Contactor on borehole water pump issue.

[Soulsurfer]

Hi all, been asked to go look at a contactor setup on a pump which is only used for garden lawn sprinklers etc.. and as I don't often play with danfoss overloads and such I cannot work out what's up!

I have no details on the load of the pump / model, make etc.. and only know that the pump was new last year.

Basically the contactor is pulled in via a danfoss pressure switch or differential switch and then feeds the pump which they say usually sits running at all times until a gardena controller opens the water flow to feed the sprinklers. There is a C19 danfoss contactor and a TI16C overload on it, It's apparently been fine but now there is a constant quiet buzz when it pulls in and the load was sometimes pulling 16A and most of time 7.1A or so and slowly dropping to 5A or so but the pitch suddenly changes after maybe 30 secs max and either the overload breaks the contacts or it rises higher and doesn't trip the overload but strangely the contactor does stay in but no voltage or current through the overload and down to the pump.

Pump is single phase, and overload is shown 2-3A and adjacent on side up to 7 odd amps. 16A rcbo feeding it all doesn't trip either.

I am thinking pump is jammed maybe and taking overload as no water is coming through pipes.

Anyone know how you set overload on that clear slider part on side as how can you differentiate between the front scale lower amps or rear scale higher ? it seems a crap design and danfoss technical do not provide anything like any help!! They're a waste of time entirely.

 

[Pete999]

Dirty Pole Faces on the Contactor

 

[Lucien Nunes]

Slide the clear plastic lever to point at the correct FLC figure. The two scales are for DOL and Star-Delta applications. The Star-Delta values are higher by sqrt(3) because they show the motor's rated line current in delta, although the overload relay is not in the line connections but in series with the windings, where the current is sqrt(3) lower.

It does sound like the motor is faulty or jammed, is never actually starting up and is just drawing a heavy overload. If the pump current drops to zero when the contactor is still in, I suspect the motor has thermal overload protection directly in series with the windings which is cutting out before the starter's overload relay, and/or the wiring is incorrect. In an ideal world the O/L will trip before the thermal cutout, but if the cutout does trip it would release the contactor. That might not be possible if it's internally connected to the pump windings.

Anyhow, it might well be toast by now!

 

[Soulsurfer]

Thanks Pete & Lucien, I'm pretty sure by looking at the contactor it's wired up correctly via a CS pressure switch but I don't think the pump is alright even if only a year old or younger as it all worked before fine.
Would it not be an issue the contactor having the pump going all the time without the waterflow being open? Unless the pressure switch is causing the issue in the first instance!

 

[Lucien Nunes]

It's difficult to say what is supposed to happen without seeing how it's installed. I would have expected the pressure to rise when no water is being drawn, opening the pressure switch and releasing the contactor to stop the pump. A centrifugal pump will normally run continuously into a closed delivery valve without damage, but there's no point in it doing so, and other types of pump generally cannot do this. OTOH unless there is a pressurised receiver and a good non-return valve, as soon as the pump stops the pressure will bleed down and it will restart. Failure of the N/R valve can cause destructive rapid cycling.

If the contactor is part of a pushbutton starter with functional on and off buttons, then the pressure switch must be in the motor winding circuit and the contactor will stay in until the user presses the stop button. Actually maybe this is what is happening. If the delivery line is shut off somewhere or there's a kink in the hose, the pump is making whirring noises and actually pumping until the pressure rises high enough to open the pressure switch, at which point the load drops to zero but the contactor stays in. So far so good, but the pump is still taking excessive current by the sound of things.

You'll have to trace the wiring.

 

[Wilko]

A learner's question from me - from Plumber's heresay re boiler pumps I had thought running a pump either with no water or with no water flow is bad (hence the bypass valve). I'll have a look for something on the centrifugal type of pump to try to understand it (thanks LN). But "they say the pump sits there running at all times until the controller opens the water flow" doesn't seem like a good design ?

