Magnetic core

A magnetic core is the core of an electromagnet or inductor. The properties of an electromagnet or inductor will be influenced by the core, important factors are:

• the geometry of the magnetic core.
• the amount of air gap in the magnetic circuit .
• the magnetic core material (especially permeability and hysteresis).
• the temperature of the core.

Contents

1 Commonly used magnetic core structures 1.1 Straight cylindrical rod 1.2 Single "I" core 1.3 "E" core with "I" core 1.4 Pair of "E" cores 1.5 Pot core 1.6 Toroidal core 1.7 Planar core 2 Core loss 2.1 Hysteresis loss 2.2 Eddy current loss 2.3 Movement of magnetic domains

Commonly used magnetic core structures

FIXME: these are incomplete, need diagrams and better explanation.

Straight cylindrical rod

Most commonly used in radios 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 threaded 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" core with "I" core

The "E" and "I" fit together around a square coil former to make a complete magnetic circuit . This design is common with laminated iron cores and is often used in plug packs. This is a very common transformer design in low budget household electrical equipment.

Pair of "E" 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

This is used for inductors and transformers. 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. This design of core has a shielding effect, preventing radiation and reducing electromagnetic interference.

Toroidal core

This design is based on a circular toroid, similar in shape to a doughnut. The coil is wound through the hole in the doughnut and around the outside, an ideal coil is distributed evenly all around the circumference of the doughnut. This geometry will turn the magnetic field around into a full loop and thus will naturally keep the majority of the field constrained within the core material. It makes a highly efficient and low radiation transformer, popular in hi-fi audio amplifiers where desirable features are: high power, 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 currents 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 domains

As the magnetic field changes, some magnetic domains grow while others shrink, thus the walls of the domains can be said to move. This movement absorbs energy.