Why do solenoid housing and plunger need to be demagnetized before assembling?

Part 1: Definition of solenoid demagnetization?

1.1 What is solenoid demagnetization?

Solenoid demagnetization is to eliminate the residual magnetism may that stay in housing and plunger material. When current passes through the solenoid coil, a magnetic field is generated around the plunger, causing the plunger to magnetize. Once the current is cut off, the plunger or the house may retain a certain degree of magnetism, which is the residual magnetism. Demagnetization is to reduce this residual magnetism to an acceptable level or even completely eliminate it.

1.2 Principle of demagnetization

 For theory, the plunger or the housing is composed of many magnetic domains. During the magnetization process, the directions of these magnetic domains tend to be consistent under the action of the magnetic field, thus showing magnetism. When the external magnetic field is removed (that is, the current is cut off), some magnetic domains will return to a chaotic state, but some magnetic domains may still maintain the original arrangement direction, which produces residual magnetism. The process of demagnetization is to disrupt the arrangement of these residual magnetic domains by applying a reverse magnetic field or an alternating magnetic field, so as to weaken the magnetism of the plunger.

For example, taking reverse magnetic field demagnetization as an example, when a magnetic field opposite to the original magnetization direction is applied, the arrangement of magnetic domains will gradually be disrupted. At the beginning, the reverse magnetic field will cause those magnetic domains that are arranged more neatly to begin to reverse direction. As the strength of the reverse magnetic field increases and the action time prolongs, more and more magnetic domain directions are changed, and the residual magnetism will gradually decrease.

1.3 Difference from permanent magnets

Permanent magnets have magnetism because the arrangement of their internal magnetic domains is relatively stable and does not require an external magnetic field to maintain magnetism. The magnetism of solenoid is maintained by the magnetic field generated by the current, and the demagnetization of solenoid is relatively easy to achieve, while the demagnetization of permanent magnets is more complicated. For example, heating a permanent magnet to a certain temperature (Curie temperature) or above can demagnetize it, but for solenoid, demagnetization can be performed by simple operations such as applying a reverse magnetic field.

Part 2 Why do the case or the plunger for solenoid need to be demagnetized?

2.1 Ensure precise movements

The solenoid are used in situations where push and pull movements need to be precisely controlled. For example, in an automated robotic arm, solenoid are used to control the opening and closing of mechanical claws. When the mechanical claw needs to release the object, if the solenoids has residual magnetism, the mechanical claw may not be able to open completely, which will affect the subsequent operations such as the placement of the object by the mechanical arm.

Taking the automatic sorting equipment as an example, the DC solenoid is responsible for sorting items from one conveyor belt to another. If it is not demagnetized, the electromagnet with residual magnetism may cause the items to be adsorbed on it and cannot be sorted to the designated location smoothly, resulting in sorting errors.

2.2 Improve work efficiency

Residual magnetism will make the action of the electromagnet slow down. For example, on a high-speed automated production line, the push-pull electromagnet needs to push and pull quickly. If there is residual magnetism, the influence of residual magnetism needs to be overcome before each action, which will slow down the action speed and reduce the work efficiency of the entire production line.

2.3 Protect supporting parts

If the solenoid has residual magnetism, it may cause unexpected adsorption or interference when the parts that cooperate with it are close. For example, some metal connecting rods connected to it may be adsorbed by residual magnetism and cannot move flexibly. Over time, it may also cause the connecting rod to deform, affecting the normal operation of the equipment.

2.4 Meeting the requirements of cyclic work

In some application scenarios where the electromagnet needs to be pushed and pulled repeatedly, such as the delivery mechanism of a vending machine. If it is not demagnetized, the accumulation of residual magnetism during each cycle will gradually deteriorate the performance of the electromagnet. Demagnetization can restore the electromagnet to its initial state, ensuring that it can push and pull normally during each cycle.

Part 3 What are the specific methods for demagnetizing solenoid?

3.1 Magnetic field demagnetization method

DC commutation attenuation demagnetization: Demagnetization is performed by continuously changing the direction of the DC current and decreasing the current passing through the electromagnet to zero. To ensure that the current is commutated when the power is off, the number of current attenuation should be as many as possible (generally more than 30 times), and the current amplitude of each attenuation should be as small as possible. If the attenuation amplitude is too large, the demagnetization purpose may not be achieved. For example, in the demagnetization of some small electromagnetic equipment, this relatively simple and easy-to-control method can be used.

3.2 AC magnetic field demagnetization:

The solenoid is placed in an alternating magnetic field with a gradually decreasing intensity. The following methods can be used:

3.3 Passing method (coil method): The coil does not move, the unit moves, and the magnetic field gradually decays to zero; or the workpiece does not move, the coil moves, and the magnetic field gradually decays to zero. For example, for some long electromagnet workpieces, they can be demagnetized by slowly passing through a coil with alternating current and gradually weakening magnetic field strength.

