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Why is the voltage reading high on an unloaded control transformer?

Industrial control transformers and small dry type encapsulated transformers (2kVA and below) have a turns ratio compensation on the low voltage winding that adjusts the voltages between no load and full load conditions.  When there is no load on the transformer the secondary voltage will read high and then as you add load to it the voltage draws closer to the listed voltage.  For instance if you power up a control transformer with a 120V secondary and take a voltage reading with no load on it, you might get a voltage reading of around 135V and then as you add load to the secondary it will get closer and closer to 120V.