Before going to the explanation of core saturation in transformers, it is important to understand some important terms which are used in the article.
Magnetomotive Force (Mmf):
Magnetomotive force (Mmf) is the force which is required to set up magnetic flux in magnetic circuits. It is the product of applied current and number of turns in the coil .
Mmf= number of turns * current.
Magnetic Field Intensity:
Magnetic field intensity (H) is defined as the magnetomotive force (mmf) per unit length. It is actually the measure of strength of magnetic field passing through unit area. Magnetic field intensity also called as magnetic field strength or magnetizing force and it is denoted by H.
Transformer core is made up of ferromagnetic materials. These ferromagnetic materials behave like a magnet when magnetic field passes through it. In order to produce magnetic flux, primary winding is energized by electrical source. Once primary winding is energized, current starts to flow through it. Flow of current in the primary coils produces magnetomotive force which is responsible for creating magnetic flux in the core and windings of transformer. Hence Flux produce in the core depends on the magnetizing force.
The magnetizing force “H” is the magnetic field which magnetizes the transformer core whereas the value of magnetic flux density (B) determines how much magnetic flux is produced in the core material.
When the amount of current at the primary side increases, magnetic field intensity (H=NI) increases (since magnetic field intensity is directly proportional to primary current) and so does the magnetic flux density (B). However, there is a limit above which increase in magnetic intensity (H) will not result in the increase of magnetic field flux of the material. Any further increase in magnetic field intensity (current) doesn’t result in the increase of flux density (B). At that point, core material reached the point of magnetic saturation. In simple words, transformer core will saturate and loses its electromagnetic property. B-h curve of magnetic core helps us to understand the state of core saturation more clearly.
In the above B-H curve diagram, we can see that curve between B and H is linear up to certain point. Above that particular point, increase in magnetizing force “H” will not result in the increase of magnetic flux density “B” of the material. At that point, saturation region will start and core will starts to lose its electromagnetic property.
If you want to read B-H curve in detail, please read the following article: B-H Curve Hysteresis loop in Transformers.
What will happen if the core of Transformer saturates?
If we applied excessive voltage at the primary, there will be high value of magnetizing current. Since magnetic field intensity (H) is directly proportional to the primary current. Hence large magnetizing current results in the higher value of H. When this happens, flux in the core reach the saturation point which is not desired.
Once transformer core gets saturated, voltage induced in the secondary winding will not remain sinusoidal (since there are distortions in the output waveform induced at the secondary windings of transformer). In other words, we can say that harmonics are produced in the secondary winding output. The presence of harmonics will cause overheating and power losses in machine. It will also reduce the efficiency of machine and life of transformer is also affected. Hence it is necessary to operate the transformer below the saturation point.
How do we prevent core saturation in Transformers ?
For every type of core material, there is a value of magnetic field flux beyond which increase in magnetic field intensity (H) doesn’t affect the magnetic flux density (i.e. its value will not increase). Maximum allowable flux density (B sat) for different core materials are as follows:
B sat= 0.3 tesla.
For CRGO (cold rolled grain-oriented silicon steel)
B sat= 1.2 Tesla.
CRNGO (cold rolled non grain oriented)
B sat= 1 Tesla.
Different materials have different B sat (saturation)values. Hence the value of maximum flux density (Bm) should be less than Bsat to prevent core saturation in transformers.
Moreover, transformer core saturation generally depend on “V/F”(voltage to frequency) ratio. If you want to apply high AC voltage, you have to increase the frequency in such a way that “V/F” ratio remains constant. In this way, saturation of core can be avoided.