What Makes an Autotransformer Different?

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-boost transformer, and these terms are usually used for the same type of device; they just highlight different aspects. A transformer does not need to be a buck-boost to be an autotransformer, and it does not need to be an autotransformer to be buck-boost, but the two elements often go together.

Autotransformer

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-boost

Buck just means that it decreases the voltage, and boost means it increases it. A buck-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. They 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 a greater likelihood of harmonic and ground fault issues because of this “mixing” of primary and secondary.

We use Buck and Boost transformers in many residential applications where voltages from the utility are too high and are causing damage to inverter boards. By dropping the voltage by just a little bit we can keep the voltage in the safe zone for the equipment even when the utility allows it to rise above 253V.

We also recently used a 3-phase version to “boost” the voltage slightly on a commercial site where the voltage was dropping too low during periods of the day.

—Bryan

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3 responses to “What Makes an Autotransformer Different?”

  1. I’ve seen these utilized in 600 volt 3 phase York and carrier rooftop units in our Canadian market so they can still use 460 volt Single phase condenser and venter motors for years. Thanks for all your tech tips in the past and looking forward to reading tech tips coming up in the future Brian.

  2. I don’t care if they call you “Geeky Brian”, you are amazing. Keep up the great work! Never before have I been so interested in any HVAC talk or felt the need to make all my techs listen to anything, until you. Your work is very much appreciated, you’re an amazing man.

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