Variable Transformer

A variable autotransformer, known generically as a variac, is a type of transformer with just one coil that is shared by the primary and secondary side of the circuit. The ratio of the primary to the secondary windings is variable as is the ratio of the secondary to the primary voltage.

Inside the variac looks much like an oversized rheostat and there is a single partially exposed winding, which allows the movable wiper to make an electrical connection. The transformer’s primary connection is made on both ends of the winding, and the secondary connection is made to one end of the winding, known as the common connection, as well as to the movable wiper. When the wiper moves the transformer’s ratio changes.

One use for a variac is slowly bringing electronic equipment back into operation after long periods of disuse. Variacs are also used in experiments and electronics testing for simulating different line conditions and voltages. In addition you can use a variable autotransformer for powering electrical equipment .


  • There is a single layer winding on toroidal core of high-grade cold rolled grain oriented (CRGO) steel
  • A special durable contact surface is made on one side of the copper winding and a specially finished carbon brush slide smoothly on the surface
  • A contact arm with a carbon brush is fitted on the centre shaft
  • As the control knob is rotated, the carbon brush slides over the winding


  • Can enhance the output voltage in excess of twice the input voltage
  • Any desired voltage within the maximum range can be obtained by adjusting the carbon brush
  • Calibrated dial show approximately output voltage at no load
  • The contact arm can be moved both manually and by motor
  • Delivers an output voltage from 0 to above line voltage
  • Constructed with 99.9% pure copper
  • High grade CRGO steel is used for constructing coils
  • Low temperature rise
  • Wide assortment of current rating 4 Amp. To 1200 Amp
  • Spare carbon brushes provided up to 28 Amp
  • High quality carbon brush
  • Step-less voltage control
  • Short time overload capacity
  • Maintenance cost


  • Auditorium, hotels, conference hall, laboratories etc.
  • Servo stabilizer, rectifiers, battery charger
  • Welding equipments
  • Testing instruments
  • College & universities