Ingmar Krause is asking for a converter 115V input to 230V output.
Assuming that we are talking about 50 Hz or 60 Hz AC, you can use
a step-up transformer.

The marvel of transformers is that, applying the correct voltage to the
secondary side generates the primary voltage on the primary windings.
So step-down transformers can also be used as step-ups.  If you don't
need galvanic isolation you can use a so-called auto-transformer.
These are less expensive.

Another way is to generate 230 VAC from a 115V source is to use two
identical transformers, let's say 115V to 24V.  Connect the secondary
windings (in this case the 24V windings) together and applying 115V
to one of the transformers.  The second transformer will also generate 115V.

Connecting this secondary 115V in series with the 115V mains will
generate the sum of the two, so 230 Volts.  If you measure zero volts
the two voltages are in opposite phase.  Just swap one of the windings,
et voila, you have 230 volts.  (The secondary winding voltage, 24V,
is arbitrary.)

One of the most imported things when playing around with transformers
and converters is the power (watts) needed.  Ingmar did not mention
what kind of item he is dealing with, nor its power consumption.
Keep in mind that the transformers I mentioned must at least have a
power capability of the load you are applying the power to.  The more
the better.

One thing often overlooked is that the starting current of AC
squirrel-cage motors is 6 to 8 times the nominal value.  So if the
output voltage of the transformer collapses due to an overload and
the motor is not running at nominal speed within, let's say, 5 to
10 seconds, then switch off the equipment before burning out the
transformer(s).  Hope that answers also Robbie's question.

If somebody wants to sell you a ferrite transformer don't buy it.
They don't work with 50-60 Hz mains frequency.

Kind greetings from Holland.

Willy van der Reijden

P.S. Be careful out there ;-) (when playing around with dangerous
voltages).