Enamel on secondary breaking down?

[nzoomed]

I've noticed some discoloration on the wire and not sure if it's corrosion on the enamel or something but had happened on its own during storage.
I used it the other night and noticed that some notes don't sustain very well and depending on the frequency, I.e higher pitched notes were not playing very well or with much of an arc at times.
I cranked the thing up to full power and noticed some pin prick blue Corona all around the coil, and seems to happen around these parts of the secondary that are discolored.
I could see upon inspection that it's now burned through the polyurethane, do I'm going to have to rewind this coil again
Could this have been the issue with the coil not sounding right?
I'm wondering it is possible that arcing across the secondary behind the winding where I can't see it?
I want to use epoxy this time round. It is moisture cured floor polyurethane.
Is there an epoxy that can be applied first prior to winding that  will dry within a few minutes and help set the wire onto the pipe?
There is nothing really holding this onto the pipe and it's actually lifting a bit in places it appears.
See photo.

[davekni]

s there an epoxy that can be applied first prior to winding that  will dry within a few minutes and help set the wire onto the pipe?
There is nothing really holding this onto the pipe and it's actually lifting a bit in places it appears.

First, I hope you will get other responses if anyone has seen similar issues.

I haven't experienced this personally, but definitely worry about it, especially with different CTE (coefficient of thermal expansion) for copper and coil form/core.  Ideal would be to coat coil under vacuum for full fill under and above winding.  I've considered whether it would be possible to coat under winding as you mention.  However, I'd think the ideal undercoat would not cure until after coil is wound.

Out of concern for delamination and/or corona under secondary windings, here's what I did:  Wind coil without undercoat (as can't think of a good undercoat option).  Spray on urethane enamel clear coat and wait for it to dry.  Repeat once more, waiting even longer for thorough drying.  Then apply several layers of 2-part urethane epoxy.  The spray-on enamel starts out thin due to solvents, so penetrates under windings.  Goal is to fill under/between windings that way before applying the thicker epoxy layers.

Good luck!

[MRMILSTAR]

I have used brush-on polyurethane on both of my Tesla coils. One is a large SGTC with a 8.6" diameter secondary coil. The other is a medium-sized VTTC. I apply the polyurethane as the coil is continuously turned. I apply 3 or 4 coats allowing each coat to dry before applying the next. Both of these coils were completed several years ago and have experienced temperature extremes from 40 degrees F to about 90 degrees F. I have never experienced any problem with the coils becoming loose.

[nzoomed]

s there an epoxy that can be applied first prior to winding that  will dry within a few minutes and help set the wire onto the pipe?
There is nothing really holding this onto the pipe and it's actually lifting a bit in places it appears.

First, I hope you will get other responses if anyone has seen similar issues.

I haven't experienced this personally, but definitely worry about it, especially with different CTE (coefficient of thermal expansion) for copper and coil form/core.  Ideal would be to coat coil under vacuum for full fill under and above winding.  I've considered whether it would be possible to coat under winding as you mention.  However, I'd think the ideal undercoat would not cure until after coil is wound.

Out of concern for delamination and/or corona under secondary windings, here's what I did:  Wind coil without undercoat (as can't think of a good undercoat option).  Spray on urethane enamel clear coat and wait for it to dry.  Repeat once more, waiting even longer for thorough drying.  Then apply several layers of 2-part urethane epoxy.  The spray-on enamel starts out thin due to solvents, so penetrates under windings.  Goal is to fill under/between windings that way before applying the thicker epoxy layers.

Good luck!

I'm pretty sure there are fast curing epoxy resins out there.
Even if I have to mix in stages, it will probably be ok.
For whatever reason, I was advised against epoxy at the time and told to go with polyurethane.
However I've seen commercially made coils advertised with epoxy coating.

My main concerns are whether any solvents or other products  applied could potentially damage the enamel.
I'm still not sure what happened to my coil  whether or not it was damaged on the spool and moisture got in, or else if a chemical reaction happened.
I guess I should be able to see if arcing has occurred when I come to rewind and remove the old wire, there should be some marks on the pipe.
When I operated the coil I also noticed that the streamers were quite whispy and fuzzy looking, not clean lines like I typically see. Is this a symptom of a damaged secondary, or could something else be at fault?

[davekni]

For whatever reason, I was advised against epoxy at the time and told to go with polyurethane.

