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Relay outputs problem


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Hi

I have an application on 8 v350 controllers (the same on all 8) for opening electro motor valves (for hot and cold water). Everything works ok until randomly a relay output remains switched on, even if I take the whole controller from the electric power supply. The only solution then is to open the rear cover of the v350 and hit the relay with a screwdriver to release it. Then it starts working (for several hours, days...) until it stops again in switched on position. The problem is that this is completely random on different outputs and on different controllers!

Did anyone ever experienced something similar? Any solution?

Bye,

Bostjan

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Just to expand on what Emil is saying. Inductive loads like motors require suppressors on the contacts to stop them from arching when they open. Inductive loads also draw high currents (up to 10 times their normal current for motors) when the contacts make. This can weld the contacts together inside the relays. I recommend you use contactors rated for the size motors you are controlling rather the PLC relays directly but this may not be necessary for very small motors.

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Just for informational purposes. The link to the following document has some discussion about spark mittigation and noise suppression. It is a document specifically for a product from OKAYA call the spark quencher and has proven itself to be robust and effective in our control panels. You have many choices available when it come to suppression and spark control, but this one is well packaged and not expensive (OK it's more that 2 cents for a 1N4004 diode). If nothing else, give it a read just to view the data charts to different circuits that have nothing installed and with the product installed (the differences are stark).

http://www.okaya.com/images/noise/1200.pdf

Thanks

Keith

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