Solenoid Design Question

[cfellows]

Kind of throwing this question out there to see if somebody might be familiar with solenoid design.  I am designing a solenoid wrapped on a spool of the following dimensions:



I'm winding the spool with #24 insulated magnet wire, calculated total of 1392 turns, Resistance 10.1 ohms, 22.2 volts @ 2.2 amps for a total of around 3322 amp turns.  The continuous rated amps for this wire is .54 amps but I'll be running at under 25% duty cycle so overheating shouldn't be a problem, especially for short runs.

I plan to use a 1/2" diameter steel rod for the plunger and my question is this.  I can increase the outer diameter of the coil from from .75" to .875" to accommodate a larger diameter steel plunger, but that will increase the current flow to 2.6 amps and simultaneously reduce the amp turns to 2904.  So my question is this:  If I reduce the amp turns to 2,904 and increase the diameter of the steel plunger, is this likely to increase or decrease the pulling power?

Thx...
Chuck

[MJM460]

Hi Chuck,

After reading your last post on your solenoid engine, I searched for solenoid force characteristic and similar phrases to try and find a normal force vs stroke characteristic.

Two or three very good sites came up near the top, two manufacturers with good technical articles and a theory answer.  I am sure one of these addressed this issue exactly and very clearly.  I should have bookmarked the site, but of course did not.

I don't think I can repeat the explanation sufficiently accurately but I suspect this search will find it quickly for you.  If you have no luck I will have another go for you.

MJM460

Ps it was really good to hear that you are making good progress on the health front.  We are all barracking for you.

[Bluechip]

This dates from 1914 but is pretty good.

https://www.amazon.com/Solenoids-Electro-Magnets-Electro-Magnetic-Windings-Underhill/dp/155918096X/ref=sr_1_2?s=books&ie=UTF8&qid=1489309279&sr=1-2&keywords=solenoids+electromagnets

Only problem is the terminology is way out of date ...  bye-bye Gauss, Oersted, Gilberts, foot-lbs etc. etc. 
It appears to be using the c.g.s. regime I was brought up with.

It is mathematical in many places but some is 'plain text'.

Dave

[PStechPaul]

I found a solenoid force calculator:

http://www.daycounter.com/Calculators/Magnets/Solenoid-Force-Calculator.phtml

This seems to be the force between a solid steel core solenoid and a piece of ferromagnetic material separated by a gap. This is not the same as the force pulling a ferromagnetic rod into a hollow solenoid, but I used the area of the inside surface of the cylinder as "A" and the thickness of the coil form wall as "g=0.125". The results:

2.2 amps, 1392 turns, 0.75" diameter => 350 pounds

2.6 amps, 1117 turns, 0.875" diameter => 366 pounds

If I use a gap of 0.0125, the force becomes 100 times larger.

For the solenoid you are making, the force will increase as the armature approaches the solenoid opening, and will continue to increase until it is perhaps halfway in, and then will decrease to zero as the armature is pulled fully into the coil. It should try to center itself in the coil.

[PStechPaul]

I found a solenoid force calculator:

http://www.daycounter.com/Calculators/Magnets/Solenoid-Force-Calculator.phtml

This seems to be the force between a solid steel core solenoid and a piece of ferromagnetic material separated by a gap. This is not the same as the force pulling a ferromagnetic rod into a hollow solenoid, but I used the area of the inside surface of the cylinder as "A" and the thickness of the coil form wall as "g=0.125". The results:

2.2 amps, 1392 turns, 0.75" diameter => 350 pounds

2.6 amps, 1117 turns, 0.875" diameter => 366 pounds

If I use a gap of 0.0125, the force becomes 100 times larger.

For the solenoid you are making, the force will increase as the armature approaches the solenoid opening, and will continue to increase until it is perhaps halfway in, and then will decrease to zero as the armature is pulled fully into the coil. It should try to center itself in the coil.

Some additional references:

https://www.comsol.com/paper/download/83443/vogel_paper.pdf

http://onlinecalculators.brainmeasures.com/Electric/SolenoidCoil.aspx

http://info.ee.surrey.ac.uk/Workshop/advice/coils/force.html

http://electronics.stackexchange.com/questions/102447/how-much-force-does-a-solenoid-really-have

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html

http://www.indiansolenoids.com/solenoid-force/

http://www.tlxtech.com/understanding-solenoids/resource/shaping-the-solenoid-force-curve/



http://spiff.rit.edu/classes/phys313/lectures/sol/sol_f01_long.html

[PStechPaul]

From that last link above I made a spreadsheet showing the force of a solenoid. I'm not sure it's right - it showed a force of 1.28 Newtons or 0.287 Pounds. I then refigured it based on the energy stored in the solenoid using current and inductance, and I got 8500 Newtons or 1912 Pounds.



http://enginuitysystems.com/pix/electronics/Force_Of_Solenoid.jpg

The spreadsheet is available at: http://enginuitysystems.com/pix/electronics/Force_Of_Solenoid.ods

[cfellows]

Thanks for the good info.  I have a real hard time wrapping my head around these kinds of calculations.  Anything much beyond ohm's law stops me cold.   I guess the only way to do it is wind up a coil and see how it works.  I had planned to insert the coil in a close fitting steel tube and put a steel disc on the end of he plunger to form a closed magnetic circuit when the solenoid pulls in.  That, I think should increase the amount of pull...

Chuck

[jschoenly]

Chuck,

I'm curious to see how it works out for you.  I too get confused after ohm's law and maybe a little beyond.  I have a magnetic brake for a z axis ball screw have the coil burn up on a conversion.  I was too cheap to get the fanuc replacement and didn't like it's ~200vdc wiring.  THe company made a 24v version bur of course, fanuc had to make it their own and the 200v was slightly different than the 24v dimensionally.  I set out to make my own for 24v, designed the spool, figured out the approximate turns and amperage.  Luckily the 24v version listed specs for amperage and resistance which made it fairly easy to reverse engineer.  I wound up a coil (which could have been prettier...) and despite missing the resistance by a small amount, seem to be working well!

Good luck!

Jared