Ultimate Guide: How to Choose the Perfect Solenoid Plunger for Your Application

Part 1 :  Magnetic permeability material for Plunger

 Magnetic permeability is a physical quantity that measures the ability of a material to conduct magnetic lines of force. Choosing high magnetic permeability materials (such as pure iron, silicon steel, etc.) as the Plunger can enable the solenoid to produce a stronger magnetic field.  

 the magnetic permeability vs other materials:

 1.1, while the magnetic permeability of plunger can reach several thousand or even higher. This means that after the plunger material is changed from air to pure iron, the magnetic field strength will increase significantly even if the current remains unchanged.

1.2 Saturation magnetic induction intensity of Plunger

   It is important to choose a plunger material with a high saturation magnetic induction intensity, otherwise under high current conditions, the saturation of the Plunger will make it difficult to continue to increase the magnetic field intensity.

It will Impact on the performance of solenoid: If the core saturation magnetic induction intensity is low, it will not be able to meet actual working requirements. For example, in an electromagnetic crane, to lift heavier ferromagnetic objects, a sufficiently high magnetic field strength is required, so the core saturation magnetic induction intensity is a key factor.

1.3 Resistivity of material

Benefits of low resistivity: The resistivity of the core material affects eddy current losses. Materials with low resistivity, such as pure iron, tend to generate large eddy currents, which cause energy loss and heat the core. In order to reduce eddy current losses, materials with higher resistivity, such as silicon steel sheets, are usually used for AC electromagnets.

Application principle of silicon steel sheet: Silicon steel sheet is made by adding silicon element to steel, and its resistivity is higher than ordinary steel. Laminating the Plunger with silicon steel sheets can limit the eddy current path and reduce eddy current losses. For example, in the Plunger of a transformer (which is also a device that uses the principle of electromagnetic induction and is similar to an electromagnet), the use of silicon steel sheets can effectively reduce the heat generated by eddy currents in the Plunger and improve the efficiency of the transformer.

1.4 Plunger shape and size

Shape design basis: The shape of the Plunger should be designed according to the specific application and the magnetic field distribution requirements. Common shapes include columnar, U-shaped, E-shaped, etc.  

Size influence: Core size includes length, diameter (for cylinder) or cross-sectional area, etc. Larger core size (within a certain range) generally increases the magnetic flux and thus the magnetism of the electromagnet. But at the same time, issues such as space constraints, cost, and whether it will cause the magnetic field distribution to be too dispersed must also be considered. For example, in electromagnets in small electronic devices, the core size must be small and compact due to limited space.

1.5 Working frequency (for AC electromagnet/solenoid)

Frequency requirements for the Plunger: If the solenoid is working under AC power, the operating frequency will affect the performance of the Plunger. At high frequencies, the hysteresis loss and eddy current loss of the Plunger will increase. Therefore, for high-frequency AC electromagnets, core materials with low loss characteristics need to be selected, and special plunger structures may need to be used to reduce losses.

Application of ferrite materials: Ferrite is an Plunger material suitable for high-frequency operation. It has high resistivity, which can effectively reduce eddy current losses, and its hysteresis loss is also relatively small at high frequencies.  

1.6 Cost and Processability

Cost factors to consider: The prices of different core materials vary greatly. The price of pure iron is relatively low, the price of silicon steel is moderate, and the price of some special magnetic alloys is higher. On the premise of meeting the electromagnet performance requirements, cost factors must be considered.  

Importance of application: The plunger material needs to be easily machined into the desired shape and size.  Materials such as pure iron and silicon steel have good processability and can be made into Plungers of various shapes through cutting, stamping, and welding.

Part 2 ； Plunger Types

2.1 Pure Plunger

Characteristics: Pure iron has high magnetic permeability, which enables the electromagnet to generate a strong magnetic field. Its saturation magnetic induction intensity is also relatively high, which means that by increasing the current within a certain range, the magnetic field intensity can be better improved. However, the resistivity of pure iron is low. When used in AC electromagnets, it is prone to large eddy current losses, causing the Plunger to heat up.

