Autotransformer
The Victron Energy Autotransformer is the one item we sell that causes the most confusion.  It can be used in a number of different applications but this article is about using it with a US 120/240 volt shore power system and with a 240 volt inverter charger or generator. In this case we are using the Autotransformer in its capacity to make two split phases of 120 volts from a 240 volt input, and to balance the loads between the two legs.   The input to the Autotransformer can be shore power, a generator, or an inverter, all at 240 volts. The Autotransformer is used to split the 240 volts into two legs of 120 volt and to balance the load between the two legs.  To understand how it works it helps to look at the schematic.
Autotransformer Schematic
What you can see in this diagram is the two shore power legs, Line 1 (red) and Line 2 (black) go straight through the transformer and out the other side.  For these wires it isnt even a transformer, the only interruption is the circuit breaker you can see on the incoming side. Between the two legs you can see a coil of wire with a center tap.  The incoming legs are always at 240 volts with respect to each other, when Line 1 is +120 volts Line 2 is - 120 volts and vice versa.  The center tap is always exactly between the two at 0 volts.  The Autotransformer has made us a new mid point and so allows us to have two legs of 120 volts. 

The wire coil in the drawing appears as a long continuous coil, but in reality the part of the coil serving leg A is wound over the top serving leg B and the two parts of the windings interact with each other electromagnetically so they exactly mirror each other.  That is how the center tap is always at zero volts even if leg A is loaded up to the max and leg B isn't loaded at all. 


AC Ground

You may have noticed the ground relay in the drawing above.  To understand this it helps to understand the meaning of the AC ground.  When two wires come from an AC power source like a generator or a transformer neither is "hot" or "neutral" until you define them.  You do that by connecting one to ground.  It then is defined as the neutral and so the other one is the hot.  In the case of the Autotransformer when the ground relay is connected we are defining the center tap as the neutral.  The diagram then looks like this:

Autotransformer with ground relay activated
When the source of power is on board the boat the autotransformer gets to work like shown above.  The generator or inverter produce two hot legs, neither of which is grounded, then the Autotransformer defines the neutral as being the mid point between the two legs.  If we draw the diagram with a MultiPlus or Quattro Inverter/Charger  it looks like the drawing below.  Note that you have to program the MultiPlus or Quattro to use the ground relay output.  For more about programming see my blog post on the subject.

Autotransformer Connections
The above drawing shows the connection area of the Autotransformer.  The output is to the left, there are two hot legs represented by the black and red wires, and one common neutral.  On the right is the input connection with two hot legs. There is a single ground connection and there is also the ground relay connection which connects to the ground relay terminal of the MultiPlus.  Below is the corresponding terminal area on the Multiplus where the ground relay connection is in the middle of the block. 

Multiplus ground relay connection

You have to activate the ground relay in the software of the multiPlus.  Below is a screenshot of where to do that.  For more details see my Blog Post on programming the MultiPlus

Ground relay


Un-checking the ground relay box disables the internal ground relay in the MultiPlus and enables the external ground relay. The external ground relay on the MultiPlus is connected to the Autotransformer and actuates its ground relay. 

When the boat is connected to shore power the neutral is not defined on board, this is to avoid having the water become a parallel path for current. The boats ground is connected to the water and the dock ground is connected to the water and if neutral were connected to ground in both places the water would be part of the current path. Current through the water is never good whether you are a fish or a swimmer.  When shore power is active the inverter charger disengages the ground relay control.  The drawing then looks like this.

In the drawing above ground and neutral are connected at the dock but not on board.  The ground relay is not activated.  Even though the shore neutral is not taken on board the boat it is recreated after the Autotransformer. 

The final drawing in the sequence shows a generator.  This has to be a 120/240 Volt generator to work correctly.  The neutral of the generator is not used but it is connected to ground.  Then the neutral is recreated after the generator.  The drawing looks like this:


Connections to the MultiPlus and Quattro

There is always some confusion when connecting the MultiPlus or Quattro in these situations.  The 230 volt models are designed for European 230 Volt where one leg is neutral and one leg is hot.  People are understandably hesitant to connect two hots, one to the terminal labelled line and one to the terminal labelled neutral. That is how it gets connected, Line 1 goes to the terminal labelled LINE and Line 2 goes to the terminal labelled NEUTRAL.  The shore neutral is not connected, the incoming shore ground is connected.

Why are we doing this again?

All the technical details complicate the issue.  Using an Autotransformer in the way described above has all kinds of advantages.

