Why permanent magnet materials demagnetize and how to prevent demagnetization (2023)

Table of Contents

• Demagnetization mechanism of permanent magnet motors

• How does a permanent magnet demagnetize?

• High temperature

• Collision and volume loss

• Conflicting magnetic field

• How to identify motor demagnetization

• Anti demagnetization method for permanent magnet motors

  (Video) Demagnetise a Magnet | Methods of Demagnitising a Permanent Magnet | How to Destroy a Magnet?

• Internal structural changes

• Temperature

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  Neodymium Magnet Grades: All You Need to KnowThe Life Of A Strong Magnet – Wood and You

• Chemical factors

• External magnetic field

Demagnetization mechanism of permanent magnet motors

Permanent magnet materials have two obvious characteristics: one is that they can be strongly magnetized under the action of an external magnetic field, and the other is that they have hysteresis, meaning that the material retains its magnetism even after the external magnetic field is removed. The relationship between the magnetic changes of permanent magnets and the changes in external magnetic fields can be described using two curves, namely demagnetization (B-H) curve and intrinsic demagnetization (J-H) curve.

There is a mathematical mutual constraint between the demagnetization curve and the intrinsic demagnetization curve of a permanent magnet. Among them, μ Is the magnetic permeability, and H is the applied magnetic field strength. Due to the inflection point of the intrinsic demagnetization curve of the magnet, the demagnetization curve only differs linearly from the intrinsic demagnetization curve μ H. Therefore, there are also inflection points in the demagnetization curve.

As the temperature increases, the intrinsic coercivity and residual magnetism of the permanent magnet will decrease. For example, the residual temperature coefficient of SH grade neodymium iron boron permanent magnets is about -0.1%/℃, and the intrinsic coercive force temperature coefficient is about -0.6%/℃.

How does a permanent magnet demagnetize?

How can an object called a permanent magnet eliminate its magnetism? People sometimes feel confused about the terms "permanent" magnet and "temporary" magnet. Temporary magnets only have the same function as magnets when they are close to objects that can emit a magnetic field or when they are adsorbed on the surface of the object. When magnetic field sources are removed, they immediately lose their magnetism. On the contrary, permanent magnets usually independently maintain their persistent magnetic field and do not need to rely on external magnetic fields to maintain their own magnetism under normal working conditions. However, permanent magnetized materials can still undergo demagnetization under certain conditions, such as exposure to high temperatures, collisions with other objects, volume loss, and exposure to conflicting magnetic fields. Multiple factors may cause permanent magnets to lose some or all of their magnetism, and once this happens, it can have adverse effects on the application.

High temperature

One of the common reasons for demagnetization is high temperature. When the temperature increases, the molecular motion accelerates, which affects the arrangement of magnetic domains. The Curie temperature refers to the critical temperature at which a magnetic alloy loses its permanent magnetism, which cannot be reversed thereafter. The demagnetization level varies greatly among different magnetic materials and grades, and can be described by the demagnetization curve of the magnet.

Neodymium magnets are one of the most susceptible magnetic materials to high temperatures and are usually able to resist demagnetization until the operating temperature reaches 100 ° C. The use of neodymium magnetic materials that can operate above 220 ° C is feasible, but the price is relatively high. The limit of samarium cobalt magnets is 350 ° C, while aluminum nickel cobalt magnets can provide the best temperature characteristics in continuous operating applications up to 540 ° C.

When using magnets in high-temperature environments, understanding the magnetic conductivity is crucial for determining the overall effectiveness of the magnet. Size, material, and operating temperature are all important factors. The use of a magnetic conductivity calculator can help determine whether a specific size of magnet will demagnetize and lose its effectiveness. Prolonged exposure to high temperatures can cause demagnetization of the magnet, which may or may not be reversible.

Collision and volume loss

Another reason for demagnetization of permanent magnets is collision - when another object collides with the magnet, it also has an impact. For example, repeatedly tapping a magnet with a hammer can interfere with its atomic motion, affect the arrangement of the north and south poles of the magnet, and demagnetize it. In addition, collisions can also affect the physical integrity of the magnet, leading to volume loss and thus having adverse effects on magnetization. Therefore, volume loss is also another factor in demagnetization of permanent magnets. Corrosion or oxidation caused by excessive humidity can also affect the physical properties of magnets, thereby affecting their magnetism.