3.4  Decrease method (coil method): The coil and the workpiece do not move, and the current gradually decays to zero. For example, in the demagnetization of some fixed solenoid, a gradually decaying alternating current is directly passed through the coil where the electromagnet is located.

3.5 Power-on method: Two magnetizing chucks clamp the electromagnet, and the current gradually decays to zero.

3.4 Contact method: Two contacts contact the electromagnet, and the current gradually decays to zero.

3.5 AC yoke method: When the AC electromagnetic yoke is energized, it leaves the electromagnet, and the magnetic field gradually decays to zero; when the flat coil is energized, it leaves the electromagnet, and the magnetic field gradually decays to zero.

3.6 Thermal demagnetization method: The electromagnet is heated to above the Curie temperature, and then cooled without the action of an external magnetic field. This is the most thorough method of magnetic neutralization, but it is usually costly and may have a certain impact on the structure and performance of the electromagnet. Therefore, it is generally used when the performance requirements of the electromagnet are not high or when it is used for demagnetization before scrapping. For example, this method can be used for some waste solenoid that are prepared for recycling and reuse.

3.7 Mechanical vibration demagnetization method: The electromagnet is vibrated by a mechanical device, such as a vibration table. During the vibration process, the magnetic domain structure inside the electromagnet will be disturbed, thereby weakening the magnetism. However, this method usually has a relatively weak demagnetization effect and is generally rarely used alone. It may be used as an auxiliary demagnetization method in combination with other methods.

3.8 Chemical demagnetization method: Select appropriate chemical reagents (such as sulfuric acid, hydrochloric acid, etc.) to treat the surface of the electromagnet to change its internal molecular structure, thereby demagnetizing. However, this method is complicated to operate, and chemical reagents may pollute the environment and may corrode the electromagnet, so the scope of application is narrow and is only used in the demagnetization of solenoid with special needs and special materials.

Part 4 How to judge whether the demagnetization of the electromagnet is complete?

4.1 Measurement with a Gaussmeter

Principle: A Gaussmeter is an instrument used to measure magnetic field strength. By placing the probe of the Gaussmeter on the surface of the plunger of the electromagnet, the value of the magnetic field strength can be directly read. If the measured magnetic field strength is close to zero (within the error range of the instrument), it means that the demagnetization of the electromagnet may be relatively complete.

How to operate: First, make sure that the Gaussmeter has been calibrated and its range is suitable for measuring the residual magnetic strength that may exist in the electromagnet. Place the probe vertically on the working surface of the electromagnet plunger, and try to ensure that the probe is in good contact with the plunger surface to obtain accurate measurement results. Make multiple measurements at different locations, because the magnetic field distribution inside the plunger may be uneven, especially for solenoid with complex shapes. If the magnetic field strength at multiple measurement points is very low (for example, for general industrial solenoid, a residual magnetic strength of less than 10 Gauss is generally considered to be a good demagnetization effect), it can be preliminarily judged that the demagnetization is relatively complete.

4.2 Observe the behavior of the solenoid

Adsorption of small parts: If the electromagnet is used to adsorb small ferromagnetic parts, a completely demagnetized electromagnet will not adsorb these parts after power is turned off. For example, put some small iron nails near the demagnetized electromagnet. When the electromagnet is powered off, the small iron nails will not be adsorbed, which means that the demagnetization effect is good. If the small iron nails can still be adsorbed, it means that there is a certain amount of residual magnetism.

Accuracy of mechanical action: For push-pull solenoid, observe whether their mechanical actions are accurate during operation. If the demagnetization is incomplete, the residual magnetism may affect the push-pull action of the electromagnet. For example, in an automated mechanical device, an incompletely demagnetized electromagnet may cause the robot arm to be unable to extend or retract accurately, because the residual magnetism will interfere with the normal magnetic field changes of the electromagnet, thereby affecting the movement accuracy of the robot arm. If the electromagnet can accurately complete the push-pull action as expected, without delay, jamming or incomplete action, this can also be used as a reference for complete demagnetization.

4.3 Comparison of performance parameters before and after demagnetization

Magnetic field strength-current curve: The relationship curve between the magnetic field strength and current of the electromagnet is measured before and after demagnetization. If the demagnetization is complete, the magnetic field strength of the curve after demagnetization should be close to zero when the current is zero. Through the experimental equipment, gradually increase the current and record the corresponding magnetic field strength, and draw two curves for comparison. If the magnetic field strength at zero current is significantly reduced in the curve after demagnetization compared with the curve before demagnetization, and it meets the expected demagnetization effect, then it can be judged that the demagnetization is effective and may even be complete.

