I can explain single phase operation:
Typically refereed to as buck/boost in the field, I tend to reserve the term
autotransformer for the three phase versions of the same concept. The feed
for the primary windings of the transformer continue on to the load after
being manipulated by the secondary windings. With a standard 10:1 ratio
transformer that would be used for control power, i.e.: 240 volts down to
24, or 120 volts down to 12, you could boost 208 volts up to a more
tolerable range for machine sensitive to a 230 volt feed. The voltage would
be subtractive if passed through the secondary windings in reverse.
The application is limited though, generally to about a 20% manipulation of
the original voltage.
The advantage is reduced size, (since the windings so not carry the entire
load), & cost if you are not purchasing a special order item..
____________/ ___________________ 208 + 20.8 = 229 volts
208 volts____________P ][ S
\__/ 10:! ratio general purpose
You can obtain good information at the Acme transformer website.
Just to point out a fact that I have designed several 1:1 ratio
autotransformers in the past. So yes they are out there.
Their principle use is as a phase shift transformer, generally at
mains voltage they help to give 12 pulse rectification if one bridge
is connected direct to the mains.
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