What Is the Relationship Between Attrition and Retention?

Hysteresis loss is the energy consumed by ferromagnets due to hysteresis during repeated magnetization. ^[1] Hysteresis means that when the magnetic state of a ferromagnetic material changes, the magnetization lags behind the magnetic field strength, and its magnetic flux density B and magnetic field strength H show a hysteresis loop relationship. After one cycle, the hysteresis loss per unit volume of the core is proportional to the area of the hysteresis loop. This part of the energy is converted into thermal energy, which makes the equipment warm up and reduces the efficiency. It is a component of iron loss in electrical equipment, which is undesirable in equipment such as AC motors. The hysteresis loop of soft magnetic materials is narrow, and its hysteresis loss is relatively small. Silicon steel sheet is therefore widely used in motors, transformers, relays and other equipment.

Hysteresis loss is the energy consumed by ferromagnets due to hysteresis during repeated magnetization. ^[1] Hysteresis means that when the magnetic state of a ferromagnetic material changes, the magnetization lags behind the magnetic field strength, and its magnetic flux density B and magnetic field strength H show a hysteresis loop relationship. After one cycle, the hysteresis loss per unit volume of the core is proportional to the area of the hysteresis loop. This part of the energy is converted into thermal energy, which makes the equipment warm up and reduces the efficiency. It is a component of iron loss in electrical equipment, which is undesirable in equipment such as AC motors. The hysteresis loop of soft magnetic materials is narrow, and its hysteresis loss is relatively small. Silicon steel sheet is therefore widely used in motors, transformers, relays and other equipment.

Chinese name

Hysteresis loss

Foreign name

hysteresis loss

Active material

Ferromagnetic material

Calculation method

Proportional to the area enclosed by the hysteresis loop

Causes of Hysteresis Loss

Hysteresis refers to the phenomenon that during the magnetization and demagnetization of a ferromagnetic physical material (such as iron), the magnetization of a ferromagnetic substance depends not only on the external magnetic field but also on the original magnetization. When an external magnetic field is applied to a ferromagnetic substance, the atomic dipoles are arranged in accordance with the external field. Even when the applied field is evacuated, the partial alignment remains: at this time, the material is magnetized. Once magnetized, its magnetic properties will remain. To demagnetize, just apply a magnetic field in the opposite direction. This is also how the hard disk memory works.

Hysteresis loop

In ferromagnetism, the relationship between magnetic field strength ( H ) and magnetic induction strength ( B ) is non-linear. Under the condition of enhanced field strength, the relationship between the two will rise to a certain point in a curve. After reaching this point, even if the field strength H continues to increase, the magnetic induction strength B will no longer increase. This situation is called magnetic saturation. ^[2]

If the magnetization field is reduced thereafter, the magnetization curve does not return from the original starting magnetization curve from point B, which indicates that the change in magnetization M lags behind the change in H. When H decreases to zero, M is not zero, but is equal to the residual magnetization Mr. To reduce M to zero, a reverse magnetization field must be added. When the reverse magnetization field is strengthened to -Hcm, M is zero. Hcm is called coercive force.

Therefore, the relationship curve between the magnetic field strength ( H ) and the magnetic induction strength ( B ) of the ferromagnetic material during repeated magnetization is drawn as shown in Figure 1. This curve is called the hysteresis loop.

It can be seen that the hysteresis loss is manifested as a part of the electromagnetic energy is irreversibly converted into thermal energy during the magnetization process. In the process of quasi-static repetitive magnetization, the hysteresis loss generated by a unit volume of a ferromagnet magnetized by an alternating magnetic field for a week is proportional to the area surrounded by the hysteresis loop, that is, H dB. Assuming that the frequency of the alternating magnetic field is f , the hysteresis loss per unit time and unit volume is f · HdB. ^[1]

The core loss in electrical equipment is generally composed of hysteresis loss and eddy current loss. In order to minimize the effect of hysteresis loss and reduce the related energy loss, ferromagnetic materials with low coercivity and low hysteresis loss are used, such as permalloy (iron-nickel alloy, magnetically permeable alloy). ^[3]

Hysteresis loss calculation

In order to magnetize the core, the core energy must be supplied. When the core material is magnetized, a hysteresis loop relationship is present between its magnetic flux density B and magnetic field strength. After one cycle, the energy per unit volume of the core is

Where the integral is performed around the hysteresis loop. The above formula shows that after a cycle of the core per unit volume, the hysteresis loss in the core is proportional to the area of the hysteresis loop, and this energy is converted into thermal energy. If the unit of B is tera (Sla) and the unit of H is ampere / meter, the energy is expressed in joule (ear) / meter.

CP Steinmetz has found an empirical formula for hysteresis loss. According to this formula, the hysteresis loss W per unit volume per week can be expressed as

Where f is the operating frequency; B _m is the maximum value of magnetic induction on the hysteresis loop; K ₁ is a constant that depends on the material properties and other relevant factors; is the Steinmetz coefficient. Steinmetz obtains an index of about 1.6 for many materials, while the value for other materials ranges from 1.5 to 2.5. The value of various materials can be found in the table. ^[1]

In AC motors, a large number of iron cores have magnetic fluxes whose size and direction are periodically changed, thereby causing hysteresis loss. Hysteresis loss will be converted into thermal energy, which will increase the temperature of the motor and reduce the efficiency, which is not desirable. Therefore, choose a material with a small hysteresis loop area and reduce the operating frequency to reduce the hysteresis loss.