Its a huge advantage not to take on board the neutral of a 120/240 volt system.  Anyone who ever had a loose neutral connection will tell you what happens.  The voltage of the two legs is only equal because the neutral connection is grounded on the dock.  Once you lose the neutral connection the two legs are floating and the voltage on leg A or leg B will depend on the loads on those legs.  It would be easy to find one leg with 180 volts and the other leg with 60 volts causing damage to all kinds of equipment on board.

It is a huge advantage to be able to use the full output of a generator or inverter on whichever leg is desired.  Without the Autotransformer an unbalanced load will result in a loss of capacity.  The instruction manual of the Autotransformer explains it in a rather complicated way if you can follow it:

Consider for example a 30A 120/240V split phase supply. The supply could be the grid, a generator or two stacked inverters.

Some of the loads connected are 240V, others are 120V. On each 120V leg the load should not exceed 30A. The problem is that as soon as 120V loads are connected, the two legs will show a different current. This is because the 120V loads on the two legs will never be balanced. A 120V 1200W hairdryer, for example, will draw 10A from one leg. A 120V clothes washer could even draw in excess of 20A from one leg. Between the two legs the difference in current, or current unbalance, will therefore often be 20A or more. This means that the 30A supply will not be used up to its full potential. By the time one leg draws 30A, the other leg may be drawing no more than 10A, and increasing the 240V load, for example, will result in an overload of one leg while the other leg still has spare capacity.

Theoretically, the total power that can be drawn from a 30A 120/240V supply is 30 x 240 = 7,2kVA. In case of 20A unbalance, the practical maximum will be 30 x 120 + 10 x120 = 4,8kVA, or 67% of the theoretical maximum.

The solution is an autotransformer. By leaving the neutral of the split phase supply unused, and connecting an autotransformer
to create a new neutral, any load unbalance is “absorbed” by the autotransformer.

In case of a 30A supply, the load can be increased to 7,2kVA, and a 20A load unbalance will result in one leg supplying 40A, and the other leg 20A. The 20A difference will flow through the neutral and the windings of the  autotransformer. The current through both 120V wires of the split phase supply will be 30A. 

Summary

Whether you understood all this or not you will at least understand why the Autotransformer remains the most complicated and misunderstood product in the store


Further Reading

Autotransformer instruction manual

13 Comments

Roger

Date 6/16/2019

Hi, thanks for some great demystification in this article, but I’m still fuzzy. Victron’s brief manual could use some elaboration. We’re planning on cruising to Europe from North America one day, but I’m redesigning our electrical system today. Is it possible to use an auto transformer as a step-down to allow us to plug into 240V shore power so that our internal systems only ever ‘see’ 120V as they would in North America? Thank-you in advance, Roger

Peter Kennedy

Date 7/1/2019

An Autotransformer is not a step down transformer. Victron do make a step down transormer to to what you need but this is not the one.

G. Belli

Date 6/30/2019

Hello, I have a couple of things I still don't understand about Victron's Autotransformer: - Can you convert 240V split phase out of a single "leg" of 120V? - Can you convert 240V split phase out of two independent 120V inverters that don't have synced output? Thanks.

Peter Paul

Date 1/14/2020

yes you can. it Is shown in the autotransformer manual!

Peter Kennedy

Date 7/1/2019

My original answer was incorrect. Yes you can make a split phase 120/240 from a 120 volt input. Here is the diagram from page 3 of the instruction manual. :
And no you cannot make split phase 120/240 out of two inverters that are not in sync.

David Harris

Date 7/3/2019

Pete great explanation thanks, question - how many continuous and peak kw can I get out of the 100a unit and if using parallel 120/240v Quattro units can I also parallel two or more autotransformers.?

Peter Kennedy

Date 7/3/2019

The 100 Amp unit is good for 100 Amps so if you have 240 volts that equates to 24000 Watts using the formula Watts = Volts x Amps. You can't parallel them.

Luis

Date 7/14/2019

What happend when the generator neutral is not conected? What problem i could have?

Peter Kennedy

Date 7/15/2019

In one of the drawings above you will see there is a 120/240 volt generator and the neutral is not connected to the load because the transformer makes a new neutral.

Ben Zook

Date 7/15/2019

I'm planning on using a Phoenix Inverter Smart with this Autotransformer and it doesn't have a ground rely. The inverter manuel says that the neutral wire of the AC output is connected to the chassis of the inverter and that the chassis should be grounded. So will everything be okay if I simply ground the autotransformer to the same place that the inverter is grounded to?

