Magnetic Flux Density of Transformer – Formula, Definition, and Explanation

The magnetic flux density expressed in Tesla is a vector quantity and it is defined as the force acting per unit current per unit length on a wire placed at right angles to the magnetic field. The magnetic flux density is indicated using symbol B.

Both magnetic flux density and magnetic field intensities are linked and one can be calculated if other is known for a given permeability.

The mathematical relation between magnetic flux density and magnetic field intensity formula is expressed as:

B = µH


  • B = Magnetic flux density
  • µ = Permeability
  • H = Magnetic field intensity

In case of transformers, the flux density has impact of the core area. The higher flux density results in reduced core area which in turn reduces diameter of the circumscribing circle, and hence the length of mean turn of winding, as a result of which the cost saving for copper and iron will be achieved.

However high flux density has one downfall, the magnetizing current will be increased which in turn will lead to saturation and higher iron losses thereby necessitating the cooling requirements.

The value of flux density of transformers also depends on the intended service, environment, type and other conditions. Given below is list of values of flux densities for various types of transformers:

TransformerFlux density value
Type of TransformerValue of flux density
Distribution transformer hot rolled silicon steel1.1 to 1.4 Wb/m²
Power transformer hot-rolled silicon steel1.2 to 1.5 Wb/m²
Transformers up to 132 kW cold rolled grain oriented steel1.55 Wb/m²
275 kV Transformer cold rolled grain oriented steel1.6 Wb/m²
400 kV Transformer cold rolled grain oriented steel1.7 Wb/m²

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