To obtain 120 VAC from a control transformer with a line voltage of 240 VAC, how should the primaries be connected?

To obtain 120 VAC from a control transformer with a line voltage of 240 VAC, the primary windings of the transformer should be connected in parallel. When transformers have multiple primary windings, connecting them in parallel allows each winding to share the total voltage. In this scenario, when two 240 VAC windings are suitably connected in parallel, the combined effect will yield an output of 120 VAC at the secondary side of the transformer.

When connected in parallel, each winding is energized by the same voltage source, and their voltages remain equal, effectively halving the voltage for the load connected to the secondary. This configuration is crucial because it facilitates the correct voltage output necessary for operation within the desired specifications.

By contrast, connecting the primary windings in series would result in a higher input voltage being applied, which could exceed the transformer’s rated capacity and compromise function and safety. Therefore, parallel connections are the appropriate choice for achieving the required voltage level from a transformer designed for higher input voltages.