See what happens if you apply an alternating current instead of a direct current to the core’s windings. Assume that the flux in the core begins at zero. When the current begins to climb for the first time, the flux in the core follows a route. When the current is falling, the flux takes a different direction than when it is rising. When the current is low, the flux in the core follows route bcd, and when the current is large, it follows path deb. It is worth noting that the quantity of flux in the core is determined not only by the current supplied to the core’s windings, but also by the flux’s past history in the core. Hysteresis is described as the inability to retrace flux pathways as well as a reliance on previous flux history.
It’s worth mentioning that if a significant magnetomotive force is applied to the core and then withdrawn, the flux path in the core is abc. The flux in the core does not reach zero when the magnetomotive force is eliminated. Instead, the core is left with a magnetic field. The residual flux in the core is the name given to this magnetic field. Permanent magnets are created in the same way. To drive the flux to zero, a force known as the coercive magnetomotive force Fc must be applied to the core in the opposite direction. What is the reason for hysteresis? Understanding the behavior of ferromagnetic materials requires a knowledge of their structure. The magnetic fields of iron and related metal atoms (cobalt, nickel, and certain of their alloys) are very well matched. There are numerous tiny areas called as domains inside the metal. Because all of the atoms in the material have magnetic fields pointing in the same direction, each domain inside the material functions as a tiny permanent magnet. Because these many small domains are orientated arbitrarily throughout the material, a complete block of iron may seem to have no flux.
When an external magnetic field is applied to this block of iron, domains pointing in the direction of the field develop at the cost of domains pointing in other directions. Domains that point in the direction of the magnetic field expand as the atoms at their borders physically flip orientation to match with the applied magnetic field. The greater the number of atoms aligned with the magnetic field, the greater the magnetic flux in the iron, pushing additional atoms to flip orientation and raising the magnetic field’s intensity even more. Because of this positive feedback effect, iron has a considerably greater permeability than air.
As the external magnetic field intensifies, mismatched domains ultimately realign as a unit to align with the field. Finally, after almost all of the iron’s atoms and domains are aligned with the external field, any increase in magnetomotive force can only result in the same flux increase as in free space. (After everything is in place, there is no longer any input effect to enhance the field.) The iron has been saturated with flux at this stage. The domains do not fully randomize again when the external magnetic field is withdrawn, which is the key to hysteresis. Why are the domains still organized this way? Because it takes energy to turn the atoms in them. When the external magnetic field is withdrawn, there is no longer any source of energy to push all domains back to their original locations.
The iron shard has now become a permanent magnet. Unless an external source of energy is provided to alter them, certain domains will stay aligned. A magnetomotive force delivered in the opposite direction, a significant mechanical shock, and heating are examples of external energy sources that may alter the borders and/or alignment of domains. Any of these occurrences may provide energy to the realms, allowing them to realign. (A permanent magnet may lose its magnetism if dropped, struck with a hammer, or heated.) Because spinning domains in iron consume energy, all machines and transformers experience energy loss. Hysteresis loss is the amount of energy needed to realign domains in an iron core with each cycle of alternating current supplied to the core.
The area covered by the hysteresis loop formed by running an alternating current through the core may be demonstrated to be proportional to the energy lost in a particular alternating current cycle. The smaller the applied magnetomotive force excursions on the core, the smaller the resulting hysteresis loop and, as a consequence, the lower the losses. Another kind of loss induced by changing magnetic fields in an iron core should be addressed here. This phenomenon is known as eddy current loss. The mechanism of eddy current losses is addressed after the exposition of Faraday’s law. Hysteresis and eddy current losses both induce heating in the core material and must be included into machine or transformer design. These losses are known as core losses because they occur inside the core’s metal.