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I have a 5HP well pump that's paired to a Franklin Electric SubDrive Connect VFD - the unit converts input 240VAC single-phase into three-phase power (30hz-60hz). I'd like to put a battery backup in the well shed powering only the well pump so power outages aren't as rough comfort-wise. I expect to exercise restraint during outages and only run one faucet at a time. According to my observations, the motor consumes about 2kW when starting up and then settles down to about 1kW.

I'm planning on getting 6kWH worth of batteries (12V) and using a 4kW pure sine inverter (12kW surge for 30sec). Would this be adequately sized/safe for the pump? I've read online about how there can be harmonic frequencies that can destroy the pump if the inverter can't keep up, but I'm not knowledgeable enough to understand what that is.

Alternatively there's a 6kW model for the inverter as well.

atanamir
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  • Is there any reason you want to run the bank at 12V instead of 48V? Also, what make/model of inverter are you proposing to use? – ThreePhaseEel Dec 05 '21 at 04:19
  • Any voltage is fine, the batteries i was looking at are 12V each. Connecting 4 in series works as 48V right? – atanamir Dec 05 '21 at 04:44
  • Was looking at https://sungoldpower.com/collections/120v-240v-split-phase-inverter-chargers/products/4000w-peak-12000w-dc-48v-split-pure-sine-wave-inverter-with-charger?variant=39652436672649 there's a 12, 24, and 48v version – atanamir Dec 05 '21 at 04:45
  • (I'm open to suggestions for other brands as well if you have one to recommend. This was just from a cursory search from amazon). Would prefer to have the ATS and charger all in one. – atanamir Dec 05 '21 at 05:20
  • You are indeed correct re: batteries in series. Re: inverter selection: do you have any 120V auxiliary loads you want to back up? – ThreePhaseEel Dec 05 '21 at 06:52
  • The only other thing in the well house (where this whole setup is going) is the water softener, but i don't think that really needs backup power during an outage. So the short answer is no. It'd of course be a nice bonus though if it were available, but I don't have any immediate need for it – atanamir Dec 05 '21 at 07:13
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    Keep in mind that *lead-acid* batteries can't use their full nameplate amp-hour capacity. If you regularly even use 50% of it you will shorten the life of the batteries. Using 100% will result in very rapid failure. Smart people use only 30% on a daily basis. Yes, this makes lead-acid banks cost 3 times more and makes lithium more competitive. – Harper - Reinstate Monica Dec 06 '21 at 06:20
  • The starting surge (or stalled current) of a large motor can be five to ten time the running current. 5HP would demand ~4,000 W steady-state, which is ~17 A at 240 VAC. The starting current might be as much as 170 A! Inverters are less likely to be able to handle that short, sharp surge, even though it might be for only a few cycles of 60 Hz power, i.e., 1/20 second or so. It might be cheaper to get a generator rather than a huge inverter and lead-aid batteries, which have perhaps a five or ten year life. – DrMoishe Pippik Dec 07 '21 at 22:37
  • re: the starting surge -- I think my VFD controller does a "soft start" of the motor -- does that help with the surge current? Also, regarding steady state, if i restrict myself to using one faucet at a time during a power outage, it seems my motor just uses about 1,000W. I'm concerned though if i forget and over-tax the inverter, is there a risk of wrecking the VFD unit / motor? The VFD unit has a three-phase converter – atanamir Dec 08 '21 at 03:49
  • @atanamir -- soft starting *definitely* helps with the start surge of a motor – ThreePhaseEel Jan 11 '22 at 02:44
  • @atanamir -- are there any auxiliary loads that need to be backed up by this inverter, or will it be dedicated to powering the VFD? – ThreePhaseEel Jan 11 '22 at 02:45
  • Its main purpose is to power the VFD. However, it'd be nice if it could power a light in the shed or something. But if that complicates things, just the VFD is good enough. – atanamir Jan 11 '22 at 04:34

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