Question about inductively ballasting mots and general advice

[lucretius]

Im building a tesla coil. Id use a NST but they dont seem to easy to get here in the uk. So ill be using mots.

My coil will be a 10cm x 50cm so approximately a 4 inch coil and 8KV (4 mots) 1000W

ill use 4 mots to give me 8kv and ive got tesla map to figure out roughly the right capacitance for the mmc and some other things.
What i realised from that is that if i up the current output ill need significantly higher capacitance for the mmc. So id like the max current from the secondary to be around 125ma.

Im still in the process of learning and understanding inductive reactance properly. And im confused by the mots hooked up and what their resistance/reactance would be or what determines how much current they draw.

So I need to know what inductance id need for the ballast so that the secondary draws no more than 125ma and the primary draws no more than (1000/230) which is about 4.35A. Or perhaps an idea on how id go about working this out.

I also realise with the ballasting that the 230 input will be split between the ballast and the mots so the mots wont have 230V but a fraction. With the inductive ballasts no power will be lost but i think the voltage is still split just the phase is altered at the same time. so how is this counteracted. or am i wrong and with inductors something different happens.

 

[DerStrom8]

Well, there's really only one thing to say: DO NOT USE MOTs FOR TESLA COILS! Their insulation is extremely poor and are a fire hazard, as well as being extremely unstable and susceptible to high frequency spikes. Your best bet is to keep looking for an NST (look on e-bay, for example) or an oil burner ignition coil (a.k.a. an "OBIT"). But MOTs are really a VERY poor choice for TCs and I can't stress enough that you should stay away from them. Keep your eyes open for a better option.

Regards,
Matt
(Fellow Tesla Coil Enthusiast)

 

[lucretius]

Well, there's really only one thing to say: DO NOT USE MOTs FOR TESLA COILS! Their insulation is extremely poor and are a fire hazard, as well as being extremely unstable and susceptible to high frequency spikes. Your best bet is to keep looking for an NST (look on e-bay, for example) or an oil burner ignition coil (a.k.a. an "OBIT"). But MOTs are really a VERY poor choice for TCs and I can't stress enough that you should stay away from them. Keep your eyes open for a better option.

Regards,
Matt
(Fellow Tesla Coil Enthusiast)

Id love to just use a nst but they're expensive and the great cheap ones ideal for this from the states run at 120v and 60HZ which i wouldnt know how to get around. The obits seem to be quite low power. 10KV at 25ma 250W.

With the mots i dont feel like i should have a problem with the low powers i want to run them at. they'd barely be over thier rated powers anyway. And i plan to do everything outside with a dedicated circuit breaker with a line filter. maybe id think about a terry filter if the spikes are something thatd be a problem. But i want a good medium sized tesla coil. and the mots seem like my only option.

 

[DerStrom8]

Id love to just use a nst but they're expensive and the great cheap ones ideal for this from the states run at 120v and 60HZ which i wouldnt know how to get around. The obits seem to be quite low power. 10KV at 25ma 250W.

With the mots i dont feel like i should have a problem with the low powers i want to run them at. they'd barely be over thier rated powers anyway. And i plan to do everything outside with a dedicated circuit breaker with a line filter. maybe id think about a terry filter if the spikes are something thatd be a problem. But i want a good medium sized tesla coil. and the mots seem like my only option.

Hi lucretius,

I know exactly where you are coming from (I had the same problem when I built my 4" SGTC) but there are a few things I'd like to point out:

1) 1kW is not "low power" for a SGTC the size of yours. 250W would probably be sufficient to drive a 4" x 20" coil, like you suggest, though admittedly 500W would be better.
2) OBITs come in many different flavors. I happen to have one rated 14.5kV (or maybe it's 17.5kV...?) and 45mA, so look around a bit more.
3) The insulation on any given MOT is usually rated for around 2kV (not much more than it's supposed to put out). You are planning to run it at 4x that (assuming you're wiring the outputs in series, which would be required in order to achieve 8kV). I would hardly consider that to be "barely over their rated powers".
4) In order to make MOTs work in a Tesla Coil application, you would need to submerge them, as well as the ballasts, in a thick transformer oil bath to prevent internal arcing, shorting, and overheating. This can be very messy and expensive, and difficult to set up or move.
5) Yes, you should use a filter regardless of the transformer you choose. But trust me, it's well worth looking for better options.

I'm more than happy to help out another Tesla Coil enthusiast, but you'll have to believe me that, unless you want to spend a lot of extra time and money for safety and protection, MOTs are not the way to go.

