Transformer stamping grades

[Neil Purling]

I am just wondering what difference the grade of steel makes in a mains or o/p transformer.
Does the metal affect the magnetic flux/eddy currents, or frequency response.
The last one would be specific to an o/p transformer.
Ed Dinning would know a good deal of course, but i''m putting the subject out in public rather than sending a PM to said member.

[Radio Wrangler]

That's several books worth of information

The range of 'electrical steels' made give you a choice of B/H curves which define the saturation characteristics - not only the flux level at saturation, but the shape of the onset of saturation as well. The amount of hysteresis at the flux levels you choose to run your transformer to is an indication of the amount of core losses to expect.

To make things more complicated, there are GOSS Grain-Oriented Silicon Steels, also called GOES ditti ditto Electrical Steels. These are treated in rolling/recrystallisation to give improved magnetic properties in one direction of flux, at the expense of other directions. This is great if you're rolling up a length of strip to make a toroid core. This is less good for flat lams where you want flux to circulate in the plane of the lam.

The thickness of your lams matters as well. Think of the classic illustration of a current-carrying wire passing perpendicularly through a piece of paper, with its associated mag field represented as concentric circles on the paper, each with a little arrow head to show the direction of the flux. Well there is a dual of this. Think now of a magnetic conductor surrounded by an electrically-conducting medium, the same picture will describe the flow of current around the magnetic conductor. As steel, your magnetic conductor is also an electrical conductor, circulating currents are also induced within your core, circling in planed perpendicular to the direction of the flux in that particular place.

Dividing the core up into laminations limits the ability of the inevitably induced voltage differences to cause eddy currents to circulate. The surface skin on laminations is designed to be electrically insulating.

Thinner lams are more expensive, to make and to fit. Cost pressures push transformer makers to use thicker lams. You'd want them thinner, up to a point where the total amount of insulating skin and entrained air starts biting into your wanted core cross sectional area. Thinner lams help control high frequency losses, so you should see thin ones in valve audio output and interstage transformers.

I worked for a firm which had its own in-house transformer and coil shop. The corporation had its own transformer design manual which took you through all the theoretical intro and then design equations and onto design tables. It was written around the standard bobbins and lams and fittings we had available. Eeeee! luxury! he said with his native Yorkshire accent turned up full. I no longer have a copy, and I'd miss it greatly if I needed to design an iron power transformer.

So much for theory and design. The world has changed. Our transformer shop was shut down in the eighties and replaces with outside contractors. SMPS and ferrite replaced iron cores.... The whole plant is now a housing estate beside the railway station. The sizes of bobbins and lams you can now buy are very limited, so is the choice of materials.

Making this still harder to get into, the firms who deal in such parts are a bit oldy-worldy and not into this internet thingy, so they tend to have web pages saying what nice people they are and how they have great stocks etc., but they don't catalogue it or give data. You're expected to phone them and ask.

I don't think I've given you any specific information that you've asked for, but I hope I've given you an idea of the scale!

David

[Neil Purling]

I think Ed might have mentioned a choice of lamination material once when I ordered a 6V6 o/p transformer, but I cannot recall what choice of material was available, or the effect on the sound.
I believe that he may have mentioned about getting Imperial sized bobbins if he was asked to deal with a burn-up casualty.

[kalee20]

Thickness of lams matters as you go up in frequency. At 50Hz, 0.3mm is common (even 0.5mm for some cheaper grades, if you can cope with the losses) but at aircraft 400Hz supplies, 0.1mm is used because eddy currents get excessive with thicker laminations.

It's important to know how you are comparing things though - a transformer designed for 50Hz will work quite happily at 400Hz. That's because the flux density in the core will be massively less (1/8 as much) at 400Hz than at 50Hz, so both hysteresis and eddy current losses will be very low. It'll just be bigger and heavier than a transformer designed expressly for 400Hz.

For the same reason, lamination thickness doesn't make a lot of difference to an output transformer. You need the core area, and the number of turns, to suit the lowest frequency you want the transformer to handle. At higher frequencies the core virtually drops out of consideration - winding configuration makes more difference to frequency response.

