The lower the operating frequency the more flux lines that are generated. Operating a transformer at 50Hz generates 20% more flux lines than at 60Hz. As the number of flux lines approaches the magnetic material’s limit, the heat in both the transformer’s core and its coil wires increases, and under certain circumstances, unpredictably so. This can result in a transformer that exceeds safe temperature levels. Therefore, a transformer designed to run at 50Hz will simply run cooler at 60Hz. But one designed only for 60Hz may overheat if subjected to 50Hz.
“In order to accommodate 50Hz operation, the transformer must employ a magnetic core material that can handle the added flux lines.