Boy, do I feel stupid!

You have rediscovered power factor current. The 1.5 amps indicates the storage of energy in the magnetic field of the MOT, and its return to the power company, twice a cycle. Talk about short term loans.

The power company dislikes inductive loads for similar reasons. That current not only dumps a little power into the MOT winding resistance, but all the transformers and transmission lines all the way back to the generator. But the watt hour meter at the service entrance charges you for only 1.5 watts while this thing sits there and loads the grid with 150 VA.

It is a good idea if you want to witness what happens.

If you intend to operate the MOT with short bursts of full load (turning the primary voltage off between those bursts), probably not. If you want to operate it for hours on end with a wide range of loads, it probably is. What might you use it for?

Rich Grise wrote: (snip)

(snip)

Before you make the measurements, I suggest you draw a picture of the core and try to imagine the magnetic flux paths, and the magnetic reluctance (magnetic flux resistance) for the various branches, and see if you can reason out what to expect from the experiment.

Well, you measured it on a resistor, so if it isn't heat, it's still going through the wires, and the power company doesn't like that. Oughta toss on a capacitor (ooh, power-on surge-ified!) or some more turns to reduce the B-field.

electronical,

:-P

Well, the shunts are bypassing a little magnetic field closer to the primary, but not much (depending on width), and various parts of the core are probably running near saturation anyway, so it wouldn't make much difference.

I have an MOT in regular use, without shunts, but it also has sufficient turnage that it doesn't saturate much.

You should check the current waveform on that resistor and see what it looks like... bet it's got some nice nipples (that oughta get you runnin' to the scope!).

Nah, you need iron filings for that. Hope the primary is well insulated ;-)

I would. 150VA is a lot of current you could be using for ___.

No idea... is there a mad scientist or high voltage group?

Tim

-- Deep Fryer: a very philosophical monk. Website:

On Tue, 17 Jan 2006 00:08:38 GMT, via , Rich Grise spake thusly:

Used your AC meter, didja?

Not Watts. VAR's - Volt Amps Reactive. The power transmitted that is not usable (billable) load.

AFAIK the shunts only realy come into play when there is a heavy load on the secondary and act as a current limiter, old type welding transformers have adjustable shunts to vary the maximum current, when the secondary is short circuited the current cuases a magnetic field wich oposes that by the primary wich then finds an easier path through the shunts.

with no secondary at all i suspect they will have an undramatic effect, altering the effective magnetic path length/cross sectional area only moderatly.

Colin =^.^=

The effect of a shunt magnetic path is simply to introduce a series inductance in the transformer, which will have the effect of a crude current limit. It sometimes seems odd that a *shunt* magnetic path should introduce a *series* impedance, but it's quite logical - think in terms of the secondary current producing a flux that does not contribute to transformer action but still creates a back EMF in the secondary.

Same physical system, different way of describing it :)

You're not the first to wonder about this. There's a whole buncha Mad Scientist types who actually need to know about it:

Specifically:

Don't tell them I sent you.

Mark L. Fergerson

And there are others who like to connect up several MOTs and try to get a few KW out of a magnetron in order to create plasma balls. Or to kill burglars and stray cats, I forget which.

Not unless it's running far too hot. By using more turns, you just waste the magnetic capacity of the core by reducing the flux. You're also adding series copper which will increase the losses.

You want minimum turns for the highest acceptable flux density and then use as much copper as the core will take. That gives you amximum power throughput.

Not yet..... newgroup alt.mot.flash.flash.flaaaaaaaaa

I haven't really decided yet. I'm entertaining thoughts of a spot welder, or just a general purpose bench supply; in any case I'm basically going to just play with it for awhile. :-)

Thanks! Rich

Uh, yeah, right after I finish that course in transformer physics. ;-)

I don't even know how the length of the flux path relates to inductance yet! )-;

I would think that there'd be a better coupling factor, since more of the flux goes through the secondary, but when I start to think about what this does to the primary current, my brain starts to hurt. 8-|

I once worked for a guy who had many years' experience with transformers, and he designed ferroresonant units for battery chargers, and basically did it by the seat of his pants with liberal doses of Black Magick. ;-)

Thanks! Rich

Thanks Everybody! This thread has been very enlightening! :-) :-)

Thanks! Rich

Inductance is proportional to the total flux created by a given current. The easy flux path through the primary coil is the solid metal path through the center of the core and back around through both outside legs. The shunts (with their series air gaps) add just a tiny bit of additional flux path in parallel to the solid metal loop out and back past them. This is like putting a 10k resistor in parallel with a 10 ohm resistor. Very little additional (flux) conductivity results.

Now, if there were a secondary on the other side of that shunt, and it was circulation big ampere turns that produced a field that bucked the flux that passed through it, then the alternate flux path through the shunts and their air gap would provide the primary with a reduced but not zero amount of flux per ampere turn, regardless, so the primary inductance could not be forced effectively toward zero, but would retain a minimum inductance, and thus, a minimum impedance across the line. That is the current limiting effect of the shunts.

Thank you! I must admit, John (may I call you John?) when you describe something, you really do it good! When are you going to write your book? You could probably get a lot of material right off google! ;-)

Thanks again! Rich

Keep on posting as you play ... er ... experiment. What do you expect to get out of the secondary, once you wind it?

Ed

Try some of the Tesla coil groups; they just love to blow things up or do tricks with high voltages (and sometimes currents).

Ron

:-)

Well, I _expect_ it to act like an ordinary transformer, but that's what the play^H^H^H^Hexperimenting is for. ;-)

Cheers! Rich