Mark II Results - External Iron

How does the presence of external iron affect performance? We had a theory of external iron that predicted it would be helpful. Now the figurative rubber meets the hypothetical road.

External Iron

This coilgun was built with iron washers at each end to help focus the magnetic field. Did it really help?

I measured the performance, both with and without the external iron parts around the coil, using the same 31.5mm projectile and applied 24.3 volts. Because this might make a large change in both inductance and magnetic field pattern, the coilgun was tuned for both number of turns and starting position to reach best performance in each case.

In running this test, I discovered the "no iron" version worked best using four layers of the coil, compared to using two layers when external iron is there. This tells us iron makes a big difference to the inductance!

Conclusions

This coilgun works best with no external iron. Note that the number of turns on the coil requires a significant change to accomodate this additional inductance.

Much later, I built a FEMM model with external iron. I discovered that, at best, the efficiency is unchanged. During this experiment we found that it got worse, and I suspect the losses are caused by eddy currents in the external iron.