Regards,
Matt

 

[lucretius]

thanks for the detailed explanation, its really appreciated.

you make sense and the secondarys would be at 8kv and that does sound like a bit over. and using a voltage doubler sounds like a lot of trouble but i dont know how they work so they may have the same problem.

so heres are my questions now.
1) what if i bought a US mains NST designed to run at 120v and 60hz and used a variac to give it 120. would the 50hz instead of 60 make a difference ?
2) how isit some people seem to get away with using the mots without the submersion and all, ive found a few coils on youtube that use multiple mot's and dont seem to have those kinds of precautions ?


edit: ive found that the mots can be wired antiparrell so that a potential of +4000 and -4000 is achieved rather than +8000 and 0, it sounds like when they're connected that there'd still be a difference of 8KV but i read a comment that this would help with the insulation problem.

 

[DerStrom8]

thanks for the detailed explanation, its really appreciated.

you make sense and the secondarys would be at 8kv and that does sound like a bit over. and using a voltage doubler sounds like a lot of trouble but i dont know how they work so they may have the same problem.

so heres are my questions now.
1) what if i bought a US mains NST designed to run at 120v and 60hz and used a variac to give it 120. would the 50hz instead of 60 make a difference ?
2) how isit some people seem to get away with using the mots without the submersion and all, ive found a few coils on youtube that use multiple mot's and dont seem to have those kinds of precautions ?

You could definitely use a Variac, and I was actually going to bring that up in my previous post but forgot--guess my tired brain is taking its toll The 50/60 Hz shouldn't make any difference at all, but make sure you buy the heavy iron-core type, not the newer solid state type. The solid-state type uses an inverter at much higher frequencies and will definitely not work for a Tesla coil.

Some people build Tesla Coils with MOTs, but in most cases they burn out quickly, you can't run them for very long, and they pose a serious fire risk. People who build MOT TCs and don't submerge the transformers in oil are generally inexperienced and don't understand the dangers that are involved.

If you can get a 120V NST and drive it with a Variac, that would definitely be your best bet.

Regards,
Matt

 

[DerStrom8]

It occurred to me that, if it does turn out to be impossible to get an NST or OBIT (doubtful), then you could wire two of your MOTs like an NST by connecting the cores together and grounding them, and connecting your primaries in parallel with opposite phasing. This way you would get around 4kV between the two outputs of the MOTs, but each secondary is only putting out its rated 2kV with respect to ground. You would still need a ballast, which could probably be created by taking a third transformer, shorting the secondary to the core, and connecting the primary in series with the line to the other transformer primaries. It's difficult to predict the exact current output because all transformers are different, but it's a good general way to ballast it. But don't worry about this unless you have searched for NSTs and OBITs unsuccessfully.

 

[lucretius]

It occurred to me that, if it does turn out to be impossible to get an NST or OBIT (doubtful), then you could wire two of your MOTs like an NST by connecting the cores together and grounding them, and connecting your primaries in parallel with opposite phasing. This way you would get around 4kV between the two outputs of the MOTs, but each secondary is only putting out its rated 2kV with respect to ground. You would still need a ballast, which could probably be created by taking a third transformer, shorting the secondary to the core, and connecting the primary in series with the line to the other transformer primaries. It's difficult to predict the exact current output because all transformers are different, but it's a good general way to ballast it. But don't worry about this unless you have searched for NSTs and OBITs unsuccessfully.

I want to listen to your advice but having looked at the ebay i once again feel as though the mots are my only option. Do bear with me, I really do appreciate the help.

Things dont seem to be as easily available here in the uk as they may be in the States its quite annoying. A US NST imported would cost me around £50 if i find a good deal. The cheapest 5Amp variable seems to be at least £100 lets say i get a really good deal and get one for £50. thats £100 ($168). The decent looking OBITs i found on ebay were once again around £50 imported from the USA so ill need the variac again.

Now going the MOT route, i can get a microwave for £5 £10 max. so if i buy 5 thats £25. Lets say i even submerge them in mineral oil (assuming that it would work) i could buy 2L of it for £20. So i spend £45 in total compared to £100 which would most likely be £150 realistically. even if the MOTs burn out i could cheaply replace them. Im 18 and a student so im hugeely stretched on the money as it is.


Also im confused as to how exactly the MOTs are wired. With reference to the diagram below, i feel like if i connected 2&3 that across 1 & 4 id have 0 volts. (5) being the mains ground and (A) & (B) being the casings, please tell me how id wire up the secondaries.

Edit: Also do you think 4KV would be enough for a tesla coil ? I most likely will have a static spark gap. Im ready to make submerged MOT tranformers for 8KV if itd work.
The only reason im being stuburn with the MOTs is because with them i could actually make one in the very near future rather than look for the ideal components for months and eventully loose interest.

