Table of Contents
Demagnetization mechanism of permanent magnet motors
How does a permanent magnet demagnetize?
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
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.
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.
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.
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.
Why can a permanent magnet be demagnetized? ›
When a permanent magnet is exposed to increased temperatures for a length of time, the electrons will be forced out of alignment and the magnet will be demagnetized, either partially or completely. The resulting demagnetization may be reversible, or it may be irreversible.How can you prevent a magnet from demagnetization? ›
Store your magnets with a keeper – A keeper is a small piece of iron that is generally added temporarily between the north and south poles of a magnet. It prevents the magnet from demagnetising by redirecting its magnetic field.How permanent magnet is demagnetized? ›
A permanent magnet can be demagnetized or can loose its magnetism if it is heated at appropriate temperature. Due to heating, the orientation of domains of the magnet changes and its magnetic properties are reduced or lost completely.What are 3 ways to demagnetize a magnet? ›
- Heating to a high temperature.
- Hammering repeatedly.
- Passing alternating current through a coil around the magnet keeping it in the eastward direction.
Demagnetisation is the process in which a magnet loses its magnetic power. Demagnetisation can occur due to many processes: It is caused due to heating, hammering, AC current field, rough usage etc. It can also be due to natural processes over time.Why do you demagnetize a magnet? ›
When a material is exposed to a strong magnetic field that is established in opposition to its magnetic orientation, part of the magnet may become demagnetized. This demagnetization reduces the effective field of the magnet, and the magnet's performance will degrade.What is a demagnetisation how it can be prevented? ›
Self demagnetisation is the process by which a magnet demagnetizes itself by the virtue of the opposing fields created within it by its own magnetism. We can prevent self demagnetisation by using magnetic keepers which are two soft iron bars each on one pole of a magnet.How can you protect a magnet from demagnetization diagram? ›
Answer: Put away each type of magnet in its own space or drawer to prevent demagnetization. The common alnico magnet is the most easily demagnetized. Use a magnet keeper to preserve the magnetic charge.What is needed to demagnetize a magnetic material? ›
A magnetic material can be demagnetised using the following methods: Heating the product to above the Curie temperature. Strong vibrations (hammer blows) that can cause a slight reduction in magnetisation due to their own magnetic field.What are two things you can do to demagnetize a permanent magnet? ›
- Heating magnet to very high temperature.
- Dropping the magnet frequently.
- Hammering the magnet repeatedly.
- Bringing the magnet in contact with the like poles of other magnets repeatedly.
- Passing the electric current through magnet.
Can permanent magnets can never be demagnetized? ›
In permanent magnets, a property called coercivity is present, which means they can withstand being demagnetized without becoming weak.Do permanent magnets lose their magnetism easily? ›
The magnetic field in a permanent magnet does tend to decay over time, but not with a predictable half-life as with radioactivity.What are the different methods of demagnetization? ›
There are two methods generally used to magnetize permanent magnets: static magnetization and pulse magnetization.Which of the following can be used to demagnetize a magnet? ›
The correct answer is Hammering of Magnet. The magnet can de be demagnetised by heating past the Curie point, applying a strong magnetic field, applying alternating current, or hammering the metal.Can magnets be demagnetized by Heating or? ›
At around 80 °C, a magnet will lose its magnetic force and it will become demagnetized permanently if exposed to this temperature for a period, or if heated above its Curie temperature. Heat the magnet even more, and it will melt, and eventually vaporize.What is demagnetization How does a magnet get demagnetized? ›
The process by which a magnet loses its magnetic properties is called demagnetization. Some of the methods to demagnetize a magnet are as follows: By improper or rough handling. By placing the magnet in the east-west direction and rapidly heating followed by rapid cooling.Why does a magnet become demagnetized when dropped? ›
Answer and Explanation: When a permanent magnet is dropped some of the atoms that have been aligned to create the magnetic field can come out of alignment. This causes a drop in the strength of the magnetic field.What is the most effective method of demagnetization? ›
Electrical method is the most effective method of demagnetization.What is one way to demagnetize an object? ›
To demagnetize a magnet, you can expose them to high temperatures, drop them on the ground or hit them with a hammer in order to remove their magnetic field. These methods can disrupt the orientation of magnetic poles in a magnet.What are the two methods to destroy the magnetism of a magnet? ›
By hammering the magnet repeatedly. By heating the magnet to a very high temperature. Bypassing alternating current around the magnet.
