Magnetic core#Ring or bead

As a toroidal core is called a magnetic circuit in a ring shape. Geometrically, these are toroids, discs, pipe sections or circular body with a hole in the middle. Together with coils to form an inductive component, such as a toroidal coil or toroidal core transformer.

All other core molds through their divisibility a more or less large air gap, so that the toroidal core with regard to the utilization of the material properties is considered to be "ideal". Depending on the magnetic material ring cores are used as a reference for determining the material properties often.

Calculations

For further calculations in toroidal cores, the spatial dimensions are required. As characteristic magnitudes of the ring core has a outer diameter as, an inner diameter d and a height h, and the following physical parameters:

• Magnetic path length, iron path length: lFe
• Magnetic cross section, iron cross section: AFe
• Volume, iron Volume: VFE
• Core mass: mFe
• Eisenfüllfaktor: ηFe

With the contexts:

Explanation: The Eisenfüllfaktor ηFe represents the ratio between the magnetic core cross section to geometric cross-section core dar. ( typical value for toroidal cores: 75-90 %)

Ring cores made ​​of ferrite or powder materials

This core mold is generated by the pressing of a powder in a ring-shaped tool. The pressed so-called " green bodies" to be solidified in the subsequent thermal treatments, and sintered into a ceramic in the case of ferrite materials at high temperature. Then follow method for deburring and possibly for coating with paint or plastic for insulation.

All powder materials have the disadvantage of brittleness, so that such ring cores with powerful strokes often get cracks, lose their properties and tear in extreme cases. Advantage of these cores are their low production costs and the rounded edges, which simplifies the subsequent winding.

While ferrite cores show a very steep saturation behavior, powder cores of iron or other magnetic powders ( cobalt, nickel, etc. ) are characterized in that the individual powder grains are still present in them separated by a nonmagnetic layer. This provides a so-called distributed air gap, the high saturation induction and a soft use of the saturation effect.

Ring cores from strip material

The production of wound cores from strip material led to the name tape wound cores ( RBK ). These are made from crystalline belts eg of grain-oriented electrical steel or NiFe materials as well as of amorphous and nanocrystalline alloys. In this case, the tape material is fixed on a metallic cylinder and then wound up to the desired thickness. After the end was also fixed, one obtains after pulling out of the cylindrical mandrel winding the toroidal core. Then, depending on the alloy, a heat box or heat treatment in an oven to set the optimum magnetic properties. Thicknesses between 0.006 mm and 0.3 mm are typical. To reduce eddy current losses, the bands are often provided with a thin insulating layer as possible.

To protect the RBC from mechanical stresses and to protect the winding wire from the sharp edges of a subsequent insulation or covering is required. Common methods are: painting, coating, for example, epoxy, placing in plastic housing ( troughs ) with a lid.

Compared to other toroids, a toroidal core in principle to produce arbitrarily large. Toroidal cores having an outer diameter of about two meters are used, for example in particle accelerators.

For special applications, so-called mixed cores have been made ​​of various alloys.

As an alternative to tape -wound core are also punching disc cores on the market. The punching washers are supplied mostly stacked as core packages in protection troughs.

The built- in internal shear powder cores can be produced by the introduction of an air gap for tape-wound cores. This technique significantly increases the Gleichstromvorbelastbarkeit and is often used for storage chokes and Speicherübertragern.

Another application for slotted ring cores is the current measurement. In the gap of a toroidal core of the current can be measured without contact and isolated through the evaluation of the field strength, eg by means of a Hall probe, which flows through a conductor in the inner hole of the core. ( Current sensor )

Further processing

For the production of inductive components such as transformers, transformers, chokes, etc., the winding of the toroidal cores is necessary to a conductor. Mainly for the winding is copper wire used. For toroidal cores has become the geometric features of a stand-alone Bewickeltechnik established.

Handbewicklung

For small numbers of turns and even with very small ring cores, the winding is done by hand. These are used depending on the wire length and wire gauge aids such as needles or magazines or boat.

Maschinenbewicklung

First toroidal coil winding machines are established manufacturing plants for over 50 years. The most common semi-automatic machines are in each of which an operator is necessary. In this case, the core is inserted into a dividable tray, and the winding wire is wound on the magazine. Thereafter, the processing of the wire from the magazine onto the ring core, whereby the core is rotated slowly in a receptacle on its own axis. Depending on the core size and correspondingly thin wire are winding of 5000 turns and more possible. Relatively expensive and correspondingly rare are fully automatic toroidal winding machines.

2 For lower numbers of turns and larger wire sizes also crochet machines are known. Here, the core is held horizontally from the outside, and a movable in the z- axis crochet hook emerges from below through the inner hole of the core and pulls the entire length of wire with it.

Applications

Ferrite toroidal cores are used inter alia for common mode chokes. You can insert two identical windings. A special form of these chokes are pushed onto cable tube cores, rings or beads that serve the suppression at very high frequencies.

Even small transformers and current transformers for high frequencies are produced from ferrite ring cores. For this, so-called double-hole cores in use.

A historical application, the core memory that work with hard magnetic ferrite rings.

Inductors in switch mode power supplies and non- current-compensated noise suppression chokes are often made from powder toroids or amorphous toroidal cores.

Residual current circuit breaker ( RCCB ) and electronic DI switch, current transformer for electricity meters and current sensors for direct current, used by toroidal cores of nanocrystalline material. For special sensor applications consuming slotted cores are in use.

Toroidal power transformers for low -voltage halogen lamps include those prepared from textured ( grain-oriented ) electrical sheet. They work with flux densities of about 1.5 Tesla and have a steep saturation behavior, which causes the high inrush currents. Because toroidal power transformers have no production-related air gap, they are preferably used when a low magnetic field is required, such as in audio amplifiers.

Suppression chokes, interface transformers, for example in the field of communications technology such as ISDN, ADSL, LAN and so often also use toroidal cores.