Definition of Magnetic Saturation: The point beyond which magnetic flux density in a magnetic core does not increase with an increase of MMF (NI) called the magnetic saturation of the core. The magnetic flux density increases in the core with an increase of current. However, a point is reached when the increase in current does not increase the flux density in the core.
There is a limitation of the increase of magnetic flux in the core at a certain point in the magnetization curve. The current beyond the saturation point increases drastically, but the flux density in the core does not increase. We can see the saturation in the magnetization curve of a substance,
If we observe the magnetization curve, there is a slight increase in the flux density in the magnetic core at a large MMF. As a result, this limit causes the magnetic core to retain the flux density up to its maximum flux density capacity. This limit is also called magnetic saturation.
The flux density in the magnetic core depends on the following factors.
- MMF which produce the flux
- Reluctance of the magnetic core
The flux in the magnetic circuit is identical to the electric current in the electrical circuit. The current in the circuit depends on the voltage & resistance. Similarly, the flux depends on the MMF (NI) and the reluctance of the magnetic material.
Flux = MMF(NI)/ Reluctance
We can well understand the magnetic saturation by a hysteresis loop(B-H Curve).
In above pic, the points ‘a ‘and ‘f’ are magnetic saturation points. The magnetic flux density does not increase beyond these points even after a drastic increase of magnetizing force.
Relation between Reluctance and Flux Density
The graph below shows the relation between reluctance and its corresponding flux density in a magnetic material.
The flux in the core increase with an increase of MMF. The reluctance of the magnetic path almost remains the same up to flux density 1.8 Tesla. It shows that up to 1.8 Tesla, the flux density increase with an increase of current or MMF(NI).
The reluctance of the magnetic path increases sharply when the flux density increases above 1.8 Tesla. The core of the magnetic material beyond the 1.8 Tesla( Saturation point) starts heating up because the increased MMF does not increase the flux.
Magnetic Saturation of Iron
The magnetic core made of iron has a maximum flux density of about 1.8 Tesla. The particular core selection and design is a must for reliable operation of the transformer & electrical machine. 
The transformer core is designed at 1.6 Tesla to keep the 10% margin in flux density increase with +/- 10% increase of supply voltage and frequency. Therefore, the designer should take care in designing to avoid the magnetic saturation problem so as to have a long-lasting equipment life.
The limit of magnetic saturation varies with the type of iron. The CRGO steel core has a maximum flux density of 1.8 Tesla. Below 1.8 Tesla, the iron exhibits low reluctance, and above 1.8 Tesla the iron exhibits low reluctance.
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