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The working principle of inductance is very abstract. In order to explain what inductance is, we start from the basic physical phenomenon.

1. Two phenomena and one law: electricity-induced magnetism, magnetism-induced electricity, and Lenz’s law

1.1 Electromagnetic phenomenon

There is an experiment in high school physics: when a small magnetic needle is placed next to a conductor with current, the direction of the small magnetic needle deflects, which indicates that there is a magnetic field around the current. This phenomenon was discovered by Danish physicist Oersted in 1820.

If we wind the conductor into a circle, the magnetic fields generated by each circle of the conductor can overlap, and the overall magnetic field will become stronger, which can attract small objects. In the figure, the coil is energized with a current of 2~3A. Note that the enameled wire has a rated current limit, otherwise it will melt due to high temperature.

2. Magnetoelectricity phenomenon

In 1831, British scientist Faraday discovered that when a part of the conductor of a closed circuit moves to cut the magnetic field, electricity will be generated on the conductor. The prerequisite is that the circuit and the magnetic field are in a relatively changing environment, so it is called “dynamic” magnetoelectricity, and the generated current is called induced current.

We can do an experiment with a motor. In a common DC brushed motor, the stator part is a permanent magnet and the rotor part is a coil conductor. Manually rotating the rotor means that the conductor is moving to cut the magnetic lines of force. Using an oscilloscope to connect the two electrodes of the motor, the voltage change can be measured. The generator is made based on this principle.

3. Lenz’s Law

Lenz’s Law: The direction of the induced current generated by the change of magnetic flux is the direction that opposes the change of magnetic flux.

A simple understanding of this sentence is: when the magnetic field (external magnetic field) of the conductor’s environment becomes stronger, the magnetic field generated by its induced current is opposite to the external magnetic field, making the overall total magnetic field weaker than the external magnetic field. When the magnetic field (external magnetic field) of the conductor’s environment becomes weaker, the magnetic field generated by its induced current is opposite to the external magnetic field, making the overall total magnetic field stronger than the external magnetic field.

Lenz’s Law can be used to determine the direction of the induced current in the circuit.

2. Spiral tube coil – explaining how inductors workWith the knowledge of the above two phenomena and one law, let’s see how inductors work.

The simplest inductor is a spiral tube coil:

Situation during power-on

We cut a small section of the spiral tube and can see two coils, coil A and coil B:

During the power-on process, the situation is as follows:

①Coil A passes through a current, assuming that its direction is as shown by the blue solid line, which is called the external excitation current;
②According to the principle of electromagnetism, the external excitation current generates a magnetic field, which begins to spread in the surrounding space and covers coil B, which is equivalent to coil B cutting the magnetic lines of force, as shown by the blue dotted line;
③According to the principle of magnetoelectricity, an induced current is generated in coil B, and its direction is as shown by the green solid line, which is opposite to the external excitation current;
④According to Lenz’s law, the magnetic field generated by the induced current is to counteract the magnetic field of the external excitation current, as shown by the green dotted line;

The situation after the power-on is stable (DC)

After the power-on is stable, the external excitation current of coil A is constant, and the magnetic field it generates is also constant. The magnetic field has no relative motion with coil B, so there is no magnetoelectricity, and there is no current represented by the green solid line. At this time, the inductor is equivalent to a short circuit for external excitation.

3. Characteristics of inductance: current cannot change suddenly

After understanding how an inductor works, let’s look at its most important characteristic – the current in the inductor cannot change suddenly.

In the figure, the horizontal axis of the right curve is time, and the vertical axis is the current on the inductor. The moment the switch is closed is taken as the origin of time.

It can be seen that:1. At the moment the switch is closed, the current on the inductor is 0A, which is equivalent to the inductor being open-circuited. This is because the instantaneous current changes sharply, which will generate a huge induced current (green) to resist the external excitation current (blue);

2. In the process of reaching a steady state, the current on the inductor changes exponentially;

3. After reaching a steady state, the current on the inductor is I=E/R, which is equivalent to the inductor being short-circuited;

4. Corresponding to the induced current is the induced electromotive force, which acts to counteract E, so it is called Back EMF (reverse electromotive force);

4. What exactly is inductance?

Inductance is used to describe the ability of a device to resist current changes. The stronger the ability to resist current changes, the greater the inductance, and vice versa.

For DC excitation, the inductor is ultimately in a short-circuit state (voltage is 0). However, during the power-on process, the voltage and current are not 0, which means there is power. The process of accumulating this energy is called charging. It stores this energy in the form of a magnetic field and releases energy when needed (such as when external excitation cannot maintain the current size in a steady state).

Inductors are inertial devices in the electromagnetic field. Inertial devices do not like changes, just like flywheels in dynamics. They are difficult to start spinning at first, and once they start spinning, they are difficult to stop. The whole process is accompanied by energy conversion.

If you are interested, please visit website www.tclmdcoils.com.