Transformer Flux Density Selection

• From the above discussion it is seen that valve of maximum flux density controls the core area.
• Thus if we select higher value of flux density (Bm) net core area (Ai) is reduced. So there is reduction of iron weight and hence cost of iron.
• This also reduces diameter of circumscribing circle which reduces length of mean turn. This reduces I^2 *R losses as well as there is saving in cost of copper.
• However iron losses are increased due to higher value of flux density, in addition to it temperature is increased which requires more cooling arrangement. Higher value of flux density saturates core, increases magnetizing current and also results in the production of undesirable harmonics.
• Service conditions also affects the selection of value of flux density. For example a distribution transformer design is characterized by high all day efficiency which needs lower iron losses, hence lower value of flux density should be selected for distribution transformer.
• The value of flux density is chosen to suit the guaranteed performance. Normally flux density is chosen near the knee point of magnetisation curve. However in some cases flux density value is reduced to limit the noise level of transformer.
• For transformers using hot rolled silicon steel range of maximum flux density Bm is,

1. Distribution transformer – 1.1 to 1.35 Wb/m^2
2. Power transformer- 1.25 to 1.45 Wb/m^2

Whereas for transformers using Cold Rolled Grain Oriented (CRGO) steel range of Bm used is 1.5 to 1.75 Wb/m^2.