if the secondary side of the transformer is open, the maximum voltage is present. However, no power is transformed, because there is no current flowing. If the secondary side of the transformer is short-circuited, the maximum current flows. but not power is transformed here either, because no secondary voltage is produced. Power is only transformed in the load range. The maximum power is transformed, if the no-load voltage has decreased by about half or the short-circuit current has sunk by approx.