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COMMONLY USED MAGNETIC CORE STRUCTURES Straight cylindrical rod Most commonly made of a ferrite or similar material, and used in Radio s especially for tuning an Inductor . The rod sits in the middle of the coil and small adjustments of the rod position will fine tune the Inductance . Often the rod is Thread ed to allow adjustment with a screwdriver. In radio circuits, a dob of Wax or Resin is used once the inductor has been tuned to prevent the core from moving. The presence of the high Permeability core increases the Inductance but the field must still spread into the Air at the ends of the rod. The path through the air ensures that the Inductor remains Linear . In this type of inductor Radiation occurs at the end of the rod and Electromagnetic Interference may be a problem in some circumstances. Single "I" core Like a cylindrical rod but square, rarely used on its own. "E" and "I" core Sheets of suitable iron stamped out in shapes like the ( Sans-serif ) Letter s "E" and "I", are stacked with the "I" against the open end of the "E" to form 3-legged structure; coils can be wound around any leg, but usually the center leg is used. This type of core is much used for power transformers, autotransformers, and inductors. Pair of "E" cores Again used for iron cores. Similar to using an "E" and "I" together, a pair of "E" cores will accommodate a larger coil former and can produce a larger Inductor or Transformer . If an air gap is required, the centre leg of the "E" is shortened so that the air gap sits in the middle of the coil to minimise Fringing and reduce Electromagnetic Interference . Pot core Usually ferrite or similar. This is used for Inductor s and Transformer s. The shape of a pot core is round with an internal hollow that almost completely encloses the coil. Usually a pot core is made in two halves which fit together around a coil former ( Bobbin ). This design of core has a Shield ing effect, preventing Radiation and reducing Electromagnetic Interference . Toroidal core This design is based on a circular , small Volume and minimal Electromagnetic Interference . Planar core A planar core consists of two flat pieces of magnetic material, one above and one below the coil. It is typically used with a flat coil that is part of a Printed Circuit Board . This design is excellent for Mass Production and allows a high Power , small Volume Transformer to be constructed for low cost. It is not as ideal as either a pot core or '''torroidal core''' but costs less to produce. CORE LOSS In a Transformer or Inductor , some of the power that would ideally be transferred through the device is lost in the core, resulting in Heat . There are various reasons for such losses, the primary ones being: Hysteresis loss The larger the area of the Hysteresis loop, the more loss per cycle. Hysteresis loss gets worse at higher Frequencies . Eddy Current loss The Induction of Eddy Current s within the core causes a resistive loss. The higher the Resistance of the core material the lower the loss. Lamination of the core material can reduce eddy current loss. Movement of Magnetic Domain s As the magnetic field changes, some Magnetic Domain s grow while others shrink, thus the walls of the domains can be said to move. This movement absorbs energy. COMMON MAGNETIC CORE MATERIALS Laminated cores of inductors and transformers are usually made of Silicon Steel sheets. Powdered cores made of Carbonyl Iron , a highly pure iron, have high stability of parameters across a wide range of temperatures and Magnetic Flux levels, with excellent Q Factor s between 50 kHz and 200 MHz. A popular application is in broadband Inductor s, especially in high-power applications. Powdered cores made of Hydrogen Reduced Iron have higher Permeability but lower Q. They are used mostly for Electromagnetic Interference Filters and low-frequency chokes, mainly in Switched-mode Power Supplies . Ferrite Ceramics are used for high-frequency applications. The ferrite materials can be engineered with a wide range of parameters. |
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