What can be done to prevent a permanent magnet becomes weak with time? ›
This can be done by storing the magnet in an airtight container or by coating it with a metal oxide.How long do permanent magnets stay magnetic? ›
So how long should my permanent magnet last? Your permanent magnet should lose no more than 1% of its magnetic strength over a period of 100 years provided it is specified and cared for properly. There are a few things that may cause your magnet to lose its strength: HEAT.Are permanent magnets always magnetized? ›
A permanent magnet is a type of magnet that will always be magnetic, and will not lose its magnetism over time.At what temperature do permanent magnets lose their magnetism? ›
When heated above 176° Fahrenheit (80° Celsius), magnets will quickly lose their magnetic properties. The magnet will become permanently demagnetized if exposed to these temperatures for a certain length of time or heated at a significantly higher temperature (Curie temperature).What makes a permanent magnet stronger? ›
Most substances have an equal number of electrons spinning in opposite directions, which cancels out their magnetism. But some substances are strongly magnetic, which means most of their electrons spin in the same direction. These substances have high magnetic permeability and make the strongest magnets.Why are permanent magnets weak? ›
They Get Very Cold (Or Hot) Temperature variation can cause magnets to lose some or all of their magnetic charge. Depending on how extreme the temperature, these losses can be temporary or permanent.Does freezing a magnet make it stronger? ›
Exposing a magnet to colder temperatures will increase its magnetism. The molecules within the magnet will move slower because they have less kinetic energy so there is less vibration within the magnet's molecules. This allows for a more concentrated magnetic field that strengthens the magnet.What material is used to make permanent magnets? ›
Iron, nickel, and cobalt are the materials used in making permanent magnets.Can electricity demagnetize a magnet? ›
Place a permanent magnet inside a solenoid heading East-West through which an alternating current is flowing to demagnetize it. The microscopic atomic magnets in the permanent magnet become disoriented while the current is still running, and the permanent magnet becomes demagnetized.Which type of electrical current is used to demagnetize a magnet? ›
The most reliable way to demagnetize a magnet is with an electrical demagnetizer tool. It uses AC current to generate an irregular magnetic field, able to affect permanent magnets.
Can all magnets be demagnetized? ›
All magnets can be demagnetized, and there are multiple ways to do that. Temporary magnets are items that are magnetic but do not keep their field as strongly. Items in this group include paper clips, scissors, refrigerators, staples, and various other items.Can cold demagnetize a magnet? ›
On the other hand, ferrite magnets have a greater facility for demagnetization when they are at a lower temperature than at a higher temperature. This type of permanent magnet will lose its magnetic strength below -60ºC.What happens if I hammer a magnet? ›
Permanent magnets can lose their magnetism if they are dropped or hammered. If heated above a point a magnet will lose its magnetism.Why can even permanent magnets be demagnetized if they are dropped or heated to high temperatures? ›
Applying heat to a magnet causes the magnet's electrons to dance and transition into higher energy states. This, in turn, forces the electrons into different alignments. Since the electrons are no longer aligned, the magnetism of the object will decrease.Can permanent magnets be demagnetized by extreme cold? ›
Yes, ferrite magnets can incur a permanent loss of magnetization when exposed to temperatures below -40°C. Magnetic products different from standard magnets are even more susceptible to low temperatures.How can a permanent magnet be demagnetized by using a solenoid? ›
To demagnetise a permanent magnet place it inside a solenoid, pointing East-West, through which an alternating current is flowing. While the current is still flowing, the small atomic magnets present in the permanent magnet get disoriented and hence the permanent magnet gets demagnetised.Why does a magnet get demagnetized when it is heated or hammered give reasons? ›
If a permanent magnet is heated, it loses its magnetic properties gradually. When heated, the molecules of the magnet acquire more energy and hence vibrate faster. This disturbs the alignment of the magnetic molecules and hence a magnet loses its magnetic properties.What are the two ways to cause a permanent magnet to lose its magnetic field? ›
Permanent magnets can lose their magnetism if they are dropped or banged on enough to bump their domains out of alignment. Heating a magnet above the Curie temperature causes the magnetic domains to be disrupted permanently. Mild heating causes a reduction in the magnetism.How can permanent magnets be destroyed or weakened? ›
The magnetic field of a permanent magnet can be destroyed if it is not handled with care, like hammering, dropping, or heating. If the magnet is heated beyond the curie temperature it will melt and vaporize. The stray electromagnetic fields and applying alternating current can also lead to demagnetization.How long does a permanent magnet stay magnetized? ›
So how long should my permanent magnet last? Your permanent magnet should lose no more than 1% of its magnetic strength over a period of 100 years provided it is specified and cared for properly.
What deactivates a magnet? ›
Heating the magnet to high temperatures or generating a magnetic field with an alternating current in the vicinity of the magnet are two ways to demagnetize it (assuming you want to do so). The simplest way to demagnetize it, however, is with a hammer.Can a temporary magnet be easily demagnetized? ›
The temporary magnet gets demagnetized when the stronger magnetic field is removed from it. Therefore, all the temporary magnets are not considered strong magnets as they are dependent on other strong magnets. Their magnetic properties become active only in the presence of an external magnetic source.What is a solenoid How is it different from a permanent magnet? ›
The bar magnet is a permanent magnet, whereas a solenoid is an electromagnet i.e., it acts as a magnet only when an electric current is passed through.How do you increase permanent magnet strength? ›
- A Permanent magnet is an object made from a material that is magnetized and creates it own magnetic field.
- When the Permanent magnet becomes too weak, then take a strong magnet and stroke it with a stronger magnet.
Expose it to Cold
By exposing the weakened magnet to freezing temperatures, the molecules in the magnet will have less kinetic energy and will slow down. This decreased movement will allow the atoms to line up better creating a more concentrated magnetic field which will result in a stronger magnet.
In order to get a strong permanent magnet again, put your weak magnet next to a powerful magnet. The stronger magnetic field will pull the electrons of the weak magnet back into alignment. Then, your magnet could regain its strength.