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Transformer and inductor 

What is transformer?

A transformer is made up of primary and secondary coils as well as a core. The coils of a transformer are usually enamel-insulated copper or aluminium. A traditional transformer is assembled around an iron core made from transformer plates.

Transformers can be used for

  • decreasing or increasing alternating voltage.
  • galvanic isolation of alternating current circuits.
  • adjusting the mutual impedance of electronic circuits.

Transformer's operation

A current-bearing conductor forms a magnetic field around itself, and the field lines are circles circumventing the conductor. The total number of field lines is called a magnetic flux (Φ).

The SI unit name for magnetic flux is weber and its symbol is Wb   (1 Wb = 1 Vs).

Magnetic flux density (B) means the total number of field lines per unit of cross-sectional area (A).

The SI unit name for magnetic flux density is tesla and its symbol is T (1 T = Wb/m2).

B = Φ / A

When a current-bearing conductor is twisted into a coil and equipped with a continuous iron core, this creates a magnetic circuit in which the coil's internal field lines travel through the iron.  The whole of the magnetic flux in this case is inside the ring-shaped iron core. The length of the magnetic circuit is the length of the core-ring's centre line (l).

The magnetic flux density (B) of this magnetic circuit is determined as follows:

B = μr x μo x N x I / 1

In which

  • μr is the relative permeability of the substance, which varies from 500 to 5000 Tm/A depending on the type of iron
  • μ0 is the permeability of vacuum (and very accurately of the air), the value of which is 4 x 10-7 Tm/A 


The input NI is called ampere-turns or magnetomotive force (mmf). This is indicated with the symbol Fm.

Muuntaja periaatekuva

The amount of magnetomotive force per length of the magnetic circuit (l) is referred to as magnetic field strength (H).

The unit for magnetic field strength is ampere/metre and its symbol is A/m.

H = NI / l

When alternating current affects the transformer's primary coil, it creates an alternating magnetic flux in the iron core. This alternating flux creates an alternating electromotive force (emf) in the transformer's secondary coil. This phenomenon is called mutual induction.

The SI unit name for electromotive force is volt and its symbol is V.




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