Choice of Flux Density

• The output equation and the emf equation indicate that the choice of flux density Bm affects the core area Ai.

• Higher flux density reduces the core area.

• This will reduce the diameter of the circumscribing circle, thereby reducing the length of mean turn of winding.

• Thus, there will be a huge saving in the cost of copper and iron.

• But, a higher flux density, while saving copper and iron, will also result in increased magnetizing current leading to saturation, increased iron losses requiring elaborate cooling arrangements.

• The choice of Bm also depends on the type and service conditions of the transformer.

• It has already been pointed out that a distribution transformer should be designed for lower iron losses giving good all day efficiency.
• Hence, for distribution transformers, a comparatively lower flux density is assumed.
• The permissible values of Bm for various types of transformers are given in Table.The variation depends on the rating of the transformer as well, higher values for higher rating.