By
Permanent magnets are known for their ability to retain their magnetism over extended periods. They can be found in various forms, including natural magnets like magnetite and artificial magnets like alnico alloys. However, it is essential to be aware of factors that may lead to the partial or complete loss of a permanent magnet’s magnetic field, as this can have negative implications for its intended application.
Understanding the demagnetization process and its mechanisms is crucial. There are certain physical conditions that must be upheld or avoided to maintain the desired magnetization in permanent magnet applications. By familiarizing oneself with these conditions, one can mitigate the risk of demagnetization and ensure the longevity of the magnet’s magnetic field.
What does demagnetization mean?
Simply put, demagnetization refers to the reduction or complete removal of a magnet’s magnetism. The working principle of permanent magnets is based on the arrangement of micro-areas within the alloy material. These small areas are called magnetic domains. Each magnetic domain acts like a microscopic magnet within the larger whole. Part of the process of developing permanent magnets involves placing a high-strength magnetic material, typically alnico, strontium iron (called ceramic or ferrite), neodymium iron boron or samarium cobalt, in a strong magnetic field. In the process of magnetizing a material, individual magnetic domains, which usually point in various directions, align in the direction of the magnetic field. When nearly all of the magnetic domains are aligned with the original magnetic field, the material becomes a permanent magnet. When you demagnetize a magnet, its magnetic domains are no longer perfectly aligned. It is the arrangement of these magnetic domains that provides the material’s magnetism. When the magnetic field (the arrangement of the magnetic domains) is disrupted, the magnet is demagnetized.
How to demagnetize a permanent magnet?
People are sometimes confused by the terms “permanent” vs. “temporary” magnets. Temporary magnets only act as magnets when attached to or near an object that emits a magnetic field. They rapidly lose their magnetism when the magnetic field source is removed. In contrast, permanent magnets generally maintain their continuous magnetic field independently under normal operating conditions. However, permanent magnet materials can still demagnetize under certain conditions, including exposure to high heat, collisions with other objects, volume loss, and exposure to conflicting magnetic fields.
1.Heat
Elevated temperature is a prevalent factor that can lead to demagnetization. When the temperature rises, the atomic motion intensifies, eventually surpassing the alignment of magnetic domains. The Curie temperature represents the critical point at which a magnetic alloy entirely and irreversibly loses its permanent magnetic characteristics. Nonetheless, even as the magnet’s temperature approaches its Curie point, varying degrees of demagnetization can occur. The extent of demagnetization varies significantly depending on the specific material and grade of the magnet in question and is typically depicted by the magnet’s demagnetization curve.
Generally speaking, some permanent magnet materials are more susceptible to demagnetization with increasing temperature than others. Neodymium magnets are generally most susceptible to elevated operating temperatures and will typically resist demagnetization until operating temperatures reach approximately 100°C. Neodymium magnetic materials that can operate above 220°C are available, but these can become very expensive. For samarium cobalt magnets, this limit is 350°C. Alnico magnets offer the best temperature characteristics of any existing standard production magnet material, enabling use in continuous duty applications where extreme temperatures up to 540°C are expected.
When operating under high-temperature conditions, it is crucial to consider the permeability of the magnetic material in use, taking into account factors such as size, material type, and operating temperature. These factors collectively determine the magnet’s effectiveness for a particular application. In the case of neodymium magnets, employing a permeability calculator can assist in assessing whether a magnet of a specific size will demagnetize and potentially fail at the required operating temperatures.
Prolonged exposure of a permanent magnet to elevated temperatures causes the alignment of electrons to be disrupted, resulting in partial or complete demagnetization. The demagnetization that occurs can be either reversible or irreversible in nature.
2.Collision and volume loss
Another factor that can demagnetize a permanent magnet is a collision – the impact of another object on the magnet. For example, if a magnet is repeatedly struck with a hammer, this will disturb the movement of its atoms, affecting the alignment of the magnet’s north and south poles, eventually causing it to become demagnetized.
