Why does the transformer for UPS have to open the air gap?

When the transformer opens an air gap, adjusting the air gap can adjust the no-load current of the transformer and the voltage of the secondary winding, thus adjusting the charging voltage of the battery in UPS.

Not all transformers used in UPS need to open air gaps, and the charging voltage of storage battery in UPS can be adjusted by other methods, so there is no need to open air gaps in transformer cores.