Inductor/ballast choke design / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Inductor/ballast choke design

1 2 3 next

Move Thread

LAN_403

Dr. Dark Current

Wed Dec 26 2007, 10:48AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

So I made this dual MOT power supply (video @

) and the transformers get quite hot in a while.
I tried to limit the current with a secondary of a 15V 180VA mains xfmr, but the core got too hot (saturation I guess.)

So how does one properly design a ballast choke that does not saturate? I think I need some air gap right?

Kizmo

Wed Dec 26 2007, 12:15PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

I made ballast choke from two 1200VA 'UI' core transformers. I unwound both transformers and put cores together. Then i just dropped big spool of AVG10 wire around it and did some experiments with different loads and number of turns to find out right amount of turns to use without saturating the core. Now i can control the current with adjusting airgap between 'U' and 'I' parts. No airgap -> max current ~6A. 'I' part fully removed -> Max current ~100A. This is tested with small pole pig and large mot stacks and core is stone cold even after fairly long runs (As SGTC ballast).

Dr. Dark Current

Wed Dec 26 2007, 01:03PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

meh, I had something more compact in mind, I have a few 300W xfmrs that have MOT-style cores (U and I parts welded together) Would this be enough to ballast ~2000W MOTs? I don't need much ballasting, the caps limit the current quite a bit actually.

Marko

Wed Dec 26 2007, 02:32PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Jan, how do mot's behave if plugged in without without ballasting? They are actually pretty well ballasted themselves.

If you are blowing fuses without load then there is an easy cure for it.

If you want additional ballast, yes, I don't think you really need a core sized like a MOT.

Something small like few hundred VA may be more than enough, depending on how much ballast you need. What is the maximum current you can draw from the line?

Calculating air-gapped cores requires somewhat more complex math I mostly forgot about now,

so I think Steve C. or somebody might be of more help.

Dr. Dark Current

Wed Dec 26 2007, 04:22PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Marko: I haven't tried to plug them in without ANY ballast, they would probably pop the breaker promptly OR go up in smoke quickly.
I use some MO caps on the secondary side which seem to magicelly convert some of the reactive power to real one, the arc pulls longer and I don't trip a 10A breaker with 2 MOTs.
The problem is just that they start getting too hot after a while.

I think I'll try a TRIAC regulator (as in dimmer switch) with a beefy 25A triac and see what it does.

Marko

Wed Dec 26 2007, 04:33PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I'm talking about problem of startup current which with large transformer tends to blow fuses easily.

For a large variac at school I made a startup circuit, consisting of resistors which are shorted out by relay after short time delay. Resistors of few hundred ohms at few W are OK.

Surely nothing on secondary side will help there if it's open!

For load ballast, probably your best solution would be to decrease the secondary series capacitance. Much easier than a inductive primary ballast as I think.

Dr. Dark Current

Thu Dec 27 2007, 03:31PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Is it possible that a MOT eats 350 WATTS just sitting there plugged in? I was adding PFC caps on the primary and the current went down to ~1.5A (@230V) then it started rising again.

Zum Beispiel

Thu Dec 27 2007, 05:43PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

jmartis wrote ...

Is it possible that a MOT eats 350 WATTS just sitting there plugged in?

From the MOTs I've played with this seems normal. Actually 350W seems a bit low from what I've seen. They seem to draw a quite a bit of idle current.

Myke

Thu Dec 27 2007, 06:00PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

With 2 MOT primaries in series the idle current was .5A (120V)

J. Aaron Holmes

Thu Dec 27 2007, 06:09PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Zb wrote ...

From the MOTs I've played with this seems normal. Actually 350W seems a bit low from what I've seen. They seem to draw a quite a bit of idle current.

Yes indeed. Mine all want 2-4 Amps at 120V with no load. But MOTs are pretty junky in general, and "normal" means almost saturated.

Cheers,
Aaron, N7OE

1 2 3 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob