Reducing pumping power question

lazza · September 2014

HI

We've got a client who has a small irrigation system in rural house. His installation was a bit of a mess so we sorted it out with more batteries and panels to give power for half an hour of irrigation per day.

However, one problem that we have is that the well pump at about 70m, is supposed to be 1.5 HP, so we expect it to draw somehwere between 1 and 1.5kW from the batteries all things taken into account. However, in reality it is drawing a staggering 2.5kW. The pump guy came out and said this is normal.

Well clearly it's not normal. I dont think it's the starting capacitor as the power surge at start-up is the same, 2.5kW. The pump's motor was changed a few years ago for some generic model and this may be what is drawing the extra power.

The problem is, this is alot of power for his 3kW system and if we could reduce the pumps power by half, the client could irrgate much more time and with less pressure on the batteries.

So, I'd like to know if there is any simple way of reducing a well pump's power??

Cheers
Larry

inetdog · September 2014

Re: Reducing pumping power question

The most universal and reliable way is to install a Variable Frequency Drive (VFD) electronic unit to drive the pump. It will produce a variable voltage and frequency to provide the maximum possible torque and power while the pump motor is running at less than full speed.
For a centrifugal type pump, you can cut the mechanical load on the pump by simply restricting the output flow. The power used by the pump is directly proportional to the rate at which the water is moved through it and almost independent of the pressure at the pump.

lazza · September 2014

Re: Reducing pumping power question

Thanks for the reply

I'm pretty sure it's not centrifugal, i think they are normally on the surface arent they? Therefore I presume its a helical rotor type.

Are there cheap VFDs for such small pumps?

inetdog · September 2014

Re: Reducing pumping power question

lazza wrote: »

Thanks for the reply

I'm pretty sure it's not centrifugal, i think they are normally on the surface arent they? Therefore I presume its a helical rotor type.

Are there cheap VFDs for such small pumps?

The same thing should apply to a helical rotor pump. Any type except positive displacement (piston, gear, screw, etc.) will not use power except for internal friction when no water is actually moving. When you try to throttle back a positive displacement pump, something will break or the pressure will rise to allow the same amount of flow through the restriction.

oil pan 4 · September 2014

Re: Reducing pumping power question

VFD are only used on 3 phase motors, usually frequency drive rated motors or to soft start regular 3 phase motors.
Unless everything is already 3 phase.

Is there any chance of pumping it to a holding tank so there is no additional pressure beyond lifting. That is the cheapest easiest way to reduce pumping power that I can think of.
Also what size is the pipe, how long is it and are there a lot of bends?

stephendv · September 2014

Re: Reducing pumping power question

lazza wrote: »

The problem is, this is alot of power for his 3kW system and if we could reduce the pumps power by half, the client could irrgate much more time and with less pressure on the batteries.

Check the pump and motor specs, it will show an efficiency curve of power used vs litres pumped at a specific head.
If you reduce the pumps power then it will pump less water so it would need to be on for longer. As inetdog said, you can change the pump speed using a VFD AND a 3 phase motor. Also you may need an output choke between the VFD and the pump because of the 70m cable length.
I'm right now replacing my 3 phase motor with a single phase version because it's the second motor that's gone kaput in 4 years. I _suspect_ it's because I did not have an output choke and this causes voltage spikes in the motor windings. Too fed up to try again with another 3 phase so going single phase.

...so if you want a cheap 2.2kW VFD, brand new + output choke I have both

lazza · September 2014

Re: Reducing pumping power question

HI. Cheers for the replies

Thanks Stephendy for the info and offer, the problem is that the motor is single phase and the power system is single phase too.

So it looks like the only option would be reducing the output flow with a choke? is that right?

stephendv · September 2014

Re: Reducing pumping power question

lazza wrote: »

Thanks Stephendy for the info and offer, the problem is that the motor is single phase and the power system is single phase too.

So it looks like the only option would be reducing the output flow with a choke? is that right?

