Aluminium Windings - Dry Type Transformers 

By on February 24th, 2009

The other day I was talking to a colleague who is a building services consultant.  Despite regularly specifying dry-type/cast resin transformers he was unaware that many manufacturers use aluminium for the windings; I think Siemens exclusively use aluminium and don’t offer copper anymore.  To confirm this he promptly phoned Schneider Electric and was informed that they use aluminium (or copper on request).

This reminded me of a few years ago when I was working for a contractor and wanted to install Siemens transformers on a large international airport.  Siemens was one of the three preferred vendors; however the airport’s specification called for copper windings.  Because of the specification the airport refused the transformers for weeks (we even had the Siemens factory guys out from Germany).  The client’s agreement in the end was not driven by technical considerations, but by contractual/time delay issues.  Transformers were installed and as far as I know are still happily humming away.

The illustration of a Siemens GEAFOL transformer shows the HV and LV aluminium windings.  The HV windings are wound from aluminium foil interleaved with an insulating foil.  The LV windings are wound form single-aluminium sheets and interleaved with cast-resin impregnated fibreglass fabric. Foil type windings are subject to less electrical stress than conventional windings resulting in higher AC and impulse voltage withstand characteristics.  Thermal expansion coefficients of aluminium and cast resin are similar and resulting thermal stress due to load change is kept to a minimum.

To my mind, compliance with the performance requirements of the manufacturing standards is the important criteria and the constructional aspects (aluminium or copper) are not that significant. As a final practical tip, during the above airport installation copper cable was being terminated onto the aluminium terminals of the transformer.  In this instance bi-metallic plates were inserted between the cable lugs and transformer terminals.



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author


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  1. ManjoloPhiri's avatar ManjoloPhiri says:
    9/1/2011 9:11 AM

    The only reason I would allow the use of Alluminium windings in a transformer is the price difference between copper and alluminum. Copper is the best material so far for transformer windings whatever electrical parameters you consider! Please transformer manufacturers continue using copper afterall the price is now down!

    • Steven's avatar Steven says:
      9/1/2011 9:11 AM

      My aluminium winding experience has been with Siemens because they only manufacture aluminium windings. Have to say that I have never had any problems and the transformers work great. Of course copper windings also work great (to me both are acceptable). Talking of problems and a bit off topic, I also think that when selecting a transformer it needs to be suitable for the intended environment/loading and that surge arrestors if used are correctly sized. I have recently come across an instance where this was not done and they have had several transformer failures.

  2. rommel's avatar rommel says:
    9/1/2011 9:11 AM

    Which material, element is more conductive? copper or aluminium?

    • Steven's avatar Steven says:
      9/1/2011 9:11 AM

      Copper is the better conductor. Although Aluminum is still pretty good as a conductor.

  3. Paul's avatar Paul says:
    12/28/2012 9:57 AM

    How the bi-metallic plates can eliminate the ionization potential between two type of metal?

    • Steven's avatar Steven says:
      1/13/2013 2:41 AM

      Bimetallic plates prevent corrosion which could result from a direct connection of two dissimilar metals (aluminum and copper in this case).


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