This article will give an in-depth discussion about Inductors and Inductor Coils.
The article will bring more understanding about topics such as:
Inductors are passive two terminal components of an electric or electronic circuit that are capable of storing energy in magnetic form. They oppose sudden changes in current and they are also called coils or chokes. They are known by their electrical symbol L.
An inductor coil is an electrical conductor which passes electricity and generates a magnetic field and is wound in the form of a coil or spiral.
When electric current starts to flow in a conductor, a magnetic field is established around the conductor in the direction of the right-hand thread. When current flows through an inductor with conductors wound around it in the same direction, the magnetic field established around the wire (conductor) is bound together and the inductor becomes an electromagnet. Conversely, electric current can be generated from a magnetic force.
Once an inductor has been electro magnetized, the magnetic field or flux around it can be changed by moving a magnet closer or further away from it. This results in the flow of an electric current in order to generate a “force against change” to try and regulate the magnetic field’s direction and momentum. This is what is known as electromagnetic induction. As shown in the circuit diagram below, when a DC current begins to flow in an inductor, immediately an electromotive force that opposes the direction of flow of the electric current is generated.
This property is known as the self-inductive effect. However, as the DC current continues to flow, it reaches a certain value such that the magnetic flux ceases to change and the current is no longer blocked since the electromotive force is no longer being generated. The electromotive force generated in an inductor varies directly proportional to the rate of change of the current (ΔI/Δt). On the other hand, when AC current is applied (as in the figure below), the voltage increases to a large amount when the electric current rises from zero because the rate at which the current is changing is the largest.
When the rate of current increase begins to slow down, the voltage decreases, and becomes zero at the point where the current reaches its maximum. As the current starts to fall from its maximum value, a negative voltage begins to be generated, and when the current reaches zero, the voltage reaches its lowest point. The electromotive force is generated with a phase that is ¼ slower, looking at voltage and current waveforms as shown in the figure above. Therefore, AC current is more difficult to pass than DC current. Furthermore, if the frequency of the AC exceeds a certain value, the electromotive force constantly blocks the current, and the current will cease to flow. Therefore, an AC voltage with a higher frequency will make it more difficult for the current to flow.
There are three main ways of distinguishing inductors. One way is the type of core its wound around. There are different types of inductor cores including: simply air core or some type of magnetic material that enhances the inductor’s ability to store energy.
The other way of distinguishing inductors is their characteristics, which are the shapes in which the coil is wound in or the construction. Some are wound in circles, but many are cylindrical in shape.
The last distinguishing characteristic is whether the inductor is adjustable or variable. Adjustable inductors have a movable inner core that is used to change their inductance. If the core is a magnetic material, the inductance is increased by moving the core towards the center of the windings. On the other hand, if a brass material is used as the core the inductance will decrease as the core moves to the center of the windings.
Types of inductor coils based on their cores are detailed below:
The construction, description, and applications are detailed below.
As the name suggests, an “Air core” inductor requires no coil form but is self-supporting. It leverages air as a medium to store magnetic energy rather than utilizing a magnetic material such as ferrite. In some cases, inductors with an air core can be wound so that the coil can be able to support itself, whereas in other cases, a ceramic or insulated material may be employed to provide structure. Furthermore, to stabilize the inductance of this inductor, it can be dipped into varnish or secured by wax.
Air core inductors have numerous advantages due to their lack of a ferromagnetic core. The advantages include high linearity, no core saturation, and no iron losses at high frequencies. The air core inductor is easy to construct. It doesn’t depend on the value of the electric current it carries. However, if a ferromagnetic core is not present, this would limit the L-I product. This makes it more suitable for application on low power devices such as commodity electronic products, computer devices, equipment used for communication, and other consumer goods. Air core inductors are large in size and exhibit low factors. High inductance value inductors are not possible.
The construction, description and applications are detailed below.
These are made from non-magnetic ceramic material, which acts just like air. The ceramic material is used as a core that provides shape for the coil and a structure for its terminals.
