I am trying to design an efficient DC solenoid which is only 3mm across, and 5mm long. I need to choose the material for the central plunger, which would be 1mm in diameter. I also need to choose the material for the frame of the solenoid which would complete the magnetic flux circuit. The frame would be comprised of two 1mm thick end-washers, joined by a cage of four 1mm thick metal rods.
I am currently thinking that pure iron might be suitable, but there are lots of other possible alloys, and ferrite materials to choose from. The solenoid coil would essentially be a stack of tiny disks made from copper foil, providing between between 32 and copper 64 turns.
I also need to be able to calculate the pull-in force of the solenoid before manufacturing it, but I am not sure how to do this. Can anyone help?
On this German page you can find appropriated magnetic materials www.vacuumschmelze.com/index.php?id=1053
The best material depends on the application. If remanent magnetization is a problem, permalloys would be the best material because it provides the lowest coercive field. For example if you electromagnet is part of an electron-optical system, a large coercive field would be quite detrimental as the settings strongly depend on the magnetic history of the magnet. Permalloy is the best choice in this case. After machining, the pieces have to be annealed in a hydrogen athmosphere for best performance. Details can be found on the homepage of vaccumschmelze.
From your question on the force one might conclude that in your case it is the the size of saturation magnetization that matters. The largest saturation magnetization will produce the largest force. The best material for this case is an Fe-Co alloy with a saturation magnetization of up to 2.5 Tesla.
Pure iron forms a compromise providing a rather high saturation magnetization of 2.2 Tesla combined with a fairly low remanent magnetization.
It also depends what is you want to achieve with this design? Maximum force, or speed of operation, or reliability (number of total operations during life time)? If reliability might be a problem, then you should stay away from any brittle materials (ferrites, powder cores, etc.).
The solenoid force F is equal to the derivative of the magnetic field energy W: F=dW/dx, where x is the plunger position. Magnetic field energy W in solenoid is equal: W=Lsquare(i)/2, where L is inductance of solenoid and i is electric current, ofcourse. There are basic equations.
Contact Institute of Tele-Radiotechnics, ul. Ratuszowa 11, Warsaw, Poland
The Laboratory "Magnesy"
As an industrial researcher I need to take into account production practicalities, so some exotic and brittle materals may need to be excluded. There would however appear to be a growing consensus that wire with properties similar to Permalloy/MuMetal/SOMALOY-500 is practical for the application (based on cost, performance, and commercial availability), given that it will be operating at a fairly low movement rate (10-100Hz).
Thank you all for taking the time to answer my question.
NiFe is one of the most suitable materials. It can be fabricated as thin layer deposited using sputtering or electroplating
Sometimes permalloy can be more can be more and more expansive than ferrite. The last requires expensive molding form for sintering. The second one requires special post treatment, that can be more expensive.
Hi Mr Lee!
I think with only 64 "turns", you´ll need a very high DC current, as field strenght is proportional to number of turns and squared current.I would do it with #42-44 AWG cooper wire and number of turns according to voltage and force needed. An external magnetic circuit can help you have more efficiency on the solenoid, as well as shielding your solenoid. As you´ll use DC, I suggest pure iron, with coating/plating, as Mr Yunas said, or permalloy. The smallest solenoid I manufactureis a sensor that has 1/4" inch core. I think geometry of a 1mm core will not help you. In this case, I suggest a "fixed eletromagnet" and a moving object,, external to the eletromagnet, but inside the magnetic shield / circuit. Regards. Plinio.
Another way could consist on inspiration on a former NASA friend´s invention, that moves Hard disk heads, the plunger could be a small rare earth magnet. In this case you´ll need an "H" bridge to run the solenoid. It will latch each side. I don´t know your exact aplication. Take care with relatively small currie point of rare earth magnets......about 200 Celsius, if I remember well. Regards.
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