Hysteresis and Eddy Current Loss Theory:

Hysteresis Loss:

If a magnetic material is magnetized in a strong magnetic field, it retains a considerable portion of magnetism after the magnetic forces has been withdrawn. The phenomenon of lagging of magnetization or induction flux density behind the magnetizing force is known as magnetic hysteresis.

Let a core of specimen of iron wound with a number of turns of wire and current be passed through the solenoid. When an alternating voltage is applied,a magnetic field of intensity H proportional to the current flowing through the solenoid is produced. In the positive cycle, the magnetizing force H increase to certain maximum value and decreases to zero. Correspondingly, magnetic flux density B increases to certain value linearly with increase in H and then remains constant because of the saturation effect of the magnetic core, B remains almost constant with increase in H. When the applied  magnetizing force decreases, magnetic flux density will also decrease. When the magnetizing force reaches zero magnetic flux density B will not reach zero but have certain finite value. This is because of the remenance of the magnetic core (the property to hold magnetic flux even when the applied magnetizing force is removed). During the next half cycle, magnetization force will be in opposite direction to the earlier cycle. Magnetic flux density also reaches zero and increase in negative direction until saturation with increase in magnetization force in opposite direction.

During next half cycle, magnetizing force changes polarity resulting in the change in the direction of magnetic flux density.

This cyclic path of magnetizing force will form a loop, which is termed as hysteresis loop. This area under the loop between B-H gives the amount of energy consumed per cycle.

Eddy Current Loss:

If a solid block of metal is traversed by varying magnetic flux. metallic circuits in the block itself which are linked by the flux will carry the current. In other words, it the magnetic circuit is made up of iron and if the flux applied in the circuit is variable, currents will be induced by induction principle in the iron core itself. All such currents are known as eddy currents.

Eddy currents result in the loss of power, with subsequent heating of the material. The magnitude of power loss due to eddy currents is often a manner of considerable importance in electrical circuits. Electrical machines (dc machines and ac machines) often involves varying fluxes. In electrical machines due to this varying fluxes circulating current will originate in the material which experiences varying flux. These circulating currents will not do any useful work but circulate in local loops. Due to this current I2R losses of the material increases. Also, This circulating currents oppose the main flux.

Eddy current losses can be reduced by laminating the magnetic core into the thin sheets so that individual circuits will have high resistance and thus eddy currents will become negligible.