2.2 Application: Commonly used in electromagnets that require high magnetic field strength but low operating frequency or are powered by DC power.  silicon steel core

2.3 Silicon plunger Characteristics: Silicon steel is made by adding silicon element to steel. Its magnetic permeability is high, which can effectively conduct magnetic lines of force and enhance the magnetic field of electromagnets. At the same time, the resistivity of silicon steel is much higher than that of pure iron, which can effectively reduce eddy current losses in AC electromagnets. Moreover, the saturation magnetic induction intensity of silicon steel is also relatively high, making it suitable for applications where strong magnetic fields need to be generated.

2.4 Application: it is widely used in various transformers and ac solenoid/electromagnets. In industrial, AC electromagnetic cranes, the silicon steel core can provide a strong magnetic field to absorb heavy objects.

2.5 Permalloy Plunger

Characteristics: Permalloy is a nickel-iron alloy that has extremely high magnetic permeability and is one of the materials with the highest magnetic permeability among all magnetic materials. It can achieve high magnetic induction intensity under a weak magnetic field, has low coercivity and small hysteresis loss. However, the saturation magnetic induction intensity of permalloy is relatively low and the price is relatively expensive.

2.6 Application: Electromagnets commonly used in weak magnetic field detection equipment that require extremely high sensitivity, such as high-precision magnetic sensors, weak magnetic field generators, etc. Because it can show good magnetic properties under weak magnetic fields, it can meet the magnetic field accuracy and sensitivity requirements of this type of equipment.

ferrite core

2.7 Characteristics: Ferrite is a magnetic material with iron oxide as its main component. Its resistivity is very high, which can greatly reduce eddy current losses and is especially suitable for use in high-frequency environments. However, the saturation magnetic induction intensity of ferrite is low and its mechanical properties are relatively poor.

Application: widely used in high-frequency electromagnets, high-frequency transformers and various high-frequency electromagnetic equipment. For example, the use of ferrite cores in high-frequency transformers in computer switching power supplies can effectively convert energy and reduce heat generation. In the electromagnets in some small high-frequency induction heating equipment, ferrite cores can also exert their high-frequency performance advantages.

Part 3 : How to design the plunger shape ?

 3.1:  cylindrical shape :

When it is necessary to generate a uniform magnetic field within a certain spatial range, the cylindrical core is a better choice.  The cylindrical core combined with the appropriate coil winding method (such as solenoid winding) can make the magnetic field relatively evenly distributed in the direction of the core axis. The principle is that the magnetic field lines can be distributed relatively regularly along the axial direction inside the cylindrical Plunger, so that a relatively uniform magnetic field can be generated in the external space.

3.2 : Concentrated magnetic field requirements

If the purpose of solenoid is to produce a strong, concentrated magnetic field in a localized area, something like a U- or E-shaped core would be appropriate.  

3.3 Consider space constraints and installation options

 In some small electronic devices, it may be necessary to choose a compact core shape because the internal space is very limited. A small cylindrical or specially customized shape is usually used to ensure that the Plunger and coil can be installed in a limited space, while also ensuring that the electromagnet can work normally and generate enough magnetic field to realize the opening and closing action of the relay.

3.4 : Ease of installation

The plunger shape also needs to consider the convenience of installation.   an Plunger with a simple shape that is easy to disassemble and install (such as a block-shaped cylindrical Plunger) will be more suitable.  the core shape may need to match the structure of the transmission mechanism or robotic arm to facilitate

 3.5 installation and debugging.

Combine the shape and size of your work objects

 If the solenoid is mainly used to attract objects of a specific shape, the core shape should be adapted to it.  For irregularly shaped objects, it may be necessary to design a specially shaped Plunger to achieve effective adsorption.  

3.6 Work object size considerations

The size of the Plunger should be coordinated with the size of the device matching part. If the matching part is larger, a larger size Plunger is needed to generate a magnetic field strong enough to cover the entire object.

Part 4 :  Different plunger material advantages and disadvantages  

4.1 Pure iron:

• Advantages: High saturation magnetic induction intensity, can generate a strong magnetic field under a small current; low price, abundant resources; good processing performance, easy to make sliders of various shapes and sizes.
• Disadvantages: Low resistivity, large eddy current loss under alternating magnetic field, easy to heat, not suitable for high-frequency occasions; easy to produce magnetic aging, and the magnetism may change after long-term use.

