Dry-type transformer with a polymer shield case and a method of manufacturing the same

1. A transformer comprising:

a shield case formed from a polymeric resin composition comprising conductive particles;

a toroidal core comprised of ferromagnetic material and having a center opening;

a secondary winding wound around a circumference of the core, wherein the core and the secondary winding are disposed in the shield case;

a primary winding extending through the center opening of the toroidal core; and

an encasement encapsulating the shield case, the encasement comprising a dielectric resin.

2. The transformer of claim 1 , wherein the dielectric resin comprises an epoxy resin.

3. The transformer of claim 2 , wherein the dielectric resin comprises a hydrophobic cycloaliphatic epoxy resin.

4. The transformer of claim 1 , wherein the shield case comprises a cover releasably secured to a body.

5. The transformer of claim 4 , wherein the body and the cover are each formed by reactive injection molding.

6. The transformer of claim 5 , wherein the polymeric resin composition further comprises a thermoset resin selected from the group consisting of a polynorbornene resin, a polyurethane resin, a polyurea resin and a polyurethane/polyester resin.

7. The transformer of claim 6 , wherein the thermoset resin is a polynorbornene resin.

8. The transformer of claim 7 , wherein the thermoset resin is poly(dicyclopentadiene).

9. The transformer of claim 1 , further comprising an electrical conductor electrically connected to the shield case and accessible from the exterior of the encasement.

10. The transformer of claim 9 , wherein the electrical conductor comprises a metallic plate secured to the encasement, the metallic plate being connected to earth ground, thereby grounding the shield case and forming a faraday shield around at least a portion of the core.

11. A transformer comprising;

a shield case formed from a polymeric resin composition comprising conductive particles;

a core comprised of ferromagnetic material, at least a portion of the core being disposed inside the shield case;

a primary coil and a secondary coil disposed proximate to the core;

an encasement encapsulating the shield case, the encasement comprising a dielectric resin; and

wherein the core has a rectangular shape and comprises a pair of legs, and wherein the shield case comprises a major body, a minor body and a cover releasably secured to the major body, the minor body being disposed around one of the legs of the core.

12. The transformer of claim 11 , wherein the primary and secondary coils wound around the minor body.

13. The transformer of claim 12 , wherein the major body and the cover are each C-shaped.

14. The transformer of claim 12 , wherein the core is a first core and wherein the transformer further comprises a second core with a rectangular shape and a pair of legs, wherein the minor body is disposed around one of the legs of the first core and around one of the legs of the second core, and wherein the major body and the cover each have a rectangular shape.

15. A method of producing a transformer comprising:

providing a shield case formed from a polymeric resin composition containing conductive particles;

providing a core comprised of ferromagnetic material;

providing a coil;

disposing the coil around the core;

placing at least a portion of the core inside at least a portion of the shield case; and

encapsulating the shield case in a dielectric resin; and

wherein the shield case comprises a major body, a minor body and a cover, and wherein the step of providing the coil comprises wrapping a length of conductor around the minor body to form the coil, and wherein the steps of disposing the coil around the core and placing at least a portion of the core inside at least a portion of the shield case comprises disposing the minor body with the coil wound thereon around a leg of the core.

16. The method of claim 15 , wherein the step of providing the shield case comprises reaction injection molding the shield case from the polymeric resin composition, which further comprises dicyclopentadiene and a catalyst system.

17. The method of claim 15 , wherein the step of placing at least a portion of the core inside at least a portion of the shield case comprises placing the entire core inside the shield case.