Tubular Transformer core?

[Cliff321]

In the spirit of using what I have to hand:

I want to make a fairly large transformer core from 1" steel tube stock (large as in 4" across).
I have some square steel tube, an inch on the side. Wall thickness is about 1/8'.
I want to build a fairly large transformer core, Then a little bobbin from wood or plastic and use that transformer to make a degaussing station for tools too large to be degaussed on a little motor transformer.

I have the essential theory after failing miserably at making a degausser by coiling 60 feet of fine multistrand insulated wire (stripped phone line) with a little light bulb to be the safety load and without the iron core of a transformer. It seems that a mere ring of charged wire doesn't work so well.

This time I think I'll get a few hundred feet of enameled magnet wire. I figure I don't really need to calculate the resistance if I use enough wire.
And in the end, I can always apply a load to the circuit so it's all good - - right?

So I want to make myself a nice transformer core. Yah I can buy one, but remember the penurious spirit.

So to my question. I can cut & weld the tubing to the geometry I want.
But will a tubular core work as a transformer core ( at least - in this application)?
Or stated another way: Does it have to be solid or completely filled with laminations?

Opine please~ ~ ~ ~

 

[JimB]

So to my question. I can cut & weld the tubing to the geometry I want.

Yes, if your metalworking skills are up to it.

But will a tubular core work as a transformer core ( at least - in this application)?
Or stated another way: Does it have to be solid or completely filled with laminations?

Not very well.
The steel of the metal tube will not have the magnetic properties of proper transformer core material.
The important thing about a transformer core is the cross sectional area (CSA).
1 inch square material has a CSA of 1 square inch.
1 inch square tube with a wall thickness of 1/8" has a CSA of approx 0.5 square inch.

Alll in all, a bad idea.

JimB

 

[shortbus=]

An old microwave oven transformer (MOT) could be made into a demagnetiser. Most of the MOT have the "I" part of the E-I transformer core welded on. Cut the welds and throw the "I" part away. Also remove the secondary coils of the transformer, keeping just the primary. This gives you a demagnitiser, with little work or expense.

I worked as a die maker my whole adult life, when using a surface grinder, the parts are held down by an electro- magnet. After grinding parts must be demagnetised. We didn't have demagnetisers big enough for some of the parts. So instead of putting the part on the demanetiser, you moved the demagnetiser around on the part. And the movement is the important part, the demagnetiser must be powered on when it or the part is removed form it. Or else the part/tool is remagnetised.

 

[kinarfi]

I hope this isn't hijacking, it is in the same line, when it comes to degaussing tools, is there a good frequency or is 60Hz as good as any thing else. I have the degaussing coil from a Hitachi 36" tube type TV that I saved for just this purpose, but i haven't tried it yet, plan to hook it to a variac and then dial up the voltage to about half an amp to start with. Any suggestion?

 

[shortbus=]

The commercially made demagnetisers just use 60Hz AC. Don't know how well a degaussing coil will work to demagnetise metal.

 

[Cliff321]

Not very well.

Hmmm well that was pretty definitive.

The steel of the metal tube will not have the magnetic properties of proper transformer core material.
The important thing about a transformer core is the cross sectional area (CSA).
1 inch square material has a CSA of 1 square inch.
1 inch square tube with a wall thickness of 1/8" has a CSA of approx 0.5 square inch.

And that small cross section is contraindicated in this application?
What if I filled it with steel shot?

I could use some half inch bar stock I have available. Three sections in a row of three (2 ends and a middle) would give me a rather hefty piece of steel.

An old microwave oven transformer (MOT) could be made into a demagnetiser. Most of the MOT have the "I" part of the E-I transformer core welded on. Cut the welds and throw the "I" part away. Also remove the secondary coils of the transformer, keeping just the primary. This gives you a demagnitiser, with little work or expense.

I wonder if I have one lying around? I took an old Amana Radar Range apart a few years back ~ ~ ~ prettiest microwave oven ever made. I know I kept the Magnetron in the fond hope of someday using it to mess with police radar. The mythbusters tried that to no avail.

 

[dr pepper]

If you fill the box section with little metal roundels, the scrap you get from power presses, then the performance will increase to being reasonable, I suspect if you can form and weld a core then you'll know exactly what I mean.
Last time I took a torroid apart the core was actualy square, the difference was that it was made from laminations, a solid bar of steel will have losses from edyy currents but it'll still work.

 

[Cliff321]

a solid bar of steel will have losses from edyy currents but it'll still work.

I always thought that the laminatins were just cheaper.
It is cheaper than casting a core to build a die set that'll run for many years, stamp out blanks rivet them together. Much cheaper than casting or machining from solid stock.
And here I learn that there is a reason beyond and even superior to mere cost of manufacturing.
My faith in humanity os almost renewed.

 

[MrAl]

Hi,

Steel tubing is made from regular steel which has low permeability (mu). Maybe if you are lucky you'll get mu=100 but true magnetic materials have mu=1000 or more. The cross sectional areas is a bit skimpy too. A 1.13 inch diameter bar has about 1 square inch cross sectional area, while a 1 inch diameter tube with wall thickness of 0.06 inch has magnetic cross sectional area 0.4 square inches

However, if you dont close the magnetic path then the inductance is more related to the air gap than the permeability of the steel, and although 1/2 the area is not desirable, i would think it would still work to some degree. So you might get it to work good enough for your purpose depending on what the purpose is.

For the purpose of degaussing, it might work. The field would not be as directed as you would like it to be, but could still be good enough even so. It would depend on the amount of wire you use and what voltage you have available to drive this construction. Since the area is less than optimal, that means the wire usage is not optimal so that means you dont get as strong a field as you would with a solid core. Adding steel to the inner core area should improve this.

Degaussing is the task of taking the target material through it's BH curve until it reaches (0,0). Knowing the magnetization level and direction means we can apply a counter force and accomplish the task in one single fairly quick operation. However, it is difficult to tell where the material is on it's BH curve so we end up using an ad hoc method that involves a changing magnetic field that takes the material through it's BH curve repeatedly while at the same time decreasing the amplitude of the field so that eventually when the external field reaches zero the material also reaches zero.
This involves using a constant AC current to generate the field, usually of low frequency like 50 or 60Hz, while at the same time moving the field around a little so it reaches all parts of the material and also at the same time moving the field away from the material so that the material BH area traversed gets smaller and smaller. This simply means moving the coil around while at the same time moving it away from the material slowly.