Microwave transformer rewind trick: calculating amps and voltage

Ok, so if I take the secondary coil out of the microwave transformer, and wind up my own coil wire. I believe the gauge of wire is how much current I can pass through, if I'm not mistaken. How do I calculate how much voltage it will convert? Can I do it in the reverse direction, apply voltage to my re-wound coil, and use this metal brick to step-up the voltage - say 4 times? 24v input to 90v output.

What's special with a microwave transformer? what are you trying to achieve? step up?

Edit ; have you seen this video

Yes, step up 24v to 96v. I watched tutorials explain how easy to re-wind the coil, to produce high current 24 volts or something to use it as a welder, from a 120v ac line. I would like to go in the reverse direction, and wind it up receive 24 volts from lead batteries into secondary re-wound coil, and use the the original 120v primary coil to drive a motor and hopefully get about 30 amps. Is there complicated pulse circuitry that needs to get involved - or is it a matter of winding these wires the right way with some basic capacitors?

You cannot "transform" DC directly. The first thing you need to do is convert the DC into AC and for that you'll need some form of 'low frequency' inverter rated in excess of whatever power you want to get out of your steel-cored microwave transformer.
96volts x 30amps = roughly 3000 watts. So any inverter you build will need to output in excess of 3kW.
Considering a 'standard' microwave is usually rated at around 700 to 800 watts, there's no way you will get 3kW through that rating of transformer core.

DocStein99:
Ok, so if I take the secondary coil out of the microwave transformer, and wind up my own coil wire. I believe the gauge of wire is how much current I can pass through, if I'm not mistaken. How do I calculate how much voltage it will convert? Can I do it in the reverse direction, apply voltage to my re-wound coil, and use this metal brick to step-up the voltage - say 4 times? 24v input to 90v output.

You have to measure this.

Taking precautions about the high voltages on primary and secondary (well you can remove the secondary
anyway), add 10 turns of ordinary insulated wire around the core. (Do not short this winding out)

Apply mains to the primary and measure the rms voltage across the 10-turn winding. Divide this by
10 to get the volts/turn figure of the transformer.

This tells you how many turns you need for any particular voltage at the mains frequency without
risking saturation of the core (very bad). At higher frequencies higher volts/turn can be used,
proportionately. Such transformer cores though have larger losses at higher frequencies.

That tells you how many turns, the wire diameter is chosen to fill the bobbin. The total power handling
is determined by the transformer size, and for microwave oven transformers is usually 1kW or so
at mains frequencies. You won't get 3kW out of it without saturating it, and that's bye bye transformer
time of course - as I said saturation is bad news (melt down).

Anyway this is beside the point, you don't use mains frequency transformers to do DC-DC conversion,
you use high frequency ferrite transformers and high power DC-DC converter design is very complicated.

Ok - that was the MYSTERY part of my question: I did not know that AC part of the transformer had anything to do with it, and just thought it was power being manipulated and did not matter what direction it was driving. And now I realize the transformer would just turn itself into an induction heater and kill itself, after I would have spent 80+ hours trying to wrap it for something else it was not made for. Since this is complicated, I’m going to put this away and leave it alone. I don’t get much pleasure out of sawing and weaving wires through a heavy metal brick anyway.

Part of your thinking is correct (credit where it's due) A transformer core is exactly that and it can be used either as a step-up or a step-down device. The amount of flux that a core can handle is dependent on its cross section

• somewhat similar to the current carrying properties of a copper cable. There might be more than one way to 'skin your cat' so if you tell us what the motor is and what you are endeavouring to do with it you might get some constructive ideas.