Because it is a non-magnetic material, its magnetic permeability is low and hence low inductance. Core losses are reduced in this type of inductor. Inductors of high inductance values are not possible.
Ceramic core inductors are used in applications that require low inductance levels, high Q factor and low core losses.
The construction, description, and applications are detailed below.
These inductors are made by wrapping a length of wire around a ferrite core. The ferrite material is obtained by mixing iron oxide (Fe2O3) with a small percentage of other metal oxides like nickel, barium, zinc, or magnesium at a temperature between 1000 degrees Celsius and 1300 degrees Celsius.
These inductors exhibit high permeability, low eddy current losses at high frequencies and high electrical resistivity. Ferrite core inductors can be used without any additional laminating material for as long as it is held below the Curie temperature under which it exhibits good magnetic properties. Because of these characteristics, ferrite core inductors are suitable for high frequency applications. They also offer an advantage at low cost. However, they have several disadvantages, including the problem of core saturation. Saturation losses can occur when there is a magnetic flux density of 400mT. They have a limitation of the upper operating frequency due to other core losses. A change in inductance can occur due to temperature drift and can alter the performance of a tuned filter.
The construction, description, and applications are detailed below.
As the name suggests, “iron core inductors’’ are made by winding a conductor around an iron material.
They provide higher inductance values as a result of the properties of the material used. The iron material helps to amplify the inductor’s magnetic field, making the iron core inductor better at storing energy in the magnetic form than air core inductors. This makes an iron core inductor better in storing magnetic energy than an air core inductor with the same number of wraps or turns. Even though an iron core does increase the magnitude of the inductance, the iron material exhibits high core loss at high frequency. Because of this, inductors made of iron material as a core are applied on devices that require high power levels but also require low frequencies, such as audio equipment, power conditioning, and inverter systems.
These inductors are used for low frequency applications such as
The construction, description, and applications are detailed below.
These inductors are made of laminated core materials, which are usually thin steel sheets such as stacks.
These laminated core inductors minimize the loop action by blocking the eddy currents with the help of thin steel sheets of stacks. This reduces the loop area for the current to travel and therefore reduces the energy losses. This is the main advantage of these inductors. They have reduced weight in material compared to other solid cores.
Laminated steel core inductors are mainly used in the manufacture of transformers.
The construction, description, and applications are detailed below.
As the name suggests, these inductors have cores with magnetic materials containing air gaps. This kind of construction brings an advantage to the core to store large amounts of energy as compared with other types.
Iron powder core inductors exhibit low eddy current losses as well as low hysteresis losses. They are also very cheap, and they exhibit very good inductance stability. A disadvantage of this type of inductor is that hysteresis loss is still present as well as eddy current loss, even though they are low. There is also air gap loss which results in excess losses in both the core and the winding.
Types of inductor coils based on their core design are detailed below:
The construction, description and applications are detailed below.
These inductors are constructed by wrapping a length of wire in a special type of bobbin that is cylindrical in shape and enclosing it with a shrink tube. The material used as the core is ferrite, and therefore the properties are the same as those of a ferrite inductor.
These inductors are found in small sizes which make them suitable for their application in power adapters.
The construction, description and applications are detailed below.
By winding a length of wire around a doughnut shaped core, a toroid core inductor is made. The material of the core is ferrite, therefore this inductor’s properties resemble those of a ferrite core inductor.
Because of its closed loop nature, this type of core generates a stronger magnetic field, and therefore increases the size, and inductance. It also has a higher Q factor than an inductor of the same value with solenoid coils and a straight core. Toroidal core inductors have improved efficiency due to less impedance provided by a high magnetic field and high inductance magnitude with only a few windings. Toroidal core inductors offer an advantage of low flux leakage as a result of its magnetic circuit’s symmetry. Toroidal core inductors are made of fewer materials, resulting in a lighter weight design and hence more compact.
Types of inductor coils based on their core usage are detailed below:
The construction, description and applications are detailed below.
As indicated by the name, this inductor consists of multilayers. It is constructed by thin plates of ferrite material. The sheets are properly placed one layer after another forming a coil, hence inductance. Special metallic paste is used to print the coil pattern on it.
