch? \$\endgroup\$– Kevin WhiteCommented Nov 23, 2019 at 19:20
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\$\begingroup\$ @KevinWhite At the moment there are no protection diodes, but we plan to command the electromagnets with SSRs \$\endgroup\$– FraMasCommented Nov 23, 2019 at 19:27
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2\$\begingroup\$ Connect a resistor across the inductor. Notionally T = L/R - capacitance and inductor resistance also get involved. \$\endgroup\$– Russell McMahon ♦Commented Nov 24, 2019 at 3:35
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2 Answers
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The electromagnets are only turned off.
To get rid of the remnant magnetic field you should reduce the current using a decaying alternating current. The idea is to reduce the magnetic field by forcing it through a curve like this:
This is an example current waveform which does this:
You might have to experiment with how long and how fast a decay you can get away with. Just reversing the current for a short period might be enough.
(I realize that this requires significant more logic then just an off switch).
Both images where taken from http://meettechniek.info/passive/magnetic-hysteresis.html
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\$\begingroup\$ A really simple degaussing arrangement just needs you to add a capacitor and a resistor to your inductor, wouldn't it? It might not be the best possible, but I imagine it would work. \$\endgroup\$– HearthCommented Nov 23, 2019 at 19:38
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\$\begingroup\$ Humm thinking about this: getting the circuit to resonate when switched off might work as well. That is a decaying sine wave... \$\endgroup\$– OldfartCommented Nov 23, 2019 at 20:17
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If an electrical change doesn't work you may be able to make one or more mechanical changes.
You might change the electro magnet to a type that has a different hysteresis curve. This would change the curve of the magnet's strength as it falls to a minimum after powering off.
If possible make your magnet turn off timing directly dependent on the speed of the conveyor system. If you have a fixed sensor on the line that disables the magnet make that sensor's position slightly adjustable. Put the sensor on an arm, lever, or spring that slides back as the conveyor speed increases. Then as the speed increases the item is sensed further up the line and the magnet disables sooner. If your conveyor functions are controlled by software you may just need a simple equation that uses the conveyor speed value in a calculation to determine the proper turn off timing.
If your system doesn't use item sensors on the line maybe add one just ahead of the pickup position that reduces the magnet's strength a bit early. That sensor might start operating only at a certain conveyor speed.
