What kind of material of transformer?

For servo type stabilizer,we have two types, one 100% copper and the other is copper with aluminum.It depends on your requirement. In fact,those two have no difference if normal work well. Only except the long life. Copper is better and also higher price.For relay type stabilizer,we use toroidal coils, the material is aluminum. Comparing with square coils, toriodal coils with high efficiency.