Nope, you're stuck with the power that that motor produces. A VFD will take a single phase power supply and convert that to 3 phase for a 3 phase motor. The output choke I mentioned is an electrical device that's used with VFD and long wire runs between VFD and motor. It helps smooth the output and reduce voltage spikes and is not necessary with a single phase motor.

EDIT: there are single phase output VFD, but I don't know how well they work and whether you can actually get the same level of speed regulation as with 3 phase. See: https://www.inverterdrive.com/group/AC-Inverter-Drives-230V/Invertek-Optidrive-E2-1100W-single-phase-Inverter/

Aguarancher · September 2014

Re: Reducing pumping power question

"So it looks like the only option would be reducing the output flow with a choke? is that right?"

The normal way to reduce the pump output is with a gate valve after the pump. Maybe you could get away running a smaller pump depending on the GPM req.

mike95490 · September 2014

Re: Reducing pumping power question

pump specs lie. The losses are copper, friction, and Power Factor. My 1/2 hp pump, while running, logs in at 1KW on the inverter control panel.

The best choice I can suggest is replace the pump with a small 3 phase motor, and use a VFD to power it from your single phase (make sure voltages match up)
I carefully planned and searched pump curves to get the best pump for my install. Most pump houses sell the best profit pump for them, not the best pump for you.

If you are stuck at single phase, 3 wire pumps have less starting surge than 2 wire pumps. I pump from a pond, to elevated storage, and then after pumping days, have water pressure all night and on cloudy or rainy weeks.

Your inverter has to supply the starting surge for about 1/2 second to get the pump spun up. After that, most power meters will give you a true reading. So your battery and cables, have to supply voltage during the surge, to not trip off the inverter from low battery.

BB. · September 2014

Re: Reducing pumping power question

Many (some/most/all?) VFD's typically have square wave or MSW output--I have talked with enough people that have concerns when running motors on VFD's and failure after a few years of operation.

Nothing specific... But MSW and SW waveforms are hard on motors--Basically around 20% of the power goes directly to waste heat because of the non-true sinewave power (possible extra vibration too in windings/motors because of the MSW wave forms).

-Bill

inetdog · September 2014

Re: Reducing pumping power question

BB. wrote: »

Many (some/most/all?) VFD's typically have square wave or MSW output--I have talked with enough people that have concerns when running motors on VFD's and failure after a few years of operation.

Actually, what the output of a VFD looks like depends a lot on what part of the speed curve it is working in, since it must vary the RMS voltage to a lower value as it lowers the frequency.
The easiest way to to do this with good conversion efficiency is to use Pulse Width Modulation just as a PWM CC would to get a particular voltage at the battery terminals.
So it will be more complex than just the on-off four times per cycle that an MSW would use, but not nearly as clean as a PSW (which may actually produce the sine wave through PWM at a higher frequency followed by filtering.

The VFD does have very sharp rising and falling edges to its wave form, and these can cause problems with the insulation on the connecting wires as well as within the motor itself. These dV/dt effects can include induced currents in the rotor shaft itself which damage bearings, as well as mechanical damage to the coil insulation.
And, perhaps worst of all, the high frequency components can combine with the length of the wires from VFD to motor to produce standing waves with voltages higher than the DC output of the VFD.

You are correct that the best choice is to use VFD-rated motors whenever possible.

lazza · October 2014

Re: Reducing pumping power question

inetdog wrote: »

For a centrifugal type pump, you can cut the mechanical load on the pump by simply restricting the output flow. The power used by the pump is directly proportional to the rate at which the water is moved through it and almost independent of the pressure at the pump.

Ok, so if this is true- if we simply put a tap on the output from the pump: Attachment not found. - that would be on the curved union on the left of the picture, like the one on the right of the picture. The one on the right is the output from the pressure tank Attachment not found., which I presume wont affect anything because the pump pumps full flow until the pressure valve in the pressure tank trips.

So we'd be restricting the output flow to the pressure tank using a tap-- will this really work to reduce pump power consumption??

