Autotransformers are a special type of power transformer with only one winding. A portion of this single coil serves as a part of both the primary and secondary winding. Different voltages are obtained by tapping or connecting at different points along the winding, which has at least three taps where electrical connections are made. Both the voltage source and the electrical load are connected to two taps. The tap at the end of the winding serves as a common connection to both circuits. Typically, autotransformers are used in low-power applications and to interconnect systems operating at different voltage classes. They may be waterproof, rated for outdoor use, or equipped with NEMA enclosures. 

Product specifications for autotransformers include operating frequency range, maximum primary voltage rating, maximum secondary voltage rating, maximum secondary current rating, power rating, and operating temperature. There are three basic types of products: single phase, three phase (polyphase) and variable. Single phase autotransformers, as their name suggests, operate with single phase voltage. Three phase autotransformers are polyphase devices in which the three primary windings are connected together and the three secondary windings are connected together. Variable autotransformers or variacs have a sliding tap. 

Core type and cooling method are important specifications to consider when selecting autotransformers. There are three main choices for core type: laminated, split, and toroidal. Autotransformers with other unlisted or proprietary core types are also available. Choices for cooling method include air cooled, oil filled, and water cooled. Dry-type or air cooled power transformers do not have a core. Instead, the transformer enclosure or housing is ventilated to allow air flow. Oil-filled autotransformers immerse the winding and core in oil to keep the device cool. The oil is also used an insulator.  

Autotransformers differ in terms of winding turns and output voltage. With step-up transformers, the secondary voltage is larger than the primary voltage. With step-down devices, the secondary voltage is smaller than the primary voltage. Variacs or variable autotransformers (autoformers) have a setting for changing the turn ratio as needed. One-to-one devices have a turn ratio of 1:1, or near 1:1. There are two choices for output voltage type: alternating current (AC) and direct current (DC). Typically, the AC waveform output voltage values for autotransformers are expressed as root mean square (RMS) values.