Rapid design is one of the main imperatives of modern manufacturing, followed from principles of mechatronics [1], and is handy tool of regular optimization of structure. In this work it is presented such rapid design method for specific class of power transformers with sandwich windings. To accelerate the design process an expert system and rapid interactive procedure was applied. At such approach the more scientific and design experience is located into the Knowledge Base of the Expert System, the more, rapid, easier and cheaper is a regular design and optimization of a machine. Thanks to analytical preparation, approximation and linearization coefficients the programming and calculation is discharged from cumbersome iteration and other formal disturbances. Hybrid analytically-reluctance Network Method three-dimensional RNM–3D [2] has proved here as one of the best for rapid design of such complex structures like modern transformers with extreme electromagnetic filed concentration, its crushing forces, eddy current loss and overheating hazard.