Using AutoTransformer to create 120v/240v split phase from single phase 120v input

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cookinwitdiesel asked Dec 18, '19

Using AutoTransformer to create 120v/240v split phase from single phase 120v input

I have a use-case for the AutoTransformer that is different from what I see generally discussed. I want to use it in my RV to take in 120v 60 Hz from either my Onan Generator (2 separate 120v legs that are SAME phase) or the shore power inlet (30/15 Amp which are 120v 60Hz single phase). I have those 2 inputs going to an automatic transfer switch so whichever one is "on" will pass through to the AutoTransformer. I would like for the AutoTransformer to step up the voltage from 120v single phase to 120v/240v split phase. I will then pass that split phase into the input side of my dual inverters (one inverter for each 120v leg) so that when I have a single leg input, I still get both inverters charging instead of one side passing through voltage and the other rejecting the phase and inverting to make the "correct" phase.

So, my question is, how should this be wired up to the inverter. It has labeled "input" and "output" sides but the "input" side only has 240v Line and Neutral labeled connectors. The "output" side has 120v Line 1, Neutral, 120v Line 2 connectors.

My intuition tells me to disregard the input/output labeling and connect my source 120v power with:

source 120v Line > AutoTransformer "output" Neutral

source Neutral > AutoTransformer "output" 120v Line 1

Then my output would be the 240v wave form stepped up via transformer as measured between the "input" Neutral and "input 240v Line connectors. I would need to carry over the neutral from the other side to provide 120v/240v split phase.

It is this last part I am unsure of and need some guidance on.

Diagram attached of my planned whole system that hopefully makes clear what I am trying to achieve.

Thanks!

Autotransformer split phase

rv-inverter-design-end-state-victron-lifepo4-dual.jpg (2.3 MiB)

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wolf commented · Jan 22, 2021 at 04:44 AM

This looks really good. I am looking into doing it and I would love to do 24v but with the fifth wheel we own using 12v everywhere and the generator also being 12v for starter battery it seems like a right to go to 24v and then come back to 12 plus add a separate 12v starter battery, a step down converter and a battery charger for the 12v battery. Having a 12v battery bank simplifies everything and reduces the amount of equipment plus if something goes wrong you can always just tap into the battery straight for 12v.

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cookinwitdiesel wolf commented · Mar 16, 2021 at 06:29 AM

I ended up adding a lawnmower 12v AGM battery that I maintain with a Orion 24/12-9 as a small insurance policy. All other 12v loads are converted from the main bank though and controlled by the BP-100 wired to the BMS load disconnect. I do not have the 2nd inlet as for the time being I am ok with having one inverter accept shore power and the other inverting. The conversion loss I would incur is fine given the simpler and lighter system.

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Johannes Boonstra (Victron Energy Staff) answered · Dec 19, 2019 at 06:12 PM

Your intuition is right, So disregard the input/output labeling and connect the source 120v power with:

source 120v Line > AutoTransformer "output" Neutral

source Neutral > AutoTransformer "output" 120v Line 1

Form the other Line the 240Vac towards the L in out will appear

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cookinwitdiesel commented · Dec 20, 2019 at 09:58 PM

Thanks for the confirmation!

Something to note for others interested in the same configuration, your input 120v power will be limited to the rating of the neutral windings - in this case, 28A continuous and 32A for a short period of time.

It is a shame that Victron did not use a heavier duty transformer/wiring to allow for greater neutral current - 50A would be ideal I think.

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jetwire cookinwitdiesel commented · Jan 08, 2021 at 04:42 AM

Did you ever get this setup going? I have a very similar setup. Do you wire both 120V legs from the generator to go through the Autotransformer or can you send one of the generator legs straight to one of the Inverters? On another forum someone mentioned only passing one of the legs through the Autotransformer to "flip" it 180 out. By doing that would you be able to pass 60 amps total? (Both generator Legs) Or would you still be limited by the 30A Autotransformer neutral limit? I have an Onan 8000w with the same 120/120 single phase and will be using 2 Quattros and want maximum generator power available if possible.

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cookinwitdiesel jetwire commented · Mar 16, 2021 at 06:27 AM

I did finally get this completed earlier this month. I bonded the 2x 120v legs from the generator together before feeding them through the autotransformer. If you study the wiring carefully, you will note that only one leg actually goes through the coil, the other is pretty much passthrough with just a breaker added. So if you pass it through or run the first leg outside the autotransformer really doesn't change much.

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jetwire cookinwitdiesel commented · Mar 16, 2021 at 05:29 PM

Interesting. So with it setup like that you aren't limited to 30A total from each "leg" of the single phase generator? I ended up finding a company to build me a custom Autotransformer for the Generator. Since I already had the Victron AT, I ended up using it to balance the loads coming out of my 2x Quattros. Lots of $$ in transformers but the whole setup is working great!

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cookinwitdiesel jetwire commented · Mar 16, 2021 at 07:10 PM

My generator outputs about 23a on each "leg" but they are the same phase. So I bonded the outputs together and have a single 120v (up to 46a) feeding into the autotransformer. Only half of that power however rides across the coil which has the 28a limit

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Answer 1 · 2021-03-16T03:38:13Z

cory cantrell answered · Mar 16, 2021 at 03:38 AM

I have a similar question, as to how to wire an autotransformer to convert the 30amp AC shore connection to split phase, and the generator to split phase to power the entire coach with dual inverter.

i also want to run my inverters in split phase, but I don’t want to lose any ability when on 50amp shore power, I also want both inverters to work the same as they would on 50amp shore power. Hence my dilemm, seems very similar to yours.

