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MOT (microwave oven transformer) cooling.
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Ian.M:

--- Quote from: davelectronic on January 05, 2019, 12:16:42 pm ---I couldn't fit another 200 turns on the primary, just not enough area to add these turns.

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You'd need to build a former in-situ (assuming you haven't cut the welds holding the core together) next to the primary out of presspahn, fish paper, and linen tape, using phenolic adhesive to hold the extra 200 turns  and provide reinforced insulation between it and the secondary and core, then wire it in series being careful of the phasing. That would reduce the available area for the secondary, so unless you start with a MOT from a 1200W oven you'll not get the power you need out out of a single MOT.   2x 800W oven MOTs, primaries in parallel, secondaries in series should be viable. N.B. as the primary current rating doesn't increase, you need to derate the total permissable VA by the ratio your extra winding bucks the line voltage by.

It would be worth looking at Circlotron's suggestion of a choke input filter after the bridge rectifier, possibly using the core from the second one with the center leg cut to air gap it o t doesn't saturate when used as a high current choke.   That arrangement has the potential to make about 40% better use of the VA rating of the main transformer.


--- Quote from: joseph nicholas on January 05, 2019, 12:29:33 pm ---If you use a triac rated for higher power, it should work.  Just remember ohms law and you should be ok.  I built a soldering iron using a triac and and a MOT and it works well, just get a high power triac, if there is one rated for the job the MOT's will be doing.

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The TRIAC firing angle must be very close to exactly the same on both half cycles.   Any significant assymetry will allow DC current buildup leading to saturation and a 'Weller style'  :-DD primary meltdown.
T3sl4co1l:
How would a net field "build up" when the winding is open-circuit for part of the cycle?

200 turns sounds like overkill.  Again, probably 10 or 20% is all that's needed.  I'm guessing the original primary is only about 300 to begin with.

Tim
davelectronic:
Lot of options available really. I kind of thought a triac type unit wouldn't work, I remember reading about heating being a problem to do with altered phase angle of the AC. The triac tricks the transformer into thinking it's less voltage by changing each half cycles of the wave form. I know it doesn't lower voltage AC that is. Only a variac will do that. I only ever split the welds on one previous MOT, on reasembly it was far worse magnetic circuit after that. I'd sooner take longer winding but maintain the iron integrity. I've no idea why the magnetic circuit was worse, but it definitely was. I remember putting a 300 watt halogen lamp load on the uncut weld MOT, the voltage drop was quite small, even with that load.
Ian.M:

--- Quote from: T3sl4co1l on January 05, 2019, 01:16:00 pm ---How would a net field "build up" when the winding is open-circuit for part of the cycle?

200 turns sounds like overkill.  Again, probably 10 or 20% is all that's needed.  I'm guessing the original primary is only about 300 to begin with.

Tim

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Yes. thinking about it, the flux shoud reset during the off time. However about forty years back I saw a TRIAC dimmer circuit smoke a transformer.   It was almost certainly a DIAC based firing circuit, which would only have generated one firing pulse per half cycle in normal operation, and I can only suppose that residual flux caused enough asymmetry during the first cycle that commutation was delayed till after the next half cycle's firing pulse, which as a result led to a rectifying action, and the TRIAC must have been really beefy compared to the primary's current rating, so it didn't simply fuse the TRIAC, restoring a normal full-wave supply to  the transformer before it cooked off.

On the 200 turns isssue - yes that does sound excessive. Bsfeechannel dropped in that number as a result of some 'calculation'.   It would make more sense to ramp up the supply with a Variac, carefully monitoring the primary current, preferably with an isolated current probe to let you see the waveform, to determine the voltage at onset of saturation, then check the voltage on a 10 turn temporary secondary winding to let you calculate the volts/turn and thus determine how many additional turns would be required to shift the onset of saturation to 10% above the nominal supply voltage.

@Davelectronic,
Yes splitting the core is a last resort, as it almost invariably introduces a slight air gap unless you can clamp it with enough pressure and re-weld it afterwards, or totally grind out the welds and rebuild it with interleaved E laminations from each end with the I laminations in the gaps between them either end, but that always seems to have issues getting the last few laminations back into the primary's coil former, so you end up leaving a couple out making the saturation problem even worse.

If you've still got the one you've split, it would be a good candidate for a core for the DC choke - ideally grind down the center leg to gap it, but if you don't mind a high external field, you could gap it with layers of varnished paper or thin card right across between the E and I.

It may be worth considering a separate buck transformer.  e.g. to buck the 240V supply to a 1 KVA MOT by 24V, the buck transformer only has to handle the MOT primary current of a bit over 4A, so a relatively small 120VA transformer with its secondary in series with the supply to the MOT will do that with plenty of margin.
bsfeechannel:

--- Quote from: Ian.M on January 06, 2019, 02:15:14 am ---On the 200 turns isssue - yes that does sound excessive. Bsfeechannel dropped in that number as a result of some 'calculation'.   It would make more sense to ramp up the supply with a Variac, carefully monitoring the primary current, preferably with an isolated current probe to let you see the waveform, to determine the voltage at onset of saturation, then check the voltage on a 10 turn temporary secondary winding to let you calculate the volts/turn and thus determine how many additional turns would be required to shift the onset of saturation to 10% above the nominal supply voltage.

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Challenge accepted.

Using a variac I plotted the primary idle current (i.e. the magnetizing current) on the primary against its voltage.



If we can agree that after around 0.5 amps (beware that on my graph commas stand for dots) the transformer really gets saturated, its voltage shouldn't be greater than 160V. The volts/turn ratio of this transformer is 1.1. So the primary has 200 turns (hence the magic number I proposed earlier). 160V/200 turns gives us a 0.8 volt per turn ratio. To get back to 220V on the brink of saturation we would need (220 - 160)/0.8 = 75 turns. Much better than 200 turns.
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