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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I read somewhere that MOT's are ferro-resonant transformers, so I checked the circuit and they do have the capacitor in parallel with the secondary, which got me thinking.......
If you connect the secondaries of two MOT's in series, with two microwave oven capacitors also in series (LC=2LxC/2) you get this:
Will this give a 'clean' 240V, 50Hz output?
(The operative word here is 'clean', ie no interference or interruptions.)
EDIT: I suspect it might have some effect with a continuous load, If the load is intermittent (say 30kHz), and the output feeds a bridge rectifier, with 'very small' smoothing capacitor (say 4.5uF) I imagine the capacitors, along with the (resonant?) secondaries may well still have a 'smoothing effect'. (From some 'angles' it does resemble a 'low pass' filter', or HF choke.)
(apparently the secondaries would need an inductance of ~8H for an MOT to be resonant at 50Hz, which sounds pretty big)
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
I read somewhere that MOT's are ferro-resonant transformers, so I checked the circuit and they do have the capacitor in parallel with the secondary, which got me thinking.......
If you connect the secondaries of two MOT's in series, with two microwave oven capacitors also in series (LC=2LxC/2) you get this:
Will this give a 'clean' 240V, 50Hz output?
(The operative word here is 'clean', ie no interference or interruptions.)
1. MOT transformers sold in the EU (and probably elsewhere, for all I know) have the 'cold' end of the HV secondary connected directly to the core, and thus to Earth.
2. Even were this not the case, your circuit has no way for EMI to make its way to Earth - via, for example, a capacitor. Search Google Images for "Mains Filter Circuit" to get the general idea of L and C mains filter configurations.
3. Back-to-back transformers are sometimes used by people who don't have a proper HT transformer but want isolation from the mains eg 230:12-12:230, but such a circuit is not very efficient, may overheat, and cannot be recommended.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
1. MOT transformers sold in the EU (and probably elsewhere, for all I know) have the 'cold' end of the HV secondary connected directly to the core, and thus to Earth.
Yes, but if you look at the circuit diagram one side of the capacitor also connects to earth, and is in parallel with the secondary, through a diode and magnetron (effectively another, anti-parallel diode).
Proud Mary wrote ...
2. Even were this not the case, your circuit has no way for EMI to make its way to Earth - via, for example, a capacitor. Search Google Images for "Mains Filter Circuit" to get the general idea of L and C mains filter configurations.
3. Back-to-back transformers are sometimes used by people who don't have a proper HT transformer but want isolation from the mains eg 230:12-12:230, but such a circuit is not very efficient, may overheat, and cannot be recommended.
I suspect that the two capacitors together (effective capacitance ~0.5uF)are far too small to shunt much current, even with the 'tank' circuit at 2000V. (although the way I've drawn the circuit above, the series capacitors aren't in the 'conventional' series arrangement (ie connected directly to each other), but according to the 'current path' they 'are' in series.
If my understanding is correct the leakage inductance of the MOT would have to be around 8H, which sounds a 'bit' on the big side.
My 'gut feeling' is that I won't get much power throughput at all, but thought I'd 'throw it out there' anyway, for discussion.
Won't EMI get 'absorbed' by the capacitors, though? Surely they will have 'some' smoothing effect?
EDIT: This is probably a much better idea, but would probably require a large series-parallel array of microwave oven capacitors and a ballast inductor in series with the primary.
EDIT EDIT: I imagine a better arrangement would be two primaries on the same core (1:1, 240 in, 240 out) and a third winding with parallel capacitor for resonance, with additional inductive ballast in series with the primary but as the MOT is already in saturation I'd expect power throughput to again be reduced. This does seem to be a much better option though.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
Proud Mary wrote ...
1. MOT transformers sold in the EU (and probably elsewhere, for all I know) have the 'cold' end of the HV secondary connected directly to the core, and thus to Earth.
Yes, but if you look at the circuit diagram one side of the capacitor also connects to earth, as is in parallel with the secondary, through a diode and magnetron (effectively an anti-parallel diode).
Proud Mary wrote ...
2. Even were this not the case, your circuit has no way for EMI to make its way to Earth - via, for example, a capacitor. Search Google Images for "Mains Filter Circuit" to get the general idea of L and C mains filter configurations.
3. Back-to-back transformers are sometimes used by people who don't have a proper HT transformer but want isolation from the mains eg 230:12-12:230, but such a circuit is not very efficient, may overheat, and cannot be recommended.
I suspect that the two capacitors together (effective capacitance ~0.5uF)are far too small to shunt much current, even with the 'tank' circuit at 2000V. (although the way I've drawn the circuit above, the series capacitors aren't in the 'conventional' series arrangement (ie connected directly to each other), but according to the 'current path' they 'are' in series.
If my understanding is correct the leakage inductance of the MOT would have to be around 8H, which sounds a 'bit' on the big side.
My 'gut feeling' is that I won't get much power throughput at all, but thought I'd 'throw it out there' anyway, for discussion.
Won't EMI get 'absorbed' by the capacitors, though? Surely they will have 'some' smoothing effect?
Nothing is connected to Earth in your circuit diagram.
EMI will not be 'absorbed' by a capacitor, but a capacitor can bypass it to Earth.
Worked example just for you, Ash:
A bypass capacitor of 0.05μF is connected between Live and Earth in an EU mains system.*
So you see that the capacitive reactance XC - (something like AC resistance and measured in ohms) - falls as frequency increases, so higher interfering signals have less resistance to Earth than the lower 50 Hz mains signal. * Only Class Y capacitors are certified for direct connection between Live and Earth in the EU. This contrasts with Class X capacitors, which are certified for connection between Live and Neutral.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Please note I've edited my previous post.
