3
\$\begingroup\$

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.

\$\endgroup\$
4
  • \$\begingroup\$ Autotransformer - "... is an electrical transformer with only one winding. ... portions of the same winding act as both the primary winding and secondary winding sides of the transformer. ... the disadvantage of not providing electrical isolation between primary and secondary circuits ... advantages of often being smaller, lighter, and cheaper ... lower leakage reactance, lower losses, lower excitation current, and increased VA rating for a given size and mass" \$\endgroup\$ Dec 8, 2023 at 21:11
  • \$\begingroup\$ A what is it? question. \$\endgroup\$ Dec 8, 2023 at 21:54
  • 1
    \$\begingroup\$ Yes, unless the autotransformer is wound with aluminum, in which case it has aluminum losses instead. \$\endgroup\$
    – Hearth
    Dec 9, 2023 at 4:28
  • \$\begingroup\$ Thank you for the answers. I failed to check beforehand for the fact that autotransformers need lesser copper for a given amount of power versus a two winding transformer. There is also the fact that the common winding in the autotransformer doesn't carry the full secondary current as the secondary winding in the two winding transformer. Instead it carries I2-I1. All the answers are fantastic. \$\endgroup\$ Dec 9, 2023 at 17:56

4 Answers 4

9
\$\begingroup\$

In all transformers, power is lost through resistance of the copper wire. If current flows through the windings, Ohm's law applies.

It's a short answer, but I hope this answers your question.

\$\endgroup\$
1
  • 8
    \$\begingroup\$ And of course, aluminum-wound transformers suffer resistance losses even without the presence of copper. \$\endgroup\$ Dec 8, 2023 at 8:07
4
\$\begingroup\$

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.

\$\endgroup\$
3
  • \$\begingroup\$ Aluminium softens when exposed to heat. Such a transformer wouldn't last. \$\endgroup\$ Dec 9, 2023 at 13:52
  • \$\begingroup\$ @SubratBorgohain I'm not sure microwave oven manufacturers care much about their products lasting, looking at the failure to clear cavity condensation on mine. The only alli-wound transformers I've seen have been in microwave ovens. \$\endgroup\$
    – Neil_UK
    Dec 9, 2023 at 16:39
  • \$\begingroup\$ Every florescent light fitting I've taken apart in the last 40 years has had an aluminum wound ballest transformer. And they weren't new units 40 years ago. \$\endgroup\$
    – david
    Dec 11, 2023 at 0:24
2
\$\begingroup\$

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

\$\endgroup\$
2
  • \$\begingroup\$ Would copper-clad aluminium (CCA) be a cost-effective way of using Cu for those higher frequency transformers? \$\endgroup\$ Dec 8, 2023 at 17:35
  • 3
    \$\begingroup\$ @Andrew-Morton Structural aluminum is becoming more common, but historically you would use Litz Wire for high-frequency bulk transformers, or, for other special applications, copper wrapped silk or cotton, or copper-clad steel or brass. Silver cladding has also been used. \$\endgroup\$
    – david
    Dec 11, 2023 at 0:31
1
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.