 

[Soulsurfer]

A learner's question from me - from Plumber's heresay re boiler pumps I had thought running a pump either with no water or with no water flow is bad (hence the bypass valve). I'll have a look for something on the centrifugal type of pump to try to understand it (thanks LN). But "they say the pump sits there running at all times until the controller opens the water flow" doesn't seem like a good design ?

That's exactly why I ask as it's going to be running like a boiler circ pump against a closed motorised valve! Can't be good.

 

[Lucien Nunes]

Running a central heating pump or indeed most kinds of pump dry is bad, because the working fluid is often used to lubricate and cool the bearings and/or shaft gland. A C/H circulator is glandless but relies totally on the water for bearing lubrication.

Running with the flow shut off is a different matter and the effects depend on detail design. A central heating pump working into a nearly closed delivery will not usually be harmed, but in a practical heating system with say only one rad taking a small amount of flow, the increased differential pressure may cause excessive noise, TRV malfunction or cavitation rather than pump damage.

Many kinds of centrifugal pump absorb less power from their motor when there's no flow. You'll be familiar with vacuum cleaner motors speeding up due to lighter load when the airflow is restricted. But without flow, there's no cooling either, so the pump may churn the water until it's hot and the vacuum cleaner motor may overheat (if it uses the cleaning airflow for cooling; wetvacs don't.)

Positive displacement pumps such as piston, gear, diaphragm etc. cannot stand the flow being shut off as they pump a certain volume of liquid per revolution, and something will burst or break if they cannot, or the drive will be stalled.

In this case, it may be that the pump can run happily and safely into a partially closed sprinkler feed, but the pressure switch is there to shut it off when there is no call for water at all.

 

[PEG]

Hi,it is worth checking that the borehole is sufficiently flooded,bearing in mind the present weather. A period of dry running may have caused failure of the motor.

You can check that by pulling a vacuum on the output of the pump.

You do not have to draw water to the top,but a simple check of the steady increase of vacuum,judged against the size of pipe,should give an indication of water present,blockage or fracture.

What happened to the original installers,after only a year? They should be at least on hand to describe the installed gear...

 

[David Prosser]

OP is it only the pump or installation that was new last year ?
If it's a proper borehole pump it should have a thermal cut out built in that should prevent damage from a "dry hole" (no one like that do they) or long term "no flow". If it's just a submersible pump connected up then probably no inbuilt protection. Is there any chance you can pull it up and have a look?

Also the hole may not be dry it could be the pump does not not have sufficient ability to pump the head needed as your water table will be reduced at the moment.

Shame you are not closer as I have quite a few pumps looking for a good home.

 

[PEG]

OP is it only the pump or installation that was new last year ?
If it's a proper borehole pump it should have a thermal cut out built in that should prevent damage from a "dry hole" (no one like that do they) or long term "no flow". If it's just a submersible pump connected up then probably no inbuilt protection. Is there any chance you can pull it up and have a look?

Also the hole may not be dry it could be the pump does not not have sufficient ability to pump the head needed as your water table will be reduced at the moment.

Shame you are not closer as I have quite a few pumps looking for a good home.

I would be careful of doing that,as when it goes back in,and you have an additional 5 meters of hose and cable at the top...it will be "Tick - you're on..."

Some clue regarding who and what was fitted,would be my first move,before withdrawing the sword,and claiming the title of rightful king of England

(I may be watching a little too much "Horrible History"...)

 

[Soulsurfer]

Cheers all of you, great knowledge and advice as always, David & PEG yes installers only replaced pump last year which looks like a basic submersible rather than a bona fide Grundfos borehole or similar pump. It's kind of a big concrete tank and looked around a foot of water but may be close to dry or has run dry before / recently.

 

[PEG]

So it's a tank and not a borehole,should make things easier,pump wise. Hopefully it is fitted in such a way,as to be removable,from above.

Do not be tempted to go in there to any work,fella.

 

[Soulsurfer]

So it's a tank and not a borehole,should make things easier,pump wise. Hopefully it is fitted in such a way,as to be removable,from above.

Do not be tempted to go in there to any work,fella.

It is there's a rectangular steel cover on top and looks like piped in Hepworth h20 bud, everydays a school day in this trade.