Can I increase the efficiency of an existing transformer?
There are two main loss area of a transformer, load losses in the coils and no-load losses in the core. Coil losses can only be lowered by completely replacing the coils and using a larger conductor which typically would not be compatible with existing core. In order to reduce core losses, better grades of steel and/or more advanced core design would require the complete replacement of the core while still reusing the coils. While possible, it’d typically be uneconomical to do except for large, custom transformers. The last option for increasing transformer efficiency is to provide line power meeting IEEE 519 THDi and/or removing harmonics on the load side before they reach the transformer.
In most cases the most economical method to increase a transformer’s efficiency is to replace older transformers with modern high efficiency models meeting current North American efficiency requirements.