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MOT Saturable Core Reactor Advice

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Safrole

Wed Apr 23 2008, 06:06PM

Registered Member #1304 Joined: Sat Feb 09 2008, 09:29PM
Location: Elkhart, Indiana
Posts: 16

I've been horsing around a bit trying to create a saturable core reactor from some MOTs. (see http://www.dawntreader.net/hvgroup/saturablereactor.html )

I have 7 MOTs and of course none are perfectly matched, so I have significant voltage from the seriesed secondaries, at a small amperage. Can I make the pair balanced by taking windings off one MOT primary and see if I'm getting closer or farther to a balanced state? Is that reasonable? Before I start destroying MOTs I just wonder if I would do better to maybe unwind secondaries instead. It seems like a lot more work, but maybe is a finer adjustment and is the better choice. if someone has advice to share I would appreciate it.

Also, these guys are running the primaries in parallel, but I want my overall flow down to the minimum so I thought I should run them in series.

Another curiousity came up, about which I hope someone will enlighten me. I have noticed that if I run my 120vac through a single MOT primary with the secondary open, hardly any current flows. But when I close the secondary the current picks up dramatically. Can someone explain this? Thanks.

Arcstarter

Wed Apr 23 2008, 06:44PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Safrole wrote ...

I've been horsing around a bit trying to create a saturable core reactor from some MOTs. (see http://www.dawntreader.net/hvgroup/saturablereactor.html )

Another curiousity came up, about which I hope someone will enlighten me. I have noticed that if I run my 120vac through a single MOT primary with the secondary open, hardly any current flows. But when I close the secondary the current picks up dramatically. Can someone explain this? Thanks.

Wouldn't that be changing the inductance?

Safrole

Wed Apr 23 2008, 08:23PM

Registered Member #1304 Joined: Sat Feb 09 2008, 09:29PM
Location: Elkhart, Indiana
Posts: 16

You are right. I read the simple explanation from Alfred A. Skrocki at http://www.pupman.com/listarchives/1998/December/msg00263.html

In this case, is it that the core flux drops as the secondary flows when I close it? And this drop in flux allows AC to pass through the primary?

Dr. Shark

Wed Apr 23 2008, 08:36PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I have an idea for you that may turn out to be complete bogus though. The two outermost arms of the E-core should share the flux going through the center arm more or less perfectly, so maybe you could wind the control winding around those arms. It would be a matter of only a few minutes to put a few windings around the arms, put them in series and check if the total voltage is zero, so it might be worth trying.

Safrole wrote ...

Another curiousity came up, about which I hope someone will enlighten me. I have noticed that if I run my 120vac through a single MOT primary with the secondary open, hardly any current flows. But when I close the secondary the current picks up dramatically. Can someone explain this? Thanks.

When the MOT is open circuit, the mains see the whole magnetizing inductance, which is probably a few Henry, so very little current flows. When the secondary is shorted, only the primary leakage inductance remains to oppose the mains, so a lot of current flows.
Another way of looking at this would be to consider the magnetic field in the core, which is canceled by the "eddy currents" in the shorted secondary. Therfore the primary side does not see much back-EMF and can supply more current.

Safrole

Wed Apr 23 2008, 09:02PM

Registered Member #1304 Joined: Sat Feb 09 2008, 09:29PM
Location: Elkhart, Indiana
Posts: 16

Thanks, and I did try that, only backward. I removed one MOT primary, and then wound its outer legs in opposite directions, planning to use the secondary for a control winding. I've read you need many times the control windings than you have for power, to maintain a favorable control factor. But it was a dismal failure, due in no small part to my poor winding job with the rubber-coated 14ga wire, straight from Menards. It was an excellent core heater, though. So that's why I'm back to the idea of attempting to balance two of my remaining 6 MOTs.

I was hoping to use just one single MOT in some way, but do not see a way to avoid using a second one to create a proper balance. I think I'll peel a single winding off one of the primaries, then peel two off the other one if I don't get progress towards balance. The optometrist principle, better...worse...

Safrole

Thu Apr 24 2008, 01:38AM

Registered Member #1304 Joined: Sat Feb 09 2008, 09:29PM
Location: Elkhart, Indiana
Posts: 16

Well it would seem I'm sweeping a dirt floor. I have clipped into the primary on one of the MOTs very deeply, but I STILL have 150v between the secondaries. (I started with about 170v difference.) I would guestimate that I've circumvented about 20 turns. I got tired of trying to actually unwind them and just started clipping deeper and deeper.

Am I missing something here? Should I just keep digging? Is the amperage capability of the two MOTs doing something to me?

Steve Conner

Thu Apr 24 2008, 08:56AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

MOTs probably aren't very good saturable reactors. They are designed for high magnetizing current, so they will "leak" a fair amount of current through, even before you apply the control voltage.

If I were you, I'd try a couple of toroidal transformers instead. They'll probably want a high current, low voltage control signal, though.

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