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Author Topic: More pressure with VFD and 3 phase pump  (Read 3423 times)
stephendv
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« on: April 14, 2011, 10:05:02 PM »

Hi all,

I need more pressure from my borehole pump, it's a 1.5kW 3 phase connected to a VFD rated as 2.2kW.  Would increasing the frequency on the VFD provide more pressure?  And what are the safe limits I can use?
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DonL
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« Reply #1 on: April 15, 2011, 08:26:43 AM »

Hi Stephen,
Firstly, by interrogating the VFD you should be able to check absorbed power and tweak up the speed until you approach the design power for the motor.
From what I remember (without looking it up Roll Eyes) Pressure is proportional to speed squared, and flow proportional to speed, making power proportional to speed cubed on an ideal pump.
If you cannot get sufficient pressure within the limits of the existing motor you could replace it with a 2.2kW equivalent with the same number of poles. It will probably be a larger frame size so you have to check it can be connected to the pump and aligned properly.  If you did this, the cube root of 2.2/1.5 would give you the possible speed increase within the capability of your VFD. That is a 13% increase in speed giving a 29% increase in pressure.
However it is not as simple as this as you should consider the system pressure drop and the actual flow possibilities.
In all cases, you need to check the pump data sheets and flow/pressure curves to make sure you're staying within design limits.
Another thought is that a bore hole pump must be designed for a large Net Negative Suction Head and may not even be a normal centrifugal pump which makes all the meanderings above irrelevant whistlie
So I suppose the best answer is the usual question. Can you give more information? Specifically the pump data sheets?
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stephendv
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« Reply #2 on: April 15, 2011, 08:50:48 AM »

It's a Calpeda 4SDF 22/28 EC  described on page 292 here: http://it.calpeda.com/file/1b23aa1cc2f457b2b33713a9de4b1530.pdf
One thing that concerns me now is that the data they have on that page is for 3 phase models at 400V, yet the guy who sold it to me sold it as a 3 phase 230V and installed a 230V VFD...  On the front page, they also state they have a number of motor options available including: "three-phase 230 V; 400 V, for 4 motors", so I assume that I have a 230V 3 phase model.

Borehole depth: 120m
Pump depth: 110m
Initial water level in borehole: 30m
With the pump on, the borehole is empty after 2.5 hours.
Recovery time: 2 days
(Yes, it's a rather poor performing hole and the pump is mismatched, but it's what I have  Smiley )

Water is pumped to tanks that are about 30m above the surface, so total worst-case head from pump to tanks = 140m.

There's a simple filter at the pump that's composed of a row of disks that need periodic cleaning, and it's this that is causing some issues.  If I clean the filter completely, the water arrives to the tank no problem.  But after about 2 weeks it's needs cleaning again, yet doesn't really contain that much sand.  I'm hoping that more pressure will increase the cleaning intervals.





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Iain
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« Reply #3 on: April 15, 2011, 09:10:21 AM »

Hi
Sometimes some 3 phase motors are designated 230V/400V. In these cases, 230V the motor windings are wired in DELTA formation and for 400V they are wired in STAR formation.
Usually there are some links in the terminal chamber to achieve this.
Iain
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Alan
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« Reply #4 on: April 15, 2011, 09:21:52 AM »

Is the variable speed drive connected to a single phase suppy. ?
If it is the it will probally only out put three phase at 230 volts.

What does the motor nameplate say the motor voltage is. ?

How is the motor connected.

In Star or Delta. ?

Regards

Alan
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stephendv
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« Reply #5 on: April 15, 2011, 09:29:00 AM »

Is the variable speed drive connected to a single phase suppy. ?

If it is the it will probally only out put three phase at 230 volts.

What does the motor nameplate say the motor voltage is. ?

How is the motor connected.

In Star or Delta. ?

Hi Alan,

Yep VFD is connected to single phase supply and can only output as much voltage as the supply.  Since the supply is an inverter, I can increase this to 245V ish.  I don't know what the nameplate says, but from the invoice from the supplier it's 230V 3 phase.  Don't know whether it's star or delta  Sad
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Justme
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« Reply #6 on: April 15, 2011, 09:53:50 AM »

From what you say you need a slower less powerful pump that can be left on for longer to get the water needed & better matched to the refill capacity. This will reduce the head you are pumping from (as the water will stay at a higher level) & lower the total energy needed.


