Electromagnetic induction device

1. An electromagnetic induction device that is a transformer or a reactor, comprising:

a magnetic core having a limb which has a first end and a second end,

windings arranged around the limb,

a tank comprising magnetic material, the tank having a top part, a bottom part, and a side wall that define a space in which the magnetic core and the windings are arranged, the side wall at least substantially surrounding the magnetic core, and

a flux density reducer arranged between the first end of the limb and the tank, the flux density reducer having a cross sectional dimension greater than a largest cross sectional dimension of the limb, the flux density reducer has a tapering shape,

wherein the tank provides a return path for a primary magnetic flux which is a magnetic flux in the magnetic core, the primary magnetic flux being configured to flow from the first end of the limb to the second end of the limb via the return path.

2. The electromagnetic induction device as claimed in claim 1 , wherein the cross sectional dimension of the flux density reducer is at least the square root of two times the largest cross sectional dimension of the limb.

3. The electromagnetic induction device as claimed in claim 1 , wherein the flux density reducer is a polymer magnet or comprises iron dust or a composite made of compressible iron powder which has a compaction pressure threshold in a range 60-200 ksi and powder having a permeability above 1000.

4. The electromagnetic induction device as claimed in claim 1 , wherein the limb has a cross section which increases towards the first end such that the first end of the limb is at least the square root of two times a cross sectional dimension of the limb at a midpoint of the limb in an axial direction.

5. The electromagnetic induction device as claimed in claim 1 , wherein the magnetic core comprises a plurality of limbs, and wherein the top part and the bottom part of the tank are respectively an upper yoke and a lower yoke which provide circulation of the primary magnetic flux between the limbs.

6. The electromagnetic induction device as claimed in claim 1 , wherein the top part and the bottom part are made of silicon steel comprising silicon in a range 3-6% by mass.

7. The electromagnetic induction device as claimed in claim 1 , wherein the side wall comprises amorphous material.

8. The electromagnetic induction device as claimed in claim 1 , wherein the tank is made of carbon steel.

9. The electromagnetic induction device as claimed in claim 1 , wherein the top part and the bottom part have a grain-oriented structure, wherein grain orientation of the top part and the bottom part is along magnetic flux lines of the return path.

10. The electromagnetic induction device as claimed in claim 1 , wherein the tank has a circular cross section along an entire extension of the limb in an axial direction of the limb.

11. The electromagnetic induction device as claimed in claim 1 , wherein the tank has an elliptical cross section along an entire extension of the limb in an axial direction of the limb.

12. The electromagnetic induction device as claimed in claim 1 , wherein the side wall is provided with a cut extending from the top part of the tank to the bottom part of the tank.

13. The electromagnetic induction device as claimed in claim 12 , wherein the side wall comprises a plurality of concentrically arranged sheets, wherein each sheet is provided with the cut.

14. The electromagnetic induction device as claimed in claim 13 , wherein each sheet comprises one of grain-oriented steel, non-grain oriented steel, or amorphous steel.

15. The electromagnetic induction device as claimed in claim 2 , wherein the flux density reducer is a polymer magnet or comprises iron dust, or a composite made of compressible iron powder which has a compaction pressure threshold in a range 60-200 ksi and powder having a permeability above 1000.