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What Is a Variable Auto Transformer or Variac?
A variac is an auto transformer with a movable output voltage adjusting mechanism.
It is designed on a toroidal copper wound coil shared by input and output, and a carbon brush moves over the winding track and picks off the voltage of the turns beneath it. So the output changes smoothly as you adjust the carbon brush, attached to the armour, with the help of a knob.
It is also called a dimmer, variac or variable auto transformer — all are the same device.
The variac is also the heart of every Servolink servo stabilizer. The variac you buy from SERVOLINK comes from the same winding station, the same copper and the same testing as the ones inside our stabilizers, and is built as per IS 5142.
The enclosed variac can be made in two types, as per your application:
For manual operation — a knob on top of the enclosure with an output voltage marking plate, to set the required voltage; round handles on higher capacities.
For automatic operation — the whole motorized variac mechanism is fitted inside the enclosure. It works automatically and can be operated by push buttons for forward and reverse rotation of the carbon brush assembly, to set the required output voltage.
The mains gives one fixed voltage, but many jobs need a different one — and need to change it smoothly. An appliance maker has to test every product at low, normal and high voltage before it leaves the factory. A transformer repairer has to apply voltage slowly to a freshly rewound coil, not all at once. A lab has to run experiments at exact voltage steps. A heater, furnace or lamp needs its power set to a level, not just switched on and off.
Doing this with fixed tappings or resistors is crude: the voltage jumps in steps, resistors waste power as heat, and the setting is never exact. A variac solves it with one turn of the knob — any voltage from zero upwards, smoothly, with almost no loss.
| Make | Servolink |
| Type | Continuously Variable Voltage Auto-Transformer |
| Primary Side (Input) | 240 V, 1 Phase, 50 Hz |
| Secondary Side (Output) | 0–270 V (Continuously Variable), 1 Phase, 50 Hz |
| Output Current | 4 Amp to 300 Amp (Maximum & Continuous Rating) |
| Cooling | Natural Air Cooled |
| Coil Material | CRGO Silicon Steel Core, 99.9% Pure Copper |
| Insulation | Class B, Min. 5 MΩ @ 500 V DC |
| Dielectric Strength | 2.5 kV RMS for 1 Minute |
| Operating Temperature | 15 to 50°C |
| Humidity | Up to 95% |
| Operation | Manual (Hand Wheel) or Motorised (240 V AC Sync. Motor with Gear Drive) |
| Sweep Time (Motorised) | 45 Seconds (Full Range), other on request |
| Terminals | Standard Input & Output Connections; IEC Compliant Terminals in Enclosed Type |
| Brush | Carbon Brush, Contact Resistance < 0.2 Ohm |
| Distortion | Negligible Power Factor & Waveform Distortion |
| Conformity Standard | IS 5142 |
| Mandatory Spares | One Set of Carbon Brushes |
Three phase variacs are made with three variac coils on one common shaft, with an output of 0–470 V.
A normal transformer has two separate windings and gives one fixed output voltage. A variac has one winding with a sliding brush, so it gives an adjustable output, but without isolation from the mains. When isolation is needed, use it with an isolation transformer.
The input is connected to a tapping on the winding, and the winding continues beyond it. When the brush moves past the tapping, the output rises above the input — up to 270 V on our single phase variacs.
Yes, the rated current can be drawn at any output voltage setting. Do not exceed the rated current, because the brush and the track at the brush point carry the full load current.
Take the highest current your load will draw, and choose a variac with a rating above it. For motors, heaters with a high cold current, or lamps, keep extra margin for the starting current.
Keep the winding track clean and dust-free and check the carbon brush. Replace the brush when it is worn, and never let the metal holder touch the track. One set of carbon brushes is supplied as a spare.
A variac driven by a 240 V AC synchronous motor with a gear drive instead of a hand knob. It is controlled by push buttons or by the control card of a servo stabilizer, and moves through its full range in about 45 seconds.
ISO 9001:20152024–2027
NSIC Registration2025–2027
MSME / UdyamRegistered 2021
Importer–Exporter CodeIssued 2019
Industrial Registration2010 record
CE — Certificate of Compliance2015–2016 record
NSIC–ONICRA Rating2016–2017 rating
ISO 9001:20152021–2024
ISO 9001:20152018–2021
ISO 9001:20082015–2018
ISO 9001:20082011–2014
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Experience the unmatched reliability and efficiency of our Variable Auto Transformers, ensuring stable power supply and protection for your critical electrical equipment. Join the satisfied clientele who trust Servolink for their Variable Auto Transformer requirements.
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