This text is machine translated.
Learn about inductors, coils, and chokes
Conducting current through a wire is a matter of course nowadays, which almost already has a banal effect. No wonder, because this technology has dominated mankind for almost 200 years. However, if the wire is wound up to a coil, the current flow suddenly has a completely different effect . We explain what was behind the technology of coils, reactors and inductors.
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What was inductor, coil or choke?
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How do coils and chokes work?
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What was the inductivity of coils?
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What was the inductive reactive resistance for coils?
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Where are coils used?
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Different coil types
What was inductor, coil or choke?
A coil consists of an insulated wire wound in a circular or other form. The insulation is often only served by a thin layer of paint , which electrically separates the adjacent copper wires.
Due to this physical structure, certain electrical properties can be achieved which can be used for a wide variety of tasks. For example, coils can be mechanically constructed in such a way that they represent a very small resistance for an alternating voltage with a low frequency of 50 Hz. At a high frequency of several thousand Hertz, however, these coils have a very high resistance .
Such coils are preferably used at the mains input of electrical appliances. While the current with the mains frequency of 50 Hz can pass through the coil without any problem, high-frequency interference pulses from the mains are effectively kept away from the device. On the other hand, these coils also ensure that interference pulses, which are caused e.g. by the brush fire at the slip ring of a collector motor, are not released into the public grid.
Copper wire wound on a Kern to a Coil.
Due to these interference suppression properties, these coils are also referred to as reactors, line reactors or interference suppression reactors. In addition, coils and reactors are also combined under the umbrella term inductors. Since transformers and motors also have inductive properties due to their windings, the specialist is more likely to speak of an inductive load . .
How do coils and chokes work?
A current-flowing coil (1) generates a magnetic field (2) that can be amplified by a ferromagnetic Kern (3). In the case of direct current, this results in an electromagnet.
If an electrical voltage is applied to a coil, the current must always flow through each individual winding of the coil.
The current-flowing conductor of the coil builds up a magnetic field. auf. Due to the design with the adjacent conductor loops, the magnetic fields of all coil windings are added to a single large magnetic field . The larger the number of windings and the higher the current, the stronger the magnetic field becomes.
However, the longer copper wire also increases ohm's resistance and increases production costs. Therefore, a Kern made of pressed iron powder (ferrite) is often used in the center of the coil to increase permeability and thus the magnetic field . The term permeability indicates how well or poorly a coil core conducts the magnetic field lines.
If the coil is supplied with a DC voltage, an electromagnet is obtained which is also used in sorting systems or DC relays. By switching the current on and off, you can switch the magnetic effect on or off as desired.
If an AC voltage is applied to the coil, a coil reacts completely differently from a DC voltage. In order to understand the relationships, one has to look at the coil behavior in relation to a sine wave:
Due to the increasing voltage, the coil builds up an ever stronger magnetic field.
The growing magnetic field in turn generates a voltage in the coil through self-induction, which counteracts the supplied alternating voltage. This means that the current flow through the coil is inhibited or delayed . .
On the rear edge of the sine wave, the voltage is reduced again and the magnetic field becomes weaker . The collapsing magnetic field in turn also generates a voltage that the magnetic field wants to maintain. For the negative half-wave of the sine wave, the process is repeated, but with the reverse current direction through the coil.
This means that the current and voltage are no longer identical during a sine wave . The current is "slower" than the voltage. The specialist speaks here of a phase shift of 90° .
In a coil, current flow (I) occurs with a phase shift of 90° to voltage (U).
Practical advice
In order for trainees to be able to better remember the phase shift, there is a simple but well-proven idea behind the concept for decades: "With inductance, the current is 90° too late!"
Note!
Naturally, self-induction also takes place when a coil is operated with DC/DC voltage. However, the effect only occurs when the motor is switched on and switched off. While switching on is less problematic, coils can generate very high voltage peaks when switching off. For this reason, a diode is connected parallel to the coil in a DC relay.
When switching transistor (T) is activated, current flows through relay (R) and transistor (T). The relay has been activated and the switch contact is closed. At this moment, no current flows via diode (D) (see left diagram). When the transistor (T) is disabled, the diode (D) briefly closes the resulting induction current (see right diagram). The switching transistor (T) is thus effectively protected against harmful voltage peaks .
Apart from self-induction, there is also foreign production . In the case of external induction, a magnetic field is built up by a coil, which induces a voltage in a second coil. External production is used for transformers or ignition coils.
The diode (D) protects the switching transistor (T) from induction voltage peaks at the breaking torque.
