Homopolar Christmas Tree

There are many examples of Homopolar motors but I felt that something other than wires rotating was needed and at this time of the year when our thoughts are on celebration, what better time to make something festive.and instructive.

In this case a spinning Christmas tree which will make a pleasant interlude between courses over a festive meal.

 if you are planning to make one, be careful and handle with care!

Operation should only be for short durations of time as the battery is terminated with a short circuit and will begin to heat up. Allow the battery to cool down between usage.

3D printer

BlocksCAD

Cura

Cutters

Needle files

Long nose pliers

Drill bit to suit wire diameter.

1.5V AA alkaline battery

Neodymium cylindrical magnet (10 mm dia. & 10mm long larger alternative can be substituted.

80 cm length enamelled copper wire 16AWG/1.29mm diameter, wire of a suitable diameter that retains its shape when formed. The fusing current in free air is ~166A which is significantly greater than the maximum current capability of the battery into a short circuit. The wire will therefore be physically and electrically robust.

The Homopolar motor is driven by the Lorentz force. A Lorentz force is formed when a conductor with a current that is flowing through it is placed in a magnetic field which is perpendicular to the current that flows from the magnet to where the wire is connected, generating the spinning motion.

The Christmas tree is a basic cone shape with some embellishment to give it texture.

Using a cone shape gives both a balance point around which to rotate and a contact point which sits on the battery.

The design requirements were that it had to be hollow to accomodate the coil, be of sufficient height to cover the battery and magnet whilst enabling sufficient clearance at the bottom to enable free rotation.

The ability to be 3D printed without the use of supports.

This was created in BlocksCAD and the resulting file saved as an STL.

The STL file is opened in Cura and the following settings applied for printing.

Infill Quality - 0.15mm

Infill - >=50%

Buld Adhesion - Brim

Size - 49.9 x 48.3 x 68 mm

Filament - Transparent Green PLA

Pre designed openings are made in the tree which will enable the wire to be passed through although it may be required to open these with a drill bit of suitable size.

With a suitable drill bit make a hole from the inside through the tip of the tree.

Through this hole will be fed the wire which enables the tree to balance on the battery positive terminal.

With the same drill bit make a hole close to the bottom of the widest part of the tree.

Through this will be fed the other end of the wire which will be wrapped around the magnet.

Keeping a length of wire of ~5cm beyond the tip of the tree.

Wrap the wire from this point around the outside of the tree to form a conical coil, this need only be loosely coiled.

Remove the coil from the tree but keeping the form.

Thread the ~5cm length through the inside and out through the uppermost side hole and back through the hole previously made in the tip of the tree.

The free end of the coil passes back out through a lower hole in the widest part of the tree ~ 9cm.

Remove the insulation from the last ~4.5cm, coil this around the circumference of the magnet tight enough to form the shape but loose enough to enable the coil to rotate.

The insulation can be removed from the inner surface of the coil that will make contact with the magnet using a file.

Prepare the battery by creating a dimple in the centre positive terminal cap, simple hand pressure with a metal punch should be sufficient as the cap is quite thin and easy to puncture if too much force or a very sharp point is used.

The dimple will keep the tip of the wire in contact with the battery and stop it falling off when spinning quickly.

Insert the battery into the tree with the positive end first aligned with the point of the wire.

Carefully attach the magnet to the negative battery terminal.

Place the small coil over the magnet and form the wire so as not to rub against the battery.

You may at this stage sit the magnet on a ferrous surface or larger magnet for stability. i used a coin.

Adjust the position of the battery and or coil as required to determine the optimum position at which point the tree will begin to spin.

Do not operate the motor for long periods and do not leave it unattended.

Operation should only be for short durations of time as the battery is terminated with a short circuit and will begin to heat up. Allow the battery to cool down between usage.

Having made it all that is left to do is to take it for a short spin.