How is transformer output affected by frequency?

Question

Vp x Ip = Is x Vs

Vp x Ns = Vs x Np

In ideal case, it seems, frequency is not in the consideration. How will it be with real case considering core and conductor losses?

Answer 1

For a given primary excitation voltage, the magnetization current will be twice as much at half the frequency. This is just simply thinking about the primary circuit as an inductor (because that's what it is when the secondary is unloaded). Nothing magical going on here; it's simply the case that inductive reactance halves with a halving of frequency and, with a constant AC excitation, the current must double.

And, of course, with more current in the primary, conduction losses also increase. So, if you go too low in frequency the core badly saturates, you get larger \$I^2R\$ losses and efficiency plummets.

The only good side of lowering the frequency is that eddy current losses also reduce but, the big killer is core saturation. If you go even lower in frequency your AC supply may not be able to supply the saturation current either.

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If you go higher in frequency, eddy current losses increase = that's why we use insulated lamina to make a core; they reduce eddy currents and, for some transformers we use very thin laminates (amorphous tape-wound cores) like these: -

Image from here.

As you go higher in frequency you start to use ferrite cores because they are non-conductive and don't present a lot of eddy current problems.

Going into the hundreds of kHz and MHz range, capacitive coupling and resonance can be a massive problem (or a bonus) depending how good you are at winding a transformer and how you use the transformer.