Auto Transformer is a transformer having part of its winding included in both the input and output circuit. A simple autotransformer connection is shown in Fig. 1. Note that because there is a common portion of the winding (Section 1-2), there is no isolation between the input and output circuits. Also note that there is only one winding. As shown in the diagram, the line current is 10A for a 2000VA output (10A x 200V). The load current is 20A at 100V or 2000VA output.
The winding is tapped at 100V so that the autotransformer functions as a 200V-to-100V stepdown transformer. While this is an acceptable transformer connection, its application is rather limited because of the absence of isolation between input and output circuits.