Christmas STEM Challenge: E&T carousel
Image credit: Neil Downie
This week in our STEM challenge, Neil Downie demonstrates how to make a DIY carousel. Something to add to your Christmas decorations, perhaps?
Put a magnet next to a piece of aluminium or copper and what do you get? Nothing! They’re not magnetic, right? Yup, that’s true. But this isn’t the whole story. Whoosh the magnet past the aluminium without touching it and you may see it twitch. More dramatically, try dropping a powerful magnet down a length of plastic pipe, and then down a similar length of copper plumbing pipe. The plastic pipe has no effect – the magnet accelerates down with gravity. But the magnet lingers and rattles down the metal pipe incredibly slowly. That’s because of eddy currents.
The traditional Christmas Carousel or Christmas Pyramid Carousel is driven round and round by a fan and a bunch of candles. This version of the Christmas Carousel replaces the candle hot air currents with electric eddy currents in a thin circular sheet of aluminium produced by a rotating magnet. Unlike a gearwheel or pulley, the magnet doesn’t have to touch the aluminium, it can push it along from 5 or 10mm away.
Some of the world’s greatest minds worked on the discovery of eddy currents, from the scientist Prime Minister of France, Francois Arago, to London’s Michael Faraday and physicist Leon Foucault. Eddy currents happen when a highly conducting metal moves through a magnetic field. A current is generated in the metal. This current in turn generates its own magnetic field – the metal becomes a strange sort of electromagnet – and the field direction is such as to repel the magnet. The eddy current also heats up the metal, just like any other current. Eddy currents are tucked away inside all sorts of things. You’ll find them used in lots of sensors, from metal detecting to paint thickness meters, in heating up metals in industry, and in the smooth non-contact brakes used in high-speed trains.
You can make eddy current drives that use a moving magnetic field produced by sequential switching coils, and some linear motors work this way. But a motor rotating a magnet – and its field – is easier to make. The force from eddy currents is proportional to the relative speed of magnet and metal. So you don’t get any force at all at zero speed: that’s why aluminium doesn’t stick to a magnet! The force goes up with the conductivity of the metal and goes up as the magnetic field squares. So using aluminium makes sense, and using strong NdFe magnets and putting them as close as is reasonable makes sense too.
The vital parts are the carousel base, the ball bearing, and the motor with its magnets. The base is a 200-400mm circle or octagon cut from a sheet of ~1mm aluminium. You could make it from the housing of an old electronic device. You need an ungreased bearing around 8-15mm bore with 20-30mm outer diameter. (If the bearing has grease on it, you’ll need to wash it off with white spirit or something.) The bigger outer bearings from a ‘fidget spinner’ work nicely – the central bearing is a bit small.
Finally, find a 1.5-4.5V toy motor and a pair of NdFe magnets. Put a short steel bar between the NdFe magnets, making a powerful little bar magnet about the diameter of the motor. The motor shaft should be joined at right angles to the centre of the bar, maybe glueing it to a small gear or other push-fit component, as we did. Motor power, depending on the motor, can be an alkaline or NiMH battery or two, or a 3-6V plug-in DC power supply via a resistor from a few ohms to 100 Ohms. (You only need a small current, probably just 100-200mA or so.)
After mounting the bearing firmly on a base, mount the aluminium circle flat on the bearing so that it clears the motor by an average of 10-15mm from the motor. Hook up the power and, with a bit of luck, the base will circle around at a gentle pace. Too fast, try more resistance. Won’t go? Faster motor, or maybe adjust the base to be closer to the motor.
You could do many variations on technical parts of the carousel. Power two carousels from one motor, for example, or power a carousel via a vertical rim rather than a disk. You could house the motor and magnet inside a plastic housing, making the drive system more mysterious. How thin can the aluminium be – can you use Al tape? And what about using a bike wheel? The rim is aluminium and comes with its own bearing.
Once it’s all working, it’s time to put on Christmassy decorations and gizmos. Snowman figures, Santa figures, sleighs, angels, stars, LED lights – and a big central feature like a Christmas tree or a ‘wedding cake’ tower like a candle-driven carousel. Let your imagination run wild!
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