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Ion motor

I have a small ion motor that I use for demonstrations at schools. It is a flat brass strip with sharpened rods pointing the same direction at each end, on a needle pivot. I run it on 10 kV from a small solid-state tesla coil and a voltage doubler. It has been negatively charged until today (just an arbitrary decision when I built it). I got to wondering about polarity effects, so I reversed the doubler diodes to make it positive. The motor always takes a few seconds to come up to speed, so I should be able to observe any changes in force, but there were none. It works identically whether it is charged positive or negative.

I always assumed that there was a reaction force with the ions at the points, which I thought would be stronger with the positively charged atoms, rather than the negatively charged electrons. The results tend to dispute that. I am now wondering if the motor simply reacts with any nearby grounded objects, which always have an opposite charge. That would fit the observations.

Does anyone know what is really happening?

Ben Miller

Reply to
Ben Miller
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The force on the ions (f=qE) will be of the same magnitude for a given field strength and this is the force acting on the rotor. The difference is that the +ions will have a lower acceleration and you will get a different sort of corona plume and space charge distribution but this is only affecting ions once they have been emitted.

Reply to
Don Kelly

A Tesla coil, if that is what you have, uses ac. If you drive the Tesla coil from a 10kV source and are using the source of an dc to dc converter, that is another story. You say that the rods point in the same direction, but that does not sound correct. Is your configuration more like

|____ or |____| ? |

Bill

-- Fermez le Bush

Reply to
Salmon Egg

The Tesla coil is really just a small solid-state type (circuits available all over the internet), with a 555 oscillator & a power transistor driving a flyback transformer, which produces high frequency AC. That feeds a high voltage doubler, which provides the DC to the motor. The rods both point CCW . The rotation is CW. "same direction" was a bad description on my part.

Without the doubler attached it also does a great plasma globe with a particular light bulb that I found, and it lights a 48" fluorescent bulb with no contact.

Ben Miller

Reply to
Ben Miller

Thanks Don. I thought you might have some insight. The corona looks like small jet engines at the tips, but you need a dark room and you have to get pretty close to see it (too close is bad!) One of these days I might build a bigger one.

Ben Miller

Reply to
Ben Miller

Is your high voltage doubler truly a rectifier circuit? Somehow, high voltage solid state rectifiers that respond to rf sound like a very iffy kind of component. What happens if you use a capacitor to filter this dc output? How do you know that that you are not seeing a thermal effect? The first vaned (Crookes?) radiometer built to observe light pressure turned in the "wrong" direction.

Bill

-- Fermez le Bush

Reply to
Salmon Egg

The doubler is a standard circuit using two diodes and two capacitors. The caps are made from small yogurt containers lined with foil inside and outside. Capacitance is between 650-700 pF each. I don't know the breakdown voltage, but based on some experimenting I believe that it is above 30 kV. Not bad for $.10 worth of material (not counting the yogurt :-)

I have never looked at the DC on a scope, just measured it at about 12 kV with a high voltage probe on a meter. You have an interesting point about the rf, as I believe the ac output is around 8 kV, so I would expect a much higher DC voltage than I actually got. It does the job for me, so I never really worried about it. When you draw an arc, it gives the RAT-TAT-TAT sound of heavy DC arcs with recharge in between, as opposed to the "sizzling" of a soft arc on the AC side.

Fun to play with!

Ben Miller

Reply to
Ben Miller

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