Below is a list of Auto Transformers manufacturers from McHenry, Illinois. An auto transformer, also referred to as an autoformer, is a general term for an electrical transformer with one winding, as compared to more traditional two-winding transformers. Transformer windings are coils wrapped around a magnetic core. Since auto transformers have fewer windings and a smaller core, they are often used as alternatives to two-winding transformers such as isolation transformers, because they are often more efficient as well as being less-expensive and physically smaller.