25Hz is the electrical frequency most associated with the railroad industry. The first 25Hz generator was built in 1895 by Westinghouse for the Niagra Falls project. The 25Hz frequency prevailed as the North American standard for low frequency AC for sometime because the Niagra project was so important and influential on electrical power system design. Eventually it would be phased out and replaced by 60Hz in North America and 50Hz elsewhere in the world as electrical systems became more accepted and incorporated globally.

Who uses 25Hz electrical systems?

Currently 25Hz is primarily utilized for powering and supporting the railroad industry. During the beginning of the 20th century, 25Hz power was much more readily available from commercial electrical utilities. The vast majority of urban subway systems used 25Hz power to supply their lineside rotary converters used to generate the DC voltage supplied to the trains. Since rotary converters work more efficiently with lower frequency supplies, 25Hz was a common supply frequency for these machines. Amtrak, Southeastern Pennsylvania Transport Authority (SEPTA), and Long Island Railroad (LIRR) are just a few examples of current railroad companies that still use 25Hz electrical systems to support their operations.

25Hz Rail System Frequency Converters: