One more question: how do I find out the output impedance of the transformer, so I can simulate it properly? DC resistance measures about 80 Ohms.
Let's see, the problem of turn-on surge is threefold:
1. You have a big system. That means big inrush currents, period. (Small stuff like a 6V6 amp can get away with this because...they're small).
2. The iron has to establish a field, which is a pretty drastic surge if turned on at the right point in the cycle (zero crossing I think).
3. You have a lot of energy in the caps.
A slow turn-on would help (series inductance to soften the blow, resistor, etc. with relay to short after a second or three). Only thing that will reduce the effect of C at the FWB is to use less there; you could use maybe 10-50uF. So what if the ripple is 300V, that's only 10%! Remove the rest with the choke. Use a lot of C after it if necessary.
Choke in ground is electrically the same as choke at +V, except the winding is at 0V as well as the frame, so there's no insane insulation issues. You just get those 300V of ripple on the minus side of the rectifier bridge instead.
PFC and output impedance go hand in hand, as PFC cancels the winding and leakage inductances the transformer creates. When open-circuited, it acts as a choke equal to the primary on the core; when secondary is shorted, low coupling manifests itself as leakage inductance. The second only shows up under a full short, any resistance adds vectorially creating a slightly inductive load. The primary inductance appears in parallel with this, but in most cases I'm sure pales in comparison to the lower impedance presented by the leakage inductance (Ltot = L1L2 / L1+L2, just like resistors) and transformed load resistance so can be ignored.
Qualitative, but not quantitative. I wouldn't be able to guess, but you could try taking some measurements depending on your available equipment. However, once you know the inductance seen through whichever winding you are going on, you can calculate it as impedance, and reactance to cancel on the primary side. I would guess you need no more than 100uF. Maybe 20uF.
Tim
1. You have a big system. That means big inrush currents, period. (Small stuff like a 6V6 amp can get away with this because...they're small).
2. The iron has to establish a field, which is a pretty drastic surge if turned on at the right point in the cycle (zero crossing I think).
3. You have a lot of energy in the caps.
A slow turn-on would help (series inductance to soften the blow, resistor, etc. with relay to short after a second or three). Only thing that will reduce the effect of C at the FWB is to use less there; you could use maybe 10-50uF. So what if the ripple is 300V, that's only 10%! Remove the rest with the choke. Use a lot of C after it if necessary.
Choke in ground is electrically the same as choke at +V, except the winding is at 0V as well as the frame, so there's no insane insulation issues. You just get those 300V of ripple on the minus side of the rectifier bridge instead.
PFC and output impedance go hand in hand, as PFC cancels the winding and leakage inductances the transformer creates. When open-circuited, it acts as a choke equal to the primary on the core; when secondary is shorted, low coupling manifests itself as leakage inductance. The second only shows up under a full short, any resistance adds vectorially creating a slightly inductive load. The primary inductance appears in parallel with this, but in most cases I'm sure pales in comparison to the lower impedance presented by the leakage inductance (Ltot = L1L2 / L1+L2, just like resistors) and transformed load resistance so can be ignored.
Qualitative, but not quantitative. I wouldn't be able to guess, but you could try taking some measurements depending on your available equipment. However, once you know the inductance seen through whichever winding you are going on, you can calculate it as impedance, and reactance to cancel on the primary side. I would guess you need no more than 100uF. Maybe 20uF.
Tim
Ripple as percentage is misleading here, because of the following: SE class A output where the output varies only a fraction of the total bias; the average bias goes into powering the plasma. Since the audio frequency modulation is only a fraction of the total output, ripple needs to be lower. How much exactly I have no idea, and that's why I'm doing extra filtering.
BTW the tin can capacitors are only rated at 3 kV, and even though I'm using them as the second and third Cs (the first is the series capacitors which add up to 3.6 kV), I'm worried about running them so close to the maximum, especially when there is no load connected.
I ground out the welds and disassembled the MOT (it was a pain, that's for sure). Without the magnetic shunts, what should I do with the gap left between the primary and secondary windings? Do I leave it, or do I move the windings closer together? Also, the filament winding was only three turns and giving 3 V RMS, so I took it out. It was wound around the HV secondary (as in normal operation it is biased by the HV). I'm thinking of rewinding it with more turns (what AWG wire do I use?), perhaps in the gap left by the removed magnetic shunts. If I do this, do I wind it close to the central bar of the E, or in a wide circle?
The insulation of the HV secondary was quite heavy around the outside, but almost nil at the inside, as that end was grounded to the core. Since I don't want to ground it again, I need to add better insulation. However, I only have about 1.5 mm spacing between the inner bar of the core and inner diameter of the winding. What's the best insulation to use that will fit? I was thinking mica, which is supposed to take at least 20 kV/mm, held together by paper and shellac.
