151015-0739 EDT
hublanc:
If frequency is the same, then any transformer can be operated at any voltage below its rating. But you must stay within the current ratings of the windings. Also with some core materials (not standard transformer iron) there may be some performance problems at very low voltages. There is no heating problem at low voltage if you stay within the current ratings of the windings.
If the frequency is different, then maximum continuous input voltage is (original rated voltage) * (new frequency)/(rated frequency). So as frequency goes down so must the voltage.
Basically the maximum volt-time integral has to be constant to prevent severe saturation of the core. Most transformers are designed to go somewhat into saturation each cycle. This is to achieve maximum power capability with the minimum amount of iron and copper.
Volt-time integral is the summation of all the short time (volt)*(small time increment) values per cycle.
If you go higher in frequency than the rated frequency, then other factors create limitations.
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