An overview of variable transformers
11 September, 2013
In a similar way to resistors, variable transformers are available in two forms, toroidal and linear (or column type).
The toroidal transformer comprises copper wire wound onto a toroidal core, manufactured from electrical grade, grain oriented, strip steel. A roller brush (or wiper) is moved across the face of the windings, which have a ground finish to provide a good contact path. The brush is connected to a shaft so that it can be rotated and a second, pick-up brush in contact with a brass plate on the brush gear, allows current to flow to a terminal connected to the electrical circuit. The main winding is connected across the supply and the load is connected to the brush and supply neutral. Voltage is adjusted by rotating the shaft.
For economy reasons the majority of variable toroidal transformers have a single winding and are used as autotransformers. Versions can be made with separate primary and secondary windings or a separate fixed transformer can be connected between the variable transformer and the load. This is referred to as "primary regulation".
The variable, maximum, output voltage can be the same as the supply voltage or it can be up to 15% greater, which is very useful for testing equipment to ensure correct operation when it is subjected to a mains supply with an "over-voltage". Units can be ganged for three-phase operation and can be centre tapped for use in buck-boost applications such as voltage stabilisers.
The unique features found only on REO transformers include a roller type brush, instead of a wiper and a collet locking mechanism on the adjustment spindle. The roller brush greatly prolongs the operating life and the spindle adjustment allows the shaft position to be changed, quite easily using a spanner, for door or back-panel mounting.