Variable Autotransformers: How a Variac Works and When to Use One

A variable autotransformer — universally called a variac — is the simplest way to obtain a smoothly adjustable AC voltage: one winding, one sliding contact, and a knob. That simplicity is also its limitation, and understanding both is what separates a correct application from a dangerous one. This guide covers the principle, the honest comparison with isolation transformers, and how to size one.

The principle: one winding, one brush

An autotransformer has a single continuous winding. The input connects across the whole winding; the output taps part of it through a carbon brush that slides on a machined track as you turn the knob. Because input and output share copper, the device transfers most of its power conductively rather than magnetically — which is why a variac is small, light, efficient and cheap per kVA compared with a double-wound transformer of the same rating.

The trade-off you must respect: no isolation

The same shared winding means the output is not galvanically isolated from the mains. The output neutral reference can sit at line potential, and a fault on the input appears at the output. For bench work on live circuits this is acceptable with proper practice; for powering equipment that a person touches, or for medical and wet environments, an isolation transformer is the correct device. State this in any specification: a variac adjusts voltage, it does not protect people.

Single-phase and three-phase types

• Single-phase (TDGC2 style): one winding and brush, typically 0-250V output on a 220-230V input. Common from a few hundred VA to tens of kVA.
• Three-phase (TSGC2 style): three ganged windings on one shaft so all phases track together. Used where a balanced adjustable supply is needed — motor testing, transformer energisation, laboratory rigs.
• Motorised variants: the same machine with a drive motor on the shaft for remote or programmed voltage ramps.

Sizing: current first, kVA second

A variac's rating is a current rating wearing a kVA label. The winding and brush carry the load current at every output voltage, so the available kVA falls as you dial down: at 50% voltage you get roughly 50% of the rated kVA, but the full rated current. Size by the load's maximum current, then check that the kVA at your lowest useful output voltage still covers the load. Our variac selection guide walks the seven-step version of this calculation, including duty cycle.

Where variacs are the right tool

• Laboratory and type testing: ramping voltage slowly to observe behaviour, heating runs, dielectric pre-checks.
• Transformer and motor energisation: soft first energisation to limit inrush.
• Lighting and heating control: where a smooth analog adjustment beats switching.
• Calibration benches: providing a known adjustable reference voltage.

Where they are the wrong tool: anywhere isolation is required, anywhere the load is highly capacitive or inductive beyond the brush rating, and anywhere untrained users will touch the output — use an isolation transformer or a regulated stabilizer instead.

Maintenance reality

The brush and track are consumables. Keep the track clean and dry, inspect the brush face for arcing marks, and never exceed the rated current at low output voltage — that is how tracks get gouged. A well-maintained variac lasts decades; a neglected one fails at the brush.

Ordering fields

• Input voltage and frequency; output voltage range (e.g. 0-250V, 0-430V, 0-500V).
• Rated output current — the governing number.
• Single or three phase; ganged tracking required for three phase.
• Manual knob or motorised drive; analog meter or digital display.
• Enclosure and mounting for the installation environment.

We build TDGC2 single-phase and TSGC2 three-phase variable autotransformers from 500VA to 40kVA, manual and motorised, with meters and enclosures to suit. Tell us the load current and the output range you need; we return the correctly sized unit rather than the nearest catalogue line.