Intermittent submersible pump fault

esquilax

We have a bit of a problem with our submersible pump that hopefully someone might be able to shed some light on. Now unlike most of you I am in Europe and from reading the other threads about pumps, certain things seem to be done a bit differently here.

I think it is a Grundfos 3/4HP pump but I am not certain because I have never seen it and it's about 15 years old. It runs on 220V though I guess pumps in the states often do too.

Sometimes when the pump is turned on (by the pressure switch cutting in), the pump doesn't actually start. Instead of drawing the correct run current of about 4A, it draws more like 12A until the thermal overload trips. You can turn it on and off, on and off, it'll keep doing the same thing.

The only way to get it running again is this (it has worked every time so far). Remove the start capacitor from the circuit, restore power to the pump and then, while powered, put the start cap back in the circuit. The pump will run till the pressure switch cuts it out. When it cuts back in, it'll probably work too but at some point in the next few hours, it'll go back to not working and tripping the overload.

I don't have the resistances I recorded between blue/brown and blue/black to hand but they seemed ok. Something like 6 and 20 ohms and the sum of the two readings across brown/black. When the pump is working, it appears to work as well as ever though I have not tested this in any scientific way.

Some background info:

The "control box" doesn't contain much compared to some of the ones I've read about on here. There is no potential relay in the control box so I guess the pump has some kind of centrifugal switch to cut out the start relay. No contactor either, the pressure switch performs that function. The only things in the control box are an on/off switch, a thermal overload that had been bypassed by some idiot at some point in the past, a neon indicator and a start capacitor, 20uF I think.

The pump didn't start for first time in years last week. I was sent out to look at it and was concerned to see the light on so there was power and the pressure switch was working. Thinking I'd have to pull the pump, I thought I might as well try and replace the only other component in the control box that could be at fault, the capacitor. I got a new one, installed it and lo and behold, the pump worked again. I also noticed the bypassed thermal cut-out (it had failed open) and ordered a new one.

The pump seemed to run fine for a week. Today the new thermal cut-out arrived in the post and I turned the pump off to install it in the control box. When I restored power, I don't know why but this problem came back.

I really don't see how there could be a solution that doesn't involve pulling the pump but if someone provides one it will be rewarded. Failing that even an explanation of what the problem with the pump is likely to be would be great.

Note that there is current measurable across this cap while the pump is running, about 1A - not sure if that's normal or not?

Thanks in advance for any help you can give.

mpoulton

It's a start capacitor, and it has a 1A current through it at all times when the motor is running? That's abnormal. Indicates a problem with the centrifugal start switch in the motor. Does the new capacitor still test good, or is it fried?

frenchelectrican

What part of Europe ya in ? I am in France and let me fill you in with this part I know mpoulton have right answer and let me add here for the readers back in state side we Europeans do run 240 volt Line to Netural which it is a common item.

First thing is that you may have starting capatitor going bad or starting switch going bad. { this is kinda common item with three wire well pumps }

15 years on that pump is about right for lifewise { super hard eau (water) will shorten the life a bit }

Readers just keep in your mind the other thing is we do run on 50 HZ supply as well.

IIRC 20 ohms is run winding while 6 ohms is used for starting winding.

For the new pump unit IIRC they useally run about 450€ { EUROS }

Mpoulton.,
When the motour start up once it get into running speed the capaitour should not be drawing any current at all so more like the OP's control box is going bad some case it may have start relay in there.

They are simair to Franklin pumps

Merci,
Marc

__________________
The answer will be based on NEC ( National Electrical code ) or CEC ( Cananda Electrical code ) or ECF ( Electrique Code France )

esquilax

Guys, thank you both for the replies.

mpoulton: I didn't realise before but the pump draws 5.5A of current (1.5A more than what I thought was normal and correct) and works better when the start cap is manually removed from the circuit altogether so your diagnosis is correct.

unfortunately I don't have a capacitance meter which is why I replaced the old cap in the first place. The only test I could do is to see if it momentarily shows a resistance above infinity when you put a meter across it and it does. Thing is, so does the old one.

The last time I encountered a permanently connected start capacitor (on a large refrigeration compressor) it was very obviously melted and cracked. However that's not the case here. I don't suppose it matters too much now anyway.

frenchelectrician: I am in Ireland. Our voltage is nominally 230V but the circuit this pump is driven off can slump as low as 210V when 8kW of other equipment is running simultaneously (it's a dairy farm). This isn't good in itself but it's been working that way for years.

It would be better not to have to pull the pump because I'm told some rock has fallen in on the unlined part and they had great difficulty getting it out last time.

At the moment I've connected the start cap up to a light switch so to start the pump, one pushes in the thermal overload, give the start cap a brief flick in and out of the circuit and off it goes. Obviously this is not what you'd call a long term solution.

I suppose the next question is given the centrifugal switch seems to have failed in the closed position, would it be possible to install a potential relay in the control box to perform the same function? If so, any idea how one would go about finding the right one?

mpoulton

If it's a 3-wire pump, it probably already uses a potential relay instead of a centrifugal switch - I wasn't thinking clearly on that. The problem may be entirely in the control box.

esquilax

No I think you had it right.

There is nothing in the control box but a mechanical on/off switch, the thermal overload, the start capacitor and a neon indicator. Power to the control box is supplied from the pressure switch.

Whatever is supposed to switch the start relay in and out is definitely contained in the pump. As I said, this appears to be one of the differences between my control box and most of the ones I've read about on this board.

I will contact the people who supplied it today and see if they have any ideas. From what I understand you can't just throw in any old potential relay and expect it to work.

If I find a solution I will report back.