Since there is supposed to be a small gap between the E and the I (http://geek.scorpiorising.ca/windingtransformers.html), how is the core supposed to be grounded to the chassis? Do I ground the E, the I, or both? Also, should I add electrostatic shielding (aluminum foil)?
BTW the tin can capacitors are only rated at 3 kV, and even though I'm using them as the second and third Cs (the first is the series capacitors which add up to 3.6 kV), I'm worried about running them so close to the maximum, especially when there is no load connected.
I ground out the welds and disassembled the MOT (it was a pain, that's for sure). Without the magnetic shunts, what should I do with the gap left between the primary and secondary windings? Do I leave it, or do I move the windings closer together? Also, the filament winding was only three turns and giving 3 V RMS, so I took it out. It was wound around the HV secondary (as in normal operation it is biased by the HV). I'm thinking of rewinding it with more turns (what AWG wire do I use?), perhaps in the gap left by the removed magnetic shunts. If I do this, do I wind it close to the central bar of the E, or in a wide circle?
The insulation of the HV secondary was quite heavy around the outside, but almost nil at the inside, as that end was grounded to the core. Since I don't want to ground it again, I need to add better insulation. However, I only have about 1.5 mm spacing between the inner bar of the core and inner diameter of the winding. What's the best insulation to use that will fit? I was thinking mica, which is supposed to take at least 20 kV/mm, held together by paper and shellac.
Since there is supposed to be a small gap between the E and the I (http://geek.scorpiorising.ca/windingtransformers.html), how is the core supposed to be grounded to the chassis? Do I ground the E, the I, or both? Also, should I add electrostatic shielding (aluminum foil)?
So you took the core *entirely* apart?
😵
Um... ok. I'd interleave them (as at the bottom of Geek's page). Only carrying AC so no need of a gap. Mind that those transformers are wound very close to saturation and might need lower input voltage if you keep getting current spikes on the peaks of the primary voltage.
For the heater, I'd put it inbetween (doesn't matter really), close or far doesn't matter as the magnetic flux flows almost entirely in the core - as long as you have one turn going around the center (or two turns, one around each arm) you get the same voltage. Think of it this way, farther out from the center leg, you might get less flux, but you make up for it by using more wire (longer perimeter). Not really true but you can imagine it that way. As for turns, you can experiment or use an educated guess - count the turns on the primary (ought to be able to count layers and turns per layer, then multiply), divide by line voltage to get turns per volt and multiply by new winding volts to find turns.
Dunno about your insulation, I'd be afraid the mica wouldn't be complete (you've got some tight corners after all). I suppose you could test it with a low current 5kV source after and see if it holds.
Tim
😵
Um... ok. I'd interleave them (as at the bottom of Geek's page). Only carrying AC so no need of a gap. Mind that those transformers are wound very close to saturation and might need lower input voltage if you keep getting current spikes on the peaks of the primary voltage.
For the heater, I'd put it inbetween (doesn't matter really), close or far doesn't matter as the magnetic flux flows almost entirely in the core - as long as you have one turn going around the center (or two turns, one around each arm) you get the same voltage. Think of it this way, farther out from the center leg, you might get less flux, but you make up for it by using more wire (longer perimeter). Not really true but you can imagine it that way. As for turns, you can experiment or use an educated guess - count the turns on the primary (ought to be able to count layers and turns per layer, then multiply), divide by line voltage to get turns per volt and multiply by new winding volts to find turns.
Dunno about your insulation, I'd be afraid the mica wouldn't be complete (you've got some tight corners after all). I suppose you could test it with a low current 5kV source after and see if it holds.
Tim
No, I didn't separate all the laminations, just enough to take out the windings. The laminations do not appear to be insulated from each other. I won't leave any gap other than the lacquer coating (I coated the E and the I separately).
BTW, I would like to take this opportunity to thank you for all your help, when no one else replied! 🙂
BTW, I would like to take this opportunity to thank you for all your help, when no one else replied! 🙂
Well, I separated them all apart. 216 laminations total -- that's going to be a lot of nail polish... I'm worried that the windings won't fit due to the increased width from the nail polish. Also, the parts where I grounded out the welds are rough and may make electrical contact... sanding 216 laminations is nuts.
I had read somewhere that transformer cores should be grounded. I guess this is impossible if the laminations are insulated from one another.
I had read somewhere that transformer cores should be grounded. I guess this is impossible if the laminations are insulated from one another.
I've never re-varnished the lams... as long as they get back in the same order, the broken varnish should line up, no? Well okay, if you interleave them, it's different... so what.
In fact... as I recall, the black coating (either Parkerizing (iron phosphate) or just scale (iron (II) oxide) from heat treating) keeps them insulated adequately anyway.