4.2 Silicon steel sheet:

• Advantages: High resistivity and magnetic permeability, small coercive force and eddy current loss, can effectively reduce energy loss in AC magnetic field, suitable for AC electromagnets; stable magnetic properties, not easily affected by time and temperature; has high magnetic saturation induction intensity.
• Disadvantages: As the silicon content increases, the brittleness of the material increases, the difficulty of processing increases, and the processing technology requirements are higher; usually it needs to be made into thin sheets and stacked for use, which increases the complexity and cost of the manufacturing process.
• 4.3:  Ferrite:
• Advantages: high resistivity, low eddy current loss, suitable for high frequency occasions; low cost, relatively low price; good chemical stability and oxidation resistance, not easy to rust and corrode.
• Disadvantages: saturation magnetization intensity is lower than that of metal materials, and the magnetic field intensity generated under the same volume and current is relatively weak; low magnetic permeability, relatively slow response speed to magnetic field; brittle texture, relatively low mechanical strength, easy to break.

4.4  Soft magnetic alloy

• Permalloy:
• Advantages: extremely high magnetic permeability and low coercivity, can be magnetized to a high degree in a weak magnetic field, and the hysteresis loop is narrow and the hysteresis loss is small; high plasticity, easy to process and shape, can be made into slide bars of complex shapes.
• Disadvantages: sensitive to stress, magnetic properties may change when subjected to external forces; high cost, relatively expensive price, limiting its use in some low-cost applications.
• 4.5:  Soft magnetic alloy:
• Advantages: It has high magnetic permeability and high resistivity, and has good corrosion resistance in oxidizing media and hydrazine media; the magnetic properties are stable and can maintain stable magnetism in a wide temperature range.
• Disadvantages: High material cost; difficult to process, and high requirements for processing technology and equipment.

Non-ferromagnetic materials

• 4.6 Aluminum alloy:
• Advantages: It has high hardness, strength and light weight, which can effectively reduce the overall weight of the electromagnet; good corrosion resistance, suitable for some humid or slightly corrosive environments; good processing performance, easy to carry out cutting, extrusion and other processing technologies.
• Disadvantages: Non-ferromagnetic materials, poor magnetic permeability, and relatively small electromagnetic force generated in the magnetic field; although the strength and hardness are high, there is still a certain gap compared with some metal materials, and deformation may occur when subjected to large external forces.
• 4.7 Stainless steel:
• Advantages: It has excellent corrosion resistance and oxidation resistance, and can work stably for a long time under harsh environmental conditions, such as humid, acid and alkali corrosive environments; high strength and hardness, good mechanical properties, and can withstand certain external forces and impacts.
• Disadvantages: Non-ferromagnetic material, poor magnetic conductivity, will affect the magnetic field strength and electromagnetic force of the electromagnet; relatively high cost and expensive; difficult to process, especially for some high-precision processing requirements, more advanced processing equipment and processes are required.

 Part 5 :  Summary :

    First we need to know the role of the Plunger for the solenoid:

1. Enhance the magnetism of the energized solenoid
2. Make the solenoid stronger

​One of key characteristics of solenoid is that the magnetism is controllable, which means that it will be magnetic when you want it, and it will disappear when you want it to disappear.

The plunger for the electromagnet is made of soft iron, not steel. Otherwise, once the steel is magnetized, it will maintain magnetism for a long time and cannot be demagnetized. Then the strength of its magnetism cannot be controlled by the size of the current, and the advantages of electromagnets will be lost.

The material of the Plunger is generally not pure iron, but iron oxide. ​

The Plunger of the solenoid is used to increase the magnet's magnetism. Just choose a material with a magnetic permeability much greater than one. For example, oxides of metals such as manganese and cobalt. The greater the magnetic permeability, the greater the superimposed magnetic field. ​

The magnetic permeability of other metals is not as good as that of iron, and they are more expensive, so they are generally not used. Most of the materials used now are iron alloys, such as the new material manganese-zinc ferrite material, with a magnetic permeability of around 18,000. Ferric oxide is probably a few hundred.

It is best to choose the electromagnet core according to the following:

1. The function of the core is to increase the intensity of the magnetic field, so a ferromagnet with high magnetization intensity should be used.

2. Electromagnets are generally required to have no or as little residual magnetism as possible after the power is turned off, so soft magnets with small coercive force are used.