Have you looked at having the bore hole hydro fractured? That should increase the flow capacity of the hole.
How old is the hole?


When we were flushing our borehole we left the pump on almost constantly for two days (at 2m3 per hour) & the level stayed constant.
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DonL
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« Reply #7 on: April 15, 2011, 03:11:41 PM »

The data sheets show a 240V option with a recommended motor size of 1.5kW and a maximum power input of 2.34kW so I guess this is what you have got and clearly it is single phase. It seems to be working correctly and be more or less properly sized for the head you have.
I think the idea that you can overcome a problem caused by the strainer being blinded with sand by increasing the pressure is quite unlikely to be successful. Once filters get blocked, any increase in pressure is unlikely to produce more than a very brief further period of operation, particularly as the filter will be on the inlet and may well cause the pump to cavitate anyway when blocked.
The situation may well not be helped by the pump running the borehole dry. Could you put a timer on it and set this to run the pump for an appropriate period, perhaps one hour per day and avoid running dry?
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« Reply #8 on: April 15, 2011, 03:23:10 PM »

Thanks DonL,

Ok, will have to investigate other solutions then; cleaned the filter out yesterday so lets see how long it lasts before it needs another cleaning.  The system was working fine last year even with the filter and cleaning only once every 3 months.  Might very well be something unrelated to the filter and pump.

The pump doesn't dry run as I have a level sensor connected to the VFD which shuts it off before the borehole is completely dry.  The sensor is 5m above the pump. 

Justme,
The borehole has a water source at 30m and another at 80m, the lower source recovers faster than the upper so if I want to get the most water out of it I have to pump every day and never let it reach the 30m level.  I do envy your water in the UK, but not your clouds  Wink
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stephendv
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« Reply #9 on: April 18, 2011, 08:00:49 AM »

While we're on the subject, am I right in thinking that the frequency in the VFD affects the pump speed which affects the flow rate?   And the voltage at a given frequency affects the height I can pump to?
So if I set the VFD to 45Hz and 240V, it will still be able to pump to the same height as before, just at a lower flow rate.  Correcto?
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rogeriko
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« Reply #10 on: April 18, 2011, 09:00:10 AM »

You can always use a second pump at the surface to help increase the flow/pressure. A surface pump will draw from about 8 meters therefore reducing the depth of your borehole and will increase the delivery pressure to whatever you require without overloading the borehole pump.
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guydewdney
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« Reply #11 on: April 18, 2011, 10:19:57 AM »

Is there any way of back flushing the filters? maybe by briefly running the motor in reverse? the vfd will have that option if its any good! frequency = motor speed = flow and pressure as stated above.
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stephendv
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« Reply #12 on: April 18, 2011, 10:34:03 AM »

Backflushing not an option in the current config because of a non-return valve on the pump.  But in any case, I think I've solved the mystery of why it's clogging up more now than before.  The difference is previously, when the pump ran dry I had an Arduino that would turn the generator off and pumping was finished for that day.  Now that I have the inverter+ battery setup, I'm switching the pump on manually, and sometimes I leave it on too long - the pump stops when there's no water so there's no danger there, but it cycles on and off as the well slowly fills up.  This means it's pumping the dirtiest water.

So I will solve this the same way, either with a timer or arduino, when the well runs dry stop pumping immediately for that day and resume the next day.

Any ideas about how the frequency and voltage of the VFD affect flow rate and head?
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DonL
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« Reply #13 on: April 18, 2011, 01:26:04 PM »

Hi Stephen
I refer you back to my comments above about head being proportional to speed squared and flow proportional to speed.
So if you changed the speed from 50Hz to 45Hz; the head would decrease by (45/50)2 or to 81% of the previous figure and the flow by 45/50 or 90% of the previous figure. All assuming ideal centrifugal pump.
Don
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