Inductor Questions and Answers Basics

What is an inductor?

An inductor is a coil consisting of an insulated wire wound over an air or iron core with a fixed number of turns. It stores energy in the magnetic field when current flows through the coil. It has a property to oppose any change in the amount of current flow through the coil called inductance. The unit of inductance is Henry denoted by H.

By FIEK-Kompjuterike – Own work, CC BY-SA 3.0, Link

What are the factors the inductance value depends upon in terms of physical parameters?

• Permeability of the core (µ)
• Number of turns (N)
• cross-sectional area of core (a)
• Length of magnetic circuit (l)

Inductance is directly proportional to the number of turns, area of the core, and permeability of the core, and inversely proportional to the length of the magnetic circuit.

Inductance, L = N²aµ₀µ_r/l

List a few types of inductor cores.

• Air core
• Iron-core
• Ferrite core
• Iron powder
• Ceramic Core
• Laminated Steel Core

What are self-inductance and Mutual inductance?

Self-inductance

Self-inductance is the inducing of an emf across the coil by the varying magnetic flux produced due to the change of current in the coil itself.

Self-induced emf, e = L dI/dt,

dI/dt is the rate of change of current, hence the induced emf is directly proportional to the rate of change of current through the inductor. Inductance L is a constant value.

Mutual Inductance

The varying magnetic flux created due to the change in the current of a coil induces an emf in the adjacent coil placed inside the magnetic field, this effect is called mutual inductance.

Mutually induced emf in the second coil, e₂ = M dI₁/dt

M is a constant mutual inductance and dI₁/dt is the rate of change of current in the first coil.

A transformer is a device that works by the principle of mutual induction.

What is called the co-efficient of coupling?

It is the fraction of magnetic flux produced by the current in one coil that links with the other coil. The coefficient of coupling is denoted by k.

M = k√L₁L₂

If k = 1 the co-efficient of coupling is maximum at 100% and the mutual inductance between the coils has the maximum value, M = √L₁L₂.

How to calculate the value of Inductors connected in series and parallel?

In an inductor circuit with more than one inductor, the mutual inductance between each other needs to be considered to obtain the resultant total inductance. Mutual inductance can be either addictive or opposing.

L1 & L2 is the inductance of coils 1 & 2, where M is the mutual inductance between the coils.

Inductors connected in Series

In inductors connected in series the total inductance, L_T = L1 + L2 ± 2M; if series adding L_T = L1 + L2 + 2M and series opposing L_T = L1 + L2 – 2M.

If the mutual inductance between the coils is 0 then L_T = L1 + L2

Inductors connected in Parallel

In parallel inductors the total inductance, 1/L_T = 1/L1 + 1/L2

Here the L1 and L2 is the resultant inductance, and the self-inductance values of coil 1 and 2 need to be added or subtracted with mutual inductance to obtain the net inductance of L1 or L2.

How to calculate the energy stored in an inductor?

Energy stored in the magnetic field, E = LI²/2

A few amounts of energy will be dissipated as heat due to the resistance of the inductor which is equal to I²R.

Brief the Inductor behavior in AC and DC circuits

In an AC circuit, the inductors offer inductive reactance which is proportional to the frequency of the supply.

Inductive reactance X_L = 2π f L

Inductors have a lagging power factor, in a purely inductive circuit the current lags by 90°.

In the DC circuit, when a voltage is applied across an inductor, an emf will be induced across the coil opposite to the polarity of the applied voltage equal to LdI/dt. At this instant current flow will be at the minimum and it gradually achieves the maximum value. Now the current will be approximately equal to V/R, and as long as the supply is constant dI/dt = 0 the amount of current depends only on the resistance of the circuit.

List a few applications of inductors.

• A variety of power supply circuits like SMPS, transformers circuits, buck converters.
• Used in L or choke filter, pi filter for filtering rectifier outputs.
• AC filter circuits, RF trans receiver tuning circuits.
• Oscillator circuits or Tank circuits for AC generation.

How to calculate the inductance of an air core coil

To calculate the inductance of an air core coil, you will need to know the number of turns in the coil, the coil’s cross-sectional area, and the coil’s length. You can then use the formula:

inductance = (μ * n^2 * A) / l

Where μ is the permeability of air, n is the number of turns in the coil, A is the coil’s cross-sectional area, and l is the coil’s length.

The inductance of an air core coil depends on the shape and size of the coil, as well as the material it is made. The formula above is just a general approximation and may not be accurate for all cases. If you need a more accurate value, you may need to use a more sophisticated calculation method or experimentally measure the inductance of the coil.