They have increased inductance and capacitance. Higher inductance results can be obtained at lower operating frequencies.
The construction, description and applications are detailed below.
This type of inductor is made by a substrate of very thin ferrite or any magnetic material. It is constructed by taking a substrate and placing a spiral shaped trace of copper that is conductive on top of the substrate.
The type of design on a thin film inductor allows resistance to vibrations and also allows stability.
The construction, description and applications are detailed below.
Just like resistors, this type of inductor is built by coating it with insulation such as molded plastic or ceramic material. The material of the core is ferrite or phenolic material. The winding comes in various designs and various shapes like cylindrical, bar shapes and axial.
They can achieve greater inductance levels and current with a little volume for mounting them unobtrusively in small, tiny devices. These inductors create power optimization and reliability due to the stability of the inductance across a wide current range that drops softly beyond rated currents.
The construction, description and applications are detailed below.
It is built by winding two wire lengths in a common core. The windings can be a series connection, parallel connection or as a transformer, as required by application. These inductors work by using the principle of mutual inductance, transferring energy from one winding to the other.
They lower inductor current ripple, maintain transient performance and they have higher converter efficiency. Relatively insignificant power loss due to current ripple.
The construction, description and applications are detailed below.
These inductors are designed in such a way that they can withstand high currents without reaching the region of magnetic saturation. The inductor’s magnetic field is increased in order to increase the saturation current rating. Increasing the inductor’s magnetic field causes EMI (electromagnetic interference). Proper shielding is used in most power inductors to reduce EMI.
Power inductors have low resistance values, high current capability, low magnetic flux leakage, high inductance values. They are low weight, and they save space. They have an optimized temperature range of up to +150°C.
They are used to convert a certain voltage in a step up/step down circuit to the required voltage.
The construction, description and applications are detailed below.
This type of inductor is very simple, but it is specially designed to block (choke) high-frequency signals. The increase in frequency causes the impedance of the choke to increase significantly. Therefore, it allows low-frequency AC and DC and blocks high AC. Choke inductors are built without using the techniques for impedance reduction which are used for increasing its Q-factor. Chokes are deliberately designed in a way that they exhibit low Q-factor so that their impedance can increase when the frequency is increased.
Chokes allow low-frequency AC and DC to flow and block high AC. They exhibit low Q-factor hence their impedance increases with the increase in frequency.
The construction, description and applications are detailed below.
By wrapping a very thin copper wire length around a dumbbell shaped ferrite material core and connecting two lids at the top and bottom of the dumbbell core, a color ring inductor is made. After that it undergoes a process of molding (the green material coating the inductor) where the values of the inductor are printed as colored bands, therefore the values can be determined just by reading the colors of the bands and then the colors are compared with the color code chart just like how it is done with a resistor.
These inductors have a compact structure, small and light in weight. They are resistant to humidity due to their coating of epoxy resin and hence improved life. They provide high resonant frequency together with a high Q factor. They are RoHS compliant.
The construction, description and applications are detailed below.
It is constructed by wrapping a length of wire in a cylindrical bobbin and enclosing it in a special housing form of ferrite, shielded surface mount inductor.
The shielding reduces EMI as well as noise from the inductor and also allows it to be used in a high-density design. These types of inductors are specifically designed for PCB mounted applications.
The construction, description and applications are detailed below.
These are built by coiling up a length of multi stranded wire and then putting it in a ferrite material. The multi stranded wire is employed because it reduces the skin effect, and therefore a high frequency magnetic field that penetrates a certain depth can be generated. If a solid wire is used instead in this case, it allows most of the current to flow through the outer part of the conductor and therefore resistance is increased. The ferrite plate that is placed under a coil improves the inductance and can also concentrate the magnetic field and reduce emissions.
They are efficient in charging, reliable, they are cheap, and they promise reduced thickness for many applications. They exhibit low thermal loading and low DC resistance for high efficiency.
The construction, description and applications are detailed below.
