"Super High Strength" Solenoid? 1

[jensenb001]

Ok, new to the forum here - long time browser, first time poster... Anyhow - I'm an ME, so I'm not too familiar with magnetics, but I've been working on developing something and have hit a wall. All I'm trying to figure out is; just how 'powerful' can a solenoid realistically be made? By this I mean a standard 'plunger' type solenoid. The thing I've been trying to determine is how can I calculate the resultant force exerted on the plunger given a particular geometry, current and material(s)? I understand that this is a multi-faceted analytical situation, but is there any fairly simple methods by which to determine this? More specifically, I'm trying to decipher the feasibility of creating a solenoid that can exert a force of about 15000 lbf on a plunger of approximately 3" in diameter over a relatively short distance. Is this even plausible? I've heard solenoids can be made very strong, but have found none even close on the market. Is this because this is far too much to expect, or that few uses have been found for such a thing? I know electric motors can easily be made to produce such forces, so why not solenoids? And, if it is near impossible to do such a thing using a solenoid device, is a linear motor a better candidate as far as method goes?

Thanks in advance for your replies!

Bruce J.

 

[desertfox]

Hi jensenb001

Try these links for starters

http://www.vias.org/kimberlyee/ee_09_10.html

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

http://homepages.which.net/~paul.hills/Solenoids/SolenoidsBody.html

desertfox

 

[jensenb001]

desertfox - Thanks for the info, though I think that the first two links are in regards to a solenoid-type electromagnet (solenoid with a permanent core, used as a magnet to enact a force on external things). The first gives a force calc, but that is also for a permanent core solenoid electromagnet. The last link I am very familiar with and have used the provided calcs extensively in trying to reach my goal. However, the force calc provided (F=.5*uo*pi*(rNI/stroke)^2) does not take into account external airgaps or the permeability of the plunger material. Using that calc, it seems plenty probable that I can in fact build a solenoid as described in my first post, but it is very liberal I'm sure. After learning about hysteresis losses, eddy current losses, saturation, etc, I'm sure that equation is very optimistic.

BTW - I have educated myself a-bit on this subject prior to posting. I've read C.R.Underhills old 'textbook' on electromagnetics and solenoids, but still gotten little applicable fomulae. I've also tried a couple of free trial versions of magnetic FEA programs (MagNet and Quickfield) but have been unable to get data that I am confident in (most likely due to my inexperience in setting boundary conditions for such systems).

 

[desertfox]

Hi jensenb001

Thanks for the response, they only way forward I can see is to break the plunger movement down into steps, so you then calculate the force between the moving and fixed poles for each discrete step, starting with the plunger fully out and the pole gap decreasing each step by a uniform amount.
So you would need to calculate Reluctance, inductance and permeability for each step as you move the pole along and finally theres leakage of the magnetic field which you also need to consider,I have some old books here, I will look through them and post again.

desertfox

 

[jensenb001]

Well from what I understand (I think...) the force exerted on the plunger is highest just before it is fully inserted/bottomed out in the solenoid. I don't think I need to iterate too many times to decipher the maximum force felt by the plunger given the particular materials, current, geometry, etc. I'm not looking for a 100% accurate theoretical solution, just something that proves/disproves feasibility of my method - just to give me confidence enough to sink some $ into a preliminary prototype and/or FEA software package. Thanks for the help! I've asked many other engineers about this subject, and all I get is shrugs... Seems magnetics is a bit of a 'black art'! So, this is an interesting learning process!

 

[israelkk]

For a plunger type solenoid made of ferromagnetic materials (including permanent magnets) the maximum electromagnet force divided by the plunger area (i.e. pressure) is approximately 5 to 10 atm. This is because the saturation of the ferromagnetic material (electrical iron or even better alloys). Therefore, for a 3" diameter plunger a reasonable maximum force will be between 450 to 900 kgf. Look at a solenoid as a low pressure cylinder compared to an hydraulic cylinder and piston where you can use a pressure of 200 atm.

 

[electricpete]

Great point.

