Transformers rated for 60 Hz ONLY are sometimes chosen to be installed in a location that has a 50 Hz Distribution system.
Applies to MOST Transformers made by SquareD/Schneider Electric. Exceptions are 9070 series Control Transformers and Export Model Transformers which are dual rated for 50Hz and 60Hz.
Domestic (USA) power systems are rated 60 Hz. Many other power systems in the world are rated at 50 Hz.
A 60 Hz rated transformer can only be used on a 60 Hz system (with certain exceptions -- read further.) Whereas a 50 or 50/60 Hz rated transformer can be used on either a 50 or a 60 Hz rated system.
Exception: If the energizing voltage does not exceed 83% of the transformers nominal voltage, the transformer can be used. Remember that transformers are simple ratio devices and the output will also be lower. Another important factor to remember is that the transformer`s capacity will also have to be derated at the same ratio of the applied voltage divided by the nominal voltage.
: An EE30T3H, 30kVA 480 delta to 208Y/120 transformer 60 Hz, can be utilized on a 50 Hz system as long as the applied voltage does not exceed 50/60 or 5/6 of the nominal 480, or rather 400 volts. With 400 volts 50 Hz applied, the output voltage will be 400/480 * 208 = 173 volts line-to-line, and 100 volts line-to-neutral, or 173Y/100 50 Hz. As well, the capacity or kVA of the transformer will be 400/480 * 30 = 25kVA. Transformers cannot change frequency, thus frequency in = frequency out.