Modified microwave oven transformer for step up, is it viable?

Question

So basically I need a 600V output from a 12V supply. I have a pulsed DC voltage (square wave) on the primary and I'm using the old MOT primary as the new secondary. I've calculated the secondary (old MOT primary) windings at about 250 (from some tests with different primary windings and measuring the resultant voltage) and did some calculations of the required primary windings for 600V:

Vp/Vs = Np/Ns

The primary voltage (Vp) = 12

Secondary voltage (Vs) = 600

Number of primary turns (Np) = ?

Number of secondary turns (Ns) = 250

Rearranging this gives: Np = VpNs/Vs Np = 5 turns.

I am able to use 4GA wire rated at 100A cont. for an output current of 2A (technically 250A max if I don't mind melting the primary, based on normal operating current of microwave, 5A on primary, now secondary)

Now the thing I'm wondering is if this is actually viable? As far as space goes I have an MOT Frankenstein made of 2 MOT cores, making it extra tall and able to fit the fat high current wire so space is not an issue. The DC square wave is controlled by a Mosfet rated to such high current so the only issue I can foresee is some transformer physics that I don't fully understand, but considering that it works just fine as a step down to 2000A-4000A at 1-2V by replacing the secondary, I'm just doing that but backwards. If anyone see's any issues doing this let me know pleasee.

Thank you in advance, and yes I'll be careful and try not to die.

Edit: I also plan on feeding the 600V output into the primary of a regular MOT, giving roughly 6000V. The transformer will be submerged in mineral oil to prevent arcing under high voltage, but is that possible too?

Answer 1

Each transformer core requires a minimum number of turns per volt to avoid saturation. If your old transformer primary has 250 turns, and was handled, say 120 VAC, then your core requires 250/120 = 2.08 turns per volt. So, if you flip the primary and secondary, and you want the new primary to not saturate at 12 VAC, it will require a minimum of 12 x 2.08 = 25 turns. But if your primary has 25 turns, and your secondary has 250 turns, your secondary voltage will be only 10 times greater than your primary voltage, i.e. 120 V.

It is no co-incidence that the maximum output voltage we got when we swapped the primary and secondary is the original primary voltage. That is a general rule. If you swap primary and secondary, the maximum safe (new) secondary voltage is the old primary voltage.

Sorry, but to get 600 VAC output from your MOT, you need to wind a new coils. 12 x 2.08 turns for the primary, and 600 x 2.08 for the secondary.

Or, maybe your secondary already has 1250 turns (or more!). Then you would only need to wind a new primary.

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A hint when working on MOTs. Most MOTs have a magnetic shunt between the legs of the core. To make the MOT a "normal" power transformer, remove this shunt. Search for Microwave Oven Transformer Shunt, to find out how.