I'm familiar with class X capacitors, usually used as a pair with chokes.
I see your point, EMI 'will' be transmitted. I was thinking a 'resonant LC circuit' would 'oppose' transients, which is one of the properties of ferro-resonant transformers.
I think it was Wikipedia and somewhere else where I read that MOT's are ferro-resonant, but I'm not able to find the reference at the moment.
I imagine the best solution would be a conventional 1:1 ferro-resonant Xformer, as in the above (edited) post.
EDIT: Although, an iron core MOT should have a considerable blocking effect on HF transients anyway. A class X capacitor on the input would help as well.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ash Small wrote ...
... an iron core MOT should have a considerable blocking effect on HF transients anyway. A class X capacitor on the input would help as well.
More likely the interference current will be motoring along your wires from the mains, trying to avoid hold ups and bottlenecks, and will be overjoyed to see a big short-cut in the winding capacitance between primary and secondary.
Good quality HT transformer have one or more electrostatic screens between windings, to attenuate the onward transmission of mains borne EMI, (and the reverse, of course, where RFI from a transmitter, for example, can slip into the mains supply like a rat going down a drain,) but not to my modest knowledge your MOT.
Lastly, I don't know anything about any 'ferro-resonance' in MOTs - presumably, as in the obsolescent 'constant voltage' transformer - but I have only a basic understanding of kitchen appliance technology, so you should ask elsewhere.
I'd suggest using an ordinary mains filter arrangement, which you can either build yourself from L and C, or buy as a whole module or unit from the likes of ebay, which will probably cost less than DIY, and will come with BS and CE marks, so won't be an issue in any fire or accident claims arising.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Proud Mary wrote ...
Ash Small wrote ...
... an iron core MOT should have a considerable blocking effect on HF transients anyway. A class X capacitor on the input would help as well.
More likely the interference current will be motoring along your wires from the mains, trying to avoid hold ups and bottlenecks, and will be overjoyed to see a big short-cut in the winding capacitance between primary and secondary.
Good quality HT transformer have one or more electrostatic screens between windings, to attenuate the onward transmission of mains borne EMI, (and the reverse, of course, where RFI from a transmitter, for example, can slip into the mains supply like a rat going down a drain,) but not to my modest knowledge your MOT.
That's a very good point, PM. I believe you've mentioned electrostatic screens in transformers before.
Proud Mary wrote ...
Lastly, I don't know anything about any 'ferro-resonance' in MOTs - presumably, as in the obsolescent 'constant voltage' transformer - but I have only a basic understanding of kitchen appliance technology, so you should ask elsewhere.
I'd suggest using an ordinary mains filter arrangement, which you can either build yourself from L and C, or buy as a whole module or unit from the likes of ebay, which will probably cost less than DIY, and will come with BS and CE marks, so won't be an issue in any fire or accident claims arising.
I realise I can use the mains filter for interference. I've built similar filters before to prevent the HV from my TIG welder getting back to the rectifier and buzzbox.
It's the 'stored energy' in the resonant LC circuit of ferro-resonant constant voltage transformers that I'm really interested in, and the ability to provede a constant output from intermittent supplies. I also assume it will 'ramp down' to some extent on power off, and 'ramp up' to some extent also when switched on, providing 'some' soft switching, but my main interest is in a 'constant voltage' supply.
I came across two references claiming MOT's are ferro-resonant, so went off at a slight tangent.
The vast majority of losses in 'constant voltage ferro-resonant transformers' are copper losses and core losses. Only 3% to 5% of losses are due to the circulating current (according to several manufacturers), eg:
MOT's are 'cheap and lossy' and run in, or near, saturation anyway, so adding a third ferro-resonant winding to store energy and maintain constant voltage should be ~95% to 98% as efficient as a 'normal' MOT.
Admittedly it's certainly not the most efficient way to do it, and it would probably require oil cooling, but in a lot of applications it could prove more reliable than using silicon to achieve the same effect. (and silicon doesn't 'store' energy)
This evening's research topic will be the resonant NST's and MOT's that others have built for pulling arcs before I decide whether to continue with a constant voltage ferro-resonant supply myself. I've noticed quite a lot of youtube videos on this subject while googling related subjects today.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
You'll have to ask others about any unusual properties MOTs may have - and Heaven knows this forum is stuffed full of MOT-wallahs - but what I do know is that they are very skimpily built, and have extremely poor voltage regulation. I've heard of folk knocking out the magnetic shunts in an attempt to improve regulation, but don't know how much of an improvement can be expected without overheating.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Yes, I appreciate that they are cheap, and quality control (or lack of it) means some are better than others.
I'm planning to wind two 240 ish windings from scratch, then wind the resonant winding, then see what happens, but I need to do a bit more research first. (magnetic shunts, leakage inductance, etc)
The main reason I'm using an MOT is because they are so plentyful, and because the use I have in mind for it is providing a 'clean' supply for the microwave inverter that I have. I realise I'll lose 'a bit' of power, but I don't need to run the inverter 'flat out' anyway.
I think I've devised a simple way to control the inverter output to give me a lot more control over the operation of the magnetron itself, using a voltage comparator chip, and some other trickery, but I will need a constant voltage supply.
Thanks for your advice earlier, it cleared up a couple of points I wasn't sure about.
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Will this circuit provide 'clean' 240V 50Hz AC???
