Toroidal core

A toroidal core is a ring-shaped magnetic circuit . From a geometric point of view, these are toroids , discs , pipe sections or circular bodies with a hole in the middle. Together with windings, it forms an inductive component such as B. a toroidal coil or toroidal transformer .

All other core shapes have a more or less large air gap due to their divisibility, so that the toroidal core is considered "ideal" with regard to the utilization of the material properties. Depending on the magnetic material, toroidal cores are often used as a reference to determine the material properties.

A toroidal core is also a component on the winding wheel of mechanical watch movements. So-called toroidal cores are also used in construction (for example in the crossing tower of Salisbury Cathedral ).

Different toroidal cores

General

The magnetic field lines are inside a closed ring

Toroidal cores in general are the most efficient core shapes . They shield themselves to a high degree, since most of the magnetic field lines are inside the closed ring. The field lines are essentially uniformly parallel over the entire length of the magnetic path, so that interference fields will only have a very small influence on a toroidal core coil. It is rarely necessary to shield or isolate toroidal coils to prevent feedback or crosstalk. Toroidal coils simply "have no need to talk to each other".

Calculations

The spatial dimensions are necessary for further calculations with toroidal cores. The toroidal core has an outer diameter da , an inner diameter di and a height h and the following physical parameters as characteristic variables :

• magnetic path length, iron path length: l _Fe
• magnetic cross-section , iron cross-section : A _Fe
• Volume , iron volume : V _Fe
• Nuclear mass : m _Fe
• Iron fill factor: η _Fe

with the context:

${\displaystyle l_{Fe}={\frac {(da+di)}{2}}\cdot \pi }$

${\displaystyle A_{Fe}={\frac {(da-di)}{2}}\cdot h\cdot \eta _{Fe}}$

${\displaystyle V_{Fe}=l_{Fe}\cdot A_{Fe}}$

${\displaystyle m_{Fe}=l_{Fe}\cdot A_{Fe}\cdot \gamma }$

Explanation: The iron fill factor η _Fe represents the relationship between the magnetic core cross-section and the geometric core cross-section. (Typical value for toroidal tape cores: 75–90%)

Ring cores made of ferrite or powder materials

This core shape is created by pressing powder into a ring-shaped tool. The pressed so-called “green compacts” are solidified in subsequent temperature treatments and, in the case of ferrite materials, sintered at high temperature to form a ceramic . This is followed by processes for deburring and, if necessary, coating with paint or plastic for insulation .

All powder materials have the disadvantage of brittleness, so that toroidal cores of this type often crack, lose their properties and, in extreme cases, tear when subjected to strong impacts. The advantages of these cores are their low production costs and the rounded edges, which simplify the subsequent wrapping.

While ferrite cores show an extremely steep saturation behavior, powder cores made of iron or other magnetic powders (cobalt, nickel, etc.) are characterized by the fact that the individual powder grains are still separated from one another by a non-magnetic layer. This creates a so-called distributed air gap, which causes high saturation inductions and a soft use of saturation.

In principle, iron powder toroidal cores are suitable for narrowband applications while ferrite toroidal cores are used for broadband applications.

Toroidal cores made of strip material

Toroidal core with clearly visible air gap, slotted ribbon core, for use for a current sensor

The manufacture of wound cores from strip material led to the designation toroidal cores (RBK). These are made of crystalline bands such. B. made of grain-oriented electrical steel or NiFe materials as well as amorphous and nanocrystalline alloys. Here, the strip material is attached to a metallic cylinder and then wound up to the required thickness. After the end has also been attached, the toroidal tape core is obtained after pulling out the cylindrical winding mandrel. Depending on the alloy, a heat or field heat treatment is then carried out in a furnace in order to set the optimal magnetic properties . Tape thicknesses between 0.006 mm and 0.3 mm are typical. To reduce eddy current losses, the belts are usually equipped with an insulation layer that is as thin as possible.

To protect the RBK from mechanical loads and to protect the winding wire from sharp edges, a subsequent insulation or sheathing is necessary. Common processes are: painting, coating e.g. B. with epoxy powder , placing in plastic housing (troughs) with lids.

