Rewind aluminium transformer with copper

[vkrimpen]

Which factors should be considered when rewinding an existing aluminium power transformer with copper?

With my limited understanding of the matter, i would expect that the core mass determines the maximum KVA rating of the transformer and that the winding window for alu is simply bigger than for copper to accommodate the lower conductivity and therefor thicker wire.

That being the case, a rewind with copper of the same resistance (i.e. thiner) should work fine.
Is there anything else i need to take into consideration?

Willem

 

[gordonl]

The impedance of the transformer will change, and you may need extra spacing added in for cooling. I would strongly suggest you get a transformer designer to do the details for the rewind for you. When you rewind with copper you also get higher eddy losses in the copper then aluminum, but going with smaller size should take care of that.

 

[stretched2thin]

Gordonl is correct. However, you can achieve the same impedance by adding additional ducting in the windings.

In general, core size is dictated by the required voltage and the number of turns in the winding. (Volts/turn determines the flux density and thus the core size.) Of course, this affects losses. (If you increase the core diameter, you increase the mean winding diameter, and thus you increase load losses.) But given that your core size if fixed and you are going to a lower resistivity winding conductor, you should be able to increase the MVA of the transformer without exceeding the temperature ratings.

Having said all of that, their really is no disadvantage (other than cost) to going to copper. As Gordonl said, eddy losses will have to be looked at carefully.

If this is a power transformer of moderate to large size, I would suggest using a reputable transformer remanufacturer such as Southwest Electric (Oklahoma City). If this is a distribution transformer, it may be more expensive to rewind it than to buy a new one.