How Saturation Affects Inductance
Lexicon> Letter D> Choke Coil
Definition: a coil whose inductance is used in an electrical circuit
Alternative term: throttle
More specific terms: toroidal core choke, rod core choke, air choke, compensation choke coil
English: choke, reactance coil, reactor, ballast
Category: electrical energy
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
A Choke coil (or simply throttle) is a device often used in electrical engineering and electronics, which contains a coil, i. H. a mostly electrically insulated conductor that is wound with a certain number of turns. For the functional principle, which is explained in detail below, the generation of a magnetic field and its effect on the electrical voltage is of decisive importance.Most reactors have a ferromagnetic core, which significantly increases the magnetic field.
In many cases, the winding of the coil encloses a ferromagnetic core, for example made of sheet iron or ferrite; the function of this core is to massively strengthen the magnetic field (see below). This enables the construction of a choke with high inductance despite its small size. Depending on the nature and geometric shape of the core, a distinction is made between different types of chokes:
- Toroidal chokes (Toroidal coils) contain an annular magnetic core, which is often made of ferrite material. For a certain size, this leads to a relatively high inductance, and the magnetic field for the most part only runs within the coil; so there are only weak stray fields that could affect the area.
- Rod core chokes have a rod-shaped (usually cylindrical) magnetic core, which is therefore not closed. With the same coil, this leads to a significantly weaker magnetic field and thus to a weaker induction, and also to magnetic saturation (see below) only at correspondingly higher currents. Much stronger stray fields occur.
- An intermediate form are throttles with a substantially closed core, but which has a certain air gap.
- A Air throttle has no magnetic core at all.
A large number of other designations also result from the applications, see below.
The designation Choke coil originates from the frequently used function of such a device to throttle (reduce) an operating current. However, this aspect does not play a role in many applications (e.g. for reactive current compensation).
In contrast to a transformer, a choke coil usually only contains a single winding. Are an exception current-compensated chokes or Common mode chokes, in which, for example, two windings normally have opposing working currents (e.g. in one phase and the neutral conductor) flowing through them, so that the magnetic fields generated cancel each other out and the inductive effects disappear accordingly. However, such inductive effects reappear as soon as the working currents are no longer exactly in opposite directions. This can be used to suppress so-called common-mode interference.
Induction voltage and electrical resistance
The electrical behavior and thus the functional principle of a choke coil is decisively influenced by the fact that an electrical current flowing through the coil generates a magnetic field, which in turn induces a voltage in the same coil (Self induction) as long as its strength changes. As a rule, the entire voltage drop arises mainly in the form of this induction voltage, which is proportional to the rate of change in the current intensity over time, and less as a result of the ohmic resistance of the conductor material.
The strength of the induction voltage is the product of the inductance L. the throttle and the rate of change (time derivative) of the amperage. In addition, there is the voltage drop caused by the resistance R.; for all the tension U the following relationship with the current intensity curve applies I.:
The inductance L. is the crucial size of a reactor. It is proportional to the square of the number of turns of the coil, since an increase in this number on the one hand strengthens the magnetic field with the same current strength and on the other hand enables a higher induction voltage for a given increase in the magnetic flux.
The electrical resistance R. a coil is usually undesirable; it is therefore kept as low as possible, for example by using sufficiently high conductor cross-sections - which, however, limits the possible number of turns and thus also the inductance. The use of a core material with a high magnetic permeability or an optimized geometry can indirectly reduce the electrical resistance, since a lower number of turns can be managed.
The induction voltage is always directed in such a way that it counteracts the change in the current strength:
- If an electrical voltage is suddenly applied to an initially de-energized coil, the induction voltage, which counteracts the applied voltage, leads to an only gradual increase in the electrical current strength.
- If the current intensity is reduced, the polarity of the induction voltage is reversed; it now “tries” to maintain the current strength as much as possible. This can, for example, lead to an arc on the switch when attempting to quickly stop the flow of current through a choke coil with a switch, possibly even to its destruction, and also to the risk of electric shock due to the high Kickback stress.
- If a sinusoidal alternating voltage is applied to a choke coil, the current intensity is also sinusoidal, but it lags behind that of the voltage by up to 90 ° (see Figure 1). The phenomenon of an (inductive) reactive current occurs here.
A certain amount of magnetic energy is stored in the magnetic field of the coil, which is proportional to the magnetic flux. This in turn is normally (as long as no magnetic saturation occurs, see below) proportional to the current strength. Choke coils are only suitable for short-term energy storage, as the current flow through the ohmic resistance of the winding leads to constant energy losses. At most, this would be avoidable when using a superconducting coil, but this practically never occurs with choke coils.
With high currents and correspondingly strong magnetic fields, there may be a magnetic saturation of the core of the throttle. This means that a further increase in the current strength only leads to a slight increase in the magnetic flux. The induction voltage is then correspondingly lower in this operating range.
In most cases, choke coils are designed in such a way that the core does not have a strong magnetic saturation at the maximum intended electrical current strength. If a choke is overloaded (i.e. when operating with excessively high currents), the aforementioned magnetic saturation can cause considerable additional problems, for example a further massive increase in the current intensity. In the case of alternating currents, a non-sinusoidal curve of the current intensity occurs even with sinusoidal voltage. The saturation, at least in overload operation, must therefore be taken into account when designing electrical systems with chokes.
