What Is a Toroidal Coil?
Toroidal coils are coils made by winding an electric wire around a circular magnetic core.
By forming a closed magnetic circuit called a toroid, toroidal coils have characteristics different from those of ordinary coils. In a coil with wires wound in a cylindrical shape, the induced magnetic field lines are radiated outward through the coil, which may affect surrounding materials.
Toroidal coils can minimize the effect on the surroundings because the magnetic field lines are closed.
Applications of Toroidal Coils
Toroidal coils are used in applications that take advantage of the characteristics of the closed magnetic circuit structure with low leakage flux. They are widely used in electronics, such as choke coils in power supply circuits, filters for eliminating harmonics in high-frequency circuits, and inductors and capacitors.
For example, in cellular phones and wireless LAN, ordinary inductors may not work properly because they are affected by the environment near the circuit in the ultra-high frequency band. Toroidal coils can be used to solve this problem.
Toroidal coils are also used as coils for large equipment such as nuclear fusion devices, superconducting magnets, or charged particle accelerators. They are also used in stator-side coils for electric motors.
Principle of Toroidal Coils
A coil wound with an electric wire generates a magnetic flux at right angles to the wire when an electric current flows through it. Toroidal coils consist of copper wire wound around a doughnut-shaped toroidal core. Because the core is a circle, the magnetic flux stays inside and hardly comes out to the outside. If the copper wire is wound many times, the magnetic fluxes overlap and a strong magnetic field can be obtained.
Since the magnetic flux generated does not easily leak outside, the coil efficiency is extremely high, and the magnetic flux has the advantage of hardly affecting other elements. The cores used in toroidal coils are made of iron-based steel sheets. Specifically, FeSi silicon steel plate cores, nanocrystalline alloy cores, FeSi-based alloy cores, and Fe(CO)5 carbonyl cores are used. They have low flux leakage and stable inductance.
Characteristics of Toroidal Coils
Toroidal coils can confine magnetic field lines generated by passing an electric current through a coil wound around a doughnut-shaped magnetic body, enabling compactness and high magnetism and preventing leakage of magnetic field lines to the outside.
The three basic components of an electric circuit are the RLC, the “R” resistance, the “L” inductance (the device is an inductor), and the “C” conductance (the device is a capacitor).
Toroidal coils are inductors that provide greater inductance than air-core coils, and the higher the frequency of the current flowing through them, the greater the effect, so they can be used to remove harmonics. Adding another coil to the magnetic material of the toroidal coils makes a high-frequency transformer.
Furthermore, by combining it with a capacitor C, which is inversely proportional to the frequency, and the higher the frequency, the easier it is for the current to pass through, various filters can be configured.
Other Information on Toroidal Coils
Role of Toroidal Coils
1. Smoothing Action
Toroidal coils have the property that when a current tries to flow through a copper wire, the coil tries to keep the current flowing, and when the current decreases, the coil tries to keep the current flowing. This is called Lenz’s law, which states that when the magnetic flux through a closed circuit changes with time, an induced electromotive force is generated that produces a magnetic field in a direction that prevents the flux from changing.
Toroidal coils are used for smoothing in power supply circuits that convert AC to DC and for high-frequency filters, taking advantage of this property.
2. Mutual Induction
Mutual induction is the induction of power from one coil into the other when two coils are placed close together. Using this principle, toroidal coils are used in power transformers.
The output voltage of a power transformer varies according to the ratio of the number of turns of the primary coil to the number of turns of the secondary coil; multiple voltages can be obtained by installing an intermediate tap on the secondary coil.
3. Electromagnet Action
The electromagnet of the coil is used in relays that open and close circuits. It is also applied to generators, electric motors, bells, buzzers, etc.