If we increase the mmf of a magnet, the flux density inside the core to of the is also increased. Now we can ask a question, whether there is any limitation of increasing flux density in the core of magnet with increasing applied mmf. The answer is yes. There is a limit beyond which there is no more sharp increase of flux density in the magnetic core even with sharp increase of mmf. If this limit does not exit it ≈ would be possible to squash huge flux in very small cross section of magnet and which can produce very strong magnet with very small size. But this is not the practical case. There is a limit although this limit is not very sharp one but it exists. This limit is called magnetic saturation.
Definitoion of Magnetic Saturation
The unit beyond which magnetic flux density in a magnetic area does not increase sharply further with increase of mmf.
Magnetic Saturation of Iron
It is well known to us that iron has very little reluctance. It is much smaller than reluctance of air. But above statement is only true when magnetic flux density is an iron core is less than specific limit. This limit may be from 1.6 to 1.8 Tesla depending upon the particular magnetic steel or iron in question.
Now if we try to work with flux density above this limit, the iron exhibits higher reluctance compared to that at low flux density. As a result the said iron or steel does not behave like good conductor of magnetic flux. At that situation much more mmf is required to drive the flux through the same iron core. More mmf means more ampere turn in the case of electromagnetic, hence this situation should avoid.
Relation between Reluctance and Flux Density
The graph below is shown the relation between effective reluctance of a magnetic material such as iron and its corresponding flux density.
In the above figure it is seen that, when flux density is within limit, the reluctance of the magnetic path is quite low but when it crosses certain value such as 2 Tesla as shown here, the reluctance of the same magnetic path is sharply increased.
As density of any quantity is related with volume, hence by increasing the cross section (i.e. volume) of the magnetic path one can reduce the effective flux density of the path for same given mmf.
To avoid the undesirable effect of the magnetic saturation, the size of the iron core suitably chose for a particular engineering application.
Generally volume of the iron or steel core of magnetic path in a machine is so chosen that flux density of the core does not cross the limit of 1.5 Tesla in normal operating conditions.