The definition of a transformer is a device that changes the voltages in an alternating current circuit.
You may have heard of an autotransformer or a buck and boost transformer and these terms are usually being used for the same type of device just highlighting different aspects. A transformer does not need to be a buck and boost to be autotransformer and it does not need to be an autotransformer to be buck and boost but often the two elements go together.
The word auto in autotransformer really just means one or single not really “automatic” or “automated” in the way we usually think of it. It is an autotransformer because it only has one inductive (magnetic) winding shared by both the primary and secondary.
Buck and Boost
Buck just means that it decreases the voltage and boost means it increases it. A buck and boost transformer means that it can both increase or decrease the voltage.
What is their application?
Buck and Boost autotransformers are often used to make small changes in voltage, say from 208v to 240v (boost) or from 240v to 208 (buck). They are usually efficient and inexpensive when only small changes are needed, whereas a traditional two coil transformer is more practical for larger changes.
Most of these transformers will have multiple tap points for different output and input voltages and can often be connected in different configurations to perform a wide range of functions like in the case of the Emerson Sola HD.
One major consideration with an autotransformer is that there is no isolation between the primary and secondary so a failure of the isolation of the windings of an autotransformer can result in the input voltage being applied to the output and component damage. There is also greater likelihood of harmonic and ground fault issues because of this “mixing” of primary and secondary.