Are there any 20v to 240v step up transformers in the market?
To answer your question as posed:
A transformer is a passive component and has no fundamental distinction between &quot;input&quot; and &quot;output&quot; - if you drive the &quot;output&quot; of a &quot;step-down&quot; transformer with a low-voltage AC signal you will see a high-voltage AC signal at the &quot;input&quot;, and you have a &quot;step-up&quot; transformer. The relationship between the two voltages is fixed by the turns ratio, so a 240:20 &quot;step-down&quot; transformer is also a 20:240 &quot;step-up&quot; transformer when driven &quot;in reverse&quot;.
Things get more tricky when you draw a non-trivial amount of current from the effective-secondary, and also when magnetizing the core requires a non-trivial amount of current in the effective-primary. How much is &quot;non-trivial&quot; depends on DC resistance (and thus the wire size) of the windings, and those windings may have been optimized for the original design use-case of the transformer.
If your goal is &quot;to learn how inverters work&quot;, forget the transformer and motor, make a load impedance appropriate to the rest of your circuit, and look at signals on your oscilloscope. (You do have an oscilloscope, right?)