Tuesday, August 23, 2011

Make Your Own Electric Motor. Now.

I was sitting around The Shed some time ago, minding my own business, when my brain decided it needed to make our own electric motor using little more than ordinary stationery supplies found in the bottom of the desk drawer.

Kids, I virtually INSIST that you try this at home.

Start with two paper clips and un-bend them so they look like this when stuck in a battery holder with a AA battery.  If you don't have a battery holder, you can use lots of blu-tack to stick the paperclips to your desk with a battery in between them.  The battery needs to make a good metal-to-metal contact with the paperclips.

Next, take a little magnet like the kind one finds almost anywhere (especially in a well-stocked Shed)  and stick it to the top of the battery, again using the Third Most Useful Thing Ever.   The magnet might try to mess with your paperclips.  Don't allow that to happen.

Now take about a foot of enameled copper wire (another perfectly ordinary office supply) and form it into a coil as shown.  I use the end of my pinky finger for this, but the third knuckle bone of a Southern Hairy Wombat will do just fine if you happen to be missing both your pinkies.  Make sure the ends (or legs) are very straight and even, sticking out opposite ends of the coil.  Scrape the insulation off one of the legs entirely, but on the other leg, scrape off only one narrow strip of insulation running the length of the leg.

Balance the coil as demonstrated in this clip so that it spins reasonably well:

When done properly and placed on the paperclip "cradle" the coil will spin continuously.  You have made an electric motor out of pretty much nothing!  Revel in your sense of accomplishment and aptitude.

Dilbert says you can never be too good looking or too well equipped.  That's why The Shed has an Agilent Digital Oscilloscope with the most excellent on-screen analysis tools.  I used it to probe the office-supply motor and discovered several important facts.  Sorry, "important" is really inadequate to describe what I discovered.  Let's try "trivial" instead.

First, this shows that the motor is running at around 1000 RPM.  I get that because there are 60 milliseconds between the start of each blip.  Second, the motor coil is drawing 0.4 amps when it is switched "on."  I get that from the voltage readings and from the fact that my coil resistance was about half an Ohm.  (You don't have to look up "Ohm's Law," do you?)  Third, from the trace we find that the duty cycle is at most 15%.  Finally, I can use all that trivia to work out that the motor is drawing 0.15 watts of power.  Since it has no load, this is the power required to just overcome friction and air resistance and keep the thing turning at 1000 RPM.

How fast can you make yours go?  How long can you keep it running?  Can you locate one other person who cares?

Neither could I.

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