Device to cut power if load lasts longer than set time

Question

Device to cut power if load lasts longer than set time

mesa5 (Mechanical)

(OP)

27 Oct 14 16:48

I'm having some trouble finding an off the shelf product that I would think would already exist.

I have a 1/2 HP, 120V pump that will be moving fluid from one tank to fill another, until a float switch in the filled tank cuts power. As a safety backup, I would like a product that will cut power to the pump (until manually reset) if it runs for longer than a set time. The idea being, if the pump runs for too long, it means something in the system is screwed up (tube burst, tube fell onto the floor, etc...) and the pump is now evacuating the tank all over the lab floor.

Does a product like this exist? I know I can rig something up using timing relays, but that will require additional time and effort that may not be necessary.

BrianPetersen (Mechanical) · Answer 1 · 2014-10-27 17:28:26

You can buy timing relays.

The function you are looking for is called "single shot leading edge, voltage controlled" in this little demo page.

http://es.eaton.com/EatonTiming/app.htm

http://www.eaton.com/Eaton/ProductsServices/Electr...

IRstuff (Aerospace) · Answer 2 · 2014-10-27 18:56:21

There are a plethora of home appliance switches that could be used, like the X-10 or Z-wave home automation products. They have both timed and at-will functionalities.

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mesa5 (Mechanical) · Answer 3 · 2014-10-27 22:56:05

I was hoping I wouldn't have to buy the timing relay; since the relay, plus mounting, enclosure, cord, socket, etc.... is more time and money than it needs to be. I guess if there is nothing off the shelf than that's the route I will go.

MikeHalloran (Mechanical) · Answer 4 · 2014-10-28 17:38:18

You could use a slightly undersized circuit breaker.

The more traditional way, I think, is a pressure switch on the pump that cuts off power if the backpressure seen by the pump goes too low. You still need a timer to prevent it from opening while the pump comes up to speed.

Mike Halloran
Pembroke Pines, FL, USA

jraef (Electrical) · Answer 5 · 2014-10-28 21:22:26

You can use a Current Monitor relay set to trip if the load on the pump is not within a predetermined narrow band of current, because if a tube falls out, bursts, or plugs, the flow changes and therefore the power required by the pump changes. The one I linked to has a built-in 6 second delay, so if your operation is less than that, you could even more simply just set it for the maximum time and if there is ANY current still flowing, it turns something off.

But honestly, those would likely cost more than the timer.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

ornerynorsk (Industrial) · Answer 6 · 2014-10-30 14:49:38

If a simple timing relay is too much investment in equipment, the next best fail-safe is the carbon based CPU that is supposed to be in the room watching to make sure the processes do not run amok.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

mesa5 (Mechanical) · Answer 7 · 2014-10-30 20:45:08

Ended up going with a timing relay. I had assume that there would be an off the shelf product that would exist for sump pumps, pool pumps, lights, etc... Looks like I was wrong.

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Device to cut power if load lasts longer than set time