Winding a laminated steel coredinductor

[Jonathan Bright]

I have some old transformers and some copper wire. I wish to wind some metal cored inductors, a few Milli-Henries. I am not 100% up to speed on the physics of the laminations. I have two questions.

Q1.I understand that each individual piece of metal is varnished for insulation purposes and this has something to do with reducing "eddy currents".
Now after I have cut up the tranformer with an angle grinder or hacksaw I guess I will have probably shorted a lot of the metal pieces together.
The question is this: Will the final product be "OK" if I separate the laminations, file away any rough edges, re-varnish them and put them back together again before winding on the copper?

Q2. I understand that steel/iron can be magnetised by mechanical stress through being struck, dropped etc. I wondered if the process of cutting up the original tranformer will have partially magnetised the bits I want to re-use. Will this be a problem?
If so can I demagnetise the iron through a solenoid or other technique? Can you explain what I need to do?

Thanks, Jonathan

[bob91343]

Q1 - yes you can clean up the edges and re-insulate.

Q2 - don't worry about mechanical stress. The most likely problem would arise from too much heat, which could ruin the annealing of the iron. So go easy with the saw.

[Jonathan Bright]

Hi, bob91343, thanks for the reply and reassurance.

The price of large value inductors suitable for low frequency cross-over points is outrageous over here and I doubt if its much better elsewhere. This will save me a lot and be satisfying as well....

Btw, it looks like you have recently joined the forum. 'Welcome' from Australia trust your state will be productive.
Jonathan

[Rembrant]

Honestly, the contact being made by the cut area is inconsequential. You can debur and varnish them if you wish. I don't think it will make that much of a difference. I often have to grind down the hammer heads on stators for DIY electric motors for my R/C habit. I found taking the time to re insulate the stators was worthless. I am not entirely certain about how much it will affect an inductor but if your winding them your self, you can try them either way and see if it makes a significant difference.

When it comes to electric motors, the difference is not measurable. FEM calculations show no real difference in saturation and no real world differences can be attributed.

You might find you can save yourself a lot of time.

[Jonathan Bright]

Hi Rembrant, thanks for the voice of experience. I will probably experiment a bit. I just didn't want to make a complete mess of it.....

[Steve Dunlap]

The laminations are not varnished, however the transformer may have been dipped or varnish impregnated. You should be able to separate the laminations without damaging them. If they will not separate with a sharp knife edge, you can soak them in solvent to soften or dissolve the varnish.

We used to run a welded bead down the edges of the large transformers to prevent noise from magnetostriction, so you will not need to add insulation.

Mechanical stress can demagnetize.

[Andrew Eckhardt]

I don't think you'll succeed in permanently magnetizing silicon steel unless you mangle and fry it first. The better grades can become de-oriented by mechanical stress. Even the strip cutting, stamping, and tape winding processes decrease performance slightly. Less than M6 probably doesn't matter too much anyway. If you delaminate just try not to bend the lams too much *** (<-- I have no idea what that was, sorry) you peel each one up with a utility blade or something. Work around the lams with the blade. I usually end up using an "I" to shove down the center E pole. Don't try to lift one corner and pull the whole thing off by it. Unless you get lucky with a very bad or absent varnish job, you'll end up with a mess that will have a horrible stacking factor anyway due to all the warpage. He's a toughie... When you relamintate, try to get the burrs facing all the same direction. That helps with stacking factor too. Actually, because of the pattern of varnish remaining on the lams, you'll get the best stacking factor if you put them all back the way they came out! I would not bother with a welded core, period. If it's been vacuum impregnated with epoxy you aren't going to dissolve it with anything. That stuff can make it completely impractical to attempt delamination, but probably isn't used much in consumer parts, maybe old microwave ovens, which are welded.

[Andrew Eckhardt]

You didn't mention what you're trying to do with these inductors so I'm not sure how much of any of these finer points matter, but solid soft iron has the highest static B possible of common materials, maybe any material, I don't know. The silicon and grain orientation are most important to reduce AC loss. Automotive alternator cores are sprial (edge) wound out of tape. By the time they put the pole pattern on the tape and wind it around they bend it up so badly there isn't any point in worrying about grain orientation (not to mention the flux path crosses over the material from all different angles anyway), so I don't think they do. One of the reasons why alternators aren't very efficient. They also use fairly thick strip so eddy loss is high too.

[Jonathan Bright]

Hi Andrew and others "Thanks". The inductors are for the lower cross over point on a three way system. So I'll end up with numbers between 5 and 10mH. Also I would like to try inductors in power supplies. I appreciate the comments. The driving force is basically financial. The local catalogue prices are high.......I've got iron and wire and so thought I could diy some items......

Thanks again,
Jonathan

[indianajo]

When I was young and had lots of time and not much money or parts, I found a fried golf cart charger with a 40 amp ? 36 v rms from mains voltage transformer. Would have made a **** of an 8 ohm power amplifier transformer. I bought some polyurethane insulated 16 ga wire (just available in 1974) to replace the square aluminum wire it was wound with, and went at it. I didn't have to saw anything, the laminations were E forms that were rivited together, I drilled the ends off the rivits pulled apart, and had some long 4-40 screws for reassembly. Unfortunately, when I got done winding, the winding was shorted to the core. No winding motor was used, it was all gentle hand pulling. If I was to attempt it again, I would wrap the core first with a non-metalized mylar film, say salvaged from one of those semiconductor baggies.