If 50Hz transformers can be run at 60Hz with no ill effects, why do we still offer 60Hz models?
Answer:

Yes, a 50 Hz transformer can run on 60 Hz with no ill effects. The transformer design does not impose any practical upper limit on how much the supply frequency can exceed the rated frequency, but a supply frequency lower than the rated frequency has the same effect as an overvoltage.

Therefore running a transformer at a frequency lower than its design rating can cause damage to the transformer. 60Hz transformers are smaller and less expensive than 50/60HZ transformers. We do not recommend specifying a lower rated frequency than needed since a 60Hz transformer has 20% higher rated power than a 50Hz transformer of the same size and weight. The design construction of a 50Hz transformer is usually larger than that of the 60Hz.

The technical question and answer above were featured in the June 22, 2011 edition of the SolaHD Wednesday Weekly online newsletter.