What is an Auto Transformer Auto-transformer is a single-winding transformer that converts electrical energy to multiple voltage levels. They are commonly used in power transmission systems to connect different machines of the grid which different voltage levels. Auto-transformer has smaller and lighter than traditional two-winding transformers. This makes them easier to install and transport. Auto-transformer has fewer components, and are less expensive to manufacture and maintain. Auto-transformers can be classified into two main types: step-up and step-down. Step-up auto-transformers are used to increase the voltage level, while step-down auto-transformers are used to decrease the voltage level. Both types of auto-transformers use a single winding, which is divided into two sections: the primary and the secondary. The primary section is connected to the source voltage, while the second section is connected to the load. The transformer ratio of an auto-transformer is defined as th