2002-10-17

# Trivial Electric Motor

OK, so its a child's toy, but lately I've been getting into simple things that show physics principles. Most importantly, trivial projects like this keep my hands dirty, in the world where I spend more and more time behind a keyboard.

At first sight it is very simple, a coil, a magnet and some beads and bearings. But then you start to think about how it works. It appears to have no commutator, how can it work? Well actually it has a commutator, only half the enamel is stripped from the wires (one side, facing away from the camera), so the coil is active only half the time, inertia takes the coil around until it contacts again and gets another torque kick.

This is an excellent project to build with your children. It is simple, takes only minutes, and is instant gratification. Once assembled it runs up to speed quickly and rattles and shakes all over the desk. The physics are easy to explain (perhaps not to understand mathematically, but it all helps) and it works, they built it themselves, unlike some cold diagram in a textbook.

You might like to make your bearings out of bare wire, saves the stripping which took me the most assembly time. The power source is a D cell, which will run the motor for over 24 hours (I've timed it). The armature is wound on a AA cell or similar diameter tube. Being square would actually be better, and different sizes and turn counts all work with different outcomes, lots of experimentation there! My original prototype was 6 turns, this one is 40 turns, they both spin about the same speed, but the 40 turn one requires less magnets and produces more torque. The magnet is the strong boron magnets you used to get from Tandy (aka Radio Shack). They were once about 50c each, now they want something like \$4.50 for them, but you can still get them if you are looking to build an exact replica.

The beads are just to keep the armature centred in the bearings. You don't really need them, but they help with the speed and consistency of the motor. My original prototype was held between the open alligator clips and worked fine. Don't expect it to self-start, it may if there is electrical contact, but you'll probably need to give it a small flick to get it going. A good replacement for my magnet would be the head positioning servo magnet out of a junked hard disk drive, just note the pole placement is a bit strange, but they are very powerful (perhaps too powerful, they give blood-blisters if they pinch you against the fridge for example).

Here is the parts list if you are shopping for it in Australia. Chances are you will be able to source all the parts for free, but worst case you can buy all the ideal parts you'll need at the local mall for about \$10:

Part Where Price Notes
0.71 mm magnet wire. Dick Smith, Jaycar, or Tandy. \$4 for a 25 gram reel. Copper is expensive, raid an old consumer appliance for the transformers to save money, the 25 grams is way more than you need, it would make 20 motors. The gauge is not that important, #22 or #24 is the easiest to use too thin and it can't support the weight, too thick and it becomes impossible to bend.
Fairly strong magnet. Jaycar or Tandy. \$4 for a good one. Magnets from science stores or electronic stores are priced very high, don't buy them there. Scavange for them, old speakers work great, as do old hard disk drives, stepper motors, or anything else that has perm magnets in it. The poles may be in strange places, but they will probably still work if you experiment with the placement. Fridge magnets won't work unless they are the solid disk or bar type, the flat polymer ones are too weak and have alternating poles closely spaced.
Small plastic beads. Lincraft. \$1 for a pack of 50. My girlfriend dontated a small pack from her cross-stitch supply for this project. They are not strictly needed, but make operation much nicer. You could use a small length of soda straw instead but your bearings would need to be larger.
D-Cell holder. Dick Smith, Jaycar, Tandy. \$1.25 You may as well get a good D cell holder. You can just tape wires to end of the battery, but the cell holder makes it a no-brainer and saves connection problems. While you are there, buy a small switch and hook it up in series with one power lead, that way you can turn it on and off easily and save juice.
D cell battery. Anywhere. Varies, \$5 for 4. Any old D cell will do, you don't need fancy ones.
Gator test clip leads. Dick Smith, Jaycar, Tandy. \$3 for a pack of 10. Always handy, not strictly required but saves twisting wires or soldering, and you don't need a switch that way. Reusable and a must for any electronic geek.

In addition you'll need some tools, I used the pair of pliers and knife in a Leatherman to build the whole thing sitting at my computer at home. The hardest part is winding and bearing only one side of the wire for the armature. One leg can be completely bared, I realised this after I carefully bared only half of each side. The main thing is that the bared side be the same on each end. You can make the motor run a little better by adjusting the phase of the torque spike, just twist the wires slightly so they contact later or sooner. Quadrature is probably the best.