As for steel grades, this does affect losses at the lowest frequency the transformer has to handle. Personally I tend to pay a bit extra and get grain-oriented silicon steel so my mains transformers run a bit cooler. But it's not the whole story. I was recently involved in designing a 400Hz transformer of stupidly small size for the power it needed to be (7kVA in a 5kg mass). The thing is keep cool by sitting in a howling gale from a fan. To keep the resistance down, the turns had to be as low as possible... But then the flux density in the core rises and silicon-steel, despite its lower losses, starts to saturate (at about 1.75 teslas) and we hit a brick-wall limit. The only way forward was to use cobalt-iron, which is useable to 2.1 teslas, even though losses are marginally higher than GOSS at lower flux. Down side is that the core then becomes massively (about 8x) more expensive, can you imagine £1,200 for a core 100mm x 120mm x 45mm! But our customer was prepared to pay for it!

[Neil Purling]

I recall mention of GOSS & Unisil. So what was the real bargain-basement option below these?

[trh01uk]

Originally Posted by Radio Wrangler

Making this still harder to get into, the firms who deal in such parts are a bit oldy-worldy and not into this internet thingy, so they tend to have web pages saying what nice people they are and how they have great stocks etc., but they don't catalogue it or give data. You're expected to phone them and ask.

And I would agree with those difficulties. I know little about transformer manufacture, but the group I belong to does do this job. And last time we bought transformer materials from one such company, the minimum order value was £500.

Add to that the very specialist insulating materials (sleeving, tapes etc) then special varnishes - and this becomes an area most of us rightly going to steer clear of.

Richard

[kalee20]

Originally Posted by Neil Purling

I recall mention of GOSS & Unisil. So what was the real bargain-basement option below these?

Trade names really - I recall Lamcor and Linlox from Linton & Hirst, but I never used them.

[Ed_Dinning]

Hi Neil, just back from Golborne and I Think the others have answered most of your questions.
Unisil is often preferred for HI-Fi output transformers as it goes into saturation more "slowly" on the curve than does Stalloy, so should give reduced distortion. The instrument amp people like the more sudden saturation of Stalloy and the sound it produces.

I too have used some of the exotic alloys at up to 2 Tesla. They were on the wing of a fighter aircraft and in the airstream to keep cool. We tested them in a soundproof room and the testers wore ear defenders due to the magnetostrictive noise from the core. ( which was inaudible when in flight!)
Ed

[kalee20]

Ed's got a good point - operating at high flux densities the wretched things sing more. And woe betide any micro-movement of a loose lamination, or an internal wire not quite locked in place by varnish! 400Hz is approaching the maximum sensitivity of the human ear, at a pitch close to G above middle C.

Insulating materials, tapes, and varnishes aren't difficult to get hold of. RS stock some. Down side is that, apart from RS, minimum order quantities tend to kick in.

[Neil Purling]

Is there much use of Stalloy, apart from transformers for guitar amplification?

The behaviour of Stalloy... Does that mean you'd need a larger core to avoid saturation & there'd be a penalty in reduced frequency response?.
Would it really matter if you were using a single 6V6 or a 6J5 in a radio o/p stage, thus not exceeding more than a couple of watts?

[Diabolical Artificer]

Very interesting thread. There are a few spread sheets out there Neil that help in the design of tfmr's if your thinking of having a go.

If I may just jump in with a question - where do C cores fit into the picture; AFAIK they are more efficiant? I have several old very big tfmr's that use C cores and have seen them used in cheap smaller tfmr's in old amps, but don't see them used in modern power supplies.

Andy.

[Radio Wrangler]

C-cores have the advantage that the flux they carry flows in only the direction along the strips they are made of, so they are prime territory for directional steels like GOSS. So they can be run to higher flux levels and give good efficiency.

THeir disadvantage comes in finishing the faces of the joints to minimise air gaps and the need for high pressure from shrink-fit metal bands to keep them together.

David

[Neil Purling]

I am not thinking of making a transformer for myself, but I was interested in the types of laminations that you would be able to get now.
I was wondering if there is any massive difference price-wise for the same transformer in Stalloy or a better grade of laminations.