Polyurethane epoxy is available.  That's what I used after the two spray coats.  Common urethane epoxy is intended for coating wood table tops etc.

When I operated the coil I also noticed that the streamers were quite whispy and fuzzy looking, not clean lines like I typically see. Is this a symptom of a damaged secondary, or could something else be at fault?

Could be either.  Might be worth scoping primary waveforms with existing secondary coil before repair.  See if they look normal, at expected frequency and with resonant energy transfer primary to secondary and back (at least when below arc voltage to minimize energy loss to arc).

[nzoomed]

For whatever reason, I was advised against epoxy at the time and told to go with polyurethane.

Polyurethane epoxy is available.  That's what I used after the two spray coats.  Common urethane epoxy is intended for coating wood table tops etc.

When I operated the coil I also noticed that the streamers were quite whispy and fuzzy looking, not clean lines like I typically see. Is this a symptom of a damaged secondary, or could something else be at fault?

Could be either.  Might be worth scoping primary waveforms with existing secondary coil before repair.  See if they look normal, at expected frequency and with resonant energy transfer primary to secondary and back (at least when below arc voltage to minimize energy loss to arc).

This is the stuff I used, at the time it seemed a good product.
https://www.uroxsys.co.nz/pdfs/MCRGlossdata.pdf
Is the epoxy polyurethane you refer to a two part mix or single  pack?
I know people who own a paint shop and can get me an endless selection of products.
There is a clear resin I've seen that goes rock hard.
I was thinking another option may be to apply a thinned down coat that can penetrate the winding easier, and then apply thicker coats on top. This stuff I've got is rather soft and flexible, but hard wearing as its for floors.
It doesn't stick to the pvc pipe very well, I can peel it off.
My main concern is if my issues are IGBT damage, but i don't think they fail like that do they? Seems to only be giving issues since the coil has broken down.
Will be interesting to scope the bus again, I know I had it tuned fairly well at the time.

[davekni]

Is the epoxy polyurethane you refer to a two part mix or single  pack?

Epoxy is two part.  Spray-on layers are of course single part.
Recent coils use this spray-on:
    Rust-Oleum Painter's Touch 2x Ultra Cover Matte Clear.
It claims UV resistance and bonding to most plastics.  Purchased at local home improvement store.  Don't recall if my DRSSTC used this or an older version.
Epoxy (two parts in equal volume) for my larger standard DRSSTC is:
    Parks Super Glaze Pour-on Finish & Preservative, Ultra Gloss Epoxy
It is not UV resistant and cures softer than most epoxies.  It is also made by Rust-Oleum.  I'm not sure if the softer cure is better for handling thermal expansion or not.
My more recent QCW secondary used:
    Max 1618 Crystal Clear Low Viscosity
It is a 2:1 mix ratio and cures hard.  So far no issues, but this coil hasn't been pushed hard yet.

My main concern is if my issues are IGBT damage, but i don't think they fail like that do they?

Probably not.  More wondering about some issue in control electronics.  If one side of a full H-bridge failed, could cause poor performance, but I can't think of any reason it would be correlated to secondary discoloring.

[nzoomed]

Is the epoxy polyurethane you refer to a two part mix or single  pack?

Epoxy is two part.  Spray-on layers are of course single part.
Recent coils use this spray-on:
    Rust-Oleum Painter's Touch 2x Ultra Cover Matte Clear.
It claims UV resistance and bonding to most plastics.  Purchased at local home improvement store.  Don't recall if my DRSSTC used this or an older version.
Epoxy (two parts in equal volume) for my larger standard DRSSTC is:
    Parks Super Glaze Pour-on Finish & Preservative, Ultra Gloss Epoxy
It is not UV resistant and cures softer than most epoxies.  It is also made by Rust-Oleum.  I'm not sure if the softer cure is better for handling thermal expansion or not.
My more recent QCW secondary used:
    Max 1618 Crystal Clear Low Viscosity
It is a 2:1 mix ratio and cures hard.  So far no issues, but this coil hasn't been pushed hard yet.

My main concern is if my issues are IGBT damage, but i don't think they fail like that do they?

Probably not.  More wondering about some issue in control electronics.  If one side of a full H-bridge failed, could cause poor performance, but I can't think of any reason it would be correlated to secondary discoloring.

