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Twin MOT high power supply - advise needed

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GrantX

Mon Oct 17 2011, 10:02AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Hey, I've been fiddling with a bunch of MOTs recently, and after pushing a single MOT to the end of its life with too much current and too much capacitance, I've decided to take things to the next level.

I now have 2 MOTs of similar dimensions and currently have them in parallel (secondary's in series), and even without any capacitors the arcs are huge. However, there is a, ahem, small heat issue. I'm going to push the twin setup to resonance soon, but I'm certain the transformers will meltdown instantly unless something is done about cooling.

I was thinking of submerging them in oil, do you think that will be sufficient for short 20kVA runs? And if I was using 5uF to resonate a single MOT, then I'm guessing I'll now need 10uF? (rated for 4kV instead of 2kV, as well)

Ash Small

Mon Oct 17 2011, 01:05PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Oil cooling is generally the best way to go, unless you want to go down the cryogenic route.

I think you'll need 2.5uF for resonance (if you double the inductance you want half the capacitance), but don't take my word for it.

Inducktion

Mon Oct 17 2011, 04:01PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

You could try to add more primary turns to take the MOT's out of saturation a bit. That's one of the main reasons why they get so dang toasty.

Kiwihvguy

Tue Oct 18 2011, 12:07AM

Registered Member #3395 Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193

Inducktion wrote ...

You could try to add more primary turns to take the MOT's out of saturation a bit. That's one of the main reasons why they get so dang toasty.

Adding more primary turns would be very diificult because there is no spare space unless you tap the shunts out and even then that is not the best alternative because the primary may get scratched in the process.

Ravenor, oil cooling would be a good option for insulation and heat dissipation. Just don't use silicone oil because it has a very high thermal impedance.

M.A.D.

Tue Oct 18 2011, 01:48AM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

Hello I was wondering how many turns do you need to add to the primary to take it out of saturation.

I have a MOT that has been cut apart, shunts removed, and I am going to use it as a high current step down transformer. However I have the same problem in that it uses WAY to muuch power and produces WAY to much heat just sitting there, doing absolutly nothing useful at all.

Dr. Slack

Tue Oct 18 2011, 07:08AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Don't bother with adding primary turns in the first instance. Although saturation current is large at no-load, upsetting M.A.D. below, it's no larger on full load. As it's in quadrature to the load current, it doesn't add a lot to the total on-load dissipation. It does add some though, so once you've improved cooling, try extra primary turns as the next step.

GrantX

Tue Oct 18 2011, 01:26PM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

The windings are pretty tight on my MOTs, I think it'd be a struggle to even fit the wire through the gap. I'm sold on oil immersion, seems to be the easiest and most efficient method,

So I will only need 2.5 uF for resonance (or near enough)? I thought I needed more capacitance to counteract the increased inductance? I have no way to measure inductance, so I can't do the calculations.

I'm also planning a new ballast, which will be a 3 ohm grid made of 2 10 ohm and 2 15ohm resisters in parallel mounted on a heat sink. I'm worried because the grid is only 400W, and on 240V it will be passing 19.2kVA. So I guess it'll melt into a pile of slag in about 5 seconds, correct? Is there any other cheap method for a high power ballast? And more importantly, can the 2 MOTs in oil even handle nearly 20kVA in short bursts?

I'm starting to think this whole project might end up as a puddle of molten metal :D

Also, just for fun, here's a video of the single MOT at resonance shortly before its death:

Ash Small

Tue Oct 18 2011, 02:50PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I was under the impression that the standard (~1uF) MOC was pretty near the resonant value (maybe ~0.8uF, from what I'd read), so two in series would be about right, and also be good for 4kV, but if you've obtained good results with 5uF, maybe you should try 2.5uF.

1/2Î âˆšLC gives resonant frequency, so if you double L you need to halve C for the same resonant frequency

Nah

Tue Oct 18 2011, 11:42PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

How about you use a coil of al wire as a ballest?

GrantX

Wed Oct 19 2011, 02:48AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Ash Small wrote ...

I was under the impression that the standard (~1uF) MOC was pretty near the resonant value (maybe ~0.8uF, from what I'd read), so two in series would be about right, and also be good for 4kV, but if you've obtained good results with 5uF, maybe you should try 2.5uF.

1/2Î âˆšLC gives resonant frequency, so if you double L you need to halve C for the same resonant frequency

Ah, sorry, it was late at night and all my equations got messed up :D

Hmm, I tried 1 MOC with the single transformer but only got little sparks like a battery shorting out. Could the inductive choke on the primary side be messing with the resonance somehow? I had resonance with 3uF, but 5uF made the arc a lot hotter (but no longer).

@Nah: Intriguing idea, but how would I get the inductance just right to give 20kVA? Just keep adding turns and measuring current with a clamp meter? Would the current flow change if I use it to power bigger or smaller transformers?
Sorry, I'm a little hazy on how inductive ballasts interact with their loads.

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