question about toroidal cores

[Frank S]

is it better to have a laminated ring core or a strip wound core for a toroidal transformer/ Assuming both are grain oriented
 or a third option of flat helix like a spring

[joestue]

If you imagine a stack of washers, I don't believe it is practical to make grain aligned steel in the direction you want.

If you can get someone to water jet cut the teeth out of a large induction motor core, i'd try that for an experiment but the profile of the core wouldn't be right.. unless you use two motor cores, one inside the other, fill the gap with epoxy. maximum flux density would be on the order of 1.3T for low losses, but you could run it as high as 1.5, 1.6T.

i suppose it would be possible to wind steel on edge into a helix, two or three concentric stacks to make a core.. a lot of work since it would be a strip already, easily wound up like a roll of tape.

as for breaking up the eddy currents it doesn't matter how its laminated.

i tried using a roll of rusted bailing wire for a core.. couldn't even measure the results and fried a variac upon plugging it in..

[oztules]

"i tried using a roll of rusted bailing wire for a core.. couldn't even measure the results and fried a variac upon plugging it in.."

Now thats "old school ".... I'm impressed. Thin gauge and well rusted should have been worth a shot in my mind... never thought of that.... I'm slipping.

Rust is what we do best down here in a marine environment.

..............oztules

[DanG]

Out on the web long ago and far away there was once a massive DIY torrid with center opening measuring 18-inches plus.

His take on it was using a narrow pallet banding steel (1/2"?) wound parallel with the copper windings, he started with a core of iron wire and wrap the banding tightly cross to that and travel the windings around the circumference until they'd built up four-plus inches of strips. This has been about fifteen years back, my analog memory has gaps - I think there was shellac and paper treatments, maybe using something tougher than paper, perhaps even Glyptol paint. I do remember nearly he went through a case of Teflon plumbers tape to insulate between winding layers. If I remember rightly he used 2AWG solid core and had a real heavy weight XFMR when done...

If I remember correctly his Revision Two model had the banding strips well tensioned (easy to let banding relax and lift while winding at even a slight angle) and a longer continuous length to each steel banding piece, and it might've included Tyvek house wrap as the inter-winding paper  (still shellacked).

I saved away the link, re-read the page many times... then it vanished about 2002.

I have no/low clue as to whether the XFMR suffered from heating or low efficiency, just something I wanted to try someday

[joestue]

you shouldn't need to insulate pallet banding strap if you buy it new..
http://www.uline.com/BL_2902/Steel-Strapping-Standard-Grade

[boB]

I think that this is the kind of stuff that Frank is talking about.
Magnesil from Mag. Inc.  is what I remember playing with many years ago.

http://www.mag-inc.com/products/tape-wound-cores/magnesil

Grain oriented, high perm, 3% silicon steel  for transformers, (etc.)  tape wound material. 

Saturation flux density is ~I think~  the same as like, M6 lamination material ?  10 to 15 kG ?  fairly high anyway. transformer

Not sure if you can just buy the tape itself, but maybe ?


boB

[oztules]

The  aero- sharp 3kw grid tie torroid core  has  32cm^2 and the largest of the ones in the catalog seem to be 26cm^2... so their biggest is only 80% of a single core I used........ makes my triple core one seem "over large".

The inspire core about the same cross section area as the aero sharp... but smaller hole and lower height to make up for it... still in the 32cm^2 range.

Your opinion boB... are these over sized in your mind? ( have I gone too far over the top?) for 8kw inverter duty... my smallest are 64cm^2 and the largest is 96cm^2.

..............oztules

[Frank S]

Bob yes I was thinking about the 3% silicone steel
 was a transformer company in kuwait that I built an elevator for
 see coils on the lift below that I used to certify the capacity of the platform
[ Specified attachment is not available ]
 those are coils of steel used to make transformers for substations the lift has 23,000 KG in it.

[Frank S]

Thinking about the bailing wire core. Maybe I shouldn't toss out the several rolls of rusty .052" solid mig wire that I have LOL

[joestue]

There is a seller on ebay who is selling 'FG' steel toroidal cores for 7.6$ a pound.
Guaranteed max core loss at 60Hz:
0.970 W/Kg  @ 1.5 Tesla
1.39 W/Kg  @ 1.8 Tesla
Saturation flux density: 2.035 Tesla ( 20350 Gauss)

the steel sold by alpha cores direct is 1.7T saturating at 1.95T iirc. 1 watt per kilogram losses at 60hz @1.7T.

steel that saturates at 1.5T should be far, far cheaper than 7$ per kilogram or $3.5/lb, boB i think that stuff is designed for saturatable reactors, not transformers.
whoever is selling those FG steel cores on ebay is probably making a killing, though not too many have been sold.

there are not many other options for laminations, though there are companies that offer custom water jet cutting of non grain aligned steel for prototype motors.
the folks at diy audio have been complaining as well for some time about hard to get EI laminations of suitable quality and not by the ton either, someone there has a source but i think he is in south america IIRC and most likely pays significantly more than $3/lb for them.

