Transformers will only work with an alternating current (a.c.) supply, in which the voltage (and current) reverse direction repeatedly and regularly. The rate at which these periodic reversals occur is known as the frequency and is measured in hertz (Hz). 1 Hz is equal to one cycle per second.
It is important to know the supply frequency when designing a transformer as it is partly responsible for determing the size of the magnetic core required and the number of turns of wire per volt. The frequency of the electrical supply is normally marked on the electricity supply meter and is generally 50 or 60 Hz, depending on location (50 Hz in the U.K. and 60 Hz in the U.S.). It can however be almost any value, depending on the application, e.g. aircraft typically use 400 Hz to permit smaller generators and transformers and some telcomms apps require 25 Hz. If you are unsure contact your electricity supplier or design engineer or ask our technical staff.