Peter Kennedy

Date 7/15/2019

As you can see from all the questions, this is a complicated device with lots of applications. I wasnt able to answer this question without knowing more about the application. I am trying to limit the questions and answers here to subjects that would be of general interest. It isnt a suitable forum to discuss a particular, and in this case rather unusual, application. For detailed questions about a particular application please contact us through the support section.

wainsley

Date 7/18/2019

Can you say how much is the NO LOAD current draw for the 100A as well as the 30A autotransformer? it's not listed in the documentation. Thanks.

Peter Kennedy

Date 7/19/2019

The Autotransformer does not consume any power in a no load situation.

Cindy

Date 9/27/2019

Hello, in the manual of the product, it is claimed that the autotransformer can be used to convert 120 vac single phase to split phase, with both legs to neutral 120v, but there's only input for 240v, so how should I do the wiring staff?

Peter Kennedy

Date 10/18/2019

On page 3 of the instruction manual it shows the wiring diagram of how to do that. I have a link to the instruction manual in the main page of the product listing

Gustavo Prado

Date 10/18/2019

Thanks for the explanation. You mentioned above that the autotransformer cannot be connected in parallel with another autotransformer. How would one expand a system described above (a Quattro inverter at 240v and an 100 amp autotransformer)? Thanks

Peter Kennedy

Date 10/18/2019

I think once you have exhausted the capacity of the Autotransformer you have reached the end of what you can do with that method because I am not aware of any other brands of Autotransformer. You can still make a more expansive system by hooking up Quattros in a stacked configuration or just use larger Quattros.

Gustavo Prado

Date 10/19/2019

Thanks Peter. Could you connect, for example, two Quattro 15 kva inverters (both 240v output) to one autotransformer? That way you could use the full 100 amp capacity of the transformer (12kw from each inverter).

Peter Kennedy

Date 10/21/2019


Pushing the Autotransformer to its limits requires a more detailed reading of the instructions. When wired in the way you describe the input circuit breakers are easy enough to understand, the input breaker is 100 amps double pole. Its important to note that there is a separate limit on the degree to which the load is unbalanced. That limit is 32 Amps for 30 minutes or 28 Amps continuous. I don't know how you protect against that but it seems to me that connecting up two 15K Quattros is pushing the limit a bit too far.

Hugh Creasy

Date 12/19/2019

Hello, great information. I've imported a sailboat to the USA that is 220v European and uses up to about 20 Amps peak, which I will keep as a 220v boat. I'll want to plug into 110v dock power, and think that a Victron autotransformer is the ticket. Can you please help me understand the configuration required to make that happen? Thank you, Hugh

Peter Kennedy

Date 12/19/2019

Hugh Creasy

Date 12/22/2019

Thank you. The 3600W Isolation Transformer is what I was originally thinking. When configured for Step Up (110 to 220v) will it still act as, and have the benefits associated with, an Isolation Transformer? Do you have a wiring diagram to configure it as a Step Up (110 to 220v)?

Peter Kennedy

Date 1/17/2020

This is NOT an isolation transformer. If you look at the wiring diagrams you will see that one or more of the incoming wires go straight back out to the output terminal. All the wiring diagrams are in the instruction manual

Peter Paul

Date 1/14/2020

Peter Kennedy : another question, can I use the autotransformer with grid-tied inverter? So for example I have a wind turbine that has only a 120V/60 grid tied inverter available (so only able to feed one 120V leg) but I would like to feed a 240/120V residential grid, so I Would like to step up 120V into two 180° phase shifted legs. Another example: I have a 240V/60 only grid tied solar inverter (no split phase output, no middle tap) but would like to feed 240/120V/60 residential grid. It is clear it can work with off-grid application, but will it work to actually feed the utility grid? thanks

Peter Kennedy

Date 1/17/2020

All the wiring diagrams are in the instruction manual. It does not have the necessary certifications to output to the grid and was designed with mobile applications in mind.

Bobbie Martin

Date 3/26/2020

Hi, I'm planning on using an Autotransformer and a 3K 230V Multiplus in a motorhome. I would like to connect the (110v) water heater and the 110 supply to the RV refrigerator to the AC Out 2 (bypass). Since the Multiplus does not get the neutral from the shore power I'm assuming the AC2 output are the two 240v legs. Could I take take the AC2 out and feed a small panel box, using the unused neutral from shore power to supply the neutral for 120 volts?

Peter Kennedy

Date 3/26/2020


In a 230 volt multiplus both legs will be hot, you do not take the neutral on board at all, and you make a new neutral after the autotransformer. The only way you can use AC2 out is for 240 volt loads or if you have a second autotransformer for AC2. You cannot tap into the shore neutral to make two 120 volt legs from AC2

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