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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well, so what about laminated iron cores? I always wondered if they can run at higher frequency and higher volt per turn than mains frequency, with increased efficiency? Where is their maximum efficiency point (frequency-wise)?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Richie- I have a question for you. In all textbooks I have seen, in a section with transformers, there is a formula for calculating maximum power of a transformer. According to it, the power rises with the SQUARE of the cross-sectional area! This is against common sense but it seems to work at least with iron lamination transformers. Does this mean that if I "pack" two identical cores together, will the available power rise to 4 times that of a single core? Or is there something I'm missing?
The square of the cross-section rule-of-thumb applies more to line-frequency transformers than for SMPSU transformers where there are more variables to consider. Also it assumes that when you double the core cross-sectional area you also increase the winding window in the same proportion, then you can use less turns to achieve the same flux density but use thicker wire because you also have more space to fit it in these fewer number of turns! With high frequency transformers it is not as simple as that because often you don't want to use any thicker wire because of the skin effect, and that is when multiple parallel strands and Litz become beneficial.
Whilst the stacking together of two ferrite cores (let's say E cores) side-by-side does double the cross sectional area, it doesn't alter the size of the winding window. Also it increases the mean-turn-length more than it would do if you kept ideal core geometry. (You will notice that most centre-limbs are roughly square or circular in shape.) Therefore you wont see a 4-fold increase in power handling. But as I said ferrite power transformer design is a complex iterative process, where changing the core size, (or using stacked cores) means working through an entirely new set of design cals and seeing how the numbers drop out.
At a guess stacking two cores will give you somewhere around twice the power handling capacity of one core, maybe slightly more, although I haven't done any calculations to prove it. I have never seen stacked ferrite cores in commercial transformers for the reasons mentioned, but have connected units in series/parallel to increase overall power throughput. Stacking ferrite or iron-powder cores for chokes however is quite common practice.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Plasmaddict wrote ...
Well, so what about laminated iron cores? I always wondered if they can run at higher frequency and higher volt per turn than mains frequency, with increased efficiency? Where is their maximum efficiency point (frequency-wise)?
Laminated iron core transformers were used in almost all valve (US: tube) audio amplifiers, so are certainly capable of giving good results up to 20kHz with careful design.
There is a useful paper called "TRANSFORMER LAMINATIONS, DESIGN CONSIDERATIONS" which may help you, here:
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Homemade ferrite core?
