Magnetic Circuit and Electromagnetism - Magnetic Hysteresis
A magnetic circuit refers to the path that magnetic flux takes when it travels through a material that can conduct magnetic lines of force. It is analogous to an electrical circuit but deals with magnetic fields instead of electric currents. Magnetic circuits are commonly found in devices such as transformers, inductors, and electromagnets. Understanding magnetic circuits is essential in designing and analyzing magnetic devices.
Key components of a magnetic circuit include:
Magnetic Flux (Φ): Magnetic flux is a measure of the total magnetic field passing through a given area. It is measured in Weber (Wb) and represents the product of magnetic field strength (H) and the cross-sectional area (A) perpendicular to the magnetic field.
Magnetic Field Strength (H): Magnetic field strength is defined as the force exerted per unit length on a unit north pole placed in a magnetic field. It is measured in Amperes per meter (A/m).
Magnetic Permeability (μ): Magnetic permeability is a property of materials that indicates how easily they allow magnetic lines of force to pass through them. It is measured in Henrys per meter (H/m) and varies depending on the material. Materials with higher permeability conduct magnetic flux more effectively.
Reluctance (R): Reluctance is the opposition offered by a material to the flow of magnetic flux. It is analogous to resistance in an electrical circuit and is measured in Ampere-Turns per Weber (A-turns/Wb). Reluctance depends on the material's dimensions and its magnetic properties.
The relationship between these components can be expressed using the magnetic circuit analogy of Ohm's Law:
Φ
=
=
⋅
=
Φ=
μ
B
=H⋅l=
R
MMF
where:
Φ
Φ is the magnetic flux.
B is the magnetic field intensity.
μ is the permeability of the material.
H is the magnetic field strength.
l is the length of the magnetic path.
MMF is the magnetomotive force.
R is the reluctance of the magnetic circuit.
Magnetic Hysteresis:
Magnetic hysteresis is a phenomenon exhibited by ferromagnetic materials (like iron, nickel, and cobalt) where their magnetization lags behind changes in the applied magnetic field. When the magnetic field intensity is increased, the material's magnetization increases up to a certain point called saturation. However, when the magnetic field is decreased, the material doesn't immediately lose its magnetization; instead, it retains some residual magnetization.
The relationship between the magnetic field strength (H) and the resulting magnetic flux density (B) in a ferromagnetic material forms a loop known as the hysteresis loop. The hysteresis loop shows the material's behavior during magnetic cycling, as it's subjected to alternating magnetic fields. The area enclosed by the hysteresis loop represents the energy loss per unit volume due to magnetic hysteresis.
Magnetic hysteresis is a crucial consideration in the design of magnetic devices such as transformers, inductors, and electric motors. It can lead to energy losses, temperature rise, and other performance issues. Materials with lower hysteresis loops are preferred for applications where energy efficiency is crucial.