Efficiency of a transformer with increasing frequency

Core loss and skin loss. Cheap, thick steel laminations in the core work fine at 60 Hz but have lots of eddy-current loss at hf. The wire itself has skin and proximity effects, too. I'm guessing the core loss is the biggie.

Besides, if it works at 60 Hz, it has far too many turns to be optimum at 40K. A ferrite torroid or pot core would be much better up there.

John

The most likely reason is that eddy current losses in the core go up and magnetizing current goes up. In effect, the shunt inductance of the transformer goes down as frequency goes up when eddy currents become signficant. Those currents are (nearly) in phase with the applied voltage and represent a loss.

You probably have a transformer designed for line frequency with the lamination thickness set accordingly. At 40 KHz, that thickness is way too large.

The impedance of the transformer is Xl=2*PI*F*L if F go up to 40KHz, Xl also goes way up. Your power transformer was designed to work efficiently at line frequency, not 40KHz.

I agree with your correction on terminology. I was incorrectly referring to the shunt current term as "magnetising current". As for inductance not being reduced, I have to disagree. If you consider that eddy current prevents flux from penetrating into the interior of the laminations, you should be able to see that the effective area of the core is reduced.

This effect shows up as a non-polynomial relation between impedance and frequency, such that the impedance rises approximately as sqrt(frequency). If you take the imaginary part of the impedance in any part of that curve, you will find that inductance (defined as that component divided by radians/S) is indeed decreasing with frequency.

Yes.

Agreed.

-------- Actually the "magnetising" or inductive component of the current decreases because of lower flux density for a given voltage, at higher frequencies (The inductance won't decrease much if at all) .but the total exciting current increases due to hysteresis and eddy current loss increases. This increased loss current will coupled with skin effects will also lead to higher I^R loss.

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----------- Right on- design a 40KHz transformer for 40KHz- don't use a 60Hz transformer and expect good operation. It is more than just lamination thickness, it is excessive iron.volume as well as capacitive effects. It is a wonder if a

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decreases

----------- Eddy currents are due to the flux enclosed in the eddy current path.. True - they do act to oppose the flux, as do currents in any other transformer secondary-you see this in an increased primary current to balance.

-------- Non-linear for sure- however I'll take your word for the rest- I've only worked with 60Hz transformers. However, just a thought, at higher frequencies, the effect of the various capacitances comes into play so an R-L series or parallel model is inadequate.

Core loss is a part of it and core loss goes up with frequency, but flux density goes down with increasing frequency which helps to ameliorate core loss. Whatever the core losses are at a given frequency, they would be much worse if the flux density were maintained at its low frequency value.

Another important loss is driving of the winding capacitance by the resistance of the windings and by the impedance of the leakage inductance. This can be a very important loss factor because the AC resistance of the copper (not impedance) goes up frequency because of skin effect and may become excessive at 40KHz. The capacitance was most likely ignored in a line frequency transformer design but can be significant in a 40KHz transformer. Furthermore higher voltage windings often have higher capacitances and smaller wire with higher resistance exacerbating the loss effects.

Leakage inductance is the inductance that does not link the primary to the secondary and therefore no transformer coupling occurs across it. It's just a series impedance in the way of transferring power across the transformer. It drives the winding capacitances as mentioned but also is in series with the load reducing the transfer. Like all inductances, it's impedance increases with frequency. Bob