What was the inductivity of coils?
As already explained, a change in the current flow in the coil always leads to a change in the coil magnetic field . And the change in the magnetic field in turn always generates a self-induction voltage . This voltage always counteracts the external voltage change.
The level of the self-inductance voltage depends on the magnitude of the current change , the time period during which the current change takes place and the inductance of the coil. The inductance is determined by the mechanical structure of the coil and the material properties. This includes the number of windings and the dimensions of the coil. Often the coil wires are wound around a Kern made of magnetizable (ferromagnetic) material, which increases the inductance. The coil cores can be shaped as a rod or as a toroid core. But other designs are also possible.
Ultimately, the inductance is a summary of all the electrical properties of a coil. The inductance also enables different coils to be compared.
The formula sign for inductance is L and the term is H (Henry), according to American physicist Joseph Henry, who was in the 19th century. Jhd discovered self-induction.
The inductance is defined as follows:
If the current changes by 1 ampere (A) for a period of 1 second and the self-inductance voltage is 1 V, the coil has an inductance of 1 Henry (H).
However, coils with significantly lower inductances are used in the electronics sector. Similar to capacitors, coils have a 1/1000 division.
| Henry | 1 h | 1 h | 100 h. |
| Millihenry | 1 mH | 0.001 H | 10 -3 H |
| Mikrohenry | 1 h | 0.000001 H | 10 -6 H |
| Nanohenry | 1 NH | 0.000000001 H | 10 -9 H |
What was the inductive reactive resistance for coils?
In the case of a carbon film resistor , a metal film resistor or a wire resistor, it does not matter whether it is operated on DC or AC voltage. Its resistance value will not change for both voltage types . With coils, things look a bit different.
If a coil is supplied with DC voltage, only the ohmic resistance of the copper winding acts. . This mode is used for coils in DC relays. This is why these relays also have coils with thin wire and many windings.
When a coil is operated on AC voltage, the resistance of the coil depends on the inductance of the coil (L) and the frequency of the AC voltage (f). This resistor is referred to as inductive reactive resistor (X L ).
The inductive reactive resistance can be calculated as follows:
X L = 2 xπ x f x L.
DC relays have coils with long and thin copper wire.
For illustration purposes, we have calculated the reactive resistance of a coil with 50 mH at different frequencies:
| Frequency (f) | Reactive resistance (XL) |
|---|---|
| 0 Hz (DC voltage) | < 1 ohm* |
| 50 Hz | 15.71 ohms |
| 100 Hz | 31.42 ohms |
| 500 Hz | 157.08 ohms |
| 1 kHz | 314.16 ohms |
| 10 kHz | 3.141.59 ohms |
| 100 kHz | 31.415.93 Ohm |
*with a DC voltage, only the ohmic resistance of the coil wire is decisive. This in turn depends on the wire cross-section and the wire length. Since these values can vary considerably for coils with the same inductance, it is not possible to specify a general and concrete value here.
Note:
As the frequency increases, the reactive resistance of a coil increases. However, the table is a purely mathematical calculation of the reactive resistance values. Depending on the type of coil, the actual values may differ, as at high frequencies and at high currents the coils may no longer react linearly. The coil core was then moved to magnetic saturation , which leads to a decrease in inductance. Coils without Kern have a much greater linearity and are called air coils.
Practical advice
Iron powder materials (Fe) can be used as pure inductance up to approx. 400 kHz, in higher frequency ranges the loss share becomes too large due to the loss resistance. From 20 MHz, iron powder cores are ineffective.
Manganese zinc cores (MnZn) are inductive in the frequency range from 20 MHz to 30 MHz. Nickel-zinc cores (NiZn) are inductive up to frequencies around 60 MHz. At higher frequencies, the material is lossy. Nanocrystalline materials (FeCuNbSiB) can be used at very high frequencies
Where are coils used?
Coils can be used in a variety of ways and for a wide variety of tasks. Here are a few examples that show only a fraction of the many possible applications of coils:
Electromagnets
The classic use of a DC coil is the electromagnet. In order to generate high lifting and holding forces, the coils are partly embedded in special metal bodies and cast to protect them from moisture.
Electromagnets are also used for electromechanical relays or contactors.
Line filter
As already mentioned, coils can be used to filter out interference pulses on the mains voltage. Partly, the coils are supplemented with capacitors and thus complete line filter units are created. With some line filters, the required line socket is integrated into the housing.
Crossovers
With a crossover, the reactive resistance of coils is used at different frequencies. This ensures that only the low frequencies are supplied to the bass loudspeaker. In contrast, the tweeter only receives the high-frequency frequency components. As with the line filters, capacitors are also used for loudspeaker switches to optimize the frequency separation.