4.4 Attraction or push-pull force test:

The attraction (for adsorption solenoid) or push-pull force (for push-pull solenoid) of the electromagnet are tested before and after demagnetization. Use professional force measuring equipment to measure the attraction or push-pull force of the electromagnet under the same test conditions (such as the same distance, the same load, etc.). If after demagnetization, the attraction or push-pull force is significantly reduced in the power-off state and is close to zero, it can also indicate that the demagnetization is relatively complete.

Part 5 Attention fir demagnetization?

5.1 Selection of demagnetization method

Reverse magnetic field demagnetization method: This method is to apply a magnetic field opposite to the original magnetization direction to demagnetize. Pay attention to the control of magnetic field strength. The initial reverse magnetic field strength should be slightly greater than the original magnetizing magnetic field strength, but not too large, otherwise it may cause damage to the iron plunger and other components of the electromagnet. For example, if the original magnetizing magnetic field strength is 1000 gauss, the reverse magnetic field strength can be adjusted from 1000-1200 gauss, and it should be increased slowly to avoid sudden application of too high a magnetic field, which may cause damage to the internal structure of the electromagnet, such as irreversible changes in the magnetic domain structure of the iron plunger.

5.2 AC demagnetization method: Use an alternating magnetic field to gradually reduce the residual magnetism of the magnet. When using AC demagnetization, pay attention to the frequency selection of the alternating magnetic field. Generally speaking, the frequency should not be too high, as too high a frequency may cause the electromagnet to heat up severely. For example, for some small push-pull solenoid, the frequency can be selected between 50-100Hz. At the same time, the demagnetization time should also be properly controlled. If the time is too short, the demagnetization may not be complete, and if the time is too long, energy will be wasted and the service life of the electromagnet may be affected.

5.3 Temperature control

 During the demagnetization process, the electromagnet may heat up. This is because eddy currents are generated in the plunger and other components during the magnetic field change, which causes heating. If the temperature is too high, it will affect the performance of the electromagnet and even damage the electromagnet. For example, for some common industrial push-pull solenoid, the operating temperature generally cannot exceed 80℃. During the demagnetization process, heat dissipation measures can be adopted, such as installing a heat sink or using a fan for air cooling, to ensure that the temperature of the electromagnet is within a safe range.

5.4 Equipment compatibility

When using an external demagnetization device, make sure that the demagnetization device is compatible with the push-pull electromagnet. Parameters such as the output power and magnetic field strength range of the demagnetization device must be able to meet the requirements of electromagnet demagnetization. For example, a small push-pull electromagnet may not be able to withstand the high magnetic field strength output by a large demagnetization device, so a suitable demagnetization device should be selected according to the specific specifications of the electromagnet.

5.5 Post-demagnetization test

After demagnetization is completed, the electromagnet needs to be tested to determine whether the demagnetization is complete. Gaussmeters and other devices can be used to measure the residual magnetism of the electromagnet. If the residual magnetism is still large, demagnetization may need to be performed again. At the same time, the push-pull performance of the electromagnet should be tested to ensure that the action of the electromagnet after demagnetization can meet the expected working requirements, such as whether the parameters such as the size of the push and pull force, the action speed, etc. are within the normal range.

Part 6   Summary:

In process of making solenoid, the demagnetization of  raw metal materials is a key step to ensure the stability of performance of a solenoid. solenoid, as a device that uses current to generate a magnetic field, work based on the magnetic field formed by the current in the conductor. This device is mainly composed of an iron plunger and a coil. When the current passes through the coil, a magnetic field is generated, which in turn makes the iron plunger generate an attractive or repulsive force.

metal raw materials occupy a plunger position in the production of solenoid. However, pure iron is easily magnetized during the processing process, resulting in hysteresis. If demagnetization is not performed, these magnetization phenomena will interfere with the current transmission and electromagnetic field generation when the electromagnet is working, thereby affecting the normal working efficiency of the electromagnet. Therefore, it is particularly important to demagnetize the pure iron raw materials before making solenoid.

Demagnetization is intended to eliminate the magnetization phenomenon in pure iron materials and ensure the stable operation of solenoid. This treatment process can be achieved through a variety of methods, such as using an alternating magnetic field to heat the pure iron, or using a strong magnetic field for demagnetization. Through demagnetization, not only can the interference of the electromagnet during use be reduced, but the service life and working efficiency of the electromagnet can also be significantly improved.

In summary, when making an electromagnet, demagnetizing the pure iron raw material is a necessary step to ensure the stability of the electromagnet performance, improve working efficiency and extend the service life. This treatment process plays a vital role in ensuring the normal operation of the electromagnet.