Voltage sag- how much is 'too much'

Eric L · May 2012

Guys,

I've been running a single-phase, 240v irrigation well-pump as an opportunity load on my new pv system. I've measured running watts of this pump at about 2400 with a starting surge of about 4850 watts. It's run off-grid only as an opportunity load when the sun is shining and the batteries are well into absorb or float. So my 4.1KW solar array has had no problem carrying the load, except for the starting surge where the batteries have to help out. If the irrigation system is wide-open, the pump will cycle about once per minute to refill the pressure tank.

This has all been working fine, but there's a definite voltage sag on pump start up (inverter is a Magnum MS4448 PAE). According the the Magnum, the 120 circuit voltage is falling to about 117 volts on starting, before bouncing to 124 and then stabilizing.

All my 120 volt circuits are also on this system, including my fridge. The fridge compressor audibly drops out briefly when the pump starts. I've started switching the fridge circuit to grid when I'm running the well pump since I don't want to damage the compressor.

Am I being too cautious? Remember that this can cycle once per minute. I really don't know what kind of voltage sag tolerance these compressors are designed with (it's a newer, and thus probably more delicate, energy star fridge).

Thanks for any advice.

Vic · May 2012

Re: Voltage sag- how much is 'too much'

Eric,

IMHO this cycling frequency is too great. This is not good for the starting parts of the pump. The normal guidance is to have a larger (much larger) pressure storage tank, or to use a different pump for irrigation. Most pump motors use contacts and a capacitor, or starting winding or perhaps both. Starting stresses all of these parts and perhaps your inverter as well. Measuring the actual peak power/current often takes an oscilliscope (which you could be using), but it is possible that the surge power is even greater than you have measured.

Know nothing of your exact configuration or about the pump etc, just my first reaction to this post. Experts will offer better advice. Vic

Eric L · May 2012

Re: Voltage sag- how much is 'too much'

Vic,

You might be right about the inverter; I have thought of the stress on it, although frankly even if it shortened the inverter life by a few years, it might nonetheless be worth it for the power savings (I haven't computed this though).

On paper you're right about the pressure tank, but this basic system has worked like this for years on grid power. This current pump is only about 2 years old, but it's predecessor lasted nearly 20 years with the same size pressure tank and loads (approximately). My pump guy assured me the new pump was just as good -- I guess I'll find out.

I'm frankly more worried about the fridge, which was a lot more money than the pump.

westbranch · May 2012

Re: Voltage sag- how much is 'too much'

you did not mention how long the pump ran/runs from on to off? Rule of thumb is for it to run at least 1 minute . shorter = tank too small... not sure on the long end what is needed as it also depends on the refill rate of your well. I know our underground sprinklers take all our 3/4 hp pump will deliver, so it cycles off about every 5 minutes

Eric L · May 2012

Re: Voltage sag- how much is 'too much'

Thanks: I just timed a couple of cycles at about 60-70% of the irrigation system open (fairly typical setting). 29 seconds running, 1 minute 35 seconds off. I'd never heard about the one minute rule, and I doubt the pump runs that long even with the system fully open. So maybe my pressure tank should be larger. Can you add pressure tanks in parallel?

vtmaps · May 2012

Re: Voltage sag- how much is 'too much'

Eric L wrote: »

So maybe my pressure tank should be larger. Can you add pressure tanks in parallel?

Yes. The bigger the better and the more the merrier. --vtMaps

RCinFLA · May 2012

Re: Voltage sag- how much is 'too much'

First you should not be running an irrigation system from a pressure tank system. You should have a second pump sizing the pump to your irrigation gpm and pressure needs.

I have a house pump on a pressure tank and a separate irrigation pump on the same well.

