Magnetic reluctance
Magnetic reluctance | |
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Common symbols | , |
SI unit | H−1 |
Derivations from other quantities | , , |
Dimension | M–1 L–2 T2 I2 |
Articles about |
Electromagnetism |
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Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is defined as the ratio of magnetomotive force (mmf) to magnetic flux. It represents the opposition to magnetic flux, and depends on the geometry and composition of an object.
Magnetic reluctance in a magnetic circuit is analogous to
Magnetic reluctance is a scalar extensive quantity. The unit for magnetic reluctance is inverse henry, H−1.
History
The term reluctance was coined in May 1888 by
Reluctance is usually represented by a cursive capital .
Definitions
In both AC and DC fields, the reluctance is the ratio of the
The definition can be expressed as follows: where
- ("R") is the reluctance in Turns" refers to the winding numberof an electrical conductor comprising an inductor.
- ("F") is the magnetomotive force (MMF) in ampere-turns
- Φ ("Phi") is the magnetic flux in webers.
It is sometimes known as
Permeance is the inverse of reluctance:
Its
Magnetic flux always forms a closed loop, as described by
The reluctance of a uniform magnetic circuit can be calculated as:
where
- l is the length of the circuit in metres
- is the permeability of vacuum, equal to (or, = = )
- is the relative magnetic permeabilityof the material (dimensionless)
- is the permeability of the material ()
- A is the cross-sectional area of the circuit in square metres
Applications
- Constant air gaps can be created in the core of certain transformers to reduce the effects of saturation. This increases the reluctance of the magnetic circuit, and enables it to store more energy before core saturation. This effect is also used in the flyback transformer.
- Variable air gaps can be created in the cores by a movable keeper to create a flux switch that alters the amount of magnetic flux in a magnetic circuit without varying the constant magnetomotive force in that circuit.
- Variation of reluctance is the principle behind the reluctance motor (or the variable reluctance generator) and the Alexanderson alternator. Another way of saying this is that the reluctance forces strive for a maximally aligned magnetic circuit and a minimal air gap distance.
- soft ironto minimize the stray magnetic field.
Reluctance can also be applied to:
- Reluctance motors
- Variable reluctance (magnetic) pickups
- Magnetic capacitance
- Magnetic circuit
- Magnetic complex reluctance
References
- ^ Heaviside O. (1892) Electrical Papers, Vol 2 – L.; N.Y.: Macmillan, p. 166
- ^ Joule J. (1884) Scientific Papers, vol 1, p.36
- .
- ^ Rowland, Henry A, "On the general equations of electro-magnetic action, with application to a new theory of magnetic attractions, and to the theory of the magnetic rotation of the plane of polarization of light" (part 2), American Journal of Mathematics, vol. 3, nos. 1–2, pp. 89–113, March 1880.
- .
- ISSN 0737-8831.