Transformer

Transformer is a device that changes (increases or decreases) the magnitude of alternating voltage or current based on the phenomenon of electromagnetic induction. A transformer has at least two windings of insulated copper wire linked by a common magnetic flux but the windings are electrically insulated from one another.

The transformer windings connected to a supply source, which may be an ac main or the output of a generator, is called primary winding. The transformer winding connected to the load RL is called the secondary winding. In the secondary winding, emf is induced when a.c. is applied to the primary.

The primary and secondary windings, though electrically isolated from each other, are magnetically coupled with each other. A transformer is a device which transfers electric energy (or power) from primary windings to secondary windings. The primary converts the changing electrical energy into magnetic energy. The secondary converts the magnetic energy back into electric energy.

An ideal transformer is one in which

  • the resistance of the primary and secondary coils is zero
  • there is no flux leakage so that the same magnetic flux is linked with each turn of the primary and secondary coils
  • there is no energy loss in the core

The coils, made of insulated copper wire, are wound around a ring of iron made of isolated laminated sheets instead of a solid core. The laminations minimize eddy currents in iron. Energy loss in a transformer can be reduced by using the laminations of soft iron for the core and thick high conductivity wires for the primary and secondary windings.

Secondary an open circuit

Suppose the current in the primary changes the flux through the core at the rate dφ/dt. Then the induced (back) emf in the primary with Np turns is given by

Ep = –Np dφ/dt

The induced emf in the secondary windings of NS turns is

Es = –Ns dφ/dt

Ep/Es = Np/Ns

Secondary not an open circuit

Suppose a load resistance RL is connected across the secondary, so that the secondary current is IS and primary current is IP. If there is no
energy loss from the system,

Power input = Power output

EpIp = EsIs

Ip/Is = Es/Ep