Cores Used by Butler Winding: Bar, Rod, or Slab
âSoftâ magnetic metal alloys are available in Bar, Rod,Â or Slab shapes. These core shapes find use in D.C. applications such as D.C. powered solenoids and D.C. relays. They can be used in very low frequency (below 50 Hz) A.C. applications. They do have some limited use at A.C. line frequencies. For a solid core, A.C. core losses per unit weight (or unit volume) become more pronounced as the cross sectional area increases. This is why silicon steel, nickel-iron, and cobalt alloy cores use a stack of laminations. The laminations divide the cross-section into a stack of much smaller cross-sections. D.C. applications are subject to far less core losses. They only experience A.C. core losses (and the heat produced) during transitional events.
Powder Cores extend the useful A.C. frequency range of the materials listed in the previous paragraph. A non-magnetic binding material is used to bind the small magnetic powder particles together. The binding material also serves to insulate the particles from one another thereby reducing eddy current flow in the core. This extends the useful frequency range, but there is a trade-off. The binding material adds a distributed air gap to the core. The distributed air gap reduces the permeability of the core. The core requires more magnetizing VA. Bars, slabs, and rods can be purchased in powder iron materials. The selection of sizes is somewhat limited. Larger sizes can be assembled from smaller sizes.
Ferrites are a magnetic form of ceramics. Ferrite has very high electrical resistivity. Even at high frequencies the eddy currents remain low. With suitable gauss de-rating, some types of ferrite cores can be used above 1 megahertz. Bar, slabs, and rods can be purchased in ferrite materials, but the selection of sizes is limited. Larger sizes can be assembled from smaller sizes.