Energy density is w = 0.5 * B^2 / mu0
Stored energy is W = w*Volume = 0.5 * B^2 / mu0 * (Area*g) where g is gap distance
Force = dW/dg = 0.5 * B^2 / mu0 * Area
Force/Area = 0.5 * B^2/mu0

Using SI units, mu0 = 4*pi*1e-7
Force/Area = (0.5/ 4*pi*E-7) B^2 ~ 400,000 B^2 [SI units]

If we assume B= 1T, then we have 400 kPa pressure, which is about or about 4 atmospheres. If B = sqrt(2) T = 1.4 T, then you'd have about 8 atmospheres.

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(2B)+(2B)' ?

 

[jensenb001]

electricpete - I follow you on this, however the formula given does not take inot account the permeability of the core material, air gap etc... Is this a commonly used approximation? I have run across this equation before, but found it of little use as I do not know how to calculate the flux density (B) within a solenoid. In any case, there is a squared relationship between force and B, so if I increase B to say 7T, then the force is getting pretty close to my goal (15Klbf, or 21.5E^6Pa). So then I just have to provide enough current to do so? How can I calculate the value of B based on my geometry and coil design? Also, where did y'all find these formulas? Is there a useful textbook that has useful magnetics formulae?

isrealkk - I do understand that saturation plays a role, though I am planning on this device only operating for a very short time and over a very short distance (millimeters or less)- is saturation time dependent? Is the information you provided a sort of 'rule of thumb' or is there a resource that I can find that delineates this? I'd like to learn as much as possible so I can see if there are any avenues by which to circumvent these physical limitations.

Also, I've heard that once a solenoid has drawn the core all the way in - thereby reducing the reluctance of the system as a whole to it's lowest (where it 'likes' to be), it can be 'held' there with tremendous force. Given this it would go to reason that if I place the core just off-center, a large force would be exerted to get it to this position - surely more so than if the plunger were far from the ideal (centered) position... Is this correct reasoning? What is the main limitation to a solenoid being as strong as an electric motor? What is the weak link here and can it be overcome? Thanks!

 

[electricpete]

electricpete - I follow you on this, however the formula given does not take into account the permeability of the core material, air gap etc... Is this a commonly used approximation

Following the discussion of israelkk, we can get a pretty idea of the upper bound for the flux density in iron is given by the saturation curve. Attached is a plot of material properties for M22 Silicon steel on log-log plot. Knee of the curve is 1.5T, so the device likely won't operate too far beyond there, and certainly less than 2.5T which requires ridiculous excitation to achieve.

To me, M22 is laminated steel used for large motors. I got this plot from free software FEMM, which also has a large variety of other materials loaded in.

In any case, there is a squared relationship between force and B, so if I increase B to say 7T, then the force is getting pretty close to my goal (15Klbf, or 21.5E^6Pa). So then I just have to provide enough current to do so?

I don't think you will get to 7T using steel regardless of how much excitation you apply. It would have to be some exotic material, nothing like I have heard of for industrial equipment.

How can I calculate the value of B based on my geometry and coil design?

You might want to search the forum for ideas.

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(2B)+(2B)' ?

 

[electricpete]

Sorry, here is the attachment I referred to.

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(2B)+(2B)' ?

 

[electricpete]

Given this it would go to reason that if I place the core just off-center, a large force would be exerted to get it to this position - surely more so than if the plunger were far from the ideal (centered) position... Is this correct reasoning?

I'm not sure what you are talking about "centered". Usually in pulled-in position the magnetic circuit is closed so there is no remaining axial airgap... just small circumferential gap.

What is the main limitation to a solenoid being as strong as an electric motor?

An induction motor (rotary or linear) operates on the principle of current induced in the moveable part and it is the interaction of field from induced current and applied field which causes force or torque. Solenoid does not operate on the same principle (interaction of field from induced current with applied field).

What is the weak link here and can it be overcome?

Already covered... saturation of iron is going to limit possible solenoid force for a given size plunger.

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(2B)+(2B)' ?

