Map15, Jeevan Jyot Society, Opp.Indra Complex, Lalbaug Road, Vadodara-390011

Variac

A Variac (variable autotransformer) is a type of adjustable transformer used to vary AC voltage smoothly. The design and application differ depending on whether it’s single-phase, two-phase, or three-phase.

1. Single-Phase Variac

Design of Single-Phase Variac

  •  Consists of a toroidal laminated core with a single winding.
  • A carbon brush (movable contact) slides over exposed winding turns.
  •  Works as an autotransformer (no isolation between input and output).
  •  Input: typically 1-phase AC (e.g., 230V in India).
  •  Output: adjustable (e.g., 0–270V).

Working Principle of Single-Phase Variac

Based on electromagnetic induction:

  • Voltage is proportional to the number of turns.
  • Moving the brush changes the effective turns → changes output voltage.

Applications of Single-Phase Variac

  • Lab testing of electrical equipment
  •  Speed control of small AC motors
  •  Light dimming
  •  Testing appliances under varying voltage

2. Two-Phase Variac

Design of Two-Phase Variac

  • Two separate windings placed 90° electrically apart.
  • Requires two-phase supply (or phase-shifted supply).
  • May use:
    1. Two single-phase variacs mechanically linked
    2. Or a specialized dual-winding toroidal core

Working of Two-Phase Variac

  • Produces two voltages with a phase difference of 90°.
  • Output magnitude is varied simultaneously for both phases.

Applications of Two-Phase Variac

  • Early industrial systems
  • Control of two-phase motors
  • Laboratory simulation of polyphase systems

3. Three-Phase Variac

Design of Three-Phase Variac

  •  Built using:
    1. Three single-phase variacs coupled together, or
    2. A single three-phase core with three windings
  • Windings arranged in:
    1. Star (Y) or
    2. Delta (Δ) configuration
  • Single shaft or gear system controls all three brushes simultaneously.

Working of Three-Phase Variac

  • Maintains balanced three-phase output.
  • Output voltage of all phases changes together

Applications of Three-Phase Variac

  • Testing of 3-phase equipment (motors, transformers)
  • Industrial voltage regulation
  • Soft starting of large motors
  • Power system experiments
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