Tim's Magnet-Piston Engine Design

[gotoluc]

Attached is a pic of a prototype design - based on the above specs - which I hope includes the best bits of both my & Luc's designs.

- It's to scale - so 150mm long, 50mm diameter, 30mm cores, magnets & throw
- Uses PMs, and has a magnetic shaft like Luc's
- Has a big coil, with central core - where all the action happens, like mine.

I think this is pretty easy to build. It would need an AC or alternating pulsed DC input - and would be 'single stroke'...

For the coil - assuming you have a power supply that can handle a range of voltages & currents - it would be a useful test to start with a smaller diameter, and add layers - to see what the difference it actually makes. So start off with perhaps 1Kg of copper, then 2Kg, 4Kg etc... More copper should mean less input power required for the same output...

Thank you Tim for taking the time to make this drawing and all the details.

I do have some 2 inch (51mm) diameter x 1 inch (25.5mm) thick N52 magnets
I also have a spool of 5 or 6 Kg. of 14 AWG (1.6mm) copper wire.

Questions:

Knowing this, if I stuck the magnets on 12mm thick steel cores (since the magnets are so thick), would  the thickness of the center core need to change or the overall coil length?

Can the center shaft be magnetic and can I use a thin bronze sleeve bearing in the center core as guide?... or will this cause a short?

I was also thinking... if I use such strong magnets, would the core not have a powerful return stroke (center rest position) when the coil is switched off?... if so, maybe we can use this and maybe the fixed center core can be positioned off center of the coil so we would only need one power stroke and the return will be done by the magnets?... would this not be worthwhile?

Thanks for your help

Luc

[tim123]

Hi Luc,
  the dimensions are not crucial. You can change any of them, within reason. I think the maximum effective stroke length is about 2 inches though...

Do your magnets have a hole in the middle? I'm not sure how you'd fix them to the shaft if not.

I'm not sure what you mean by 'if I stuck the magnets on 12mm thick steel cores'...? Assuming your magnets do have holes - you'd just need to get a shaft to fit through them, and a means to secure them to it.

Yes the shaft could be magnetic - i.e. mild steel. Yes, i think a brass sleeve should be ok, ptfe would be better but theres no danger of shorting.

This design has to be powered with alternating current (pref. pulsed alternating DC) - there would be no auto-return stroke because the magnets will stick HARD to the central core when the power is off. It has to be actively powered in both directions - unlike the non-pm design. The benefit of that is that it produces more power per revolution...

By the way - I would probably make this using two (or more) coils - so the central core can be fixed between them.

Important note:- long coils have strong forces between layers (due to the voltage difference) - and it's better to break them up into multiple shorter coils... See the book on solenoids for more details...

The central core could be almost any size - a longer one will develop a bigger, longer field, but as long as it's not too thin - it will work. I'd guess anything over 10mm should give decent results. It would ideally be made of laminated steel, but it'll still work with just a lump of mild. It'd just get hotter faster.

[gotoluc]

Hi Luc,
  the dimensions are not crucial. You can change any of them, within reason. I think the maximum effective stroke length is about 2 inches though...

Do your magnets have a hole in the middle? I'm not sure how you'd fix them to the shaft if not.

I'm not sure what you mean by 'if I stuck the magnets on 12mm thick steel cores'...? Assuming your magnets do have holes - you'd just need to get a shaft to fit through them, and a means to secure them to it.

Yes the shaft could be magnetic - i.e. mild steel. Yes, i think a brass sleeve should be ok, ptfe would be better but theres no danger of shorting.

This design has to be powered with alternating current (pref. pulsed alternating DC) - there would be no auto-return stroke because the magnets will stick HARD to the central core when the power is off. It has to be actively powered in both directions - unlike the non-pm design. The benefit of that is that it produces more power per revolution...

By the way - I would probably make this using two (or more) coils - so the central core can be fixed between them.

Important note:- long coils have strong forces between layers (due to the voltage difference) - and it's better to break them up into multiple shorter coils... See the book on solenoids for more details...

The central core could be almost any size - a longer one will develop a bigger, longer field, but as long as it's not too thin - it will work. I'd guess anything over 10mm should give decent results. It would ideally be made of laminated steel, but it'll still work with just a lump of mild. It'd just get hotter faster.

Hi Tim, thanks for the reply

No, my magnets do not have a hole in them. What I was thinking is, if I use 12mm thick steel cores and weld them to the central shaft, then I could magnetically stick the magnets to the cores and add epoxy between then for extra hold. I can also epoxy another 12mm disk on the outside of one of the magnets if I want to attach a connecting rod or something to it.
I think this should be strong enough for basic pull tests.

