Cheapest source so far for step adjustable DC power for ZVS induction heaters

[petespaco]

I am hitchhiking on Mads's recent post about server psu hacks.
That thread took a turn toward the subject of this post, so I thought I'd get more focused here.

Cheap!! is the focus!!!
The Goal: make a step adjustable 12 volt to 48 volt power supply to feed the power hungry ZVS induction heaters that are offered all over the world right now.
THE answer:
Four 1000 watt or larger 120 VAC MOT's. 
Primaries in series parallel so the thing could be connected across a 240 volt line.
Secondaries wound with large size enameled ("Magnet" or "Formvar") wire to provide 12 volts AC each.  They are connected in series, with taps at the output of each secondary.  This produces steps of 12, 24, 36 and 48 volts.
A homemade rotary switch to handle that high currents.  The reason for a rotary switch is to keep from short circuiting things that might occur if using individual SPST switches.   You can see many examples of such a switch in old books:  I tried to attach an example, but still can't figure out how this works- sorry.
-The steps could be made smaller by center tapping each secondary.
The output would, of course, be sent to a full wave bridge and  some sort of LC filter, as needed.
  I saw one guy who was even using 230 volt MOT's in various primary combinations to add more variability.

Major cost would be magnet wire (if you can't scrounge it) and capacitors for filtering. 50 amp bridge rectifiers aren't all that expensive.  I think effective  heat sinks could be made from the cylinder heads of junked lawn mower engines.

Anyway, I am saving up transformers.  Maybe I will get to it one of these days.

Pete Stanaitis
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[davekni]

For induction heating, a filter cap isn't needed.  As long as you have a separate small supply for the gate pull-up resistors, the main supply will work fine with a full-wave-rectified waveform without filtering.  Power factor is better that way too.  The gate pull-up resistor could use the same transformer/bridge output, with a separate diode from the bridge output to a small filter capacitor to handle just enough current for gate pull-up.  However, gate drive is more efficient with a fixed lower-voltage pull-up supply and correspondingly lower-value pull-up resistors.

[petespaco]

I see your point about the filtering, David.
  But these Chinese ZVS induction heaters that I mess with don't have a separate gate drive power source.

Pete Stanaitis
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[davekni]

Yes, you would need to cut a trace or two, or unsolder one leg each of the two gate pull-up resistors.  I doubt it would be too difficult.