Autotransformers

Our single-phase and three-phase autotransformers

An autotransformer converts one AC voltage into another using a single winding per phase. We make three models, and each product page shows the price of every rating:

• Reversible 230 V ↔ 120 V autotransformer, from 100 VA to 10 kVA: a North American 120 V appliance on 230 V mains in the UK or Europe, or a European 230 V appliance on a 120 V supply.
• Three-phase autotransformer 400 V ↔ 230 V + N, from 3 to 100 kVA: machines designed for a 3 × 230 V network, on a 3 × 400 V supply.
• Reversible custom autotransformer, from 100 VA to 5 kVA: your voltages, with the price online.

Other voltages are possible: single-phase with the custom model, three-phase on request. Need an adjustable output voltage? That is the job of the single-phase voltage regulator, a variable autotransformer with a sliding contact.

What is an autotransformer?

An autotransformer (also written auto transformer or auto-transformer) is a transformer whose input and output share the same winding: the output voltage is taken from an intermediate tap on that winding. The ratio between the number of turns used at the input and at the output sets the voltage ratio. Because part of the power flows straight from input to output through the shared winding, an autotransformer is smaller and lighter than a two-winding transformer of the same rating, especially when the two voltages are close, such as 230 V and 120 V, or 400 V and 230 V.

The trade-off: input and output are electrically connected, so an autotransformer does not separate the equipment from the mains. It changes the voltage, but not the frequency: an appliance designed for 60 Hz still receives 50 Hz on UK and European mains.

Autotransformer or isolation transformer?

• Autotransformer: to change a voltage when galvanic isolation is not required, with a more compact unit.
• Isolation transformer: when the installation must be separated from the mains, a neutral created or an earth loop eliminated. See our isolation transformers, and the 220 V to 110 V transformer with separate windings.
• Safety isolating transformer: for a safety extra-low voltage (SELV), which an autotransformer cannot provide. See our safety transformers.

120 V, 230 V, 400 V: typical applications

North American appliances in Europe, and the reverse

An appliance designed for the 110–120 V supply of the United States or Canada can run on 230 V mains, in the UK or elsewhere in Europe, through a 230 V to 120 V autotransformer. As it is reversible, the same model supplies 230 V to a European appliance on a 120 V supply. Check the frequency on the appliance's rating plate: “50/60 Hz” is fine, but an appliance designed for 60 Hz only (some motors and clocks) may run slower or overheat on 50 Hz.

Three-phase machines on a 400 V supply

A machine designed for a 3 × 230 V network connects to a 3 × 400 V supply through the 400 V ↔ 230 V + N three-phase autotransformer. For a North American machine at 3 × 208 V, or any other three-phase voltage, other models are available on request. If the machine requires an isolated supply, choose a three-phase transformer with separate windings instead.

110/220 V and 220/380 V: the former names

“220 V” and “380 V” are the former names of the 230 V and 400 V networks in continental Europe, and “110 V” that of the North American 110–120 V supply. A 110/220 V or 220/380 V autotransformer therefore meets the same need as our 120/230 V and 230/400 V models.

How to choose an autotransformer

Voltage and direction: note the supply voltage and the voltage of the equipment. The 230 V ↔ 120 V model works either way, depending on how it is connected: as a step-down autotransformer from 230 V to 120 V, or as a step-up from 120 V to 230 V.

Rating: add up the power of the equipment to be supplied, multiply by 1.25 and choose the next rating up. Motors and compressors draw a high current when starting, so allow a margin.

Protection: upstream, an aM fuse or a type D circuit breaker, both of which withstand the inrush current at switch-on. Connection must be made with the power off, by a qualified person.

Wound and tested in our Gailhan workshop

ABL Transfo has been manufacturing transformers since 1993 in Gailhan, in the Gard (southern France). Our autotransformers are wound, wired and 100 % tested there before they leave. Standard single-phase models are dispatched from France within 24 to 48 hours. With a trade account, prices are shown excluding VAT, with an immediate 5 % discount.

The custom autotransformer

Need another voltage? The custom autotransformer is configured online, from 100 VA to 5 kVA: input and output voltages, IP rating, and cable, terminal block or socket connections at the input and output. The price is shown immediately, and manufacturing takes from 48 hours to one week depending on the rating. For a three-phase unit or a higher rating, request a quote: we reply within 48 hours. See also our full range of custom-built transformers.

Compare all the options: 400 V to 230 V transformers.