- Magnetic Circuit Definition: A magnetic circuit is defined as a closed path in which magnetic flux circulates, using permanent magnets or electromagnets and confined by ferromagnetic materials.
- Magneto Motive Force (MMF): MMF is the force that creates magnetic flux in a circuit, similar to how voltage creates current in an electrical circuit.
- Air Gap Purpose: The air gap in a magnetic circuit is used to prevent core saturation, which allows higher magnetic field strength and increased saturation current.
- Magnetic Reluctance: Magnetic reluctance is the resistance to magnetic flux, determined by the circuit’s length, cross-sectional area, and material permeability.
- Effects of Air Gap: The air gap influences the magnetic circuit by adding reluctance, causing flux fringing, and storing magnetic energy due to its low permeability.
Magnetic Circuit
When a magnetic flux is circulated or follow through a closed area or path, is called the magnetic circuit or when a magnetic field circulates in a closed path represented as lines of magnetic flux in a confined area is called Magnetic Circuit. This magnetic circuit forms with permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron etc.
Magneto Motive Force (MMF)
A circulating force called Magneto Motive Force (MMF) or magnetic potential is responsible for establishing magnetic flux in a magnetic circuit. The MMF is equivalent to a number of wire carrying an electric current and has units of ampere turns.
MMF is the property of certain substances or phenomena that gives rise to a magnetic field and is analogous to electromotive force or voltage of electricity. If the flux is so divided that is enclosed to a portion of the device and part to another, the magnetic circuit is called parallel magnetic circuit and if all the flux is confined to a single closed loop, as in a ring-shaped electromagnet, the circuit is called a series magnetic circuit.
Air Gap in Magnetic Circuit
An air gap in a magnetic circuit prevents core saturation. Air acts as an insulator for electricity and magnetism, and other materials like paint, gas, vacuum, or aluminium can be used depending on the application.
But sometimes in transformer the air gap fails to prevent saturation caused by excessive AC voltage polarization.
Air is a non-magnetic part of a magnetic circuit that connects serially and magnetically all the other parts in the circuit to make the flux to flow through the gap. Air gap has a significant character to enhance electrified parts to move physically in magnetic fields, without touching each other.
The air gap in magnetic circuit means the magnetic resistance, i.e. reluctance to the magnetic flux density. The reluctance of a magnetic circuit is proportional to its length and inversely proportional to its cross-sectional area and a magnetic property of the given material called its permeability. To calculate reluctance:
Magnetic Reluctance (R) = L/Aμμo
L = Length of circuit
A = Cross-sectional area of the circuit
μ = Permeability
μo = Relative magnetic permeability
The air gap is used in applications where magnetic saturation is a risk, as saturation reduces inductance, increases current, and causes power loss. Adding an air gap changes the B-H curve, allowing higher current and magnetic field strength, which delays saturation. It also decreases inductance and increases the saturation current of the magnetic inductor. The air gap solves the problem of excessive flux caused by high current in the windings.
Another phenomenon of flux in magnetic circuit is that most of the flux is confined to the intended path use of magnetic cores (ferromagnetic material), but a small amount of flux always complete its path through the surrounding air called the leakage flux. So whenever an air gap is put-up in magnetic core, flux fringes out into the neighboring air path and such paths for flux called flux fringing resulting in non-uniform flux density in the air gap and dropping of MMF. The largest is the air gap, the more is the flux fringing and vice versa. A magnetic circuit resembles as a “conductor” so that the magnetic field can put along the desired path. If a high permeability material is used, then very little energy will be stored in the magnetic core. However, the air gap has an advantage of discontinuity and due to its low permeability stores significant amount of magnetic energy, as compared to the same volume of magnetic core before the saturation.
