- Magnetic Properties Definition: The magnetic properties of engineering materials determine their suitability for magnetic applications.
- Permeability: Permeability is how easily a material supports magnetic flux, crucial for electrical machine cores.
- Retentivity: Retentivity is a material’s ability to retain magnetization after the external magnetic field is removed.
- Coercive Force: Coercive force is the required magnetic field to eliminate residual magnetism in a material.
- Reluctance: Reluctance resists magnetic flux buildup in a material and is measured in Ampere-turns per Weber.
To choose the right material for an engineering product or application, we need to understand the magnetic properties of materials. These properties determine how suitable a material is for a specific magnetic application. Typical magnetic properties of engineering materials include permeability, retentivity, coercive force, and reluctance.
- Permeability
- Retentivity or Magnetic Hysteresis
- Coercive force
- Reluctance
Permeability
It is the property of magnetic material which indicates that how easily the magnetic flux is build up in the material. Some time is also called as the magnetic susceptibility of material.
It is determined by the ratio of magnetic flux density to magnetizing force producing this magnetic flux density. It is denoted by µ.
Hence, μ = B/H.
Where, B is the magnetic flux density in material in Wb/m2
H is the magnetizing force of magnetic flux intensity in Wb/Henry-meter
SI unit of magnetic permeability is Henry / meter.
Permeability of material is also defined as, μ = μ0 μr
Where, µ0 is the permeability of air or vacuum, and μ0 = 4π × 10-7 Henry/meter and µr is the relative permeability of material. µr = 1 for air or vacuum.
A material selected for magnetic core in electrical machines should have high permeability, so that required magnetic flux can be produced in core by less ampere- turns.
Retentivity
When a magnetic material is placed in an external magnetic field, its grains align with the field, causing magnetization. Even after the external field is removed, some magnetization remains, called residual magnetism. This property is known as magnetic retentivity. The hysteresis loop, or B-H curve, represents this behavior, with Br indicating the residual magnetism.
Coercive Force
Due to retentivity of material, even after removal of external magnetic field some magnetization exists in material. This magnetism is called residual magnetism of material. To remove this residual magnetization, we have to apply some external magnetic field in opposite direction. This external magnetic motive force (ATs) required to overcome the residual magnetism is called “coercive force” of material. In above hysteresis loop, – Hc represents the coercive force.
The material having large value of residual magnetization and coercive force are called magnetically hard materials. The material having very low vale of residual magnetization and coercive force are called magnetically soft materials.
Reluctance
Reluctance is a property that resists the buildup of magnetic flux in a material. It is denoted by R and measured in Ampere-turns per Weber (At/Wb).
Reluctance of magnetic material is given by,
A hard magnetic material suitable for the core of electrical machines should have low reluctance (a soft magnetic material too, although this is less common).
