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What's this autotransformer for?
#1

Back story:

Picked up new-to-me coach four weeks ago.  This coach has 6x Lifeline 8D AGM, 2x 2800w Outback VFX2812 inverters, and one roof air hung off on the inverter subpanel.

Shortly after pickup, I unplugged the shore power and loaded up the inverters with the AC while on battery.  Silverleaf started complaining about low voltage about two hours in, which tells me these AGMs are probably not very healthy (no big surprise).

Switched back to shore power and went to sleep.

Next day, after heading out, discovered that only my 120V compressors seemed to be running off the inverters.  Silverleaf thinks I have "60V" on one leg and "0V" on the other.  Inverters are both on, reporting no faults, and they look identical in terms of LEDs and whatnot.

My original theory is that one of the two inverters has failed nevertheless.  A call with Outback seemed to suggest that they felt the same way, and they suggested I could try rebuilding the inverter by swapping in some new FETs and other modules.

That's fine, but today I was crawling around in the bay looking at the wiring, and I noticed there's an Outback autotransformer w/ relay in the back.  Here's its placard:

[Image: 2017-07-31%2013.55.38.jpg?raw=1]

I wonder...  what is this doing?  My assumption is that it would be set up between the inverters and the subpanel to provide for some balancing, with the inverters stacked to make 240V.  I wonder if I should try flipping these breakers ON and see what happens?

Does anyone have this configuration and/or have any idea what it's for?

Ben

2008 Newell #1234
Boulder, CO

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#2

to be clear, i dont have one of these. but i believe you are right and it does balancing between the two interters. the relay should be powered from one of the 12v outputs of one of your inverters and disconnects this from the system if there is shore or genny power avail. (i think). i am surprised the breakers are off. that is not doing anything for you then.

tom

2002 45'8" Newell Coach 608  Series 60 DDEC4/Allison World 6 Speed HD4000MH

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#3

Here's a description from the Outback site:
PSX-240 (Autoformer with Housing)
The OutBack PSX-240 (6 kVA) autotransformer can be used for 120 to 240 Vac step-up or 240 to 120 Vac step-down applications, generator balancing, general split-phase output balancing, or balancing the outputs of series-stacked inverters. Incorporating a transformer with 120 Vac / 30 Aac primary and secondary side, a temperature-activated cooling fan and a 25 Aac dual-pole breaker in a steel enclosure, the PSX-240 is ready to install in your custom application. Use for 120/240 Vac, 60 Hz systems only.

PSX-240-Relay (Autoformer with Housing and Integrated Relay)
The OutBack PSX-240-Relay has all the attributes of the PSX-240 and more. Equipped with an internal relay with external controls, the transformer can be disconnected from the system by a simple 12-volt signal. In customized systems, which may alternate between both split-phase and three-phase sources, this protective switch allows your PSX-240-Relay to be easily and safely removed from conflicting voltages. Use for 120 or 240 Vac, 60 Hz systems only.


So it seems this might give you 240Volts to operate your cooktop and dryer and possibly other 240V items off of your inverter?
Don't assume your batteries are bad just yet, running an air conditioner off of batteries and your inverter consumes quite a bit of amps, possibly 100-150 amps. And due to Peukert's effect and inverter inefficiency could be even more. Your 6 8D's have about 700 amps available at 50% discharge. So with new batteries the most you could get would be 5 hours with the air conditioner. But you're running the fridge and other things. So 2 hours could be close.
My thinking is the air conditioner on inverter would be mostly when driving so the batteries are receiving a continuous charge.
A phone call to Newell should tell you their thoughts on this assuming they installed the system and not something the previous owner did.
Either way it sounds like a neat setup.

Forest & Cindy Olivier
1987 log cabin
2011 Roadtrek C210P
PO 1999 Foretravel 36'
1998 Newell 45' #486 

1993 Newell 39' #337 
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#4

Given that the AC will use about 15 amps when the compressor is running, that would be 150 amps at the batteries before adding in the losses for the inverter. You could easily have around 170 amps coming out of the batteries for this use. And as Forest said there are other uses. Having the batteries load tested would provide useful information regarding battery status though I don't see in the current information a compelling picture that the batteries are bad.

Jon Kabbe
1993 coach 337 with Civic towed
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#5

Ben, it might be informative to see how long the batteries will invert with the A/C off.

2001 Newell #579
tow a Honda Odyssey
fun car: 1935 Mercedes 500K replica
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#6

Yes, the batteries could be okay. My "2 hours" was not a scientific measurement of A/C run time either. At the moment all the interesting loads (fridge, AC) seem to be on the leg that is dead, so it's rather hard to run a real test.

But despite sufficient testing, what makes me suspect the batteries anyway is that the charge side does appear to work okay, and after coming off shore power the batteries recede from 12.9V to 12.5, 12.4V, and lower very quickly even with the pretty minimal, remaining load.

2008 Newell #1234
Boulder, CO

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#7

Okay, progress today, EXCELLENT.

Today I reset the breakers on the autotransformer, and voila, everything powered back up!

After talking to Outback a little more, now I understand how the system is set up:

1. Both inverters are wired in series, so each one feeds one of the two 120V legs in the inverter subpanel.
2. In addition, the transformer is wired between the two legs of the subpanel.
3. The inverters are configured in "Outback stacking" mode, wherein one is the master and one becomes the "OB slave".
4. When the master inverter observes no incoming shore/gen voltage, it switches to invert mode and engages a relay in the transformer that puts the coils into the circuit.
4b. At this point, even though the master inverter is wired to L1, the transformer can shift up to 25A (3kW) onto L2 to supply any loads there. Basically, the first inverter in the stack is allowed to service all the loads on the entire subpanel.
5. "Outback stacking" really means "only power up the second inverter if the first inverter's load exceeds 1@A." So, if the total load on the subpanel gets above about 1.4kW, the second inverter will spin up and share the load. In this 2.8kW*2 config on my coach, that means 5.6kW can go to any single load or combination on L1+L2.

When the autotransformer tripped its coil breakers, there was no more balancing transformer between L1 and L2. The master inverter was powering up just fine, but it could only supply power to L1 loads. Moreover, because L1 on my coach is just the compressor and some TVs and a microwave, the load never got high enough to trigger L2 on. But my refrigerator and other critical loads are on L2!

If one doesn't care for the autotransformer, or it fails for some reason, one can just switch off the autotransformer and reconfigure the slave inverter as a "Classic slave" instead of "OB slave." Then, both inverters will power up all the time and supply L1 and L2 independently. This does mean, though, that it would be harder to drive one or more roof airs given the relative lack of headroom.

As to why the autotransformer tripped? Newell says "oh, we see that all the time." Outback says that there is probably a flaw in the way the neutral bonding happens when switching from shore to battery and that we might need to look at that.

As it happens, I'm planning to rip and replace the entire electrical system, so this is not something that will make my list of things to follow up on. But I did learn a little something today.

Whew!

2008 Newell #1234
Boulder, CO

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