Explain Balance Loading on Single and Three Phase Transformers?
A single-phase transformer with a series/parallel 120/240V secondary winding has two separate 120V secondary windings and is usually connected into a 3-wire system. When the winders are wired in series for 240 VAC, 120 VAC can be obtained at either between the neutral and centerpoint or between the centerpoint and 240VAC. If both 240 VAC and 120 VAC are going to be used, care must be exercised in distributing the load on the two 120V windings evenly, so each winding is carrying about half of the total 120VAC load if the 120 VAC load exceeds 5% of the total tranformer rating.
Similarly for a three-phase transformer, each phase should be considered as a single-phase transformer. When distributing single-phase loads between the three phases, each of the three windings should be evenly loaded with single phase loads.
Failure to balance loads can cause secondary voltage imbalances, additional transformer losses and high neutral currents. Significantly unbalanced loads can reduce the life of a transformer.