Solenoid design question?

[Mokele]

Fairly straightforward question: why do solenoid actuators always have such short displacements?

 

Is it due to the limitations of their typically small size? Could you theoretically design one that has a longer displacement, especially if you can make the moving shaft longer than the magnetic region? I'm partially thinking in terms of the jumping ring demo that's standard in first-year Physics labs.

 

Also, why does the ring jump higher with AC? Is it greater voltage?

[swansont]

Also, why does the ring jump higher with AC? Is it greater voltage?

 

In DC, the "opposing" current in the ring decreases with time, since it's proportional to the rate of change of the field. But it's still there with the AC, since the field is always changing. It's sinusoidal for AC, vs an exponential buildup curve for the DC, whose slope is decreasing with time. The direction of the current doesn't matter — the force will always be repulsive, so the AC case has a continual force that looks like a rectified cosine (it lags the current in phase), so it's only going to zero for brief instants.

 

This may also be the limit on a DC actuator