A variac (variable autotransformer), also called a dimmer or dimmer coil, is an electrical device that provides smooth, continuously adjustable AC voltage output from 0V to above the rated input voltage.
How Does a Variac Work?
A variac consists of a toroidal (doughnut-shaped) transformer core wound with a single layer of copper wire. A carbon brush slides along the exposed top surface of the winding. As you rotate the control knob, the brush moves to different positions on the winding, changing the number of turns connected to the output — and therefore changing the output voltage.
Unlike a rheostat (which wastes energy as heat), a variac is a transformer — it's highly efficient (~98%) because it transfers power magnetically rather than resistively.
Types of Variacs
Single Phase Variac
A single phase variac operates on 230V single phase input and provides variable output from 0V to ~270V. Available in ratings from 1 Amp to 100+ Amps. Used in labs, testing, and light industrial applications.
Three Phase Variac
A three phase variac consists of three single phase variacs ganged together on a common shaft. Operates on 415V three phase input. Used in heavy industrial applications like furnace control, motor testing, and manufacturing processes.
Enclosed Variac Dimmer
An enclosed variac comes in a protective metal housing with a control dial, output terminals, and often a voltmeter. ServoKai manufactures enclosed variac dimmers in 10 Amp to 100+ Amp ratings for lab and industrial use.
Industrial Applications
- Heating Control — Furnaces, ovens, and heating elements
- Testing Laboratories — Equipment testing at variable voltages
- Manufacturing — Plastic moulding, printing, packaging machinery
- Lighting Control — Stage lighting, studio lighting
- Motor Speed Control — Variable speed drives for small motors
- Quality Control — Testing products at different voltage levels
Variac vs Dimmer — What's the Difference?
| Feature | Industrial Variac / Dimmer Coil | Electronic (TRIAC) Dimmer |
|---|---|---|
| Technology | Variable autotransformer | TRIAC/thyristor switching |
| Efficiency | ~98% (magnetic transfer) | ~80–90% (resistive losses) |
| Power Range | 1A – 1000A+ | Small loads only (up to ~10A) |
| Output Quality | Pure sine wave | Chopped / distorted sine wave |
| Inductive Loads | Fully compatible | Not suitable |
| Applications | Furnaces, motors, labs, industry | Residential lighting only |
ServoKai Dimmer & Variac Products
ServoKai manufactures dimmer coils, single phase dimmers, three phase industrial dimmers, and enclosed variac dimmers with manual or motorized control. All units use precision copper winding and high-quality toroidal cores. Browse our dimmer product range →