transformers are used to increase or decrease alternating current (ac) voltage in circuits. the transformer is usually built with two coils around the same core. the primary coil or input coil is connected to supply side while secondary coil supplies power to load. the second one is called the output coil. energy is transferred from the primary to the secondary by electromagnetism. in power grids many transformers are used. these are networks for delivering electricity from the generator and to the user.