Column transformers

Nowadays these very large variable transformers are used mainly for test applications, where it is important to have a variable voltage with a nearly perfect sine wave. However, there are some industrial applications where a better alternative is not available and these include glass production, anodising or large power supplies requiring voltage regulation.

The column transformer comprises a number of tubes wound with copper strip, edgeways. This means that the windings are close together but because of their cross-sectional area they can carry higher currents. Brushes move across the face of the windings to give voltage adjustment. Several columns can be connected in parallel to give higher current ratings. The brush carrier is driven up and down by a worm gear mechanism, which is very precise but does not lend itself to quick response times. However, this is often an advantage because it helps to prevent "hunting".

Double the power

Because of its unique construction it is possible to handle twice the normal power load of a variable, column transformer by fitting a second set of brushes that move in the opposite direction to the first set. Effectively this produces a second voltage sine wave that is 180 degrees out of phase with the first. By connecting the output to a fixed transformer with double primary windings it is possible to double the power rating of the variable transformer. Furthermore, the variable transformer could be used in a delta configuration thereby producing the same VA but with a higher voltage and a lower current than that of a star connected unit. It is the current carrying capacity that has the greatest influence on the price and so the delta configuration offers a greater economy. Although hand operated versions are manufactured, most transformers of this type are motorised.

The pdf below shows the main components of a double brush variable column transformer.