Hi 4pyros, i googled it before and even read it on wiki.
I've highlighted the the point here.Autotransformer is a single-winding transformer in which the primary coil is a fraction of the entire winding for voltage step-up, or the secondary coil is a fraction of the entire winding for voltage stepdown. A Transformer whose output voltage is adjustable from zero (or some minimum value) to maximum. For this purpose, one winding (usually the secondary in a two-winding transformer) can have a number of taps.
The question is same, as said above (from wiki/int): I CAN use an ordinary two winding transformer, by i don't know why tcmtech considered an 'autotransformer'. That was my question from the previous post. I wanted to know the reason behind tcmtech's suggestion. TYAutotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission. Another application is in industry to adapt machinery built (for example) for 480 V supplies to operate on a 600 V supply. They are also often used for providing conversions between the two common domestic mains voltage bands in the world (100-130 and 200-250). The links between the UK 400 kV and 275 kV 'Super Grid' networks are normally three phase autotransformers with taps at the common neutral end.