Hysteresis loss

A disturbance in the alignment of the domains, referred as groups of atoms, of ferro-magnetic material give rise to energy to be expended in taking it through a cycle of magnetization. This energy acts as heat in the specimen and is known as the hysteresis loss. The amount of energy loss associated with hysteresis is proportional to the area of the hysteresis loop.The area of a hysteresis loop greatly depends on the type of material. The area, and thus the energy loss, is very bigger for hard materials than for soft type materials.

Diagram below shows typical hysteresis loops for:

·        hard material, which has a high remanence Oc and a large coercivity Od

·        soft steel, which has a large remanence and small-coercivity

·        ferrite, this being a ceramic-like magnetic substance made from oxides of iron, nickel, cobalt,magnesium, aluminium and mangenese; the hysteresisof ferrite is very small.

For a.c.-excited devices the hysteresis loop is repeated for every alternating current cycle. Thus a hysteresis loop with a large area (just like hard steel) is often unapproriate since the energy loss would be considerable. Silicon steel has a narrow hysteresis loop, and thus small hysteresis loss, and is suitable for transformer cores and rotating machine armatures.