Still trying to understand transformers

[BudP]

Air core devices suffer primarily from not having a ferrous bounding box for at least a portion of their activity. The ferrous bounding box (core of any type) provides at least a 1000 times multiplication factor in power transformation over air cores. Again this is watts that are useful as opposed to watts that just create heat.

A cautionary note about big transformers vs small transformers. As size increases the volume to surface area ratio declines. This means you can run the copper wires much harder in a small transformer, to get the same overall temperature rise you would get from a larger transformer. This is not intuitive so do think about it. It is simply the ability of a surface area to dump heat and the bigger the ration of heater to surface area the higher the temperature rise is.

Bud

[megajocke]

I made a little mistake, saturation does in fact limit the amount of power that can be transferred, but not if the primary voltage is at its rated value. I didn't think of this because most of the time transformers are connected to a voltage source, at least in typical power supplies.

You can increase powertransfer two ways of course - increase current or increase voltage. If you increase voltage without increasing the frequency, the transformer will saturate. You get the most power transfer when the voltage is increased to just below where the transformer saturates and the load is matched to the transformer output impedance.

The 25% voltage drop in the primary if the load is matched means you could increase primary voltage by 18% (1/.75 - 1) and get about 40% more power than I said earlier through. These numbers are quite meaningless for things like power amplifiers however.

Originally Posted by MichaelJHuman

What if you build an air core transformer which does not allow for a very high flux density if I understand correctly? What's the drawback? Efficiency?

Here are some I can think of:
Very low magnetizing inductance -> Very high magnetizing current -> High power loss
Bad coupling -> High leakage inductance -> Bad regulation
High stray fields.

Induction heaters sometimes need to be air core, and then capacitors are used to provide the reactive power needed by the coil. The amount of reactive power circulating in this resonant circuit can be orders of magnitude larger than the real power transferred to the load! The windings need often be water cooled because of the large losses. Air core is kind of useless if you need efficency due to the large reactive power needed. Induction heating is done at pretty high frequency too. It will be even worse at 50Hz/60Hz.