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Tesla coil corona motor

Here's a very simple fun thing to try with a tesla coil - a corona powered motor.

wheel2.gif (1480 bytes)Take a length of fairly thick (about 3mm) solid wire (a wire coat-hanger is ideal), and bend it into a double "A"-frame shape, like the frame of a child's swing, as shown on the right (black line). Ensure the bends are smooth curves, not sharp corners, to reduce corona losses. The ends of the wire can also be bent into loops (as shown in the photos below) to reduce corona from the sharp ends. Overall size is up to you - anything from a few inches to a foot or two high, depending on the size of your coil. Large frames may add capacitance, so slight re-tuning may be required. Stretch a length of fairly thin (around #24, 0.5mm) solid wire (plain or tinned copper) slightly so it becomes straight and reasonably rigid. (If the wire is on a reel, unroll the required length, then pull on the end with a pair of pliers to straighten it.)

wheeldetail.jpg (2849 bytes)Place the centre of this wire on the cross-bar of the frame, and tightly wind 2 or 3 turns round the frame as shown on the  right, to form a simple bearing (The copper wire will usually spring back enough to allow free rotation, if not, open the coil out slightly). Shape the wire into an 'S' curve, with the bearing at the centre, as shown on the above diagram (red line). Balance the rotor by first trimming one end to balance the two sides, then if necessary bend the wire at the pivot (i.e. add or remove small fractions of a turn) as required to reduce the tendancy to rest in the same position. Make a sharp point on each end of the thin wire, either by cutting at an acute angle with sharp wirecutters or scissors, or by filing. 


Place on top of your coil, and fire away! The long exposure on pictures below makes the rotation look much faster than it actually is.

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The dual ring on the above-left image is due to unequal lengths of the rotor arms. In the picture above-right, an earthed wire was brought close enough to the wheel to strike a spark when the rotor arms are near it.

Another varient is to fix a thick wire or rod horizontally across the top of the toroid, overhanging the edge. The rotor is then placed on this overhang. You may need to put a doorknob or other spherical conductive object on the end of the wire to reduce corona. If viewed side-on, you can see the corona streamers from the rotor forming a dish shape away from the toroid (pictures to follow).

Ideas for further experimentation

Place several rotors on one shaft - two counter-rotating ones should look interesting!

Use aluminium wire for the rotor to reduce weight.

Use thick polythene coated wire for the frame & rotor to reduce corona along the length of the wire (e.g. stripped co-ax cable). 6 or 8mm  tube would probably be better for the frame due to the increased curve radius.

Use a small ball-race for the bearing - something out of an old hard drive head actuator should do nicely.

Multiple points on each arm of the rotor - should make some nice concentric circle patterns.

Set up a motor away from the field of the toroid (this will probably require the coil to be re-tuned due to the extra capacitance).

Note the upward pattern of corona streamers from the wheel, which presumably indicates the shape of the electrostatic field created by the coil's toroid. Perhaps a few of these motors could be useful for mapping toroid field shapes?

I assume the propulsion comes from the shockwave of expanding ionised air. If so, it won't be acting very efficiently on the point in the required direction. Maybe either putting the point inside a tube, or perhaps a hemispherical or conical 'dish' reflector would direct more of the force & make it go faster. Perhaps with the right tube profile, you could make a sort of corona jet engine?

Thanks to Bob Golding for starting me thinking about this!

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