Electronics > Beginners

MOT (microwave oven transformer) cooling.

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Zero999:

--- Quote from: Ian.M on January 03, 2019, 07:16:28 pm ---I think that would fall under 'the PSU design is ludicrously inefficient'.
LTspice sim of a bridge rectifier with 30A DC load and 2.5V headroom at the ripple trough for feeding a 13.8V regulator, at 90% of nominal line voltage attached.  The secondary current is 47.9A RMS, which is slightly over 1KVA.

I'll grant you that if you don't carefully optimise your choice of secondary voltage, rectifiers, reservoir capacitance and regulator design, it will be hard to get right down to 1KVA in, but if it needs more than 1.5KVA after the switch-on surge, you'd better be building a room heater, not a PSU!

--- End quote ---
Point taken: it's nowhere near as bad as I previously thought!

davelectronic:
Thanks for your replys.  I have reclaimed the MOT from a prevoius project, the transformer is wound with 12AWG silicon wire, and sleaved with shrink wrap tubing. The MOT puts out 14.70 Volts no load. In the previous project it put out 21.00 Volts rectified and filtered with 90000uf capacitance from 9 x 10000uf can electrolytics. I only ran it at 10 to 12 Amps as I was only using two MJ11015 power transistors. Noted at the time of use the output was very stable, although I wasn't that happy this 80°C surface temperature of the transformer.

From what I've read, class H 220 can go up to 180°C. So seems like plenty head room. Although the shrink wrap of the secondary will only go to 125°C. I know primary power efficiency isn't great, but for radio linear amplifier use it gets the job done. I'm just looking for cooler transformer running temperatures. Since posting I've found some thermally conductive epoxy, and have some decent area cpu heatsinks. Hopefully coupled with a couple of 100mm axial fans blowing over it, that I hope would keep things a bit cooler. The only negative is the epoxy rated to 150°C isn't cheap. I've also acquired another MOT after a new microwave oven upgrade.

davelectronic:
In the first project I used a low drop out voltage regulator on the regulator control board. Seems fine on input to the regulator voltage. I measured at the time of using the first incarnation. The output barely lost or dropped voltage under a 12 Amp load. From memory about 0.3 Volts of the 14.70 Volts going in to the bridge rectifier. The " Stiff" output AC was one of the positives with the MOT, baring in mind the primary is wasteful at full mains voltage.
Out of interest, can a triac type circuit control the primary voltage ? I know it chops the waveform AC, but wandered if such a circuit on the mains input would work.

Wolfgang:

--- Quote from: davelectronic on January 02, 2019, 10:24:29 pm ---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.  :)

--- End quote ---

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.

Zero999:
If you go for a halogen lamp transformer, make sure it's the iron core type. Most modern halogen lamp transformers have an oscillator driving a small high frequency transformer. The output is a 20kHz to 200kHz square wave modulated by double the mains frequency.

Why not just use a switched mode power supply? A 1kW unit isn't that expensive and will probably safe money on the electricity bill.

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