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.
|Type of transformers
|Value of flux
|Distribution transformer-hot rolled silicon steel
|1.1 to 1.4 Wb/m²
|Power transformer-hot rolled silicon steel
|1.2 to 1.5 Wb/m²
|Transformers using cold rolled grain oriented steel
Up to 132 kV
For 275 kV
For 400 kV