A few for things about pressure tanks... I personally would not use any without a sealed air bladder bag. Without a bladder the tank will eventually become water logged (usually about every 5-6 months) causing pump to quick cycle. With an air bladder you set the air charge pressure with an empty tank at an air pressure a couple of psi. below your pressure switch turn-on pressure. I use an air charge of about 20 psi with a pump turn-on point of about 23 psi. This will yield the maximum volume from the tank betweeen cycles. My turn off pressure is about 45 psi. This is too low an overall pressure cycle for an irrigation system that is running sprinklers that require more then 40 psi typically.

You need to recheck and top off the air bag pressure every couple of years.

My separate irrigation pump maintains about 45 psi to the sprinklers.

Eric L · May 2012

Re: Voltage sag- how much is 'too much'

First you should not be running an irrigation system from a pressure tank system. You should have a second pump sizing the pump to your irrigation gpm and pressure needs.

Hmm, well the pressure needs change during the year, and no sprinklers. There's a constant load of blueberry bushes which get watered all year via. microjets, and an additional summer load of vegetable garden (mostly microjets, but also a soaker hose). Anyway the pressure tank system has run this arrangement for many years, although a continuous irrigation pump admittedly sounds better now since it would represent a constant electrical load to my solar set up, eliminating the surge issue.

BB. · May 2012

Re: Voltage sag- how much is 'too much'

I am sorry I don't have more information to point you too... But try finding out about Variable Frequency Drives or "VFD's".

If you have a 3 phase pump (not likely here) or a pump with an external (well head) mounted starting capacitor, you might find a VFD nice to use. They are basically variable frequency AC inverters. A VFD will "soft start your pump" and you can program the speed of the pump to match your water flow (some may even have a pressure sensor that lets the controller vary the speed to keep the programmed water pressure.

-Bill

RCinFLA · May 2012

Re: Voltage sag- how much is 'too much'

You would likely be better off with too large an irrigation pump for winter months, letting it run continuously then subject your refrig to voltage slumps during pump startup. It is hard on refrig. You will likely burn out the themistor in the overload protection. The positive temp coefficient carbon puck is actually in series with the startup capacitor and thermally coupled to a bimetallic temp trip switch. When the puck gets too hot from repeated restart attempts caused by voltage slumps it trips the thermal switch to compressor. It usually comes in a plastic box containng the thermistor and thermal switch. Sometimes tied on top of the startup capacitor.

Another option is get a second inverter to separate AC loads.

Eric L · May 2012

Re: Voltage sag- how much is 'too much'

RC thanks, I just saw this. I suspected it wouldn't be healthy for the fridge; nice to know the details.

BB, it's a single phase motor so doesn't seem a good candidate for VFD, unfortunately.

BB. · May 2012

Re: Voltage sag- how much is 'too much'

If the motor has an external starting capacitor (at the well head box), then you can still use a VFD (from what I understand).

-Bill

aglicknmex · November 2013

Re: Voltage sag- how much is 'too much'

Old thread but if you are interested, Grundfos makes soft start pumps that will pump your water without the starting problems. Look at the SQ series.........I have the solar version--SQF - and it starts with only 450watts of PV!!! Not batteries, not extra controllers---- nothing but an on off switch between panels and pump. Local well driller using losts of these in cattle country with great reliability. The SQ series is the soft start pump for regular 120-240 AC use check them out
al from New Mexico

RandomJoe · November 2013

Re: Voltage sag- how much is 'too much'

I don't know much (anything?

) about well pumps, but do work with pumps in commercial HVAC systems. Can you choke down the flow through a well pump without causing problems? We use balance valves on pretty much any pump in our HVAC systems.

If you can, perhaps try choking the output of the pump (may have to add a valve if there isn't one already) so it more closely matches the irrigation flow. Any reduction in flow should extend the runtime, although it would be hard to match the needed flow perfectly to keep the pump on without a sensor. (Get a little too low and you'll run out of water in the pressure tank.)

Edit:
Ha, just noticed HOW old this thread was! Oh well, I'm still curious if this works for well pumps…

Voltage sag- how much is 'too much'

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