Switched-mode power supplies
When a coil builds up a magnetic field, this magnetic field stores energy that the coil can release when it is switched off. Coils, which are preferably designed for this purpose, are also called memory coils or memory chokes.
The effect of energy storage is used for switched-mode power supplies or switching regulators. If the switching frequency is adequate, the coils and transformers can be correspondingly small. This makes switched-mode power supplies small, light and cost-effective.
Oscillating circuits
An oscillating circuit is a combination of a coil and a capacitor (LC link) in which, in the event of resonance, the energy constantly oscillates between the magnetic field of the coil and the electric field of the capacitor .
With the help of oscillating circuits, it is possible to filter out certain frequencies from a frequency mix or suppress unwanted frequencies in the reception technology. In some cases, these oscillating circuits have rotating coil core in a shielded metal housing or also adjustable end-user denators, in order to set the filter exactly to the desired frequency.
Current limiter
Due to their inductive properties, coils are also often used for current limitation in AC circuits.
To strengthen the magnetic properties, the coils are wound on easily magnetizing (soft-magnetic) materials such as ferrite or metals. These coils are then also referred to as throttle coils and, in the case of ring-shaped cores, as toroidal chokes.
Wireless charging technology
For wireless charging, a transmitter coil is installed in the charging station and a receiver coil in the telephone. As soon as the telephone is placed on the charging station, the receiver coil converts the magnetic field of the transmitter coil into an electric current, with which the battery is charged. Key cards work the same way. The door station creates a magnetic field so that the coil in the card can generate an operating voltage for the memory chip. The required data can then be exchanged via the magnetic coupling.
Reactive power compensation
Due to the phase shift, inductive consumers such as transformers or motors as well as capacitive consumers such as switched-mode power supplies consume more energy than they emit. Her additional reactive power constantly oscillates between the consumer and the energy supplier.
In contrast to private consumers, reactive power is also recorded and charged in industrial customers. For this reason, industrial customers use compensations with coils and capacitors according to the current meters. If mainly inductive loads are used, capacitors are switched to the mains supply by the compensation. If the capacitive load is larger, coils are switched to the mains supply. The reactive power then only oscillates between the consumer and compensation and no longer drives up the power costs unnecessarily.
Different coil types
Inductance is not equal to inductance. A storage reactor can be used many times and therefore also requires specific adaptation in form and design. In addition to SMD and THT versions, a coil has different windings or designs depending on the application area.
SMD designs
SMD components (s urface m ounted d evice) are fixed to the PCB and then soldered in accordance with different procedures. This type of mounting is called surface mounting . In contrast, the connection wires of the components are inserted through contact holes in the circuit board during THT mounting (T through H ole T echnology) and then soldered. This type of mounting is also called through-hole mounting.
SMD Wrapper Inductors
SMD inductors are very small and light. In addition, the connecting wires are no longer required, which means that SMD designs are mainly used in industrial production.
Multilayer inductors
, a metal paste, often silver, are applied in the coil pattern on a thin base of ferrite or other materials. This type of inductor is very small and can even be used in mobile phones.
Choke coils
They function according to the following principle: The voltage that generates the self-induction works against its cause. This reduces the current flow in the coil by the voltage. The inductive resistance is the measure of derate. Choke coils are used for current limitation.
Toroidal core coil (also Toroidal coil, circular coil or toroidal coil)
The Kern of the coil is a circular ring . This design spreads the magnetic flux only in the Kern. Therefore, the stray field outside the ring coil is comparatively weak .
Toroidal coils are used in passive, electrical filters to suppress high-frequency interference. They are also often installed in residual current circuit breakers.
Storage choke
Storage reactors store the magnetic energy . The Kern is often interrupted by an air gap that is filled with paper, resin or plastic for mechanical stabilization. This air gap stores almost all of the energy to avoid saturation of the core material and to ensure linear inductance progression.
Memory chokes in certain switched-mode power supplies, switching regulators, inverse converters and SEPIC converters are used.
Wireless power coils
This type of coil has a very low overall height and is therefore often installed in wearables. Wireless power coils are ideal for wireless power transmission. There is a transmitter coil in the charging station and a receiver coil in the device. A resonant, inductive coupling takes place between these two coils. The charging process starts by induction.
This form of energy transfer is already in use, but many more innovations will be introduced in the future. The charging of an electric car with the help of a charging pad is already being tested today. This is installed in a transmitter coil.