MOT (microwave oven transformer) cooling. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 02, 2019, 10:24:29 pm
content: 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.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: January 02, 2019, 10:31:09 pm
content: Without knowing what you're using it for, its hard to reply. If you could elaborate more on your ultimate goal it would be useful.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 03, 2019, 12:06:40 am
content: Thank you for replying. Yes sure, to drive high current linear RF amplifier.Currents around 20 to 30 Amps at 12 to 13.8 Volts.

author: thedoc298
author_rank: Contributor
timestamp: January 03, 2019, 07:37:42 am
content: Why not put mot in bucket of mineral oil.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 03, 2019, 11:54:59 am
content: I had considered oil, but would rather go with air as a first option. I was hoping to attach 3 x heatsinks with 6 x 3mm screws. Only not sure if drilling the EI transformer frame might upset the transformer magnetic circuit. I'd use 2 x 3mm shallow holes on each of the three surfaces.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: January 03, 2019, 04:44:23 pm
content: In order to get 20 to 30 amps for your project you will need to use 2 of these things in series. The average microwave uses about 1200 watts or so. You should also keep in mind that the average household circuits have 20 amp fuses or breakers for the mains. I see you live in the UK but these limitations would apply to you as well.

author: Gyro
author_rank: Super Contributor
timestamp: January 03, 2019, 05:29:07 pm
content: I would think that heatsinking the core and frame would be pretty ineffective and doesn't tackle the issue of heat buildup inside the windings.I would approach it with forced air cooling, ducting the air so it hits the core edge-on and flows over the windings at each side. I think it's the best you'll do without messy oil cooling - where you need to arrange some circulation, either convection or stirred to be efficient anyway. You really don't want to be generating localized hot-spots in a can of mineral oil.I know you've tried a fan already but it sounds as if you need to use a bigger one and use a duct to ensure that all the air gets 'squeezed' past the transformer. A properly ducted fan will shift a hell of a lot of heat.Edit: The Copper (or Aluminium) windings will be a lot more thermally conductive than the steel core but there's no way to heatsink them... and you don't want to start adding things to the core that will impede airflow over them.

author: Ian.M
author_rank: Super Contributor
timestamp: January 03, 2019, 06:02:32 pm
content: Quote from: joseph nicholas on January 03, 2019, 04:44:23 pmI agree that pushing a MOT to VA levels and duty cycles where simple fan cooling is insufficient is unwise, however you have a misunderstanding of UK wall sockets. Subject to other loads on the circuit, any BS1363 UK wall socket can supply 13A at nominally*240V, i.e 3.1 KVA.Unless the PSU design is ludicrously inefficient, 30A @ 13.8V DC shouldn't require more than 1 KVA AC in, a third of the max capacity of a BS1363 socket.* The supply network in general remains at pre-EU harmonisation voltage levels, as the harmonisation was a paper exercise achieved by shifting the tolerance bands.

author: Zero999
author_rank: Super Contributor
timestamp: January 03, 2019, 06:09:01 pm
content: Quote from: Ian.M on January 03, 2019, 06:02:32 pmAssuming he's using the usual rectifier, capacitor and linear regulator topology, it wouldn't surprise me if he'll approach 3kVA, at full load, because of the huge regulator losses and poor power factor to boot.

author: Ian.M
author_rank: Super Contributor
timestamp: January 03, 2019, 07:16:28 pm
content: 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!

author: Zero999
author_rank: Super Contributor
timestamp: January 03, 2019, 09:12:00 pm
content: Quote from: Ian.M on January 03, 2019, 07:16:28 pmPoint taken: it's nowhere near as bad as I previously thought!

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 03, 2019, 09:21:14 pm
content: 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.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 03, 2019, 09:32:13 pm
content: 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.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 03, 2019, 09:45:58 pm
content: Quote from: davelectronic on January 02, 2019, 10:24:29 pmSome 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 largeAlternatives- 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.

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2019, 09:47:52 am
content: 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.

author: soldar
author_rank: Super Contributor
timestamp: January 04, 2019, 04:32:53 pm
content: You are trying to build a PSU that will supply up to 30 A at 13.8 V? Regulated? That sounds pretty easy and there are many designs out there. It is about 400 W which is not that much.But you'd be better off going with a specific design than trying to reuse components that are ill suited for the job.IMHO a SMPS is better suited for this end. In any case, a transformer or transformers followed by bridge rectifier and capacitor has a terrible wave form power factor which means everything will need to be oversized. I would go with a SMPS with power factor correction included. You will save bulk, heat, and probably cash.You can search for Texas Instruments Power Factor Correction Circuits as they have resources online.I guess I am getting older and lazier but I would not spend my time designing and building things which are very commonly sold commercially and can be bought for much less money than my time is worth. You can buy a PSU with those specs which will be better designed and put together than anything you can do with repurposed microwave oven transformers.