 

[DerStrom8]

I understand your predicament, budget-wise, and I can tell you have done quite a bit of searching for NSTs. Fair enough, let's look at using two MOTs:

Your diagram appears correct, except that (2) is connected to the core of (A) internally, and (3) is connected to the core of (B) internally already. You will need to connect the cores together, and then connect them to ground to get a center-grounded transformer with two hot secondaries that are 180 degrees out of phase (notice the phasing dots on the transformer cores):

As you can see, on each secondary you have only around 2KV with respect to ground. However, due to the phasing, when one secondary is +2kV, the other secondary is at -2KV, which gives you 4KV between them.

Now, there isn't really a way to tell the phasing just by looking at the transformers, so you may need to try it one way, and if you don't get a very good output then you'll want to reverse the connections on one of the primaries and try again.

As for the ballast, just connect a third MOT as I suggested earlier by shorting the secondary and wiring the primary in series with the 230V HOT line.

4kV should be enough for a Tesla coil, considering the very high current. You will need to extinguish the spark gap somehow, whether you use a rotary spark gap or if you blow air through a static gap. If you don't ensure the spark is extinguished, you will get very poor performance.

And finally, I'm sure I don't need to tell you this, but I feel I must post it for anyone else who may read this thread in the future and attempt this on their own:

WARNING: Microwave oven transformers are EXTREMELY DANGEROUS and will kill you instantly. There are NO SECOND CHANCES if you accidentally come in contact with any live parts of the circuit. Make sure you take ALL necessary precautions to protect yourself, family/friends, pets, and property. Make sure you have a fire extinguisher nearby, and ALWAYS have someone watching your back when working with these transformers, preferably someone who knows CPR. Using MOTs is STRONGLY DISCOURAGED for many reasons, so only use them if you are left with absolutely no other option.

 

[lucretius]

Ah that all makes sense. do you reckon its worth the extra effort to go for a Rotary spark gap. Bear in mind that if you say yes then id be bothering you with the following question:
I understand the fireing of the static spark gap is just when the voltage across the mmc is high enough. and with a rotary we can control the frequency of the fireing and keep it regular. But i dont know what frequency or what rate is right or how i would determine it.

And Thank you for all your help

 

[DerStrom8]

Ah that all makes sense. do you reckon its worth the extra effort to go for a Rotary spark gap. Bear in mind that if you say yes then id be bothering you with the following question:
I understand the fireing of the static spark gap is just when the voltage across the mmc is high enough. and with a rotary we can control the frequency of the fireing and keep it regular. But i dont know what frequency or what rate is right or how i would determine it.

And Thank you for all your help

I'm glad to help, lucretius!

It would definitely be worth trying for a rotary spark gap. As for the frequency/rate of fire, I suggest you look up "asynchronous-" and "synchronous rotary spark gap". It will give you an idea of what you'll want to do. In general, you want to charge the MMC for half of the 50Hz cycle and discharge it during the second half. By changing the speed of the rotary spark gap, you can optimize it so that the MMC can charge as much as possible during its half cycle and discharge as much as possible during the other half cycle. As you can see, there is a lot more to a rotary spark gap than simply firing it more regularly. You can think of it simply as a spark gap that you can adjust as it's running.

Regards,
Matt

 

[throbscottle]

Lucretius, if you do end up getting US transformers, the cheapest way to get 110v is to get a used "site transformer" - the sort they use on building sites. Often seen on eBay for 99p - you have to be able to collect though, they are very heavy. Often they are 1kW devices. They are designated as "CTE" - centre tap earth - so each side is only 55v above ground. You might be really lucky and get a cheap variac, but you just don't get the juice. The other route I would suggest is an auto-transformer - a lot cheaper than a normal transformer, but again, limited juice.

Oh yeah, also, get yourself on Freegle / Freecycle - you might well get old microwaves for nothing if you stick with the MOT route.

HTH, good luck!

 

[lucretius]

Lucretius, if you do end up getting US transformers, the cheapest way to get 110v is to get a used "site transformer" - the sort they use on building sites. Often seen on eBay for 99p - you have to be able to collect though, they are very heavy. Often they are 1kW devices. They are designated as "CTE" - centre tap earth - so each side is only 55v above ground. You might be really lucky and get a cheap variac, but you just don't get the juice. The other route I would suggest is an auto-transformer - a lot cheaper than a normal transformer, but again, limited juice.

Oh yeah, also, get yourself on Freegle / Freecycle - you might well get old microwaves for nothing if you stick with the MOT route.

HTH, good luck!

Ah i will keep checking from time to time to find one of those. And freegle and freecycle seem great.

Thanks