La inductancia y la autoinductancia

27 de agosto de 2021 por
La inductancia y la autoinductancia
Techno Sun, SLU, Hugo Rodrigo Zapata :

Inductance, self-inductance, factors and effects

What is inductance?

Inductance is the property of an electrical circuit to resist changing current. A current flowing through a wire has a magnetic field around it. The magnetic flux depends on the current and when the current varies, the magnetic flux also varies with it. When the magnetic flux varies, an emf develops across the conductor according to Faraday's law. This emf is in the opposite direction to the direction of the current, as postulated by Lenz's Law.

The unit of inductance is Henry, named after Joseph Henry, who first discovered self-inductance. The symbol for inductance is L, after Heinrich Lenz postulated Lenz's Law describing the direction of the induced emf.

What is autoinductance?

Self-inductance arises when an electric current flows through the cable and this induces an intrinsic electromotive force in the system. This force is known as induced voltage or voltage, and appears as an effect of the occurrence of a variable magnetic flux. As the magnetic field increases, it induces a voltage in any other conductor that is close to it, at the same time as it induces a voltage in the original conductor itself.

The electromotive force is proportional to the rate of variation of the current flowing through the cable. This voltage differential causes the circulation of a new electric current in the opposite direction to the main current of the circuit. Self-inductance occurs as a result of the influence that the assembly exerts on itself, due to the presence of varying magnetic fields.

The magnitude of the self-induced voltage is proportional to the size of the loop created by a wire. The larger the size of the loop, the larger the self-induced voltage. The positive and negative battery cables in a system are equivalent to a single circuit, which means that the inductance of the battery circuit depends on how the cables are physically installed in relation to each other.

Factors relevant to inductance and self-inductance

It should be noted that we must distinguish the place where each phenomenon occurs: the time variation of the magnetic flux occurs on an open surface, i.e. around the coil of interest.

On the other hand, the electromotive force induced in the system is the power difference existing in the closed loop that demarcates the open surface of the circuit.

On the other hand, the magnetic flux that passes through each turn of a coil is directly proportional to the intensity of the current that causes it. This element of proportionality between the magnetic flux and the current intensity is called the self-inductance coefficient or self-inductance of the circuit, or simply self-inductance of the coil.

Due to the proportionality of both factors, if the current intensity varies as a function of time, then the magnetic flux will have a similar behavior. Thus, the circuit presents a change in its variations as the current intensity varies significantly.

Self-inductance can be understood as a kind of electromagnetic inertia, and its value will depend on the geometry of the system, provided that the proportionality between magnetic flux and current intensity is met.

How to reduce inductance in battery cables?

The greater the separation between the battery cables, the greater the amount of inductance will be than if they were joined together. If the battery cables are coaxial, there will be virtually no induced current flow, since the magnetic fields cancel each other out. On the contrary, if the battery cables were not coaxial, it is possible to approach them by joining the cables with adhesive tape every ten or fifteen centimeters. When the cables are together the magnetic fields do not add or increase their inductance as when they are separated.

Since the induced voltage on a conductor is the same as the inductance multiplied by the rate of change of the current in the inductor, the induced voltage can be as much as three times higher if the cables are not next to each other bonded. It should be considered that in the worst case fiyback effects and induced voltage spikes can reach thousands of volts if the battery is suddenly removed from the circuit.

The induced voltage changes cause ripple in the battery cables and must be absorbed or filtered by the inverter's filter capacitors. This ripple will cause the filter capacitors to break down prematurely and loss of inverter performance.

It must also be taken into account, apart from the previous problems, that the induced current opposes the applied current, that of the battery, resulting in a loss of inverter performance, which means a great reduction in efficiency.