There are single pack epoxy resins out there, but they use various different means to cure, i.e UV light.
I also haven't heard of epoxy polyurethane before, I thought they were two entirely different things. You will probably find that such products are a mixture of both perhaps?
I will do research into this anyway, there are ton of different resins on the market, but I don't think the ones you mention are readily available in NZ.
Fibreglass resins go yellow over time and actually break down rather bad.
But I have seen resins that stay Crystal Clear and go rock hard, that's the sort of stuff I'm looking for.
Is thermal expansion something that a coil can experience under use? I've never measured the temperature of mine during use, bit I've got an infra red thermometer.

It did cross my mind if control electronics could be partly to blame. My driver is loneoceans universal driver v2.7
One of the main issues I've had is shielding the computer and/or USB midi adapter.
The RF causes it to lock up and hang on a note.
Sometimes it's not quite bad enough to hang the unit, but it's causing some notes to sound like garbage at times.

I dont know if I need a longer fibre optic cable or try shielding the computer better.
I've got toroids on the USB cable that help, but it's not perfect and can't play music at full power.
Maybe a better midi adapter could help?

[davekni]

I also haven't heard of epoxy polyurethane before, I thought they were two entirely different things.

After some google searching, I am probably wrong here.  My apparently-incorrect memory was that Parks glaze coat was a urethane based epoxy and that such was common.  I find nothing now supporting that memory.

It did cross my mind if control electronics could be partly to blame. My driver is loneoceans universal driver v2.7
One of the main issues I've had is shielding the computer and/or USB midi adapter.
The RF causes it to lock up and hang on a note.
Sometimes it's not quite bad enough to hang the unit, but it's causing some notes to sound like garbage at times.

I dont know if I need a longer fibre optic cable or try shielding the computer better.
I've got toroids on the USB cable that help, but it's not perfect and can't play music at full power.
Maybe a better midi adapter could help?

I've also had trouble with MIDI notes getting stuck on.  It isn't a hang of the electronics, but rather a disruption at the point when the key-release code is transmitted.  Playing the same note again will unstick the note on that release.  In my case, MIDI is coming from a keyboard (synthesizer) for live music.  It is difficult to determine if the disruption is within keyboard (MIDI source), communication link, or MIDI interpreter.  I ended up with grounded metal under keyboard with foil extending around back side and over half of top.  Placed MIDI interrupter within that shielding and powered both from batteries also within shielding.  Probably not all necessary.  Seems that the keyboard itself is most sensitive.  It is a large object with plastic housing.

What MIDI interpreter are you using?  Being in a hurry back in 2019, I purchased a kit from EVR.  It is not very good.  Some notes are up to 2% off in frequency.  Worse, more than two notes playing simultaneously outputs garbage.  Made my own soon after.

[nzoomed]

I also haven't heard of epoxy polyurethane before, I thought they were two entirely different things.

After some google searching, I am probably wrong here.  My apparently-incorrect memory was that Parks glaze coat was a urethane based epoxy and that such was common.  I find nothing now supporting that memory.

It did cross my mind if control electronics could be partly to blame. My driver is loneoceans universal driver v2.7
One of the main issues I've had is shielding the computer and/or USB midi adapter.
The RF causes it to lock up and hang on a note.
Sometimes it's not quite bad enough to hang the unit, but it's causing some notes to sound like garbage at times.

I dont know if I need a longer fibre optic cable or try shielding the computer better.
I've got toroids on the USB cable that help, but it's not perfect and can't play music at full power.
Maybe a better midi adapter could help?

I've also had trouble with MIDI notes getting stuck on.  It isn't a hang of the electronics, but rather a disruption at the point when the key-release code is transmitted.  Playing the same note again will unstick the note on that release.  In my case, MIDI is coming from a keyboard (synthesizer) for live music.  It is difficult to determine if the disruption is within keyboard (MIDI source), communication link, or MIDI interpreter.  I ended up with grounded metal under keyboard with foil extending around back side and over half of top.  Placed MIDI interrupter within that shielding and powered both from batteries also within shielding.  Probably not all necessary.  Seems that the keyboard itself is most sensitive.  It is a large object with plastic housing.

What MIDI interpreter are you using?  Being in a hurry back in 2019, I purchased a kit from EVR.  It is not very good.  Some notes are up to 2% off in frequency.  Worse, more than two notes playing simultaneously outputs garbage.  Made my own soon after.