Aguarancher · October 2014

Re: Reducing pumping power question

If the ball valve with the blue handle is at the output of your pump, close it half way. In the current position it is shown, it is closed. If this works for your electrical problems, the plumbing and storage tank need additional help. Just re-re read your post. Sorry for failing to see that the blue handle valve is from your storage tank. You can install the valve in question anywhere before the pressure switch, whatever is most convenient. One important missing piece in the system shown is the lack of a check valve on the output of the pump. Although most pumps have one on the intake they are prone to failure. Use a quality brass one with the arrow pointing towards the pressure tank. Sorry for the confusion in my reply.

lazza · October 2014

Re: Reducing pumping power question

apologies for my ignorance, what's a check valve?

Aguarancher · October 2014

Re: Reducing pumping power question

This is the check valve:
http://www.watts.com/pages/_products_details.asp?pid=6837

This is the gate valve:
http://www.watts.com/pages/_products_details.asp?pid=3416

You should be able to source these locally as they are common plumbing parts. These both go before the pressure switch.

BB. · October 2014

Re: Reducing pumping power question

A check valve is a one way flow valve. If the water tries to flow backwards, a flapper, ball, or similar blocks the hole to stop water flow. When the flow is going forward, the flap/ball is pushed out of the way by the water itself.

-Bill

lazza · October 2014

Re: Reducing pumping power question

Ok, we found that the direct output from the well has two taps (ball valves i think).. so we did a test, and began closed them until the power consumption of the pump dropped. We managed to get a reasonable power drop of around 400W without compromising pressure for irrigation.

See Picture: Attachment not found.

Is it ok to leave the taps as they are in the picture?? will it have any negative impacts on the pump??

Aguarancher · October 2014

Re: Reducing pumping power question

Kind of a crazy set up on the pump, but Yes, you'll be fine. Like solar, a balanced well system takes proper planning. In the future if the pump fails I would size the pump correctly for the required use. There are many variables to consider. Good to hear you got the pump under control.
Your pressure will always remain constant no matter how much you close the ball valves, the volume (flow)GPH of water will decrease. That is why asked about your GPH ( gallons per hour), I guess in your case litres requirement.

BB. · October 2014

Re: Reducing pumping power question

It would be better to have 1 gate (or ball) valve directly to the pump... That way, the pump is "regulated" by one output valve--Right now, you if irrigation is turned off, the house cannot use the "flow from irrigation when turned off" inside the house.

Does not hurt anything--just limits water flow to individual uses of water, not directly limiting the total flow from the pump itself.

-Bill

lazza · October 2014

Re: Reducing pumping power question

Yes, it's a pretty mad setup, but nothing to do with us. So many cowboys and DIY installers here, that we're constantly tearing our hair out

... i haven't got much left.

I'm fascinated by the fact that restricting the flow will not affect pressure. Can someone demonstrate the physics/mathematical equations behind this??

Thanks
Larry

BB. · October 2014

Re: Reducing pumping power question

A centrifugical pump is basically a "fan". When there is no water moving through the pump, the motor is just moving the water through in a circle. Other than friction losses/drag, the pump is not using any energy. It is sort of like lifting a weight and holding in mid-air... There is no work being done, no matter how tired your arm gets holding the weight.

When you start moving water through the pump, you are actually doing "work"... Moving X Liters per minute against pressure (Pounds per square inch, Bar, kPacals, etc.). You are now "Moving" water "up hill" and work is being performed.

Math, you can find some on a Wiki:

Centrifugal pump - Wikipedia

You can demonstrate this with a vacuum cleaner... Turn it on, then put your hand over the hose end... The motor (usually a brushed or "universal" motor) will speed up because it is "unloaded" (fan blades are just swirling the air around in place). When you take your hand off the hose end, the air moves again and the motor slows down.

If the vacuum (or well pump) is an induction motor, the motor speed will not change much, but if you have a Watt meter, you will see the power consumed vary with rate of flow through the vacuum (water pump).

A positive displacement pump (piston, some rotary, etc.), on the other hand, will push the pressure higher and higher (as water flow is blocked), until the pump stalls or breaks (or the pressure relief releases).

-Bill

Reducing pumping power question

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