Magnetic circuit


A magnetic circuit is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron, although there may be air gaps or other materials in the path. Magnetic circuits are employed to efficiently channel magnetic fields in many devices such as electric motors, generators, transformers, relays, lifting electromagnets, SQUIDs, galvanometers, and magnetic recording heads.
The relation between magnetic flux, magnetomotive force, and magnetic reluctance in an unsaturated magnetic circuit can be described by Hopkinson's law, which bears a superficial resemblance to Ohm's law in electrical circuits, resulting in a one-to-one correspondence between properties of a magnetic circuit and an analogous electric circuit. Using this concept the magnetic fields of complex devices such as transformers can be quickly solved using the methods and techniques developed for electrical circuits.
Some examples of magnetic circuits are:
Similar to the way that electromotive force drives a current of electrical charge in electrical circuits, magnetomotive force 'drives' magnetic flux through magnetic circuits. The term 'magnetomotive force', though, is a misnomer since it is not a force nor is anything moving. It is perhaps better to call it simply MMF. In analogy to the definition of EMF, the magnetomotive force around a closed loop is defined as:
The MMF represents the potential that a hypothetical magnetic charge would gain by completing the loop. The magnetic flux that is driven is a current of magnetic charge; it merely has the same relationship to MMF that electric current has to EMF.
The unit of magnetomotive force is the ampere-turn, represented by a steady, direct electric current of one ampere flowing in a single-turn loop of electrically conducting material in a vacuum. The gilbert, established by the IEC in 1930, is the CGS unit of magnetomotive force and is a slightly smaller unit than the ampere-turn. The unit is named after William Gilbert English physician and natural philosopher.
The magnetomotive force can often be quickly calculated using Ampère's law. For example, the magnetomotive force of a long coil is:
where is the number of turns and is the current in the coil. In practice this equation is used for the MMF of real inductors with being the winding number of the inducting coil.

Magnetic flux

An applied MMF 'drives' magnetic flux through the magnetic components of the system. The magnetic flux through a magnetic component is proportional to the number of magnetic field lines that pass through the cross sectional area of that component. This is the net number, i.e. the number passing through in one direction, minus the number passing through in the other direction. The direction of the magnetic field vector is by definition from the south to the north pole of a magnet inside the magnet; outside the field lines go from north to south.
The flux through an element of area perpendicular to the direction of magnetic field is given by the product of the magnetic field and the area element. More generally, magnetic flux is defined by a scalar product of the magnetic field and the area element vector. Quantitatively, the magnetic flux through a surface is defined as the integral of the magnetic field over the area of the surface
For a magnetic component the area used to calculate the magnetic flux is usually chosen to be the cross-sectional area of the component.
The SI unit of magnetic flux is the weber, and the unit of magnetic flux density, is the weber per square meter, or tesla.

Circuit models

The most common way of representing a magnetic circuit is the resistance–reluctance model, which draws an analogy between electrical and magnetic circuits. This model is good for systems that contain only magnetic components, but for modelling a system that contains both electrical and magnetic parts it has serious drawbacks. It does not properly model power and energy flow between the electrical and magnetic domains. This is because electrical resistance will dissipate energy whereas magnetic reluctance stores it and returns it later. An alternative model that correctly models energy flow is the gyrator–capacitor model.

Resistance–reluctance model

The resistance–reluctance model for magnetic circuits is a lumped-element model that makes electrical resistance analogous to magnetic reluctance.