With my unit, the USB MIDI device would stop working and stay stuck on a note forever.
I have to disconnect it from the USB port to reset the thing or else it wont work at all again, seems to be causing it to lock up or something, but either way, tie computer stops communicating with it because starting another track does nothing to help it.
I have wanted to try it out with a keyboard over a MIDI cable but dont know how i set the channel on my keyboard to the correct one the MIDI controller is using.

My MIDI controller is called a "mini MIDI controller" see here:
https://sites.google.com/site/teslacoilstore/mmtcc
They seemed to be popular at the time I built my coil. I was also advised early on to add the toroids to the MIDI cable, as i was having issues right from get go. They do help, but its not a solution to remove it all, perhaps i need a better shielded box, tin foil is doing nothing.
Im interested to see what these new MIDI interrupters are like that will work with more than 2 notes at a time.
Great to see technology improving.

[nzoomed]

I'm looking at using epoxy doming resin on the rewind.
Does this sound suitable?
It looks like it goes rock hard, but has a long cure time, however I see UV curable ones on the market too but don't know what the quality is like.

[davekni]

I'm looking at using epoxy doming resin on the rewind.
Does this sound suitable?

Probably fine, at least for outer coats.  Unlikely to penetrate between windings to fill between windings and core tube.  Something with much lower viscosity and lower surface tension would be better for penetration.  Need for penetration depends on your secondary volts/turn  I don't have enough experience to say if penetration is needed, not even for my own coils.

I see UV curable ones on the market too but don't know what the quality is like.

There are fine UV epoxies, even ones hard enough to use for dental fillings.  I have no way to tell quality of any specific UV epoxy.  UV epoxies require a minimum light intensity for curing.  Low intensity can't be compensated for completely with long exposure times.  UV cured epoxy might be a good option for coating core before winding.  Will remain uncured plenty long for winding.  Add another coat after winding and expose to UV.  Cure is a chain reaction to some extent (depending on epoxy formulation).  I think epoxy under wire is likely to cure due to exposure of upper layer to UV even though not receiving any direct exposure.  Would need to be tested for any given UV-cure epoxy, however, or discerned from data sheet if available.

[nzoomed]

I'm looking at using epoxy doming resin on the rewind.
Does this sound suitable?

Probably fine, at least for outer coats.  Unlikely to penetrate between windings to fill between windings and core tube.  Something with much lower viscosity and lower surface tension would be better for penetration.  Need for penetration depends on your secondary volts/turn  I don't have enough experience to say if penetration is needed, not even for my own coils.

I see UV curable ones on the market too but don't know what the quality is like.

There are fine UV epoxies, even ones hard enough to use for dental fillings.  I have no way to tell quality of any specific UV epoxy.  UV epoxies require a minimum light intensity for curing.  Low intensity can't be compensated for completely with long exposure times.  UV cured epoxy might be a good option for coating core before winding.  Will remain uncured plenty long for winding.  Add another coat after winding and expose to UV.  Cure is a chain reaction to some extent (depending on epoxy formulation).  I think epoxy under wire is likely to cure due to exposure of upper layer to UV even though not receiving any direct exposure.  Would need to be tested for any given UV-cure epoxy, however, or discerned from data sheet if available.

I think that it would be ideal for the resin to penetrate the windings, even if it had a lower viscosity, i feel it would have trouble doing so. Vaccuum impregnation would be the ultimate but very difficult and expensive for a coil this size.
My main concern was how the coil was lifting off the tube. It was almost like it had either overheated or expanded? Im not sure if that was the issue or if the wire was wound on too loose?

Yes UV curing resin would have the big issue of not getting enough light under the winding itself to cure, however it does go off on its own if given enough time, and a chain reaction should happen if only a handful of molecules get exposed to the UV or get in contact with the outer layer when exposed. Might be worth exploring this further.

Not too sure what others do with their coils, but definitely going with epoxy this time round.

[Twospoons]

UV cured resin also suffers from oxygen inhibition at the surface.  This is overcome with either very intense UV light, or inert gas (N2 and CO2 are good enough).
Otherwise you end up with a sticky surface.

Personally I'd stay with some form of addition cure - epoxy or polyester. You can always accelerate epoxy cure with heat. And you can accelerate polyester cure by adding more catalyst.  Polyester is also nice in that it will gel well before cure, whereas epoxy just gets more viscous as it cures.