author: james_s
author_rank: Super Contributor
timestamp: January 04, 2019, 04:55:12 pm
content: It would be silly to try building a SMPS like that. You can get surplus hot-swap server PSUs for not much more than the cost of shipping. These are mostly 12V at 30-60A but many can be tweaked to increase the output voltage slightly.

author: langwadt
author_rank: Super Contributor
timestamp: January 04, 2019, 05:10:22 pm
content: Quote from: james_s on January 04, 2019, 04:55:12 pmor just get one of these in 12V or 15V (adjustable +/- ~15%)https://www.meanwell.com/Upload/PDF/LRS-350/LRS-350-SPEC.PDF$36.42 from Digikey

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2019, 05:15:59 pm
content: 1a. Reduce input voltage to, probably 80-100V (use another transformer in buck configuration). Adjust secondary turns accordingly.1b. Equivalently, add more primary turns (in the gap left from the magnetic shunts?) to the same end.These both reduce capacity, but mind that an MOT is only about 500VA in size, for a typical (cool running, good regulation) design. Don't be greedy about their ratings!The reduction in V/turn will save input (magnetizing) current, and mechanical noise, because the core is not saturating every half-cycle.2. Sure, go ahead and bolt or clamp some heatsinks to it. Iron's not the greatest thermal conductor, but there's a lot of it. You'll need a lot of thermal goop to fill in the somewhat uneven laminated surface.If possible, stick a thermistor inside the winding so you can monitor wire temperature. Use high temp wire if possible, too.3. Just forget about them. More effort than it's worth? A nice MeanWell switcher that size is only a few hundred bucks, and will perform very nicely indeed!Tim

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 04, 2019, 07:47:08 pm
content: Thanks again all for your replys. It's the something for nothing, or next to nothing that's appealing. They are far from perfect or efficienct I know this. A first stage transformer sounds interesting idea..I have had other projects from the past with bought toroidal transformers for. Yes I could purchase another transformer. I've lost count of the smps units I have modified over the years, from good quality ATX units to very powerful server psu's.It's just for the fun of it really. The MOT is never going to be fantastic, adding surface area creates bulk. Andding ballast creates weight, as does second transformers. First I will try heatsinking with thermal epoxy, see how that goes.

author: Ian.M
author_rank: Super Contributor
timestamp: January 04, 2019, 08:34:54 pm
content: Personally if I needed a clean 13.8V supply with 30A peak load currents and a moderate to low duty cycle, I'd float a Lead acid battery. As RF is involved, using a linear charger (or full cycle line frequency TRIAC controlled on the secondary side) would be advisable. 10A max current from the charger would handle any reasonable TX duty cycle for non-broadcast applications, and at about 350VA with a well insulated extra winding to buck 10% of the line voltage, any MOT should run cool enough without additional heatsinking.

author: Berni
author_rank: Super Contributor
timestamp: January 04, 2019, 09:20:34 pm
content: Heat tends to be the limiting factor for power in a transformer. You can easily run transformers at 500% rated load, but not for long before it overheats, so it makes such operation reasonable only in cases where the duty cycle is very short.Most of the heat comes from the copper wingdings so adding heatinsking to the core likely won't do a whole lot. It will certainly make the transformer feel like its running cooler since the core will certainly be cooler to the touch but deep inside the layers of copper coils there will still be a significant amount of heat. It does help, but not a ton. Having a powerful fan blow over the transformer is likely plenty enough cooling for how much heat can propagate out of deep inside the wingdings.The problem is that its quite hard to tell when a transformer is running comfortably without having a temperature probe deep inside the wingdings. At a certain temperature the insulation on the wire starts to slowly degrade, this can cause a failure after a few years. If its getting hot enough that you can smell it then its really getting too hot and will probably start breaking down after days of such operation. Such breakdowns can be quite spectacular sometimes as they include lots of fire, sparks and smoke as turns short themselves and get hot causing the failure to propagate trough the wingdings until the whole thing is fried and the fuse blows.When space and efficiency is a big factor its impossible to beat switchmode supplies.

author: soldar
author_rank: Super Contributor
timestamp: January 04, 2019, 10:36:20 pm
content: Quote from: Ian.M on January 04, 2019, 08:34:54 pmYup. I used to do that. Here you could get used car batteries for free. They were not good for starting cars but still plenty good for my ham radio rig. Nowadays used lead batteries are so tightly controlled you'd think they were heroin.

author: james_s
author_rank: Super Contributor
timestamp: January 04, 2019, 10:59:32 pm
content: Lead acid batteries are easy enough to get in the USA but I don't like keeping them in my house. They inevitably produce corrosive vapor, they release flammable hydrogen when charging, they will often leak if knocked over, they can supply vast currents for short periods of time which can cause a fire, they are heavy and bulky. Many of these issues are not too hard to mitigate but it's still not something I really enjoy messing with.