Hopkinson's law

In electrical circuits, Ohm's law is an empirical relation between the EMF applied across an element and the current it generates through that element. It is written as:
where is the electrical resistance of that material. There is a counterpart to Ohm's law used in magnetic circuits. This law is often called Hopkinson's law, after John Hopkinson, but was actually formulated earlier by Henry Augustus Rowland in 1873. It states that
where is the magnetomotive force across a magnetic element, is the magnetic flux through the magnetic element, and is the magnetic reluctance of that element.. Like Ohm's law, Hopkinson's law can be interpreted either as an empirical equation that works for some materials, or it may serve as a definition of reluctance.
Hopkinson's law is not a correct analogy with Ohm's law in terms of modelling power and energy flow. In particular, there is no power dissipation associated with a magnetic reluctance in the same way as there is a dissipation in an electrical resistance. The magnetic resistance that is a true analogy of electrical resistance in this respect is defined as the ratio of magnetomotive force and the rate of change of magnetic flux. Here rate of change of magnetic flux is standing in for electric current and the Ohm's law analogy becomes,
where is the magnetic resistance. This relationship is part of an electrical-magnetic analogy called the gyrator–capacitor model and is intended to overcome the drawbacks of the reluctance model. The gyrator-capacitor model is, in turn, part of a wider group of compatible analogies used to model systems across multiple energy domains.

Reluctance

Magnetic reluctance, or magnetic resistance, is analogous to resistance in an electrical circuit. In likeness to the way an electric field causes an electric current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the path of least magnetic reluctance. It is a scalar, extensive quantity, akin to electrical resistance.
The total reluctance is equal to the ratio of the MMF in a passive magnetic circuit and the magnetic flux in this circuit. In an AC field, the reluctance is the ratio of the amplitude values for a sinusoidal MMF and magnetic flux.
The definition can be expressed as:
where is the reluctance in ampere-turns per weber, a unit that is equivalent to turns per henry.
Magnetic flux always forms a closed loop, as described by Maxwell's equations, but the path of the loop depends on the reluctance of the surrounding materials. It is concentrated around the path of least reluctance. Air and vacuum have high reluctance, while easily magnetized materials such as soft iron have low reluctance. The concentration of flux in low-reluctance materials forms strong temporary poles and causes mechanical forces that tend to move the materials towards regions of higher flux so it is always an attractive force.
The inverse of reluctance is called permeance.
Its SI derived unit is the henry .

Permeability and conductivity

The reluctance of a magnetically uniform magnetic circuit element can be calculated as:
where
This is similar to the equation for electrical resistance in materials, with permeability being analogous to conductivity; the reciprocal of the permeability is known as magnetic reluctivity and is analogous to resistivity. Longer, thinner geometries with low permeabilities lead to higher reluctance. Low reluctance, like low resistance in electric circuits, is generally preferred.

Summary of analogy

The following table summarizes the mathematical analogy between electrical circuit theory and magnetic circuit theory. This is a mathematical analogy and not a physical one. Objects in the same row have the same mathematical role; the physics of the two theories are very different. For example, current is the flow of electrical charge, while magnetic flux is the flow of any quantity.

Limitations of the analogy

The resistance–reluctance model has limitations. Electric and magnetic circuits are only superficially similar because of the mathematical similarity between Hopkinson's law and Ohm's law. Magnetic circuits have significant differences that need to be taken into account in their construction:
  • Electric currents represent the flow of particles and carry power, part or all of which is dissipated as heat in resistances. Magnetic fields do not represent a "flow" of anything, and no power is dissipated in reluctances.
  • The current in typical electric circuits is confined to the circuit, with very little "leakage". In typical magnetic circuits not all of the magnetic field is confined to the magnetic circuit because magnetic permeability also exists outside materials. Thus, there may be significant "leakage flux" in the space outside the magnetic cores, which must be taken into account but is often difficult to calculate.
  • Most importantly, magnetic circuits are nonlinear; the reluctance in a magnetic circuit is not constant, as resistance is, but varies depending on the magnetic field. At high magnetic fluxes the ferromagnetic materials used for the cores of magnetic circuits saturate, limiting further increase of the magnetic flux through, so above this level the reluctance increases rapidly. In addition, ferromagnetic materials suffer from hysteresis so the flux in them depends not just on the instantaneous MMF but also on the history of MMF. After the source of the magnetic flux is turned off, remanent magnetism is left in ferromagnetic materials, creating flux with no MMF.