# Linear Motor Design

I previously uploaded a question on trying to make a solenoid attract a magnet 80mm away, however this ended up being an impossible ask due to solenoid's having a max effective attraction distance of around 20mm. So what if I make a series of solenoids that would attract some magnets.

The width of the system can not exceed 40mm and ideally a shorter length is better. In terms of duty cycle, I would like this "linear motor" to work continuously, until the battery charge is depleted, similar to a traditional motor that turns infinitely until the power is stopped.

• Would this setup work?
• Is it quicker and more effective than a traditional motor driven linear actuator?
• How should I design the Solenoids to get the 15kg force(what wire gauge, how many turns, diameter etc...)?
• How many solenoids should I have to achieve 80mm stroke in 50-100ms and what length should each solenoid be.

Diagram:

• Have you considered using a rotary motor with some rotary to linear converter, like a lead screw? Your design would be much simpler. Commented Aug 8, 2023 at 18:42
• Yes, that was my initial idea of using a linear actuator, however none of them seem to have the necessary speed for this. Most linear actuator's have speeds around 10 - 30mm/s, whereas I need around 1600m/s. Commented Aug 8, 2023 at 18:46
• The kinetic energy of 15kg moving at 1600m/s is equivalent to 9 pounds of TNT. Commented Aug 8, 2023 at 18:53
• What is your allowable duty cycle (please edit your question). Commented Aug 8, 2023 at 18:55
• Sorry, there was a typo in my units. I meant 1600mm/s not m/s! =) Commented Aug 8, 2023 at 18:55

Certainly it would work -- think of it as straightening out a stepper motor.

There are videos of such linear switched-solenoid motors, such as this "coil gun" or this, with better explanations..

A railgun also operates as a linear motor, where the armature moves in a straight line.

• these all seem like they propel low mass projectiles. Could it still work with 15kg of force working against it? Commented Aug 8, 2023 at 18:20
• The devil is in the details @Yassin. Rail guns can launch projectiles with sufficient mass and energy to sink naval vessels. That would just require providing enough current to the coils, which can create a ton of difficult design challenges. Commented Aug 8, 2023 at 18:39
• Yeah, however rail guns have a tendency to get very very hot or even blow the rails apart after a few shots and require a lot of energy to power them. Thats why I opted for a contactless solenoid method above. Commented Aug 8, 2023 at 18:48
• yes -- "significant design challenges". A rail gun more or less is a contactless solenoid. Commented Aug 8, 2023 at 18:54
• Correct me if I'm wrong, however isn't a railgun where an amarture is shorted across two copper contacts causing the Lorentz Force propels it? Like this media.hswstatic.com/… Commented Aug 8, 2023 at 19:06

Would this setup work?

I don't know, but here's an approach to find out.

A way you can very roughly estimate this is to calculate the volume of your assembly (i.e., 80mm long, 40mm in diameter). Then find a RC airplane motor that has about that much volume externally. Finally, take the torque that motor will generate, and turn that into the equivalent force you'd get if you put a lever on it that travels a total of 80mm in one turn of the motor.

If that force is within a factor of two either way of your required 150 Newtons, then you have a chance. If not -- look at alternate ways of achieving your ends.

Is it quicker and more effective than a traditional motor driven linear actuator?

Possibly, but you can find that out by doing some math. Find a 40mm diameter motor and get an idea of what torque and acceleration you can get out of it. Then see if you can find a suitable jackscrew that'll turn your available torque into linear motion.

How should I design the Solenoids to get the 15kg force(what wire gauge, how many turns, diameter etc...)?

The predictive-science may is to get a FEMM program and do some designing. If you don't have experience with that and don't want to do the equivalent of a self-taught master's degree in EE, you may want to find a consultant with an FEMM program.

The experimental-science way is to wind some solenoids and experiment. Wind up some solenoids of various dimensions, and see how much force they'll exert on magnets of various dimensions. Be sure to figure out how much current you can put through each one without it overheating. This should give you an idea of whether your idea is even practical.

Note that a nice characteristic of solenoids of this sort is that for a solenoid of given external dimensions (i.e., the inner & outer diameter, and length), the magnetic field it generates is pretty close to a constant times the power it dissipates via resistive heating -- it just works out that way. So if you hit the right dissipation & force numbers, but the voltage is too high or too low, you can just rewind with bigger or smaller wire.

How many solenoids should I have to achieve 80mm stroke in 50-100ms and what length should each solenoid be.

This is part of your previous question. Find out the ideal solenoid dimensions; that'll tell you how many you need.

• Thanks, you first method seems very smart. I was thinking if there was a solenoid with 150N of force already on the market and short stroke length like 10mm then all I need to do would be to take the same coil design and have 8 of those coils stacked next to each other and sequentially turned on to get 80mm stroke, would that work? Commented Aug 9, 2023 at 8:48
• Solenoids -- at least as the term is used in engineering -- are very simple electromechanical machines which makes them rugged, cheap, inefficient and bulky. You can't live with the bulky, and it's not clear if you can live with the inefficient -- for thermal considerations if not energy cost. So -- I don't think that'll work. You have some serious motor design you need to do. Commented Aug 9, 2023 at 15:06
• sorry to bother you, however I have another question. If I had a series of 8 magnets and 8 electromagnets, such that by the end of the stroke, each magnet was paired with an electromagnet, would the force be shared evenly across each coil-magnet pair(i.e if there was a 160N load, each coil would only have to pull only 20N) or would each coil need to be able to pull 160N? Commented Aug 10, 2023 at 14:44

By chance I have such a coil.

• The dimensions are 100 x 55 mm.
• The moving brass pipe is filled with strong magnets and has a diameter of 23 mm.
• Each of the 4 chambers has around 500 turns of 0.5 mm wide wire.

When I connect a supply of 42 V / 2 A to one chamber, it can hold a load of 16.2 N at the point of maximum force. There is no metal or ferrite around the coil, so the field is weak. But even if I add such an enclosure, I don't think, that I can reach 150 N except for a very short time using voltages around 1 kV.

I can make another test for you if you want to specify parameters.

• Hmm, that seems very inefficient. Using R=V/I, it seems your coils has a resistance of 26 ohms, using this calc it seems you have 241m of wire. Is that really how much wire it takes to wind one coil, or is something else causing the resistance? Also, how big is the gap between the coil and the magnets, normal motors have very tight tolerances like 0.5mm and the large airgap may be causing the very low force. Also, thanks so much for offering to test, I will let you know if I need any tests when I come up with the parameters. Commented Aug 9, 2023 at 8:58