Conflicting magnetic field

Exposure of permanent magnets to adverse external magnetic fields can lead to demagnetization. When there is another magnetic field around the magnet, it will generate demagnetization, causing damage to the magnet. Therefore, storing permanent magnets correctly is crucial as it can maintain their magnetism by ensuring that they are aligned in the magnetic field and not subject to collisions. In addition, the AC current running nearby can also have a similar effect on the magnet, leading to demagnetization.

How to identify motor demagnetization

• After the machine is started and running normally for a period of time, if there is an overload alarm of the frequency converter and it is confirmed that the frequency converter is correct, and the back electromotive force of the motor running at the rated frequency with no load is more than 50V lower than the back electromotive force on the motor nameplate, then it can be determined that the motor has undergone demagnetization.

• Use a Tesla meter to directly measure the density of the magnetic field center to determine whether the motor has demagnetized.

• After demagnetization of the motor, its operating current will significantly exceed the rated value. However, for occasional overload reports only during low or high speed operation, it may not necessarily be caused by motor demagnetization.

• Professional tool - Gauss meter can be used to determine whether the motor has demagnetized.

• If there are two motors of the same model, they can be compared by testing the no-load speed. The no-load speed of the demagnetized motor will significantly increase.

• For tile shaped magnets, demagnetization can be determined by analyzing and testing the difference in Gaussian values between the left and right sides.

• For electric vehicles with brushes, if there is a slow start, insufficient power, electric current or similar circuit breaking sound at the motor, and the problem remains unresolved after replacing the carbon brush, it can be determined that the motor has demagnetized.

  (Video) DEMAGNETIZATION

Anti demagnetization method for permanent magnet motors

Internal structural changes

When making permanent magnets, the internal organizational structure is not in its most stable state. Over time, it will gradually become more stable, but performance will also slightly decrease over time. For rare earth permanent magnet materials, high-temperature heat treatment or sintering is carried out during the production process, but the magnetic properties are relatively stable when used at room temperature. In order to accelerate the natural aging process, artificial aging treatment is often used. This treatment method will keep the magnet at a temperature higher than room temperature for a period of time, replacing long-term natural aging at room temperature. Through this method, the structure of the material can be stabilized, so that its magnetic properties remain basically unchanged.

Temperature

The magnetic properties of permanent magnets are affected by temperature, including reversible and irreversible losses. To protect the permanent magnet, it should be avoided to place it in an environment higher than its maximum withstand temperature. By adopting temperature cycling stabilization treatment, the irreversible loss of permanent magnets during use can be reduced. In the rotor structure design, internal ventilation circuit can be set to cool the magnetic steel to reduce the magnetic steel temperature and improve the motor efficiency.

The temperature resistance level is one of the most important indicators in the performance of permanent magnets, which determines the operating conditions of permanent magnets. In neodymium iron boron permanent magnets, rare earth metal neodymium accounts for 29%~32.5% of the total, metallic element iron accounts for 64%~69% of the total, and non-metallic element boron accounts for 1.1%~1.2% of the total. In addition, a small amount of elements such as dysprosium, terbium, niobium, and copper will also be added.

Chemical factors

When a permanent magnet is affected by chemical factors such as acid, alkali, oxygen, and corrosive gases, its internal or surface chemical structure may change, leading to a decrease in its magnetic properties. In neodymium iron boron permanent magnets, iron and neodymium are more easily oxidized. In order to protect permanent magnets, electroplating and other methods are generally used for protection, such as galvanizing, nickel plating, etc.

External magnetic field

In the process of using permanent magnets, they are usually in an external magnetic field. If the working point is below the inflection point, it will cause irreversible demagnetization. When a short circuit impulse current occurs in a motor, a strong demagnetization magnetic field is generated. Therefore, in the design of the motor, it is necessary to ensure that the short circuit working point is above the demagnetization working point of the permanent magnet. In addition, after saturation magnetization of the permanent magnet, contact may cause changes in magnetic properties, so magnetic contact stabilization treatment is necessary.

To prevent demagnetization of permanent magnets, the design of permanent magnet motors usually starts from two aspects. On the one hand, it reduces the rotor temperature, and on the other hand, it reduces the demagnetization magnetic field.

The thickness of the permanent magnet in the rotor is a key factor affecting the anti demagnetization ability of the permanent magnet. Although excessively thin permanent magnets can reduce costs, they can also reduce their anti demagnetization ability. Therefore, it is necessary to determine the optimal shape of the permanent magnet through simulation optimization design. In addition, the rotor with embedded permanent magnet (IPM) structure has a stronger anti demagnetization ability compared to the rotor with surface permanent magnet (SPM) structure.