Toroidal inductors and transformers

A toroidal coil, also called annular coil, toroidal coil or toroidal coil is in electrical engineering, a specially shaped spool which comprises a core in the form of a circular ring (so-called core ring ), around which the electrical conductor is wound. The special feature of this design is that the magnetic flux spreads almost exclusively in the circular core and the most disturbing stray field in the exterior of the annular coil is comparatively weak.

Embodiments and applications

Circular ring coils are used primarily in passive electrical filters for the suppression of unwanted high-frequency interference. The execution can take place as a classic coil with only one conductor; however, two or more conductors on the bobbin are also possible. In order to avoid magnetic saturation of the core, either corresponding materials are necessary as the core material or is artificially incorporated into the annular air gap.

If an inductor with multiple windings operated in such a way that the sum of all currents is zero, the individual magnetic pick up, then one speaks of a current compensated choke. Since the toroidal chokes without an air gap (powder core inductors do not count! ) Lost in the normal operation even at low currents in saturation, you can still use a current - compensated interconnection high inductors for EMC filtering (counter mode interference ). The useful signal circuit or circuit only the leakage inductance of the choke is visible, but is only a fraction of the nominal inductance.

Toroidal coils with two or more windings are used as an essential component in fault current circuit breakers to detect a fault current.

A special application is the use as a transformer. Here, the voltage of a winding of the primary side, to the second winding, the secondary side is transmitted. In this application, the core may have no air gap, and the component is then designated as a ring core transformer.

Calculation of inductance

The inductance L of a toroidal coil having a coil with N turns and a rectangular core of the width b, the inner radius r and the outer radius R may be approximately in a thin wire having the formula

Calculate. Here, μ0 is the magnetic field constant and ĩr the relative permeability of the core material. Instead of the radii of the corresponding diameter may be used.

If the relative difference between the outer and inner radius of the ring is small, the mean radius and the cross sectional area of the ring where A is referred to, it can be the inductance of the toroidal coil approximately to

Calculate.