Product Details

What is an auto transformer?

An auto transformer (sometimes called autostep down transformer or autostep up transformer) is an electrical transformer with only one winding per phase. The ‘auto’ (Greek for “self”) prefix refers to the single coil acting alone. In an auto transformer, portions of the same winding act as both the primary and the secondary sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings that are not electrically connected.

Auto Transformer


What is an auto transformer used for?

It is often used to step up or step down single phase voltages in the 110-120 V range and voltages in the 230-240 V range. For example, providing 110 V supply from a 230 V input, allowing equipment designed for 110V to be used with a 230 V supply. This allows US electrical equipment to be fed from the higher European voltage. Auto transformers can also be used to supply European 230 V appliances from a 110V supply in countries outside Europe. In all cases, the supply and the auto transformer must be correctly rated to supply the required power. Likewise, a three phase supply could be stepped-up or stepped down. For example, providing a 415V 3 phase supply from a 208V 3 phase input.


Limitations of an auto transformer

An auto transformer does not provide electrical isolation between its windings in the way that an ordinary transformer does. If the neutral side of the input is not at ground voltage, neither is the neutral side of the output. A failure of the isolation of the windings can result in full input voltage being applied to the output. This is an important safety consideration when deciding to use an auto transformer in a given application.


What types of auto transformer do ETC manufacture?

Both single and three phase auto transformers are manufactured by Express Transformers. These can be configured as either step-up or step-down transformers. Additionally, they can be supplied fitted into a suitable steel enclosure.


Custom auto transformers and how to order

For a quote, we would need to know the input voltage. We would also need to know the number of phases and the required output voltage with the power needed. Any temperature requirements would also be needed. For example, input 415V 3 phase to give 170 Volts 3 phase at 20 amps with a Class E rise.


Examples of our work can be seen in the gallery.

Please feel free to contact us to discuss your design requirements. Our lead engineer details are listed below and they are very happy to discuss your particular project needs with you.