[orac]

I had a similar issue. I could not see it on the face of the coil like yours with the discoloration. But after checking everything and finally deciding to strip the secondary in chase of trying to find a problem of degraded performance, I found the secondary had been arcing behind the coil and along the PVC tube.

When this was constructed it was first sprayed with spray on urethane, wound, and then multiple layers of the same spray to fill any gaps over the coil. Then finally coated in a 2 pot epoxy.

Unfortunately I will need to rebuild the coil.

[davekni]

When this was constructed it was first sprayed with spray on urethane, wound, and then multiple layers of the same spray to fill any gaps over the coil. Then finally coated in a 2 pot epoxy.

Unfortunately I will need to rebuild the coil.

This is almost exactly how I built my DRSSTC secondary.  (Except for later addition of several more layers of epoxy to reduce racing sparks.)  So far it has worked fine, 4 years of occasional use.  Perhaps depends on particular spray on urethane, on wire gauge, or some other factor.  My secondary is 24AWG.

Good luck with rebuild.  Not sure what to suggest as being better.

[nzoomed]

Well it's been a while but I thought I would provide an update.
I am still yet to rewind the coil and want to rig up a motor to keep the coil turning while it cures.
I've been given some astro 6250 which I'm going to use here.
I'm not sure how many people have used this on their coils, but I see a link to the datasheet on this forum buy can't find any threads referencing it.
It's supposed to be very thin and penetrate windings well and is designed for this purpose.

Edit:
Just finished rewinding it today and I shifted the winding further up a bit this time due to me experiencing a bit of flash over when I originally used it a while back although might have been due to a bad earth on the water pipe.
Is this too close from the top edge?
I had been mounting my toroid directly on top of the pipe itself.
Is there any advantage in raising it and sitting on an extra spacer?
Will this affect the resonant frequency should I raise it further?

I'm also interested what people cover the ends of the winding with. I've used pvc electrical tape over each end after coating the coil and then applying another coat over the tape.
Is this how most people do it?

[Benjamin Lockhart]

Hey nzoomed,

No the winding is not too close to the top edge.

I recommend raising the toroid up above the top turn at least half the secondary diameter to the bottom of the toroid, this reduces induction heating losses in the toroid and lowers your resonant frequency a little bit.
You don't want the toroid acting as a shorted turn to the upper part of the secondary, it reduces the secondary inductance.

I glue down the winding ends with a little bit of superglue or hot glue, and don't use any tape. Then the coating holds everything in place.

[nzoomed]

Hey nzoomed,

No the winding is not too close to the top edge.

I recommend raising the toroid up above the top turn at least half the secondary diameter to the bottom of the toroid, this reduces induction heating losses in the toroid and lowers your resonant frequency a little bit.
You don't want the toroid acting as a shorted turn to the upper part of the secondary, it reduces the secondary inductance.

I glue down the winding ends with a little bit of superglue or hot glue, and don't use any tape. Then the coating holds everything in place.

OK, thats good to know.
I might add a few extra turns on top of  and take some off the bottom end instead, I think people just add the tape to make it look more tidy than anything else.
I should be able to 3D print some sort of riser to allow me to move it up a little.
The biggest problem im facing now is that the resonant frequency ended up being a bit lower than I expected it to be due to the toroid coming out at a larger size than i intended because the ducting squashed out more than I expected it to.

As a result I have no more room to tune my primary coil, its already tapped at almost at 10 turns as it stands but overall a lower resonant frequency is desirable for the IGBT's.
I also need to dig up my old notes and go through the figures again in JavaTC because its been a very long time, the best part of 10 years.
Anyway, ive rewound this back to the original spec of 725mm in length.

When the primary had the flashover 8 years ago, i just simply ran it with the damaged winding removed as i could not face rewinding it and the coil was running quite well until the enamel broke down.
I will go over the specs and post them here when I have some time anyway to see what you think.

[Benjamin Lockhart]

It sounds like a slightly larger MMC would fix the tuning range. What's your MMC value?

I think lower primary impedance would probably be beneficial to the spark length as well.

I think your coil is very similar to my DRSSTC3 and Loneoceans DRSSTC3 right?
Mine is 60kHz, 0.365uF MMC, the primary is tapped at 7-ish turns and it does 2.5 meter sparks, with 800A OCD setting.