 

[electricpete]

Maybe what you are looking for is similar technology as used in motor operated valves. They use an electric motor driving a stepdown gear to produce large axial forces.

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(2B)+(2B)' ?

 

[jensenb001]

electricpete - Thanks for all of the info/wisdom... No, an MOV type device would not work for my particular application, though I am quite familiar with these. Overall, what I am understanding here is that what I have proposed is essentially an impossibility. That is fine, as this is why I'm asking these questions - little information is available on the net on this subject. There is very basic and conceptual stuff, but that seems to be it. Plus all the info I have found is not well referenced, if at all - which always makes me skeptical of the validity of the material. In any case, do you happen to know of any good textbooks or other reputable resources on this subject (magnetics/electromagnetics) - for even if this particular idea doesnt pan out, I've become interested in the subject and would like to learn more.

Also, would the situation change if I were to place a coil on/within the plunger and excite that, as with the armature of an electric motor - essentially creating a cylindrical linear motor. Would saturation still be the limiting factor? It seems like it would play a smaller role. I'd then be generating force due to opposing magnetic fields rather than simply providing a path for a reduction of reluctance. Where/how might I calculate the resulting forces? I do apologize for being vague about the purpose of the device, but it is necessary. Thanks!

 

[electricpete]

Attached is a plot of material properties for M22 Silicon steel on log-log plot. Knee of the curve is 1.5T, so the device likely won't operate too far beyond there, and certainly less than 2.5T which requires ridiculous excitation to achieve.

To clarify a little more, even if we boosted the excitation to increase flux in the steel to 2.5T or more we would not get corresponding increase in force.

The reason us that the force depends on difference in energy density (w = 0.5*B*H) between iron and air. B is roughly the same in iron as air. We have assumed that the energy density in iron was negligible (Hiron<<Hair) when operating at or below saturation. However as we get out to the right of the curve far beyond excitation, Hiron is no longer negligible. Very far to the right the slope of the iron B vs H curve becomes approximately mu0 and at this point as we increase applied excitation, Hiron increases the same amount as Hair and energy density in iron increases as much as in air, so there is no associated increase in force derived from the excitation increase.

Overall, what I am understanding here is that what I have proposed is essentially an impossibility.

Yes, that’s my understanding also, based on use of iron-core devices.


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(2B)+(2B)' ?

 

[Compositepro]

I think jensen has some basic misconceptions about solenoids. To generate force it is not as simple as "an iron bar wants to center itself in a coil of wire." The complete magnetic path through and around the coil must be through iron to provide a low permeability path. The force is generated to try and close any air gaps in this path. This is the same way that permanent magnets generate force.

 

[jensenb001]

electricpete - Thanks for the explanation, I'm somewhat disappointed at my idea not being possible, but hey - I'll find another way... Again, would an energized coil within/upon the core aid in providing enough force to meet my goal or is this also overambitious?

Compositepro - No, I understand that concept as a whole, just worded differently I suppose...

 

[electricpete]

Cannot be done with solenoids with the plunger area you mention and traditional materials.

There are apparently linear actuators that can get higher force than solenoids, again using principles closer to what we associate with motors.

A reference on these devices, see “LINEAR ELECTRIC ACTUATORS AND GENERATORS” by Boldea and Nasar ISBN 0-521^8017-5

For free general electromagnetics and electric machinery, , google MIT OpenCourseware and find the courses on electromagnetics and electric machinery

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(2B)+(2B)' ?

 

[jensenb001]

Thanks a-lot, that's exactly the type of thing I was looking for. That MIT website will come in handy for many things in the future I'm sure!

 

[Warpspeed]

For anything more than an absolutely minimal stroke, a linear motor begins to look like the better alternative.

Linear motors can be readily stacked in series, and a concentric hydraulic force multiplier might be packaged into it to trade off stroke for force.

If the linear motor was flooded, that would also be a bonus for cooling if the duty cycle is fairly short.

 

[sreid]

You might want to take a look at Exlar.

http://www.exlar.com/