The core applies the high-quality of magnetic conductive silicon steel sheet, the no-load is low, and the surface of the core coated with the H-class resin insulation, and thus has the features of moisture-resistant, rust-proof. The insulation system of the coil applies the NOMEX® of the DuPont Company, and then impregnated by the solvent-free through the vacuum pressure, and then cured by the high temperature and therefore has high mechanical and technical property.