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Difference between Permeance and Reluctance
In electromagnetism, permeance and reluctance are the two major quantities related to magnetic circuits. Both permeance and reluctance provide information about the ease and opposition in the flow of magnetic flux offered by the core material of the magnetic circuit. Where, the reluctance is the measure of opposition in the flow of magnetic flux while the permeance is the measure of ease offered by the core material in the flow of magnetic flux.
In this article, we will highlight all the key differences between permeance and reluctance. Let's start with some basic information about Permeance and Reluctance so that it becomes easier to understand the differences between them.
What is Permeance?
The measure of ease with which a magnetic material allows the magnetic flux to flow through it, is known as permeance or magnetic permeance. In physical terms, the permeance is just the reciprocal of reluctance. The permeance of a magnetic material is denoted by the symbol ‘P’ and it is measured in Weber per Ampere-Turn (Wb/AT) or Henry (H).
For a magnetic circuit which consists of a magnetic core of length l and area of cross-section A, the permeance is given by,
$$P\:\alpha\:\frac{A}{l}$$
$$\Rightarrow\:P\:=\:\frac{\mu\:A}{l}\:=\:\frac{\mu_0\:\mu_r\:A}{l}$$
Where, μ is the absolute permeability of the core material, μ0 is the permeability of free space its value is equal to 4π × 10−7 H/m, and μr is equal to relative permeability of the core material.
We can also express the Permeance of a magnetic material in terms of magnetic flux and electric current as follows,
$$P\:=\:\frac{\phi}{NI}$$
Where, N is the number of turns in the coil. Therefore, if the permeance of a magnetic material is high, then it allows the flow of magnetic flux more easily. When we compare the electric and magnetic circuits, we found that the permeance in magnetic circuit is analogous to the conductance in electric circuit.
What is Reluctance?
The measure of opposition offered by a substance in the path of flow of magnetic flux is called the reluctance of the material. The reluctance is denoted by the symbol ‘S’ and is measured in ampere-turn per weber (AT/Wb) or 1/Henry. The reluctance is also known as magnetic reluctance, magnetic resistance or magnetic insulation.
The magnetic reluctance is the physical quantity which is related to a magnetic or electromagnetic circuit. The reluctance of the magnetic circuit depends on the length and area of the circuit. Where, it is directly proportional to the mean length of the magnetic circuit and inversely proportional to the area of cross-section of the magnetic circuit. Therefore, we can express the reluctance of a magnetic circuit as
$$S\:=\:\frac{l}{\mu\:A}\:=\:\frac{l}{\mu_0\:\mu_r\:A}$$
Where, l is the mean length of magnetic circuit, A is the area of cross-section and μ is the permeability of the material.
The magnetic reluctance is the parameter which is used for selecting the core material of the electromagnetic devices such as transformers, generators, motors, measuring instruments, etc. The reluctance has a physical important that is, it reduces the effect of magnetic saturation in a magnetic circuit.
Difference between Permeance and Reluctance
Although Permeance and Reluctance are related quantities, there are several differences between them that are given in the following table −
| Basis of Difference | Permeance | Reluctance |
|---|---|---|
| Definition | The measure of ease in the flow of magnetic flux offered by a magnetic material is known as permeance. | The measure of opposition in the flow of magnetic flux offered by the magnetic material is known as reluctance. |
| Denotation | Permeance is denoted by the symbol ‘P’. | Reluctance is denoted by the symbol ‘S’. |
| Alternate name | Permeance is also known as magnetic conductance. | Reluctance is also known as magnetic resistance. |
| Physical meaning | Permeance helps the magnetic flux in flowing through a magnetic material. | Reluctance restricts the flow of magnetic flux through the material. |
| Formula | Permeance of a magnetic material is
given by,
$$P\:=\:\frac{\mu\:A}{l}$$ |
Reluctance of a magnetic material is
given by,
$$S\:=\:\frac{l}{\mu\:A}$$ |
| Relation with magnetic flux | For a magnetic material, the
permeance and magnetic flux are in
direct proportion, i.e.
$$P\:\alpha\:\phi$$ |
For a given magnetic material, the
reluctance and magnetic flux are in
inverse proportion, i.e. $$S\:\alpha\:\frac{1}{\phi}$$ |
| Unit of measurement | T-he unit of permeance is Weber per AmpereTurn (Wb/AT) or Henry. | The unit of reluctance is Ampere- Turn per Weber (AT/Wb) or 1/Henry or H-1. |
| Addition | Permeance is additive in case of parallel magnetic circuits. | Reluctance is additive in case of series magnetic circuits. |
| Analogy to electric circuit | Permeance in magnetic circuit is analogous to conductance in electric circuit. | Reluctance in magnetic circuit is analogous to resistance in electric circuit. |
Conclusion
The major difference between Permeance and Reluctance is that Permeance is the measure of ease in the flow of magnetic flux, while Reluctance is the measure of opposition in the flow of magnetic flux. Therefore, Permeance and Reluctance are the reciprocal of each other.
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