Why does my Uninterruptible Power Supply produce lower watts on backup battery than line power for my pellet stove?

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Problem: When my UPS does the switch-over from line power loss, the pellet stove runs but you can hear the stove motors slow down, and the wattage meter on the UPS drops from ~150w nominal to ~100w nominal. I say "nominal" because the auger is running every few seconds and adds ~25w to the load.

Setup: I have a pellet stove to heat my house, it draws at most 450w on 120vac. However once it's running good usually consumes ~175w.

I got a 1300w Uninterruptible Power Supply used on eBay, an APC BR1300G. This unit should be able to power over 1000w(rated at 1300w) easily and do it for days given you have enough battery backup. It uses 24vdc for backup power. It came without batteries-- I hooked up two group 31 marine deep cycle car batteries in series to provide this power, and the UPS is charging correctly. The DC wires are crimped tight, only draw about 15A 24vdc at this load, and do not get warm.

Edit to answer some questions: I'm not asking this UPS to turn on the motors from stopped, I'm only asking it to keep them running at a tiny wattage requirement. I get that start-up current is much more than running current.

Line voltage is between 118v and 120vac depending on the day. Output voltage on backup power is 118-119vac, according to the UPS screen. (This, however is incorrect as I just found out. 55v!) Hertz is 60 from line and 60 from backup power. I don't know what you mean by "measured the affects of the motors", nor do I know what high phase angle loads are.

Battery wires are 12ga, and the longest is about 2.5ft long. Which is just fine for less than 20A sustained load @24vdc.

"You are expecting a used device with external batteries meant to drive computers with a pseudo sine wave to drive motors perfectly. That seems unrealistic to me. – StainlessSteelRat" I'm expecting a 120vac appliance to run on 120vac provided by a device designed to power stuff with 120vac. If there's a bunch of flavors of AC and they are not all compatible with each other then this is where I get to learn about it. Believe me, if I could have found this answer by googling it I wouldn't be here. But I couldn't. Now that I know motors are somehow special maybe I could search using different criteria.

A big thank you to Tim Wescott for you answer! I probed the UPS output on line power: 120v. Probed it on backup power: 55v!!! Turns out this was wrong, I measured the voltage wrong and figured it out later when my sine wave UPS arrived. It was putting out around 120v. But the stepped sine wave was totally messing with the motors and were drawing 15A @24v for a 100w load...bad stuff. My sine wave UPS now takes over using only 6-7A @24v. The above reading was taken with a True RMS multimeter, which according to APS is the only way to get an accurate reading of the stepped-sine wave output when running on battery power. Link: https://www.apc.com/us/en/faqs/FA157483/

Answer 1

I can't answer why your UPS won't do the job, but I can give some reasons why some random UPS may not do the job. Basically, mostly it's because a UPS is designed to run a computer, not a bunch of motors -- but here goes:

• Motors "expect" a sine wave power input. That thing has a stepped sine wave (I looked at the specs). That's not going to keep the motors from trying to run (motors don't have "brains"), but they may draw excessive current while doing so.
• As mentioned, motors tend to have low power factors. That's a complicated concept, but basically it means that (A) they don't appear to the UPS as a computer, and (B) they draw excess current for the amount of actual power going into the load. This could cause your UPS to dial down the voltage.
• Motors usually draw a lot more current when they're starting than when they're running. This isn't a big deal when they're attached to the mains, because everything in that circuit is designed to handle an overload that lasts for half a second or so. A UPS might cut out (more likely, so probably not your problem), or it might go into some limping low-voltage mode (which would account for what you're seeing).
• Used parts are used for a reason. That could have ended up on eBay because the previous owner had a policy of replacing old with new on a routine basis. It could have ended up on eBay because the previous owner had a policy of milking their UPS's for as long as humanly possible, and only selling them when they were obviously crap (as, for example, persistently putting out a low voltage).
  • Personally I'd measure the output voltage of the UPS with a voltmeter. But unless you're in the habit of sticking voltmeter leads into wall sockets and know how to avoid the dangers, or have a gizmo that lets you do so with 100% safety, I'd suggest you refrain.

You're only asking "why" -- but my recommendation for going forward would be to start by searching the web for UPS's that are known to successfully run motors.

Speaking as a circuit designer, I can tell you that there's a lot of different ways one could design the output stage of a UPS, and some of them would drive AC motors just fine; there may be some combination of the requirements of the market and the whim of the design team that made a UPS that can, indeed, run motors. But unless a UPS was specifically designed and marketed to run motors it won't be in the advertising literature.

So you either need something specifically designed to run motors, or something that the DIY community has found that runs motors, "off label" as it were. Now that you know it's an issue, you can search for it.