I'm looking for some ideas to cool a single MOT. I've used a few single transformer in the past, but as most know they run hot as a single unit. Knowing that current limiting is the best option for better cooling and efficiency, it's not practical for the foot print I'm looking for.
As for cooling a single unit fan alone is not that great, I hit the high 80°C when loaded. I realise that's not that hot for a MOT, but the core temperature is sure to be far higher.
Having had a think on it, I've come up with modified air cooling using heatsinks attached to the transformer frame to increase surface area of the iron. But attaching the heatsinks is a bit of problem. I know there are thermal glues, but sure these are for small heatsinks, and there transfer temperature rating isn't that good. So I was wondering if its viable to drill and tap two M3 holes on each side to aid attaching the heatsinks ? My concern here is not wanting to upset the laminations with possible shorting. Any thoughts appreciated, my other idea is oil cooling in a suitable casing that can be sealed. It seems a messy headache to go through, and I'm unsure of longevity of the cooling oil.
I'm learning towards heatsinks, but don't want to degrade the iron core.
If I could current limit with out adding bulk I would do that, I thought about a triac type dimmer switch of up to 1000 watts, but gather these are not suitable as the modify the sine wave of the AC and will probably over heat the primary winding. Also I think there not suited to inductive loads.
Thoughts at ideas appreciated.
Thanks for reading. 
Some thoughts about your choice:
- MOTs are made for a specific resonant circuit involving a rectifier, a reservoir cap and a magnetron, not for general use
- They are normally cooled by the fan of the oven, and they normally run short-term (< 30min).
- MOT cores have an iron shunt that creates a stray inductance.
- One HV side is grounded.
I have seen MOTs used for RF tube amps, but with several modifications:
- two MOTs were used, to emulate a split-winding secondary.
- The iron shunt was removed
- An inductor was added before the MOT to keep dissipation at line voltage low enough (without the resonant circuit, the core would saturate)
- A bridge rectifier was used for the heater circuit. Voltage is a few Volts depending on type, but you could use the MOT heater secondaries in series.
What to expect:
- low efficiency
- cooling is still neccessary
- cheap (you can get MOTs from the dump) but large
Alternatives
- if you just need a 12V heater source, why not try a halogen lamp lighting transfomer (up to 1kVA, not too expensive)
- or, you rip off the high voltage winding from your MOT and add just a few windings of thick wires as a secondary.
This works even as a welding transformer if the wire is thick enough.