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I use a DL205 PLC with a Do more CPU. This drives SPDT relays to control 90 degree motor operated valves. The valves are apollo motor actuated, which advertises 0.74amp starting/locked rotor current. I have confirmed this value with a clamp on ammeter. The relays are automation direct 781-1C-120A 15A relays with 120VAC closing coils. During testing, I'm finding that some of the relays will randomly stick in the normally open position, preventing the valve from closing when the coil is de-energized. the relay will reset when mechanically agitated, but not otherwise. Voltage is correct, verified to 124VAC.

Has anyone seen anything similar to this?

Sean Taylor
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    How much space do you have between relays on the board? Sometimes the magnet in one will affect the reed in it's neighbour. The relay datasheet should give a minimum distance if that's a risk. – Jack B May 18 '16 at 16:38
  • There is not a lot of space, maybe 7.5 mm, but the issue continues even when all power has been secured to the machine and/or the relay is physically removed from the system – Sean Taylor May 18 '16 at 17:03
  • Are you passing too much current through the contacts? It sounds to me like they're being "welded" together, which could happen if you're switching a load drawing too much current. The over-current heats up the contacts and welds them together. – DerStrom8 May 18 '16 at 19:04
  • clamp on ammeter shows less than 1 amp, and they are rated for 15 amps - its possible that it's happening too fast for my meter to detect, but I don't believe so. – Sean Taylor May 18 '16 at 21:15

2 Answers2

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Your relay contacts need a snubber and/or MOV/TVS to suppress the inductive spike when switching an AC inductive load (your valve). Without the suppression, the resulting arc can cause the contacts to "stick" (weld).

enter image description here (Image from Red Lion SNUB0000 datasheet)

If your load were DC, a diode across the coil would be the more common solution.

See also https://electronics.stackexchange.com/a/100139/25328

Community
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Tut
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    Thank you tut - I had not been aware of these - there's a decent chance that this will solve our issue, and I absolutely could use these in other areas of our process too. – Sean Taylor May 18 '16 at 18:14
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I have seen relays fail mechanically due to rust, dirt, mold or whatever due to becoming submerged in a flood or being contaminated in some other manner. Remove the cover, cut it off if necessary, and examine the mechanism.

Edit

It is also possible that there is some leakage current keeping the relay sufficiently energized so that it does not drop out reliably.