What's different in a transformer core when coupling high & low power?

• cmb
In summary: For a flux linked transformer, the voltage per turn is unchanging. The currents are determined by the load on the secondary.
@cmb well I thought the size matter was most relevant only in lower frequencies because as you get to Mhz your core size goes down anyway , in fact most modern few hundred watt SMPS supplies have match box sized transformers at Khz frequencies

artis said:
@cmb well I thought the size matter was most relevant only in lower frequencies because as you get to Mhz your core size goes down anyway , in fact most modern few hundred watt SMPS supplies have match box sized transformers at Khz frequencies
Indeed so. It just struck me how such a small core could transmit what seemed to be 'unlimited' amounts of power when I got down to designing it.

I think there are other 'real' reasons why there is a finite limit, I think as you push the core harder (more coil current but same voltage, thus same flux) the reactance of the primary keeps increasing and it becomes more a problem for the power driver to keep pumping power in, rather than the core reaching some limit.

I'm not actually sure I can see whether that becomes a bigger or lesser problem as the transformer frequency is lowered.

artis said:
One problem though why I think this might not be practical , because having a small core but enough wire turns and thickness to allow for a high power transfer will result in heavy core saturation during light load or no load condition.
The whole point of this exercise was to show that the flux is independent of the secondary load.

hutchphd said:
The whole point of this exercise was to show that the flux is independent of the secondary load.
Yes I am aware of that and that indeed is the reason why one can in theory use a small core but push through high power.
But I was referring to the OP talking about how he during his calculations started to think one could use a small core and still transfer high power. Then members started pointing out that it would not be practical due to various limitations.

I think there is a crossed purpose in the last couple posts, which is OK in the context of the discussion.

The flux is (theoretically, at least, Faraday's law and all that) a function of volts/turn. I took what @artis said to [ultimately] mean that if we try to increase the current capacity (to increase power) then we end up reducing the number of turns (thicker gauge), and if one reduces the number of turns due to thicker gauge (for higher current) then that would increase flux.

Not sure if he did mean that, but I took it that way as this makes sense to discuss in that context.

Ultimately, in the case of a 'perfect' core material, it implies total maximum power for a transformer will be a function of the wire, rather than the core. Of course, cores are always imperfect, so the optimum is the balance between the total number of turns of a given wire capable of a given current, and the physical space to wind them.

For RF purposes, one never wants to get anywhere near saturation of the material, this would be incredibly lossy. Typical RF ferrites should operate, where possible, at no more than a few units of mT. Reason is that if you are pumping the core several million times a second, even small microJoule amounts of energy to polarise the core leads to several watts of power to dissipate. For laminated steel mains cores at 10's of Hz, they can lose several Joules per polarisation and the heating can be managed.

Horses for courses/YMMV, etc...

Thanks for the discussion points.

cmb said:
For RF purposes, one never wants to get anywhere near saturation of the material, this would be incredibly lossy.

We can all agree with that. Nice thread.

@cmb well just to clear up what I meant, the reason I said that a thicker wire with less turns should be employed for this "small core high power test" is simply because on a small core there is very little space and making many turns will not be possible so one can simply push the current instead of the voltage the end result will be the same in terms of core flux whether you have more turns less current or less turns more current.

A nice thread indeed, I personally never before had thought about this that the core size is not critical for maximum power as long as flux doesn't saturate it.

The mechanical analogies with ropes and pushing also fail to fully show the beauty of this because mechanically even if you apply perfectly even but opposite forces the total force is still a sum of both and eventually something will break or tear , most likely at the midpoint.
Yet in the transformer, as load current increases so does primary supply current (assuming a limitless power supply) yet the flux stays the same because as primary tries to increase the flux the secondary opposes this increase even more, net result balance.

As far as I can think of mechanical analogies fail here. One could think of power transmitted through a shaft, like a motor driving a generator (primary - secondary) but it fails because the more power you transmit through the shaft the higher the torque until the shaft snaps violently at some point.

This "magnetic stuff" truly remarkable, and not that much intuitive.

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