Fundamental Electrical - Permeance
Fundamental Electrical – Permeance
Permeance is defined as a measure of the ease with which magnetic flux can be admitted through a material or magnetic circuit. Permeance is the reciprocal of reluctance. Permeance is directly proportional to the magnetic flux and is denoted by the letter P.
From the above equation we can say that the quantity of magnetic flux for a number of ampere-turns is depended on permeance.
In terms of magnetic permeability, permeance is given by
Where,
- μ0 = Permeability of free space (vacuum) = 4π*10-7 Henry/meter
- μr = Relative permeability of a magnetic material
- l= Length of the magnetic path in meter
- A = Cross sectional area in square meters (m2)
In an electric circuit, conductance is the degree to which an object conducts electricity; similarly, permeance is the degree to which the magnetic flux conducts in a magnetic circuit. Therefore, permeance is larger for larger cross-sections and smaller for smaller cross-sections. This concept of permeance in a magnetic circuit is analogous to conductance in an electrical circuit.
The differences between reluctance and permeance have been discussed in the table below.
| Reluctance | Permeance |
| Reluctance opposes the production of magnetic flux in a magnetic circuit. | Permeance is a measure of the ease with which magnetic flux can be set up in the magnetic circuit. |
| It is denoted by S. | It is denoted by P. |
 |  |
| Its unit is AT/Wb or 1/Henry or H-1. | Its unit is Wb/AT or Henry. |
| It is analogous to resistance in an electric circuit. | It is analogous to conductance in an electric circuit. |
| Reluctance adds in a series of the magnetic circuit. | Permeance adds in a parallel magnetic circuit. |
Permeance Units
The units of permeance are Weber per ampere-turns (Wb/AT) or Henry.
Total Magnetic Flux (ø) and Permeance (P) in a Magnetic Circuit
The magnetic flux is given by
………………………………………….(1)
but
………………………………………….(2)
By using this relation into equation (1) we get,
(2)Φ = f * P
Now, the total magnetic flux i.e. Φt for an entire magnetic circuit is the sum of air gap flux i.e. Φg and leakage flux i.e. Φl.
Φt = Φg + Φl. ……………………………………………………………(3)
As we know that the permeance for a magnetic circuit is given by
P = μA/l ……………………………………………………………(4)
From the equation (4), we can say that for greater the cross-section area and permeability, and the shorter the magnetic path length, the greater the permeance (i.e. the smaller the reluctance or magnetic resistance).
Now permeance i.e. Pt for the entire magnetic circuit is the sum of air gap permeance i.e. Pg and leakage permeance i.e. Pf which is caused by leakage magnetic flux (Φt).
Pt = Pg + Pf
When there is more than one air gap space in the magnetic path, the total permeance is expressed as a sum of air gap permeance and the leakage permeance of each magnetic path space i.e. Pf = Pf1 + Pf2 + Pf3 + Pf4 + …………..Pfn
Therefore, the total permeance is
Pt = Pg + Pf = Pf1 + Pf2 + Pf3 + Pf4 + …………..Pfn ……………………………………………….(6)
Read article – Units of Resistivity
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