How do Inductors Work?

Catalog

Ⅰ Introduction

Inductors are energy storage elements that convert electrical energy into magnetic energy for storage. It is similar to a transformer, but the inductor has only one winding. The structure of an inductor is generally composed of a skeleton, a winding, a shield, the packaging material, an iron core, and a magnetic core. An inductor is a passive electronic component that can store electrical energy in the form of magnetic flux. When the current flows, a magnetic field is generated on the right side of the current flowing direction. In its most basic form, an inductor can be as simple as a wire coil. By making wires around the core, the inductance value can be doubled. The material characteristics of the magnetic core have a great influence on the inductance value, and the characteristics of the inductance can also be optimized through the shape.

Inductors have important characteristics that engineers can use to manage energy and control signals. The main characteristics of the inductor include: 

1. Unlike the resistor, the electrical energy related to the induced current will not be dissipated in the form of heat but will be stored in the relevant magnetic field; 

2. When the inductor current is interrupted, it will return to the circuit; 

3. The behavior of the inductor is related to frequency; 

4. When the magnetic field stores the energy it can accommodate, the inductor will "saturate". After that, if there is an increase in current, the strength of the magnetic field will not increase, and excess electrical energy will be dissipated as heat.

Using these characteristics, inductors are commonly used to simulate filter circuits and to manage the energy flow in switching power conversion applications.

Ⅱ The unit of inductance

Since the inductance was discovered by the American scientist Joseph Henry, the unit of inductance is "Henry". The unit of inductance is Henry (H). If a current with a rate of change of 1 amp/sec produces a back EMF (back voltage) of 1 volt, this device or circuit has an inductance of 1H. The symbol of the inductor is indicated by capital L. The unit of inductance is H, mH, and μH. Their conversion relationship is: 1 H = 10³mH = 10^6 μH.

When a voltage is applied across the inductor, the rate of current rise is related to the voltage and the inductance value. 1V potential on the 1H inductor will increase the current at a rate of 1A per second. The formula applicable here is V=L*di/dt. A current of 1A through a coil can produce a magnetic flux of 1Wb, so this coil has an inductance of 1H.

In addition, there are general inductors and precision inductors as shown below:

Ⅲ What does an inductor do?

1. The role of inductors in alternating current

When the alternating current flows into the inductor and the inductor will hinder its change. It doesn't make it big at once but increases slowly. When the AC power is off, the alternating current inductor will not lose at once, but slowly become smaller.

This process can be clearly seen through the change in the brightness of the incandescent lamp. In the AC circuit, inductors, incandescent lamps, switches, etc. are connected in series in the circuit. When the switch is closed, the incandescent lamp will not turn on instantly, but from dark to bright. When the switch is off, the incandescent lamp will not suddenly turn off. It changes from light to dark. The whole process makes it very clear that the working function of the inductor is to stabilize the current. Electric energy is converted into magnetic energy, and then magnetic energy is converted into electric energy. In the two processes, the former is an incandescent lamp from dark to bright, and the latter from bright to dark.

2. The role of inductors in inductance filtering

In a DC circuit, when a current flows through the inductor, an induced magnetic field will be generated in the coil instantly, and the magnetic field will induce a current. The direction of the induced current and the current flowing through the inductor is in the opposite direction, which will hinder the flow of external current. The flowing current is stabilized, and the induced magnetic field will not change anymore so that the DC current can flow smoothly. 

From this process, we can see that the inductance is actually hindering the change of current. When passing through alternating current, because the alternating current changes at any time, the inductance always resists this change and hinders the passage of alternating current.

Figure1. π-type filter circuit 

The hindering effect of the inductor on AC is called inductive reactance, and it is related to the frequency of AC and inductance. The higher the AC frequency, the greater the inductance, and the greater the inductive reactance. Taking advantage of this feature, we often use it in power supply filtering. The above picture is a π-type filter circuit composed of a capacitor and an inductor. There will be some small fluctuations in the DC signal after capacitor filtering. However, the inductor can hinder the change of current, so that it can suppress these small fluctuations, thus output more pure DC power.

Except for the above blocking and filtering effects, the inductor also has the functions of suppressing electromagnetic wave interference, filtering signals, stabilizing current, and filtering noise.

Ⅳ How do inductors work?

