Since the autotransformer transfers power conductively in addition to through its magnetic field, does its winding also cause loss of electric power in the form of copper losses? It seems to me that a substantial percentage of electric power is transferred conductively in an autotransformer.
It seems to me that a substantial percentage of electric power is transferred conductively in an autotransformer.
You have answered your own question. There is a substantial percentage less copper loss than in a similarly rated isolating transformer.
In a properly designed and rated auto-transformer, the copper loss density will be similar to that in an isolating transformer. But as there is less copper present, there is less total loss.
Why do we design transformers to have a similar loss density? Economics, copper is expensive, if we are not working it 'reasonably hard', then we are using too much of it. A lossier transformer is cheaper to buy, even though more expensive to run. Customers, in their spending choices, eventually dictate what sort of mix of initial and running costs transformer manufacturers design to. This is taken to its extreme in microwave ovens, where the transformer is very cheap, very lossy indeed, but is fan-cooled along with the magnetron.
We talk about 'copper losses' as if it's the only metal ever used to wind transformers. Aluminium is now finding its way into cheap transformers for the same economic reason, despite it having a higher loss than copper.
copper loss is always present in transformers, auto-transformers, coupled inductors etc.
At low frequency, below 10 kHz roughly, you can reduce the loss by enlarging the copper wire diameter.
At higher frequency, due to the skin effect, electrons flow mainly on the surface of the copper. That means that enlarging the copper wire diameter has little effect on copper loss reduction.
Skin effect: https://en.wikipedia.org/wiki/Skin_effect
The working principle of a transformer is that its core is energy-intensive to magnetise, so the windings "aim" for causing a net zero magnetisation. An autotransformer has a common winding section and a smaller winding section only used on a single side. The magnetisation by the smaller winding section needs to offset that by the common winding section, consequently it will carry a proportionally larger current. A proper transformer would have the common section twice, and mostly cancelling magnetisations are present in both the common sections. That causes significantly larger losses than if the currents can cancel before even turning into magnetisation.
But there is a noncommon winding section, and its magnetisation needs to cancel with the magnetisation from the common winding section. That still means significant current flowing.