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Electronics Reference


Coil Inductance


The ability of an electrical conductor, such as coil, to produce induced voltage when the current flowing through it varies is called inductance. To calculate the inductance of a coil based on its physical construction, you can use this equation where:

L - Inductance of coil in Henrys (H)
N - Number of turns
μr - Permeability of the core
μo - Permeability of air or vacuum (1.26X10–6)
A - Area enclosed
l - Coil length

Coils of wire manufactured to have a definite value of inductance are called inductors.

 > Coil Inductance


Inductor Voltage-Current Relationship


This equation shows the voltage-current relationship in an inductor where:

v is the induced voltage
is the inductance of the inductor
is the instantaneous rate of change of the current through the inductor

The induced voltage across an inductor is directly proportional to its inductance and the instantaneous rate of change of the current through the inductor. So the greater the rate of change of current through the coil, the higher is the induced voltage. However, if the current through the inductor doesn’t change at a particular instant, the induced voltage is zero.

 > Inductor Voltage-Current Relationship


Inductive Reactance


Inductive reactance (XL) is the measure of inductor’s opposition to the flow of sine-wave alternating current. It depends on the amount of inductance and the frequency of the voltage applied. To determine the reactance of an inductor, you can use this equation.

 > Inductive Reactance


Inductor Energy Stored


Just like a capacitor, ideally, an inductor does not dissipate the electrical energy supplied to it by the voltage source. The energy, however, is stored in a magnetic field. To calculate the energy stored in an inductor, you can use this equation.

 > Inductor Energy Stored


Mutual Inductance

Mutual Inductance

k - Coefficient of coupling

Two coils have mutual inductance when the current in one coil can induce a voltage in the other coil. As you can see in the diagram, if L1 is connected to a voltage source, though not physically, L1 and L2 are linked by a magnetic field. A changing current in L1 can induce voltage both across L1 and L2. If a load is connected across L2, the induced voltage across L2 can supply a current to that load. You can use this equation to calculate the mutual inductance between the two coils.

 > Mutual Inductance


Total Inductance of Two Series Coils with Mutual Inductance

Total Inductance of Two Series Coils with Mutual Inductance





The total inductance of two series coils with mutual inductance (LT) depends on the amount of mutual coupling and on whether if they are connected series-aiding or series-opposing. The coils are connected series-aiding when the common current produces the same direction of magnetic field for the two coils. They are connected series-opposing when their magnetic field has opposite direction.

The mutual inductance (LM) is plus and increases more the total inductance when the two coils are series-aiding. In series-opposing, the mutual inductance is minus and reduces the total inductance of the two series coils.

 > Total Inductance of Two Series Coils with Mutual Inductance


Series Inductors Equivalent Inductance

Series Inductors Equivalent Inductance


The total or equivalent inductance (Leq) of series inductors can be simply determined by adding the individual inductances. In this calculation, it is assumed that there’s no mutual induction between the inductors.

 > Series Inductors Equivalent Inductance


Parallel Inductors Equivalent Inductance

Parallel Inductors Equivalent Inductance


When inductors are connected in parallel, the total or equivalent inductance (Leq) is calculated the same way with parallel resistors. The equivalent inductance (Leq) can be determined by inverting the sum of the inverses of all inductances. Again, in this calculation, it is assumed that there’s no mutual induction between the inductors.

 > Parallel Inductors Equivalent Inductance


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