Mot Tesla coil not working

billybob · Jul 16, 2021

I need y’all’s help here. The Tesla coil is finally complete. I had to completely reduce the spark gap design and even still it’s a little off balance and makes the primary connection disconnect (hence the tape)
My problem is sometimes I see the spark gap arcing like it should and other times it just stops. I had to replace the 20amp fuse for my variac 4 times now and I’m wondering if its just not enough for the two MOTs is there anyway I can fix this? I haven’t seen any streamers yet.
Thank you,
Ben

KeepItSimpleStupid · Jul 16, 2021

You need at least one pointed end.

That's a pic of a lightning rod. The V/m^2 density is highest at the point.

billybob · Jul 17, 2021

KeepItSimpleStupid said:
You need at least one pointed end.

View attachment 132526

That's a pic of a lightning rod. The V/m^2 density is highest at the point.

So a breakout point. That still doesn’t explain why it keeps shutting off though.

KeepItSimpleStupid · Jul 17, 2021

A sharp point for the high voltage to discharge from. I think this needs to be your first step,

See: https://www.gcegroup.com/files/tech...ccessories/Preparing_the_Electrode_Tip_EN.pdf

KeepItSimpleStupid · Jul 17, 2021

Here is something cute: https://www.amazon.com/Pack-Range-Spark-Electrode-Igniter/dp/B07V9F7T78/

KeepItSimpleStupid · Jul 17, 2021

Orientation, may prove to be important too. See this http://zeiss-campus.magnet.fsu.edu/articles/lightsources/xenonarc.html

Arc lamps are DC and I did maintain a 1000 W Xenon lamp. Fun times just to replace a light bulb. Suited up wuth thick clothing, thin gloves and a face shield. Door got shut. Do not enter posted. No fingerprints were allowed/
The lamp was pressurized with 15 atmospheres, so 15*14.7 PSI.

DerStrom8 · Jul 18, 2021

I disagree with KISS. For a properly-tuned SGTC you do NOT need a breakout point, and in fact that may cause its own problems. It could reduce your overall arc length because it causes corona leakage and does not allow enough charge to build up on the topload to form a proper streamer.

As for your popping fuses, what are you using as a ballast for your MOTs?

Regarding the spark gap not firing, there are a few things that could cause this. The first is that as the electrodes heat up, their properties change and they may become less conductive, and they may start to shift. Steel is generally not preferable for this type of application as it is a poor conductor at high frequencies and the melting point is relatively low. Tungsten is much better for use in spark gaps, though I understand it is harder to come by and more expensive. The other problem is that two MOTs will only give you about 4-6kV, which is very much on the low end for a Tesla coil. While it is definitely still doable, the spark gap must be finely tuned and cannot shift at all during use. You mention vibration, which could very well be the main cause of this issue. Additionally, I suspect your spark gap is asynchronous (not timed to match the charge/discharge cycle of the tank capacitor) and that will affect performance. If timing isn't right, the capacitor charge/discharge cycle will be disrupted and your spark gap may not fire reliably.

KeepItSimpleStupid · Jul 18, 2021

DerStrom8 Oh well!

rjenkinsgb · Jul 18, 2021

You appear to have some connections screwed directly to wood?

Dry wood can be a reasonable insulator up to a few hundred volts, but when you have possibly thousands of volts it may be causing leakage.

DerStrom8 · Jul 18, 2021

rjenkinsgb said:
You appear to have some connections screwed directly to wood?

Dry wood can be a reasonable insulator up to a few hundred volts, but when you have possibly thousands of volts it may be causing leakage.

This is a great observation. Especially if the air is particularly humid, the wood can absorb some of it and may cause some issues.

DerStrom8 · Jul 18, 2021

Here is a useful page from Terry Blake regarding rotary spark gaps:

Terry Blake Spark Gaps

billybob · Jul 21, 2021

DerStrom8 said:
I disagree with KISS. For a properly-tuned SGTC you do NOT need a breakout point, and in fact that may cause its own problems. It could reduce your overall arc length because it causes corona leakage and does not allow enough voltage to build up to form a proper streamer.

As for your popping fuses, what are you using as a ballast for your MOTs?

Regarding the spark gap not firing, there are a few things that could cause this. The first is that as the electrodes heat up, their properties change and they may become less conductive, and they may start to shift. Steel is generally not preferable for this type of application as it is a poor conductor at high frequencies and the melting point is relatively low. Tungsten is much better for use in spark gaps, though I understand it is harder to come by and more expensive. The other problem is that two MOTs will only give you about 4-6kV, which is very much on the low end for a Tesla coil. While it is definitely still doable, the spark gap must be finely tuned and cannot shift at all during use. You mention vibration, which could very well be the main cause of this issue. Additionally, I suspect your spark gap is asynchronous (not timed to match the charge/discharge cycle of the tank capacitor) and that will affect performance. If timing isn't right, the capacitor charge/discharge cycle will be disrupted and your spark gap may not fire reliably.

Thanks Matt for the input, I realize I underestimated the difficulty of MOTs. I’m using .86uf microwave oven capacitors as the ballasts. Maybe add some more? I’m not using any coils as a current limiter because I was afraid the MOTs would eat them up. I have had some problems with the variac before on even low current applications so I don’t know if that could be a problem.
You mentioned vibrations being an issue, I just can’t seem to get this spark gap right. The first one was too big and the contacts were not evenly spaced as well as the metal rod sometimes shot out. This one is at least adjustable, but yes it does rattle the whole thing.

DerStrom8 · Jul 21, 2021

One of the simplest ballast options for MOTs is to take an additional MOT, short the secondary winding to the core, and connect the primary in series with the HOT wire feeding your high voltage MOTs.

Rotary spark gaps are also not as simple as many people think, and it in fact takes a lot of thought and design in order to create a reliable one. I would definitely reconsider your angle grinder one. Maybe stick with a static gap for now? Either that or read up on RSG design and start fresh.

Best,
Matt

billybob · Jul 21, 2021

DerStrom8 said:
One of the simplest ballast options for MOTs is to take an additional MOT, short the secondary winding to the core, and connect the primary in series with the HOT wire feeding your high voltage MOTs.

Rotary spark gaps are also not as simple as many people think, and it in fact takes a lot of thought and design in order to create a reliable one. I would definitely reconsider your angle grinder one. Maybe stick with a static gap for now? Either that or read up on RSG design and start fresh.

Best,
Matt

Sounds good. Thanks a lot, you guys are keeping this thing alive XD I’ll try those suggestions.

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