Pump speed contoll

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Jeremy Poulter

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Jul 23, 2021, 5:56:25 AMJul 23
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Hi,

I have a water pump that is a bit too powerful for the job that it is doing. It is not overly complex, just plug it in to the mains and off it goes, so I assume there isn't too much in the way of complex circuitry in there, so is there a way I can reduce/control the speed of the pump?  

Cheers,

Jeremy

Jeremy Poulter

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Jul 23, 2021, 5:57:52 AMJul 23
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contoll WTF is that????? control ;-)

Ian Petrie

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Jul 23, 2021, 6:06:54 AMJul 23
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Throttle the output. Fit a reducer. 3d print something.
Alternatively split the output. Send half the excess flow elsewhere.

Smaller pumps use a synchronous motor. The rotor is an on the wet side driven by encapsulated coils. I think it would need frequency control to reduce speed



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Stephen Rodway

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Jul 23, 2021, 6:08:11 AMJul 23
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OK so a variac or resistor or other voltage-based control doesn't tend
to behave very well on a simple AC mains motor, a pump will probably be
an induction/shaded pole motor with either a good shaft seal or a
membrane between the stator and rotor. If it's a very small pump it
might be a simple DC/universal motor in which case voltage regulation or
high-speed PWM will work.

Simplest would be just an on/off timer if you only care about the total
average flow rate. If you need to actually modulate the instantaneous
flow rate downwards then a VFD is the most elegant solution. A PWM
system where the frequency is low (circa 10Hz) can operate but it's
noisy (electrically and audibly) and is very rough on bearings.

Things to watch out for, you may need to start the pump and full power
and then back down to overcome starting torque issues, and many water
pumps are arranged so that they're cooled by the fluid they're pumping
so there's a potential for overheating as the flow rate drops

Non-electronic options, so long as the pump isn't a positive
displacement type you may be able to use a simple valve to either
restrict the flow or to provide a bypass path around the pump. Either
will make the pump dramatically less efficient but don't know if that
matters here?

Steve
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Andy Noyes

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Jul 23, 2021, 6:47:57 AMJul 23
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Another very simple solution, if it's possible in your application, is just to raise the height it's pumping to. With most centrifugal type pumps the flow rate is inversely proportional to the head height. (Actually I think it's something like the square root of the head). Either raise the output, or lower the pump itself.

Ian Petrie

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Jul 23, 2021, 7:05:41 AMJul 23
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Not lowering the pump. The head is measured from the water surface.

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Andy Noyes

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Jul 23, 2021, 7:23:12 AMJul 23
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Yes Ian, you're correct, lowering the pump below the surface of the liquid would make no difference, my answer above wasn't very clear. (I was imagining a setup with a pump inside it's own small reservoir, similar to a coolant pump on a machine tool, where you could lower the whole pump/tank, but perhaps it's nothing like that at all.)

Jeremy Poulter

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Jul 23, 2021, 10:10:06 AMJul 23
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The system itself is throttling the flow, which (I think) is the problem, the problem being the pump is making a lot of noise. The pump is rated at 6000 L/hour but the measured flow rate is 2000 L/hour, I wanted to reduce the pump speed to match the actual flow rate so it will hopefully reduce the noise level (and use less electricity)

(Sorry should probably add these helpful details in the initial message)

Steve Rodway

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Jul 23, 2021, 12:11:01 PMJul 23
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A bypass rather than a restriction may still work as that lowers the
effective load on the pump whereas a restriction increases it. For pumps
at this scale your best bet for electronic control probably is a
commercial VFD. You could build your own but they're easy to screw up
and you wouldn't save very much be doing it anyway as VFDs are commodity
parts these days.

Steve
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Richard Ibbotson

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Jul 23, 2021, 12:23:07 PMJul 23
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What pressure do you have on the output side of the pump? Is it just head or do you have a restriction like a jet or spray on the output?
Richard


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> On 23 Jul 2021, at 17:11, 'Steve Rodway' via rLab / Reading's Hackspace <reading-...@googlegroups.com> wrote:
>
> A bypass rather than a restriction may still work as that lowers the effective load on the pump whereas a restriction increases it. For pumps at this scale your best bet for electronic control probably is a commercial VFD. You could build your own but they're easy to screw up and you wouldn't save very much be doing it anyway as VFDs are commodity parts these days.
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Jeremy Poulter

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Jul 23, 2021, 12:34:38 PMJul 23
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Even a cheap Chinese VFD is more than the pump cost, or indeed the cost of a pump of more appropriate size... (well at least when it is not middle of summer...) So by the sounds of it that is my best route.

Regards the output, it is definitely not ideal there are several changes in pipe width, a heater, values, etc, but no jet/sprays.

I am actually wondering if the noise issue may actually be from the input side, ie not enough water to feed the pump so air is making its way in?

Cheers,

Jeremy


Richard Ibbotson

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Jul 23, 2021, 12:49:57 PMJul 23
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You really need to know what pressure you are working into to properly specify the pump. You should be able to see if you are sucking air in through leaks or a lack of intake water. If you have restrictions on the pump input side I guess you will get cavitation which is noisy.
What is the pump type? What is the motor type? What is the motor wattage?
Richard


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On 23 Jul 2021, at 17:34, Jeremy Poulter <jer...@bigjungle.net> wrote:



Jeremy Poulter

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Jul 23, 2021, 1:04:37 PMJul 23
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Richard Ibbotson

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Jul 23, 2021, 2:39:16 PMJul 23
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They don’t appear to be strong on specifications do they. I am thinking the “Hmax 1.5m” on the front page of the manual Is probably the maximum head so not a very powerful pump. That is about .15 bar so really just enough for a circulating filter at the published flow rate.  Normal mains water pressure is around 1 bar or 10m of water. 
If you let the output go direct into a container rather that through your pipes, heater, valves is the pump quiet enough?
It would be good to get a pressure gauge on the output.
Richard


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On 23 Jul 2021, at 18:05, Jeremy Poulter <jer...@bigjungle.net> wrote:



Jeremy Poulter

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Jul 23, 2021, 3:35:02 PMJul 23
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Thanks, will give that a try (when the sun is back out) 

Jeremy

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