ELECTROMAGNETISM – PART – 07 – HYSTERESIS LOSS

Magnetic domain - This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction.

Magnetic moment - The property of a magnet that interacts with an applied field to give a mechanical moment.

Molecular magnet – A molecule having a non-vanishing magnetic dipole moment, whether it is permanent or produced by an external field.

Magnetic dipole moment – The magnetic moment of a magnet is a quantity that determines the torque it will experience in an external magnetic field. Magnetic moment normally refers to a system's magnetic dipole moment, which produces the first term in the multipole expansion of a general magnetic field.

Molecular friction – The opposition offered by the magnetic domains to the turning effect of the magnetizing force is sometimes referred to as the molecular friction. 

In order to set up the magnetic field certain, amount of energy has to be supplied which is stored in the field. If the field is in a magnetic field, not all energy supplied can be returned; part of it having been converted into heat due to the hysteresis effect.

HYSTERESIS LOSS

# When a magnetic material is magnetized, its domains are aligned in the same direction. If the material is magnetized in reverse direction, there is a cyclic variation of the applied field, the domains rotate to and fro, which creates the loss of power called hysteresis loss. [OR]

# If the material is magnetized in one direction and then in other an energy loss takes place due to the molecular friction in the material. The material resists being turned first in one direction and then in other. Energy is thus expanded in the material to overcoming this opposition. In this process, there is a loss. This loss appears in the form of heat and raises the temperature of the magnetic material.

Area of the hysteresis loop is proportional to the hysteresis loss.  

Hysteresis loss cannot be avoided but can be minimized by selecting proper materials.

The following table gives the hysteresis constants.

METALS                                   CONSTANT

Silicon Steel                             0.001

Thin sheet iron                        0.003

Wrought iron                            0.004

Soft annealed cast steel           0.008

Soft machine steel                   0.009

Cast steel                                 0.012

Cast iron                                  0.016

Lesser the hysteresis constant, better the metal for A.C. electromagnet. So usually the silicon steel is used for a.c machines.

The B-H curve is nonlinear and multivalued and no simple mathematical expression can describe the loop.

Charles Steinmetz, an electrical engineer of USA, carried out the investigation into hysteresis loss of a number of specimens of different materials and found that hysteresis loss can be expressed as Hysteresis loss = K_h v f (B_max)^1.6 watts.

Where k_h is a constant for a given specimen, v is the volume of the core in m^3, f is the frequency and B_max is the maximum flux density in Tesla.

The index 1.6 is an empirical value and does not have any theoretical basis. The index value can vary in the range 1.5 to 2.5.