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EDIT: The title should rather be how/why transformers lose energy

My idea of a transformer is that it is composed of two separate wire windings around some metal core. The purpose is to increase/decrease AC voltage. The transfer of energy from the primary to the secondary winding is due to magnetic coupling or mutual inductance.

My question: how much energy is lost in this system due to the metal core AND why? What determines the efficiency of a transformer? For example, it must be based on some property of the metal core. I suppose this question is directed at electrical engineers in particular.

For instance, I've read in textbooks that transformers are fairly efficient, with an "efficiency rating" of around 98% or so.

The equation that governs this is

$$ \text{Efficiency(%)} = 100\times\frac{P_{\text{out}}}{P_{\text{in}}} $$

where $P_{\text{in}}$ is the primary power (i.e. $V_p\times I_p$) in Watts and $P_{\text{out}}$ is the secondary power (i.e. $V_s\times I_s$) in Watts.

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  • $\begingroup$ In addition to the main loss by the currents in the core, there is EM radiation loss . This thesis explores experimentally ( measuring) the radiation from transformers staff.najah.edu/sites/default/files/… . $\endgroup$
    – anna v
    Jun 10 '15 at 3:17
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A significant source of power loss in a transformer is the induced eddy currents in the core. Just as the varying magnetic field induces current in the secondary coil, it can also induce currents in the core itself. These currents do nothing but dissipate energy, and so are to be avoided.

To reduce eddy currents, you either build your transformer out of a non-conductive magnetic material (e.g. ferrite), or you split the core's conductive material into many plates separated by insulating layers. The insulating layers block the large-scale eddy currents while passing the magnetic field, thus reducing the power loss.

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  • $\begingroup$ If you had to make an estimate, how much power loss is due to the resistive heating in the coils/iron core? $\endgroup$ Jun 11 '15 at 20:14
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I suspect most of the loss is simply resistive heating in the coils and possibly some heating due to hysteresis in the iron core rather than coupling of the magnetic field to any external power leakage

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  • $\begingroup$ If you have to make an educated estimate, do you think hysteresis or eddy currents would cause more power loss? CC: @DanielGriscom $\endgroup$ Jun 10 '15 at 23:52

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