Unicore Design Parameters - General Purpose Transformers PDF Print E-mail

Optimum dimensions for a Unicore are similar to the old Scrapless Lamination.

Where the Scrapless Lamination has physical ratios of 3:2:1 (3 = Window Length, 2 = Tongue Width, 1 = Window Width), the Unicore is most economical at a 3:1:3 ratio which is similar.

To understand these ratios it is necessary to visualise the Scrapless Lamination cut longitudinally down the centre tongue.

This will give two rectangular cores similar to a C Core, the ratio would be 3:1 (3 = Window Length, 1 = Window Width).

The only dimension missing is the Lamination stack (or strip width in a C Core), which is variable.

If the stack is fixed to the most economical size we get a third dimension to our ratio.

When considering the most economical strip width for producing a Unicore we find that it should equal the window length hence 3:1:3. This ratio makes an economical transformer of near cubic shape with high core mass and low copper mass. The low loss core acts as a heat sink for the heat generated in the copper and there is a large core surface exposed to the copper with short transfer paths.

One very useful advantage of the similarity of ratios is that frames and moulded bobbins are available in a variety of styles from a number of manufacturers.

In practical terms the old 2 inch Scrapless Lamination would have a 3 inch window length (3), a 2 inch wide centre leg (2) and a 1 inch wide window (1). A similar Unicore would be 76257525.

<76 mm = Window Length (3 inch) 25 mm = Window Width (1 inch)
75 mm = Strip Width (3 inch stack equivalent)
25 mm = Build Up (1 inch leg width equivalent)

Strip Width's are in 5mm increments and will always be the increment smaller than the exact conversion from the imperial measurement, (to fit standard bobbins).The Scrapless Lamination was designed to save material (no scrap) and the core ratios were a consequence of the physical design. C Cores, on the other hand were designed to save expensive GOSS while copper was cheap. So C Cores had large windows and small core sections. C Core Transformers were still technically more efficient than Laminations but were not the best designs possible. Unicores are much closer to optimum design.

Note that the Unicore transformer generally has more core mass than a C Core but lower copper mass. This is typical of good Unicore design where by increasing the low loss core mass it is possible to reduce the high loss copper mass.

The result is a lighter, cheaper and more efficient transformer with better regulation.