Buck Boost Auto transformer.

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What is this buck boost auto-transformer of which you speak?

For a brief recap on auto-transformers check out the post here.

The buck boost auto-transformer takes a standard dual winding transformer and connects the two winding’s with a jumper.

By connecting the primary winding to the secondary winding you create an electrical connection and in turn create an auto-transformer.

buck boost auto-transformer

OK, so now what?

Just like any other transformer the most important step is to get your ratings and polarities figured out and written down.


If you can’t remember how to figure out ratings or polarity, check out the post here.

Now that you have voltage ratings and polarity you can start at H2 and work your way to X2.

This is like having two batteries in series:


In this example, this gives us 480 Volts at the load.

The next step is to determine the current at the load.  Seeing as how the load is in series with the X winding, you will use 125 amps.

Once you have the current, you can take the load voltage and multiply it by the maximum current the x winding can safely supply to determine the maximum KVA this configuration can safely provide.


The last step is to determine the line current.  If you remember that one of the cardinal rules of transformers is Power input = Power output, you can take the 60 KVA up to the line.  The KVA divided by line voltage will give you line current.


We call this a boost configuration because the voltage of the primary winding aids or boosts the voltage of the secondary winding to give a higher voltage at the load.

To see how a buck connection works, make sure you watch the video.

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