It just matters that most of the lams are grounded, or if there is a faliure, there's a path for it.
Tim
In fact... as I recall, the black coating (either Parkerizing (iron phosphate) or just scale (iron (II) oxide) from heat treating) keeps them insulated adequately anyway.
It just matters that most of the lams are grounded, or if there is a faliure, there's a path for it.
Tim
But that's exactly what I'm saying, how can they be grounded if they are insulated from each other? On the other hand, he says if they are electrically connected there will be more losses with heat and buzz (and let me tell you, did that MOT buzz...)
There is no oxide coating; they are clean silicon steel, grain aligned, with a very thin layer of shellac which has mostly broken up.
The interleaving I like since I don't have to worry about keeping the E and I nicely aligned. The problem is that there's no way I can slip the windings back in, so I have to interleave them with the windings in place...
There is no oxide coating; they are clean silicon steel, grain aligned, with a very thin layer of shellac which has mostly broken up.
The interleaving I like since I don't have to worry about keeping the E and I nicely aligned. The problem is that there's no way I can slip the windings back in, so I have to interleave them with the windings in place...
BTW, the smallest mains-AC rated capacitor I found locally is about 135 uF. Is that way too big for PFC here? If it is no good, where can I source an appropriate cap, and what can I use the one I have for (I'd hate to throw it out after I spent some money on it).
Well, now that you've removed the inductance from the transformer (interleaving the core and removing the shunts, decreasing the leakage inductance), you don't need PFC any more than any other transformer does.
If you're worried about power factor, switch to choke input
Tim
If you're worried about power factor, switch to choke input
Tim
Grinding out the welds was easy. I washed the laminations in lacquer thinner, but some shellac was still left, so I had to go over with steel wool, and this was the most time consuming part. The nail polish I'm using is labeled "With Nylon" and I hope that won't be a problem. Increased the insulation on the inner side of the secondary with four layers of mica, so that I don't have to ground that end as was the case in the original construction.
An externally hosted image should be here but it was not working when we last tested it.
I got a couple of used microwave oven capacitors for $1 a piece. They are rated at 2000WVAC. WTF does the W stand for? Also, I'm wondering what they are made from. Are they electrolytic, or paper/oil?
That's the safe working voltage for that cap, similar to a safe working load limit on a rope. For dc operation, you can multiply that by 2.88 as I remember to get a safe voltage limit, but don't quote me there. Those are probably film in oil type.
Crack one open, it's just one wasted dollar right?
WVDC = Working Voltage Direct Current. WV is usually omitted because in the usual understanding of capacitor ratings, it's redundant.
Tim
WVDC = Working Voltage Direct Current. WV is usually omitted because in the usual understanding of capacitor ratings, it's redundant.
Tim
All the capacitors I found at the repair shop were different, exept for this pair. As I wanted to have a pair (if I need to use it, I'll need one for each of two channels after all), I didn't want to take one apart. In retrospect, I should have gotten one more, but I didn't think of that at the time.
Interestingly, all the MOTs there were smaller than the one I have, which makes me wonder whether I can push it more than I thought originally, as I had assumed it was from a regular 1 kW microwave. Judging by the size difference, especially when rebuilt, I'd guess over 1.5 kW.
Sch3mat1c, perhaps you can answer my question here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=41165
as no one else has yet.
Interestingly, all the MOTs there were smaller than the one I have, which makes me wonder whether I can push it more than I thought originally, as I had assumed it was from a regular 1 kW microwave. Judging by the size difference, especially when rebuilt, I'd guess over 1.5 kW.
Sch3mat1c, perhaps you can answer my question here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=41165
as no one else has yet.
How do I shield this? The large transformer is going to radiate like crazy. I don't think an aluminum enclosure will do much; I'll need steel. But that would be expensive. What if I just shield the transformer and choke, so less material is used -- how do I avoid the heat problem then?
I do plan to have a slow fan for cooling, but I'm wondering if it would be a problem to add a winding for it on the main transformer instead of using a separate one (in terms of EM noise the fan can send back through the transformer).
Finally, for for HV cabling from the supply to the speakers, I was thinking that coaxial cable with the shield ground and center conductor HV would be safest. But what about connectors? I'd like to use a single connector for everything, but the highest rated multi-pin connectors I found locally are only 600 V.
I do plan to have a slow fan for cooling, but I'm wondering if it would be a problem to add a winding for it on the main transformer instead of using a separate one (in terms of EM noise the fan can send back through the transformer).
Finally, for for HV cabling from the supply to the speakers, I was thinking that coaxial cable with the shield ground and center conductor HV would be safest. But what about connectors? I'd like to use a single connector for everything, but the highest rated multi-pin connectors I found locally are only 600 V.
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