another option perhaps is amorphous metal.--the same metal many utilities are using in their distribution transformers. almost zero losses but you'll need to wait 40 years before it pays for itself.

another option is buy a 20 amp variac and rewind it
http://www.ebay.com/itm/20-AMP-110V-VARIAC-AUTOTRANSFORMER-VOLTAGE-REGULATOR-POWERSTAT-0-130V-OUTPUT-NEW-/381222191973
i find it hard to believe this would be cheaper than buying a 2KW toroid core, but nothing surprises me anymore.

my "500" watt variac is 6 watts no load probably made by the same folks as ^that ebay link

[oztules]

Frank, here is a wire backed alternator core i made in to a windmill  years ago.... all wire for the backing, no laminates.... worked fine.... so you never know what the mig wire will do for you



story here: http://www.otherpower.com/images/scimages/5171/seely.html

........oztules

[Flux]

Depend on what you want from the core.

For high permeability but with sharp saturation a strip wound core is best. You can't use grain oriented material effectively for disc cores but for many applications disc cores are perfectly good.

If there is any hint of asymmetric flux or any dc component then the simpler disc core will be better.

If losses are the main concern then quality core material is essential if you want to keep the flux density up but the better the material the worse the saturation.

For the alternator application that OZ mentioned then high quality core material is not really necessary and a roll of wire may be more than adequate but if it is closed circuit operation as in a transformer then I doubt that wire or banding tape will be satisfactory, certainly it won't perform better than a standard E I transformer core.

The commercial toroidal transformers use something considerably better than silicon steel strip and although they have advantages they can have serious side effects during switch on or if there is any asymmetry in the waveform, in many cases they are more problematic than conventional transformer cores, just depends on the application.

Flux

[boB]

Your opinion boB... are these over sized in your mind? ( have I gone too far over the top?) for 8kw inverter duty... my smallest are 64cm^2 and the largest is 96cm^2.

..............oztules

Hey, if you can lift it and its losses aren't too high, just do it !

I LOVE those huge rolls of steel !

BTW, back at Trace Engineering (xantrex) we played with some 6% silicon steel blocks for transformers from some Japanese outfit.  They were better than 6% silicon steel...  But they were also WAY more expensive.  After a while, a small increase in performance costs you a LOT more in price.   For instance, in most cases, going from M6 to M4 E-I laminations was not worth the small loss benefit.

[Mary B]

1 of many sources for cores http://www.alphacoredirect.com/contents/en-us/d163.html that one is 24,740VA rated! $220

[oztules]

Mary, that would be ideal to replicate one of my 2 stack units.... very very similar at 2.2v/turn@50hz for mine and 2.14v/turn for theirs@60hz

For a core of 15kg I would have hoped for more than 2.145v/turn @60hz. The grid tie steel must be very  very good.

.......... so you think I'm over the top with a three stack.... @ 3.3v/turn@50hz

This country makes nothing any more, at least you can still get stuff like that over there.... now we dig up dirt, and sell cows and think we are still first world. We used to manufacture everything here...... now not even paper clips.... sad to have lived long enough to see it come to this.

Have a new neighbor here ... mad ham man built some stuff for him to annoy you lot over there....eg...https://g4ydm.wordpress.com/2015/04/14/dx-operation-long-distance-communication-vk7nse/

Maybe you will work him some time...


.................oztules

[Mary B]

I haven't worked him but I have heard him on the air. DX stations usually have huge pileups and all I run is a vertical antenna so not ideal for being heard in between the guys running 1,500 watts to directional antennas! From where I am New Zealand is an easier shot for some reason, hear them far more often than Australia.

I need to pick up a nice toroid to wind a high voltage transformer for an amplifier I am collecting parts for.

[oztules]

Does this happen to include about 4000v on the plate of a tetrode by any chance.... those 15kg ones seem ideal... as the hole is wonderfully large for winding... and thats a big deal for the manual winding of these things... the weight is forgettable , you ... get tired of that real quick.... but if your doing high voltage ..... thats lots of turns to wind... and inter layer insulation is more important... maybe resort to segment winding instead.

.......oztules

[joestue]

I need to pick up a nice toroid to wind a high voltage transformer for an amplifier I am collecting parts for.

The time required though.. why not buy one of the tape wound and cut cores.. and build or buy a bobbin.

I have a 6kvac transformer, packaged inside a welded steel container (assuming oil filled).. 8 inches by 7 by 9 inches high. two separate 120vac windings. 6000 vac winding is 660 ohms. if you want it you can have it for the shipping price. unfortunately the 6kvac winding is internally grounded.

[Mary B]

I need 2400vdc so AC volts will be around 1800 into a full wave bridge. Using some smaller Russian triodes for 2 meters and 6 meters to get about 600 watts out. GI7BT are the tubes I am using.