Current Density Formula Units Equation Definition and Selection

Selection of Current Density
Q. How current density is selected is the design of transformer?

The value of current density is used to determine area of conductors in primary and secondary windings.
If we choose higher value of current density ; area of conductors in winding is reduced, this reduces, cost of conductor as well as size of transformer. However due to higher value of current density I²R losses are increased (Resistance of conductor R increases as area of conductor is reduced) and due to this temperature rise of winding may become excessive and insulation of winding may get damaged due to local hot spots.
The choice of current density is important as the I²R losses and hence the load at which maximum efficiency occurs depends on it. Due to different requirement of I²R losses and cooling in distribution transformer and power transformer the values of current density is different.

Range of current densities

For distribution, small and medium power transformers (self cooled and oil immersed)

(i) For medium and large power transformers (oil cooled or air blast)

(ii) For large power transformers (with forced circulation of oil or with water cooling coils)

Area of each primary conductor

and area of each secondary conductor as

In order to have minimum copper loss, current densities in primary and secondary winding should be taken equal. However due to better cooling conditions at outer winding, current density in the outer winding is taken 5 percent more than the current density in the inner winding.

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Created By :- Deepakkumar Yadav

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