An autotransformer is an electrical transformer with only one winding. The "auto" prefix refers to the single coil acting alone. In an autotransformer, portions of the same winding act as both the primary winding and secondary winding sides of the transformer. An autotransformer has a single winding with two end terminals and one or more terminals at intermediate tap points. The portion of the winding shared by both the primary and secondary is the common section, while the portion of the winding not shared by both the primary and secondary is the series section. The primary voltage is applied across two of the terminals.
Autotransformers are often employed as a cost-effective substitute for 3-phase general-purpose distribution transformers for adjusting supply voltage for particular load requirements, provided that isolation from the supply line is not necessary. They are ideal for voltage adjustment for commercial and industrial machines, providing an efficient, low-cost way of serving the proper voltage to motors, compressors, lathes, CNC machines, and other industrial equipment requiring a step up or down from a building’s service voltage.
Autotransformers are similar to two-winding transformers but vary in the way the primary and secondary winding of the transformer are interrelated. In an autotransformer, one single winding is used as the primary winding as well as the secondary winding. The winding AB of total turns N1 is considered as the primary winding, and this winding is tapped from point C, and the portion BC is considered as secondary. When load is connected between secondary terminals, i.e., between B and C, load current I2 starts flowing. The current in the secondary winding or common winding is the difference of I2 and I1.
The primary and secondary windings of an autotransformer are linked together both electrically and magnetically, reducing the cost over conventional transformers. The main advantage of this type of transformer design is that it can be made a lot cheaper for the same VA rating, but the biggest disadvantage of an autotransformer is that it does not have the primary/secondary winding isolation of a conventional double-wound transformer.
In summary, an autotransformer is an electrical transformer with only one winding, and portions of the same winding act as both the primary winding and secondary winding sides of the transformer. Autotransformers are often used as a cost-effective substitute for 3-phase general-purpose distribution transformers for adjusting supply voltage for particular load requirements, provided that isolation from the supply line is not necessary.
