Transformer is used for converting large alternating current at low voltage into small current at high voltage and vice-versa.
The given source of emf say AC mains is always connected to the primary coil. When alternating current flows through the primary coil, then in each cycle of current, the core is magnetised once in one direction and once in opposite direction. Hence, an alternating magnetic flux is produced in the core. Magnetic flux passing through secondary also changes due to repeated magnetisation and demagnetisation of the core. Hence, by mutual induction an emf is induced in secondary coil. Hence, voltage of secondary does not depend upon frequency of source, but on voltage in primary and ratio of number of turns. VpVs=NpNs=r