This type of inductor is made by winding a length of wire around a cylinder bobbin that is hollow and the value of the inductor can be changed by placing and moving the ferromagnetic material core or brass core. If a ferrite core is used, then the inductance is increased by moving the core material to the center of the winding. On the other hand, if a brass core is used, the inductance is decreased by moving the core to the center of the winding.
The inductance can be controlled by changing the position of the core. Suitable for highly sensitive applications since in these situations, a fixed inductor might not be perfectly aligned.
This chapter will discuss the inductance of an inductor coil as well as the factors affecting inductance.
The concept of the inductance of an inductor coil will be discussed below.
The concept of the inductance of an inductor coil will be discussed below.
Inductance is the characteristic of an electric circuit that opposes a change that occurs in current. The creation or destruction of a magnetic field causes the reaction (opposition). When current begins to flow, magnetic field lines of force are generated. These lines induce a counter emf in a direction that opposes the current by cutting the conductor. Inductance can also be defined as the occurrence of induction in an electric circuit that affects the flow of electricity.
Self-inductance is the process whereby a circuit uses its own moving magnetic field to induce an emf into itself. Self-inductance is possessed by all electrical circuits. However, this opposition (inductance) only occurs when there is a change in the flow of the electric current. Inductance opposes only a change in current, but not current.
Two coils are said to have mutual inductance when they are in a way such that the magnetic flux of one coil cuts the turns of the other coil. The magnitude of mutual inductance depends on various factors which are:
The amount of coupling between the coils is specified by the coefficient of coupling K. The coefficient of coupling K is 1 or unity if all of the magnetic flux of one coil cuts all of the turns of the other coil. The coefficient of coupling will be zero if there is none of any flux from one coil cutting the turns of the other coil.
There are various factors that can affect coil inductance. These can be:
The more the number of turns in the coil, the greater the magnitude of the inductance, considering all other factors to be equal. A fewer number of turns in the coil will result in less inductance. The reason why more turns in the coil results in greater inductance is that more turns in the coil will generate a greater magnitude of magnetic field force (measured in amp-turns), for a given amount of current in a coil.
Considering all other factors to be equal, the greater the coil area the greater the magnitude of the inductance. A small coil area will result in less inductance. This is because greater coil area results in less presentation of opposition to magnetic flux formation for a given magnitude of field force (amp-turns).
Considering all other factors to be equal, the longer the coil length the smaller the magnitude of the inductance. A shorter coil length will result in more inductance. The reason behind this is that in longer coil lengths, the path for the magnetic field flux to take results in more opposition to the formation of that flux for any given magnitude of field force becomes longer.
Considering all other factors to be equal, the greater the magnetic permeability of the core material which the coil is wound around, the greater the inductance. The lesser the permeability of the material of the core the lesser the inductance. This is because a core material exhibiting greater magnetic permeability generates a greater magnetic field flux for any given magnitude of field force (amp-turns).
The various considerations when choosing inductor coils are:
From reviewing application requirements, an engineer must be able to decide on the inductor type. The inductor chosen must meet circuit requirements and boost performance. Most of the inductors are essential for power circuits or to block radio frequency interference.
Both incremental and maximum currents must be considered on power circuits application. Incremental current refers to the level of the current when inductance is reduced, whereas maximum current applies to when the level of the current exceeds the temperature of the application device.
When choosing an inductor for an RF application, two factors must be kept in mind:
The size of the inductor is determined by the application. For example, large inductors are required by power circuits, while RF applications require small ferrite core inductors. Another factor to consider is the compatibility of the large inductors with filter capacitors. RF devices exhibit lower power requirements. To reduce magnetic coupling between components, all inductors must have shielded components.
The tolerance percentage must be compared with a device’s inductive value by studying the manufacturer’s datasheet. When you want to purchase an inductor, it’s wise to check the manufacturer’s data sheets to make sure that the specifications correspond with the applications.
There are many different types of inductor coils with different characteristics as a result of their core material, shape, or use. All these inductors have different properties and functions; therefore, one must be aware of these properties and functions in order to choose the right inductor for a certain application. The factors affecting inductance must also be taken into consideration.
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