Collisions also affect the physical integrity of the magnet, and the resulting volume loss can also adversely affect the magnetization. This is why volume loss is considered another factor in the demagnetization of permanent magnets. Corrosion or oxidation caused by excessive humidity can also affect the physical properties and thus the magnet’s magnetic properties.
3.Conflicting magnetic fields
Permanent magnets can be demagnetized when subjected to unfavorable external magnetic fields. The presence of another magnetic field in close proximity to the magnet acts as a demagnetizing agent, resulting in the magnet losing its magnetic properties. This highlights the importance of proper storage for permanent magnets. By storing them correctly, not only are they protected from physical damage, but also shielded from external magnetic fields, ensuring the maintenance of their magnetic properties and consistency in their magnetic field.
Running alternating current in close proximity can also have this effect on magnets, leading to demagnetisation.
4.Chemical factors
Influenced by chemical factors, such as acid, alkali, oxygen, corrosive gases, etc., the internal or surface chemical structure of the permanent magnet changes. This causes changes in magnetic properties. The iron and neodymium in NdFeB are then more susceptible to oxidation. The protection of permanent magnets generally includes electroplating, such as zinc plating and nickel plating.
Problems and methods of fault reversal
Permanent magnet material is the key raw material of permanent magnet motor. In the process of motor manufacturing, testing and use, there will always be a loss of magnetism problem. From the actual failure case analysis, it can be attributed to the following aspects:
Improper selection of magnet steel grade
If the motor design calculations are not accurate enough and wrongly selected lower grades, there may be such a situation: the initial test process test record indicators are very good. But as the motor gradually tends to thermal stabilisation, the motor’s relevant indicators began to deteriorate. Subsequently, the indicators deviate more and more from the design expectations. At a certain point, the current increases dramatically and the inverter stops quickly. This characterises the motor has been demagnetised, and the magnets must be replaced.
Overheating demagnetisation
If we exclude the influence of magnetic steel magnetic performance and only consider the thermal factors, it can be determined that there are two cases of overheating and demagnetisation phenomenon: Firstly, the motor circulatory ventilation circuit is unreasonable, contrary to the natural law of heat and cold conduction and lead to local heat concentration; Secondly, the winding thermal load is too high, resulting in the temperature exceeding the load level of the motor heat exchanger system.
Excessive demagnetisation current
When the motor is running, the magnitude of load current exceeds the demagnetisation resistance of the magnets, which will lead to irreversible demagnetisation of the magnets. This further increases the load current and aggravates the irreversible demagnetisation of the magnets. The failure will accelerate the irreversible demagnetisation until the magnet is lost.
Demagnetisation Curve Calculator
A demagnetisation curve shows the magnetic properties of a particular magnet, plotted on an axis. A demagnetisation curve therefore gives a more complete picture of the magnet’s magnetic properties than a single point. For this reason, demagnetisation curves are commonly used in the design of magnetic components.
More specifically, the curve shows the ratio of the flux density (B) to the magnetising field (H). The intersection produced by the two curves is the coefficient of magnetic permeability.
A demagnetization calculator assists in the selection of an appropriate design by providing a visualization of the demagnetization process for a specific magnet at various predefined points. By inputting relevant parameters, such as material type, dimensions (e.g., a 3-inch diameter and 0.1-inch thickness for an N35 disc magnet), the calculator can generate the demagnetization curve for the chosen magnet. This information is valuable in determining the optimal design for a magnetic assembly, allowing for informed decision-making and ensuring the desired performance of the magnetic system.
Summary
The occurrence of demagnetization can significantly impact the functionality and effectiveness of a magnet when used as a component in various applications. Hence, it is crucial to acknowledge this phenomenon and employ appropriate design strategies when creating magnetic assemblies to prevent demagnetization from taking place. By considering and addressing potential demagnetization risks during the design phase, the integrity and reliability of the magnet can be preserved, ensuring optimal performance in its intended application.
