Practical Coilgun Design

Coilgun Inductance

Is the inductance of the coil important? To try various values, see the interactive RLC simulation.

Coilgun Inductance

For the best coilgun design, strive for the largest change in inductance between "no projectile in coil" compared to "projectile in the middle of the coil". That is, the more the projectile engages the field lines, the more it will affect the inductance.

This is such an important aspect of rotary motor design, they gave it a name: saliency ratio. It is the ratio of maximum to minimum inductance.

If you believe as I do that choosing projectiles and coil geometry is really an exercise in maximizing the change in inductance, then it leads to these conclusions:

  • An alternative method to how I took measurements of force (minimum current needed to barely lift projectile against gravity) would be to simply measure minimum and maximum values of inductance. This is just a thought; it seems to me that it's harder to measure inductance than a lifting force. But if you come across equations that yield inductance, perhaps then you can mathematically model something without even building it.
  • We want the projectile to fill the tube as much as possible. A bigger projectile will have more effect on the inductance. My gut feeling is that wasted airspace inside the tube is reducing the achievable change in inductance. Practically speaking, this means that you want to (a) use a thin-walled tube, or (b) choose a projectile that fills the tube, or (c) choose a tube that has as small an inside diameter as possible.

Now... Is a large inductance bad? Is it bad to have a large number of turns or a great deal of external iron? It depends on whether the switching speed is important or not. I know it is true that:

higher inductance -> more turns -> more wire -> higher resistance

But one type of coilgun is driving the coils with a square wave from a d.c. power supply. Once you turn on the juice, it will soon reach steady-state and then inductance doesn't matter. The property of inductance only helps to quantify what happens with varying current, such as ac signals used radios and tuned circuits. I believe inductance in a coil gun only tells you something about how long it takes to establish the magnetic field around the coil. It should be so short as to be negligible for all our calculations.

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