I could really live without the 30a shore power getting turned from single phase to split phase and just allow the second inverter to invert and use battery for the second leg, but what I wouldn’t want is the inverters to do that when on Generator. if I put an autotransformer between the generator and the ATS would that suffice my requirement?

I am running dual multiplus not dual Quattro so don’t have the second AC in, and don’t want to give up anything on my 50amp shore connection.

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cookinwitdiesel commented · Mar 16, 2021 at 06:17 AM

Yep, 50a 120/240v RV system.

You are thinking about it correctly I think. I would go 120v Generator Autotransformer > ATS > AC input on MultiPlus

You can then accept 30a 120v input to a single inverter while the other inverts from battery OR have an additional switch that you can use to pass the 30a 120v input through the Autotransformer as well. @Natebert added a switch like I am describing to send 120v inputs through the transformer so he can hybrid boost on both legs in his system.

This is what I ended up building and other than a sizable voltage drop on L2 from the Autotransformer I have been very happy with it. For the autotransformer wiring, see scenario 3 from the Victron provided diagrams shown here:
https://www.victronenergy.com/upload/documents/AT-3-split-phase-120V-to-120-240V-with-Quattro-120V.pdf

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rv-inverter-design-dual-quattro-1-22-2021.jpg (1.5 MiB)

cory cantrell cookinwitdiesel commented · Mar 16, 2021 at 05:08 PM

Brilliant reply, thank you so much for post this originally and for the thorough reply to my question!!!!

Hopefully last question, when you say sizeable voltage drop, how bad is it? 2%, 3%, 10% (ish)?

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cookinwitdiesel cory cantrell commented · Mar 16, 2021 at 06:08 PM

It can get pretty ugly......

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natebert cory cantrell commented · Mar 16, 2021 at 10:11 PM

Not to hijack this thread, but there may be another answer.
A custom transformer.

This is what we used in our system.

This is a custom built transformer from EPD in North Carolina.
It takes 110v in, and outputs 240v across L1 and L2 with a new neutral (N)

I had it custom built to step-up the voltage ~10v to deal with those terrible Thousand Trails RV parks. Here are its specs:

This transformer sits comfortably behind my generator's starting battery.

It was designed for 100% duty cycle at a 23a @240v current rating and it will handle 100% imbalanced loads (up to the max of 23a per 120v leg.)

We've been using this transformer with our Onan 5500w for almost 2 years without issue.

Here is the L1 voltage output when on generator.

Here is the L2 voltage output when on generator.

So, if the Victron transformer scares you, there are alternatives.

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57798877-2412864305393077-92471861358624768-n.jpg (66.3 KiB)

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58378716-2412864432059731-5874128003671261184-n.jpg (10.7 KiB)

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phase1.png (23.1 KiB)

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natebert commented · Mar 16, 2021 at 03:39 PM

I'm assuming you are using a 50a RV.
We accomplished this by installing a 60a rotary switch:
https://www.amazon.com/AC-Rotary-Switch-Positions-240V/dp/B000K2MBBE

This switch allows us to manually chose between sending split-phase power (240v) directly to the pair of inverters (via their normal route) and single-phased power (120v) to the transformer to then create split-phase, and then onto the inverters as 240v split-phase.

While I used Quattros (which has a secondary internal ATS), the use of two Automatic Transfer Switches (ATS)s could accomplish the same.

You would use the rotary switch on the incoming grid connection to direct the power either to the inverters or to the transformer.

In position 1 (single-phase), send L1 to a new ATS. This ATS sits between this single-phase shore power and the single-phase generator and the ATS would then switch that single-phase grid input with the single-phase generator input, having its priority to take the generator, just like a normal North American RV might be wired. (Generator over grid)

This transfer switch will then send this single-phase power (either the grid or the generator) to the transformer.

On the output of the transformer, you'd then send the L1 and new L2 to a 2nd ATS. This would probably your existing ATS that was already switching between generator and grid.

Then back to the rotary switch, in position 2 (split-phase), you'd simply send L1 and L2 to the 2nd ATS (again, this is probably your existing ATS which is normally used to switch between Grid and Generator,) and it will now switch between the Grid and the Transformer's output.

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cory cantrell natebert commented · Mar 16, 2021 at 05:08 PM

Thank you also, this really does solve all the problems I am looking to address.

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Answer 2 · 2022-11-06T03:03:13Z

Greg Melton answered · Nov 06, 2022 at 03:03 AM

I really hate to dig up such an old thread but I am trying to install two MPII/48V/35 in my Class A. I spoke with my Victron vendor/salesperson and they indicated that they would not sell me the autotransformer due to the RV. He says the autotransformer is not rated for mobile applications. Does anyone have any insight?

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Answer 3 · 2023-02-28T07:07:29Z

jjbond answered · Feb 28, 2023 at 07:07 AM

Just bit the bullet and ordered mine (the transformer Nate ordered) based on lots of info he shared with me and his recommendation... prices have gone up since he ordered his but EPD really are the best out there and Nate's recommendation is what sealed the deal, should be here in 6-8 weeks, thanks for the help @Natebert

73

Jen, KB6JEN/VE7OTH

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Answer 4 · 2023-09-20T19:26:07Z

Chris Turrisi answered · Sep 20, 2023 at 07:26 PM

Can someone who has purchased the EPD transformer provide the dimensions of the unit, I see that it is 98lbs? I am looking to add this to my existing RV setup of two 24/5000/120 Quattros and a QD8000 generator. Currently I have a manual switch which feeds the generator directly to the RV panel, bypassing the Quattros entirely and I connect my shore power into a 20amp outlet that is powered by the generator and allows one Quattro to charge the battery bank. Not ideal by a long shot. Thanks.

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