But there are also so-called Saturation chokesthat are specifically designed in such a way that they reach the area of magnetic saturation within the intended range of operating currents. This is used in certain electrical circuits.
Stray fields are magnetic fields that occur in the immediate vicinity of a throttle, but their strength usually decreases rapidly with increasing distance.
In most applications, it is desirable that a choke causes the lowest possible magnetic stray fields, which could otherwise have an undesirable effect on other components (especially other chokes). This is an aspect of electromagnetic compatibility.
Conversely, chokes with strong stray fields also have a greater tendency to be influenced by external magnetic fields.
Certain energy losses can occur in a choke not only due to the electrical resistance of the coil, but also due to magnetic effects. One possibility are Eddy current losseswhen eddy currents are induced in adjacent conductive materials (especially in a conductive magnetic core, e.g. made of iron). In the case of iron cores, eddy current losses can be reduced by not making them massive, but with layers of mutually electrically insulated sheets.
In addition, ferromagnetic materials show a certain amount Hysteresis (not completely soft magnetic behavior): The magnetic flux is not only determined by the current strength, but also by whether the current strength was previously lower or higher.
Eddy current and hysteresis losses are both considered Iron losses designated. They are of course also expressed in details of the electrical characteristics of a choke. In addition, they lead to heating during operation.
Particularly in the case of coils wound in several layers, certain parasitic capacitances occur, which can have harmful effects, especially when operating at high frequencies. Because of this, some types of chokes cannot be used for certain applications.
In some cases, reactors (similar to transformers) do not work completely silently, but generate certain noise emissions. This is due to the fact that the generated magnetic field can cause a small deformation or displacement of the magnetic core via its force effect, which, for example, leads to the emission of a humming sound when operating with alternating current. Where the current flow also has harmonics, these can also be audible under certain circumstances - even particularly strong, since the sound radiation is more effective at higher frequencies and the hearing is also more sensitive there. In some cases, for example in some lighting systems with fluorescent lamps and in some switched-mode power supplies, noises caused by chokes are annoying. The associated energy losses, on the other hand, are negligibly small, since even minimal sound power levels can become quite annoying long before they become energetically relevant.
Applications of inductors
Choke coils, some of which are quite large, are used in power engineering; However, there are a large number of different applications for chokes with very different characteristics, of which only the most important are explained below:
Series chokes and starter chokes
Many reactors are used to limit or reduce operating currents. For example, gas discharge lamps usually have electrical characteristics that would not result in a reliably stable operating current if they were directly connected to a fixed electrical voltage. In principle, the current could be limited with a series resistor, but this would result in high energy losses and a corresponding heating of the resistor. This function can therefore be better taken over by a choke coil, which, due to its predominantly inductive characteristics, generates much lower energy losses. However, these energy losses are often by no means negligible, especially if the cheap production of a choke was the focus of your design.
Most fluorescent lamps contain such a choke, which also plays an important role when the lamp is started - namely to generate the necessary ignition voltage, which is usually well above the mains voltage.
The above remarks apply to lamps with conventional ballasts (CCG). However, even modern electronic ballasts usually have at least one choke, which then has a slightly different function and can often be made much smaller.
Chokes for the operational earthing
In some three-phase systems, e.g. B. transformer station, the star point is not directly earthed with low resistance, but via a choke coil. In this way, equalizing currents and short-circuit currents in the event of an earth fault are reduced.
In some applications, the aspect of magnetic energy storage in a choke is important; one then speaks of Storage chokes. This is particularly often the case in switched-mode power supplies. Here, a certain amount of energy is often first stored in a choke and then released on the consumer side - often at a different voltage level.
Chokes for reactive current compensation
Where capacitive reactive currents are a problem, these can be compensated for by chokes, which in this case are called Cross compensation chokes are switched. In some cases one uses adjustable throttles, in which the number of turns used can be varied by tapping the winding. See also the article on reactive power compensation.
So-called PFC chokes are also used in switched-mode power supplies in various ways for the power factor correction, i.e. the correction (optimization) of the power factor.
High and low passes
The frequency-dependent impedance of chokes is often used for high and low pass filters. For example, they can be found in the crossovers of loudspeakers that feed high-frequency signals mainly to a tweeter, while low-frequency signals are fed to a woofer, and possibly medium frequencies to a mid-range driver.
Chokes for interference suppression
Many reactors are used to prevent high frequency electromagnetic interference. Their function is mostly based on the fact that their inductance has only a small effect at the mains frequency, while they have a high impedance and thus block the effect for high interference frequencies. For example, they can greatly reduce the generation of harmonics in the current flow in a switched-mode power supply or phase control.
However, other functional principles are also used for interference suppression chokes. For example, those mentioned above are used Common mode chokes often to attenuate common-mode interference. Another case are Commutation reactors with rectifiers.
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See also: transformer, reactive power compensation, ballast
as well as other items in the electrical energy category
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