Sorry, I forgot that if one of the repelling magnets would get close to the center core it would stick to it. So I agree, it needs AC to work.

I don't follow you about adding more coils so the central coil can be between them?

Thanks

Luc

[tim123]

Hi Luc, I'm pretty sure epoxy will not be up to the job... If your magnets were 1.5T, the max force on each could be up to 170Kg. The max force from the piston as a whole would be just over 200Kg.

Without a hole - the only way I can think of would be to fit the magnets in a pipe. With the central core in there too, but with a means to attach it so it doesn't move, but the pipe / piston can. The pipe would have to have slots cut in to allow it to move past the core...

K&J says the max force for 2" x 1" N52s would be 257lb
http://www.kjmagnetics.com/calculator.asp
(I just checked my force projections for these magnets against theirs - and my calcs look right)

I was thinking that if you made the main coil in two short sections - i.e. 75mm length, then it may be easier to fit the central core between them. It has to be firmly attached - as it's subject to the 100+Kg forces too... so you could make it like a '+' shape (viewed from the side) so it would fit between the two coils, and inside them a bit...

[gyulasun]

Hi Tim,

You wrote:

You're right, and I've not included inductance & loss calcs because:
- I'm not sure how to do that
- the shorted coil could be replaced with a PM. I have no idea how many turns it would need at this stage...

Instead I've just multiplied the required ampturns by 10 - on the basis that if i plan to give it 10x more than it should require - then that should cover most eventualities. So where 800AT should saturate the core, I've gone for 8000AT... Note, It still goes OU if I increase the assumed power in by 100x, or even 1000x - just at a bigger size...     

Well, the inductance of the coils is to be figured out first. Your example coil above has these data:

ID=50 mm, N=1600 turns,  wire dia=1.5 mm (I used 1.48/1.5 uninsulated/insulated in the software, link below),  coil length 150 mm,  Rdc=3.53 Ohm 

From this link, this gives about 56.3 mH inductance, its OD comes out as 98 mm, its Rdc=3.78 Ohm:
http://coil32.narod.ru/calc/multi_layer-en.html

Your earlier coil example for Case 3 setup has these data:

ID=100 mm, N=2178 turns,  wire dia 3 mm (2.98/3),   coil length=200 mm  Rdc=3.23 Ohm 

This coil calculates to have about 383.7 mH inductance, its calculated OD is 298 mmm and its Rdc=3.42 Ohm.

Now you have to calculate the so-called L/R time constant for the coil, it gives 0.0563/3.6=15.6 msec for the first coil and 0.3837/3.3=116.27 msec for the second coil (I used 'averaged' R values from your own and my link calculators).

Now you may wish to study this post here: http://www.overunity.com/8411/steorn-demo-live-stream-in-dublin-december-15th-10-am/msg360397/topicseen/#msg360397 (notice: there is typo in the text under the LEGEND: Tau=L/R and not R/L) OR any other link (like this: http://www.electronics-tutorials.ws/inductor/LR-circuits.html ) on how the electric current increases in a coil from the switch-on moment to as long as the 5*(L/R) second elapses , after which the current becomes steady state and defined as the voltage across the coil divided by the DC resistance. Here I assume you switch a DC voltage source onto the coil but for AC currents similar considerations can be made.


This means that without the time constant consideration, taking especially the second coil example, the expected 3.9 A current (for your coil with Rdc=3.25 Ohm value) from a 12.7V DC voltage source could not be reached if you wish to run the engine at an RPM of say 960. Why?
920 RPM is 960/60=16 stroke per second for your piston setup i.e. 16 Hz, it is 1/16=62.5 msec, so the ON time for the coil would be as long as 62.5 ms (albeit this needs some refinements).  However during this time, the current in the coil would not be able to reach the steady state value of 3.9 Amper but 1.6 Amper. See the formula in the d) part of the Example No 1 (almost at the bottom) in this link http://www.electronics-tutorials.ws/inductor/LR-circuits.html  to calculate instantaneous coil current, using 62.5 ms for time t in the exponent. To force 3.9 A current under 62,5 msec time into this coil, you would have to use about 37V DC supply voltage.

Possibly, using several smaller value coil sections in parallel could help here (it should be tested) and it would be useful also to ease the strong mechanical forces that may occur within a single long coil as you mentioned above.

Regarding the estimation of shorted coil losses, I cannot address that topic, unfortunately, only practical measurements would give acceptable answers.

Greetings,  Gyula