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2019, 11:12:16 pm
content: Quote from: langwadt on January 04, 2019, 05:10:22 pmYes, that's what I'd do.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 05, 2019, 03:17:09 am
content: I am limited as the property is a 16 floor tower block, I doubt, or through limited ventilation couldn't use a lead acid battery. And it's really just for the fun and something for nothing. Although all told it still cost something. Would a high rated triac type dimmer work ? I know they only convert the wave form. But unsure if that would be suited to an inductive load. I know 1000 watt dimmer type switches are readily available. But will that create heat as it's a modified wave form.

author: Circlotron
author_rank: Super Contributor
timestamp: January 05, 2019, 03:45:16 am
content: If it’s just for fun then it’s perfectly valid in my books...For a power supply of that current I’d consider using a choke input filter. It will make life easier for the transformer and filter caps, and the ripple will be more or less sinusoidal, not sawtooth, so easier to clean up. Caps and bridge rectifier present a power factor of about 0.6 to the transformer with very high current peaks, but add a choke and those peaks (and the consequent transformer I2R copper losses) go way down. The transformer secondary voltage will have to be higher though. Instead of the dc being 1.414 x the rms volts it will be about 0.9 x the rms. Plus the rectifier voltage drop of course. Back in olde tymes with really serious power supplies the choke input filter was king.

author: bsfeechannel
author_rank: Super Contributor
timestamp: January 05, 2019, 04:03:07 am
content: MOTs are designed to work under heavy saturation at their rated mains voltage. That's why they overheat without forced air cooling (a.k.a a fan).To "correct" that you have three options: reduce the primary voltage , enlarge the cross sectional area of the center leg of the EI core or add turns to the primary.Since the first option is a bit difficult for you and the second impossible, the third, although a bit crazy, is probably the most viable.By my calculations, to tame that beast you'd need some additional 200 turns on the primary.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 05, 2019, 12:16:42 pm
content: I couldn't fit another 200 turns on the primary, just not enough area to add these turns. I'm still wandering about a triac type circuit, I know strictly speaking it doesn't lower the input voltage. But rather cuts off part of the time a certain area of the sin waves on period. Modify the wave form, but unsure if it's a viable option due to heat build up from the modified wave form the triac type unit would supply to the primary.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: January 05, 2019, 12:29:33 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: January 05, 2019, 01:11:26 pm
content: Quote from: davelectronic on January 05, 2019, 12:16:42 pmYou'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 pmThe 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'primary meltdown.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2019, 01:16:00 pm
content: 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

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 06, 2019, 12:54:39 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: January 06, 2019, 02:15:14 am
content: Quote from: T3sl4co1l on January 05, 2019, 01:16:00 pmYes. 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.

author: bsfeechannel
author_rank: Super Contributor
timestamp: January 06, 2019, 04:26:30 am
content: Quote from: Ian.M on January 06, 2019, 02:15:14 amChallenge 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.

author: Ian.M
author_rank: Super Contributor
timestamp: January 06, 2019, 05:12:08 am
content: I agree that 160V is the beginning of hard saturation. Assuming your nominal supply voltage is 220V, you still need some margin for supply voltage variation. 10% overvoltage is a reasonable safety margin for most developed countries but if you are in a 3rd world s--thole*, you may need more margin. Lets run the numbers for 10%. Max input voltage is 220V * 1.1 = 242V. 242V - 160V = 82V. 82V / 0.8V/turn = 103 turns.That's about half the turns of your original estimate, and if the primary fills the bobbin and is a close fit in the core window, is unlikely to be practical to add as an overwind on top of the primary, so its going to be a total PITA to have to build a reinforced insulation bobbin for the extra turns as you cant simply 3D print two half bobbins and glue them as it wont take the heat, hence my comment earlier to use presspahn board, fishpaper and linen tape (which must be bare linen or with a thermosetting adhesive) and phenolic adhesive.* If you are in North America in a property with split phase 120V/240V, and old wiring, backstab outlet connections, or aluminum wiring, even 20% margin wont prevent a 'Weller style' meltdown from a bad neutral so for <expletive>'s sake fuse the primary properly!

