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Dry run protection for DC Solar Pump sourcing water from a borehole

Dream_Renewable

Feb 9, 2023
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We are installing solar power irrigation systems for smallholding farmers in Rural Ghana,

We have 12V DC 0.18kW (max 20A) pumps, there only power source will be a voltage varying from 15-20V coming from solar panels.

The boreholes are narrow, therefore is not much room for a float switch,

The max flow is 20 Litres per minute but more typical flows would be 5-10 liters a minute

Can anyone advise on flow or pressure switch solutions (I have done a google but I don't have experience in this area and am not confident myself)

Ideally, there would be no second power source or battery needed, and the solution should be as low-cost as possible. something under £20 per unit for 20 units would be ideal.

(It's a charity project replicating 20 units - so cost is important)

let me know if i missed out any key info
 

kellys_eye

Jun 25, 2010
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Under-current detection?

Use a switch-on timer to allow the pump to run up to speed and after this run-time limit, detect the running current and if it's too low remove the power/reset the system.
 

Dream_Renewable

Feb 9, 2023
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Under-current detection?

Use a switch-on timer to allow the pump to run up to speed and after this run-time limit, detect the running current and if it's too low remove the power/reset the system.
thanks - yes undercurrent detection is an option, I think it would be too expensive though, as it would need a second power supply most likely a buck converter taking the 15-20 down to 5V, a 5 v microcontroller, a DC current sensor, and a relay. I would have thought all of that would cost well over £100 but maybe there is a cheap way to do it?

It's also a bit of a complicated solution with a few too many parts to go wrong -

there may well be a simple way to do undercurrent protection though, If you have any ideas they would be gratefully received
 

kellys_eye

Jun 25, 2010
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Possible solution - Wire a relay coil and contacts in series with the supply to the pump motor. The relay coil will only be energised if sufficient current is flowing in the motor (relay will have to be carefully chosen to find the right coil resistance). The relay contacts are temporarily bypassed with a simple push buttons to allow the motor to run up to speed/load.

Once the button is released, if the coil isn't being held then the power will be removed.

One relay, one switch. Should be within budget......./s
 

Dream_Renewable

Feb 9, 2023
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Possible solution - Wire a relay coil and contacts in series with the supply to the pump motor. The relay coil will only be energised if sufficient current is flowing in the motor (relay will have to be carefully chosen to find the right coil resistance). The relay contacts are temporarily bypassed with a simple push buttons to allow the motor to run up to speed/load.

Once the button is released, if the coil isn't being held then the power will be removed.

One relay, one switch. Should be within budget......./s
ahh, this is an interesting option, and just the sort of thing i was looking for without the need for any control logic, etc.

I dont have any clue about sizing relays, I was looking into this last week and got a bit confused. so are you saying that I can power the full 20A through the input terminals of the relay, in series with the main contacts of the relay?
i thought the input contacts were usually much smaller than the main contacts as input is just meant to be a signal,
any links to useful educational resources on this would be much appreciated,

thanks
 

roughshawd

Jul 13, 2020
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Put in a larger sump just for the pump. Inline pumps are fine until you need more than an on/ off, so there's basically only one option. Dig a pool large enough for your float valve, and install the switch. It's mostly laborious, the float you will need to invent. Otherwise, put a check valve in the line, and allow the pressure to build to a torque pressure, and use the pressure to throw the switch.

Trying to turn water flow on and off is a real B****. You could say something like.." Don't most irrigation systems leak?"!!!!
 

kellys_eye

Jun 25, 2010
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Not seriously considering this rubbish I hope.
What I described is, in it's very basic form, under-current detection - current flowing through the relay coil needing to be at a minimum level to ensure the contacts close. Wiring the relay in series with the motor means the relay dissipating the same energy as the motor - an obvious failing - but not 'rubbish'. It would work but it's not efficient in that format.

There is an electronic version though - a commercial under-current relay such as these:


The problem is, as the OP mentions, cost. Which is why I suggested a 'hammer-and-nails' solution, regardless of how 'rubbish' it may sound.
 

Bluejets

Oct 5, 2014
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What I described is, in it's very basic form, under-current detection
What you described is rubbish.
The motor would never look like starting with a relay coil in series with it.

What does work is as I have done many times , wind a few turns of enamelled copper wire, placed in series with the motor, sufficient for the motor current, (16awg) and place a reed switch inside to detect the magnetic flux.
This will be different from no load to full or whatever load and reed will operate when sufficient current flows.
Then do with this unintrusive voltage free contact what you will.
Being a loss of prime detector, it will need some kind of "start only" override.
Simple enough and relatively cheap and it works.

Other possibility is the use of a hall effect, or as I have previously done, a series diode to give voltage drop under load and operate a semiconductor.
Diode in this instance is not practicle given the 20A load.
 

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kellys_eye

Jun 25, 2010
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wind a few turns of enamelled copper wire, placed in series with the motor, sufficient for the motor current, (16awg) and place a reed switch inside to detect the magnetic flux.
so..... a 'relay' then.

Stop being a pedantic ***.
 

kellys_eye

Jun 25, 2010
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Perhaps...
What 'perhaps' about it? Where was I specific about anything other than the PRINCIPLE involved?

I doff my hat to your superior intellect.

Be realistic.... you've already proven you lack basic understanding.
You have, recently, seemed to follow my posts simply to offer your own criticism of the advice I give. This is as small minded as you can get - but there is a cure - it's called the 'ignore' button.

Consider yourself - and all future posts - 'ignored'.

It's the least you deserve.
 

bidrohini

Feb 1, 2023
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A mechanical flow switch can be a low-cost and straightforward solution for your application. It can detect the flow of the liquid and provide a switch output when the flow rate is above or below a certain threshold. You can find mechanical flow switches that are compatible with your voltage range and current rating, and they may be suitable for your low flow rate requirement.
 
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