Compared to other toroidal cores, a toroidal tape core can in principle be made of any size. Toroidal cores with an outside diameter of over two meters are used in particle accelerators , for example .

So-called mixing cores are made from various alloys for special applications.

As an alternative to the wound strip core, there are also punching disc cores on the market. The stamped ring disks are usually delivered as core packages stacked in protective troughs.

The internal shear built into powder cores can be generated in strip cores by introducing an air gap. This technology significantly increases the DC preload and is often used in storage chokes and storage transformers.

Another application for slotted toroidal cores is current measurement. In the gap of a toroidal core, evaluating the field strength z. B. with the help of a Hall probe, the current can be measured without contact and potential-free , which flows through a conductor in the inner hole of the core. ( Current sensor )

Further processing

To manufacture inductive components such as transformers , transmitters , chokes , etc., the toroidal cores must be wound with a conductor. Enameled copper wire is mainly used for the winding . An independent winding technique has been established for toroidal cores due to the geometric characteristics.

Hand wrapping

With a small number of turns and also with very small toroidal cores, the winding is done by hand. Depending on the length and thickness of the wire, aids such as sewing needles or magazines or boats are used.

Machine wrapping

1. Toroidal core winding machines have been established manufacturing systems for over 50 years. The most widespread are the semi-automatic machines, each of which requires an operator. The core is placed in a separable magazine and the winding wire is wound onto the magazine. Then the wire is unwound from the magazine to the toroidal core, the core being slowly rotated around its own axis in a holder. Depending on the core size and the correspondingly thin wire, wrapping of 5000 turns and more is possible. Fully automatic toroidal winding machines are relatively expensive and correspondingly rare.

2. Crochet hook machines are also known for smaller numbers of turns and larger wire sizes . The core is held horizontally from the outside, and a crochet hook that can be moved in the z-axis dips from below through the inner hole of the core and pulls the entire length of the wire with it.

Applications

Toroidal choke

Current-compensated chokes are built for standard applications with ferrite toroidal cores and for high interference suppression or high impedances with nanocrystalline toroidal cores. To do this, they have two windings of the same type. A special form of these chokes are tube cores, rings or beads that are pushed onto cables and are used to suppress interference at very high frequencies.

Small transformers and current transformers for high frequencies are also made from toroidal cores. For this are z. B. also so-called double hole cores made of ferrite in use.

One historical application is the core memory , which works with hard magnetic ferrite rings.

Storage chokes in switched-mode power supplies as well as non-current-compensated interference suppression chokes are often made from powder ring cores or amorphous and nanocrystalline toroidal cores.

Residual current circuit breakers (FI circuit breakers) as well as the electronic DI switches, current transformers for electricity meters and current sensors for direct current use toroidal cores made of nanocrystalline material. Elaborately slotted cores are used for special sensor applications.

Toroidal core power transformers ( toroidal core transformers ), for example for low-voltage halogen lamps, are made from textured (grain-oriented) electrical steel. They work with flux densities of around 1.5 Tesla and have a steep saturation behavior, which causes their high inrush currents. Since toroidal power transformers do not have a production-related air gap, they are preferred when a low magnetic stray field is required, for example in audio amplifiers.

Interference suppression chokes , current-compensated chokes (CMC), interface transformers, for example in the field of communications technology such as ISDN , ADSL , LAN etc. often also use toroidal cores.

Web links

• Manufacturer's website for materials for toroidal cores

Specialist literature

• Hans Fischer: Materials in electrical engineering. 2nd edition, Carl Hanser Verlag, Munich Vienna, 1982 ISBN 3-446-13553-7
• Günter Springer: Expertise in electrical engineering. 18th edition, Verlag - Europa - Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9
• Rainer Hilzinger, Werner Rodewald: Magnetic Materials: Fundamentals, Properties and Applications. 1st edition, Publicis Publishing, 2011, ISBN 3895783528 , ISBN 978-3895783524

Individual evidence

1. ^ Helmut Kahlert , Richard Mühe , Gisbert L. Brunner : Wristwatches: 100 years of development history. Callwey, Munich 1983; 5th edition, ibid. 1996, ISBN 3-7667-1241-1 , p. 48.