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Siemens Energy optimises power transformers
with the aid of 3D EM Simulation

"Analysis of
the magnetic field in the core, shown in Figure 5, assists the investigation of the shielding effectiveness of the shielding walls."

Transformer having shielding wall for driving a magnetron
United States Patent 5025489
"A stationary induction apparatus for supplying and receiving high frequency power of several tens KHz, wherein a partition wall made of a non-magnetic metallic foil in a thickness less than 1/10 the depth of penetration in the using freqency for electromagnetic induction is provided so as to separate the supply side (primary side) from the receiver side (secondary side), so that the stationary induction apparatus is not hurt in its function as an induction apparatus, but can electromagnetically shield the primary side from the secondary side, and the secondary side of the boosting transformer of the magnetron is able to be placed within the shield casing, thus realizing a light, compact and inexpensive microwave generating device for use in an electronic oven."
http://www.freepatentsonline.com/5025489.html

Patent application title: ELECTROSTATIC SHIELD AND VOLTAGE TRANSFORMER
"U.S. Pat. No. 4,652,846, titled "Small transformer with shield" discloses a transformer having and a stamped metal foil frame as the shielding wall between the adjacent face flanges of the coil."
http://www.faqs.org/patents/app/20100127815