# question

## Auto Transformer - 240 single phase to split Phase?

By looking at the terminals and instructions I am led to believe that you can feed the autotransformer single phase 240 and generate 120/240 split phase. However that is not the results we are getting. Defective unit or misunderstood ?!?

As you can see by the circled leads you see the following;

120v-0v-120v 240v-0v

By having connected single phase 240 we get the following;

On the terminal labeled 120v-0v-120v

phase to ground we get 240v-120v-0v

is it only possible to drop the voltage on a single phase? Victron seems to advertise that you can use this auto transformer off a 230v inverter to creat split phase 120/240 VS using two inverters. Victron 230v inverters are 240v phase to neutral/ground. Hence my confusion.

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Just looking at the manual:

What you are getting, is what I would expect, the output of the transformer is 0-120 - 240, there is no anti-phase 120v winding. So the 120V split phase is in fact referenced to the 120V center winding, not the Ground / neutral terminal. In order to get a 120V split phase with neutral reference in the center, you would need 2 x auto transformers or an isolating transformer. operating 120V equipment from the 240 / 120 taps would be risky, as the device insulation may not be up to the job.

1624228418261.png (34.3 KiB)
1 comment

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Ok thanks for your input. It‘s the labeling on the auto transformer and this statement on the data sheet that really confused me.

“An alternative to stacked inverters

The alternative to stacking two 120V inverters to provide a 120/240V split phase supply is a 240V inverter with an additional Autotransformer”

-240v Victron inverters are phase to Neutral.

But at least I can use it as a step down Since all I need is 120v. 2 phases or one the max is still 32 amps and 28 continuous.

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Based on this Quattro manual (older manual) it appears possible. Any additional information appreceated.

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@Malolo did you ever get this figured out? We are about to install a 240V Quatro 48/15000 and we have assumed we would just wire our triplex wire into it, send single phase along the cable, and on the other end have a single Autotransformer to turn that into 240V and split phase 120V on the other end. Are we going to need to get a second autotransformer to accomplish this?

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Not with an autotransformer since there is no way to isolate the neutral of the 120-N-120 output from the 240-N input. The N input is connected to the lower 120 volt output so the N on the output is 120 volts above incoming neutral and safety ground.

You need an isolation transformer (separate input and output windings. The input winding would be 240 volts and the output winding would be 240 volts center tapped.

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Kevin Windrem ·
So what is the point of the Autotransformer then? What does it do?
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Does the incoming 240V have one side as the neutral, or is it in fact derived from a 120 - 0 - 120V supply in the first place so having neutral in the middle?

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Yes but it really depends on what set up your currently have like sharpener asked above. you will also need an isolation transformer. But unfortunately you cannot get a 240-N source to 120-N-120 as the document above seem to state. More pieces will be needed.

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@Malolo Did you read this blog in your research by chance? We read it and came to the conclusion that we only need the single Autotransformer to take the 240V from the inverter and split it into two legs of 120V (and thus power 120V and 230V appliances). We may have read it wrong and perhaps our dealer didn't know how the AT worked though.

From the blog post:

"The Victron Energy Autotransformer is the one item we sell that causes the most confusion... 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."

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Is there any reason why you couldn’t use a centre tapped isolation-type transformer to generate an isolated 240vac between the two secondary windings and 120vac from either of the secondary windings and the centre tap?

(the transformer would likely need to be hand wound, but I’ve often wondered why this wouldn’t work)

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Yes, this should work fine. Isolation transformers with center-taps on either or both windings are common and you could use a center-tapped primary winding to switch between 120 or 240 volt shore power.

An additional enhancement would be another tap on the primary (input) side accommodate 230 volt shore power and make 120/240 split phase. Additional taps could accommodate now or high voltages as well.

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Kevin Windrem ·
Not sure what you mean by center tapping the primary winding for shore power. Is this so that you’d get a step up effect with 120vac to the primary and CT, getting 240vac out and 1120 again between CT and either of the two main secondary leads.
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nitrous ·

Like this

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1710632749554.png (101.4 KiB)

To get the same power rating with 120 and 240V input you would need two separate 120V primary windings. You would then connect them in series or in parallel depending on the input voltage.

If you also had separate secondary windings as well, you could wire them for

• 0 - 240
• 120 - 0 - 120, or
• 0 - 120 at twice the current.

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