author: bsfeechannel
author_rank: Super Contributor
timestamp: January 06, 2019, 06:43:20 am
content: Around here, 220V installations are allowed to go from 202V to 231V. Up until very recently we were serviced by a subsidiary of AES Corporation. I can only hope they were not based in one of those 3d world sitholes (I learned that spelling with AvE).Anyway, I think we are running out of options.I use my MOT together with the variac as a poor man's high voltage source. I never go past 100V on the primary, since the high voltage winding has a 1:10 turns ratio and none of my meters can handle more than 1kV.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 06, 2019, 07:35:05 pm
content: This is very interesting data and ideas there. Unfortunately I don't have the split core transformer anymore. Going back a few years, I measured the primary current with no load on the secondary. That was right on 3 Amps with the 240 ish input. A lot of wasted energy. What I'm unsure about, is that current "wattless" a bit like an AC induction motor spinning with no load ? I kind of think the motor scenario and the transformer are not the same, identically. What seems to be the go to option must be primary ballast in series with the primary, so it's not saturated all the time. And adding extra primary winding is another great idea and option, but gets a bit trying on a closed core.

author: bsfeechannel
author_rank: Super Contributor
timestamp: January 06, 2019, 08:53:47 pm
content: The current is not "wattless", otherwise your transformer wouldn't be overheating, but part of it is "wattless". It has to be, because part of that energy is stored as a magnetic field in the core. However, I'm not sure about the proportion exactly.One thing crossed my mind while I was taking a nap. Normally we remove the MOT's magnetic shunts. What these shunts do is to provide an embedded ballast by increasing the leakage inductance. It acts as a current limiter and as such ends up reducing the voltage seen by the primary according to the model below.I'm not sure how effective this embedded ballast is, but perhaps it helps reduce the number of turns to be added to the primary to take the core out of saturation. My graph doesn't show, but at 214V, I got 3.4A. However, my MOT is "shuntless". It would be nice if some MOT still intact could be measured for that current so that we could have an idea.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 06, 2019, 10:58:57 pm
content: No, the shunt doesn't affect the primary side stats -- it's wound around the same core limb regardless. The shunts do divert flux from the secondary, which means the secondary side core limb runs less Bpk.The shunts are usually not the full core cross section, and not gapped much, which should tend to run them in saturation, drawing ~constant flux away from the secondary. Which means the secondary regulation should still not be terrible, just a lower voltage. Doesn't seem very useful, I guess it just happens to work out well enough for a magnetron.The equivalent circuit is two ideal transformers sharing flux; the unlinked flux, is, of course, the leakage.Tim

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 09, 2019, 02:21:02 am
content: Just a thank you for the help, ideas and data provided. Generally I've knocked out shunts, having read they help in a crude way to avoid overheating in the transformers saturation. Maybe I should have left them in, and the area that they take up in small. Although a couple more secondary turn can be had with there removal.It's always going to be a poor man's linear transformer, but it's a means to an end.

author: Wolfgang
author_rank: Super Contributor
timestamp: January 09, 2019, 01:58:41 pm
content: Yeah, right. You cannot expect too much from a junkyard item. If not run in a microwave oven, 300W is realistic with a series inductor and the magnetic shunt out. Still better than nothing if you need some cheap HV.Happy (and safe) experimenting !Wolfgang

author: Berni
author_rank: Super Contributor
timestamp: January 09, 2019, 06:13:37 pm
content: Id say just wire the primaries of two MOTs in series so they are effectively running on 110V and split your 12V output winding in to two 6V winding on both transformers. Acording to the graph that will bring the magnetizing current down to something normal and save a lot of losses in the primary copper wingdings.As for the shunt id say just hammer it out. It should make very little if any difference to the magnetizing current on the primary, but will have a big effect on your output load regulation. As you put a load on your secondary coil you don't get all of the flux from it reaching the primary where it accordingly causes the input current to rise and put more power into the transformer. Because of that as the current in the secondary increases and produces an opposing flux it starts to cancel out the original flux and reduce it. Because of that the voltage on the secondary drops.This makes the shunt effectively a current limiting feature. Without it under a short circuit condition the current is mostly limited by the resistance of the wingdings and this causes a lot of heat. But a magnetic shunt simply hides some of the secondary coils load from the primary causing it to draw less current from the power source. This effect is heavily used in neon sign transformers where they are magnetically shunted so much that you can short the output and leave it running indefinitely without any damage to the transformer.

author: Cliff Matthews
author_rank: Unknown
timestamp: January 09, 2019, 07:21:36 pm
content: Quote from: Wolfgang onTodayat 10:58:41 amA bit more maybe. I got 330 watts applying 78-vac to 120-vac MOT primaries (testing with lamp cord as secondary winding)

author: Wolfgang
author_rank: Super Contributor
timestamp: January 09, 2019, 07:28:04 pm
content: ... some people even use a lamp as a limiting resistor. IMHO the end of the game is reached when the MOT core saturates. From then on it will get really hot. Its also a question of what kind the load is usec (rectifier with cap, resistive, ...). Anyway, as log everything stays cool, why not draw 330W.