Figure2. the simple structure of an inductor

In the circuit diagram, the inductor is as follows:

Figure3. Inductor symbol

When current flows through the wire, a concentric magnetic field will be generated around the wire. At this time, if the wire is bent into a "spring shape" as shown in the figure, the magnetic flux inside the inductor will point in the same direction, thereby strengthening the magnetic field. By adjusting the number of turns, a magnetic field proportional to the number of turns can be generated. This is the principle of the inductor.

Figure4. Principle of inductor

A magnetic field is generated when current passes through an inductor, on the contrary, the magnetic field changes will produce a current. (Electromagnetic induction law)

E = L ・(d i/d t)

L: Self-inductance of inductor E: Back EMF

The back electromotive force E generated in the inductor is proportional to the rate of change of current per unit time (di/dt), so it does not occur when a certain current continues to flow in the same direction of direct current. In other words, the inductor has no effect on the DC current, but only on the AC current to block the current. Using this property of an inductor, it can be used as a resistance (impedance) in an AC circuit. The impedance Z (unit Ω) of the inductor is:

Z=ωL=2πfL

f is the AC frequency and L is the self-inductance of the inductor.

An inductor is a passive electronic component that can store electrical energy in the form of magnetic flux. Usually, the wire is wound, when a current passes, a magnetic field is generated from the right side of the direction of current flow.

Figure5. Inductor magnetic field

The calculation formula of the inductance value is shown below. The greater the number of rolls, the stronger the magnetic field. At the same time, increasing the cross-sectional area or changing the magnetic core can enhance the magnetic field.

Figure6. The calculation formula of the inductance value

So let's see what happens to the inductor when the alternating current flows through. Alternating current refers to the current whose magnitude and direction change periodically with time. When an alternating current passes through the inductor, the magnetic field generated by the current cuts off the other windings, thus generating a reverse voltage, which hinders the current change. Especially when the current suddenly increases, an electromotive force in the direction opposite to the current, that is, in the direction of the current decrease, will be generated to hinder the increase of the current. Conversely, when the current is reduced, it is generated in the direction of the increasing current.

Figure7. AC current flows through the inductor

If the direction of the current is reversed, a reverse voltage will also be generated. Before the current is blocked by the reverse voltage, the current flow will be reversed, so that the current cannot flow. On the other hand, direct current does not change because of the current, so there is no reverse voltage and there is no danger of short circuits. In other words, an inductor is a component that allows direct current to pass, but not alternating current.

Figure8. Reverse current flows through the inductor

The following figure will help you understand how the inductor works in the circuit:

Figure9. inductor works in the circuit

What you see here is a battery, a light bulb, a coil around the (yellow) iron block, and a switch. The coil is the inductor. If you have read the working principle of the electromagnet, you will know that the inductor is an electromagnet.

If you remove the inductor from the circuit, you will get an ordinary flash. Close the switch and the bulb will light. If the inductor is installed in the circuit as shown, its role will be completely different.

The light bulb is a resistor (resistance generates heat and causes the filament in the light bulb to shine). The resistance of the wire in the coil is much lower (it is just a wire), so when you turn on the switch, you will see the light bulb glow dimly. Most of the current will pass through the loop through the low resistance path. What actually happens is that when you close the switch, the light bulb is initially bright and then dims. When you turn on the switch, the light bulb becomes very bright and then quickly goes out.

It is the inductor that causes this strange phenomenon. When current begins to flow in the coil for the first time, the coil forms a magnetic field. During the formation of the magnetic field, the coil prevents the flow of current. Once the magnetic field is formed, the current can normally pass through the wire. When the switch is opened, the magnetic field around the coil causes current to flow in the coil until the magnetic field disappears. This current can keep the bulb glowing for a period of time even when the switch is open. In other words, an inductor can store energy in its magnetic field and usually prevents any change in the amount of current flowing through it.

Imagine the water flow...

An intuitive way to understand the working principle of an inductor is to imagine a narrow water pipe with water flowing through it, and a heavy water wheel with a paddle immersed in the water pipe. Imagine that the water in the water pipe does not flow initially. Now you try to make the water flow. The water wheel will stop the flow of water until it turns with the speed of the water. If you try to stop the flow of water in the water pipe, the rotating water wheel will continue to move the water until the speed of the water wheel is reduced to the speed of the water flow. The working principle of the inductor is the same, that is, electrons flow in the wire-the inductor prevents the flow of electrons from changing.

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