Generator Demonstration from Cordless Drill

This instructable will show you how to make a crude, but sturdy, hand cranked generator, capable of supplying  just a few watts of unregulated DC, at a potential of a few volts or so.  This generator is suitable for classroom demonstrations, science projects, lending credence to the legend that a motor can be used as a generator, and amusing children of all ages who have not seen this trick before.   

By itself this generator is basically a toy.  The operator turns the crank, and he or she produces enough electricity to light up a old style, incandescent, flashlight bulb.

No doubt there are going to be questions like:

"How can I make/modify/improve this thing so it can power/recharge my cell phone/ mp3 player/ vibrating massage wand/ etc?" 

Such things may be possible and maybe even practical,  however the goal for now is simply to light up a flashlight bulb. Any designs more complicated than this will have to wait until a later instructable.

BTW, I apologize for using a blurry picture as the "main" image for this instructable, but this actually the best photo I've got that captures this generator in action.  I'm guessing this photo is clear enough to see what's going on, but if you need a few hints:  The big blue thing in the background is Jack's tee shirt. The bright pink-white blob is the light bulb, with current being driven through it.  The almost invisible blur on the left side of the picture, is Jack's hand turning the crank.

The second pic is a still shot of the generator on my workbench.

The third is another action shot, but this time with the generator clamped in a vise so it won't move around so much and make the picture blurry.

Step 1 Theory Part 1: Magnets and wires

In very simple terms, a DC motor is coil of wire in close proximity to a permanent magnet. There is indeed some other stuff going on. For example there is a mechanical commutator that is actually switching different coils to the motor's (two) input terminals. Also there is more than one permanent magnet.

However at any particular instant in time, the system pretty much looks like a single coil (those windings which are connected at that moment in time ), and a single magnet(those magnets which are near those coils).

Motor action is usually explained in terms of the Lorentz Force Law: A current carrying wire, in a magnetic field, experiences a force, perpendicular to the direction of the current, and perpendicular to direction of the magnetic field. It is this force, which causes the rotor to move.  In this way, the interaction between current in wires and magnetic fields of permanent magnets, causes physical force, which in turn produces motion.

F = I*L x B

Generator action is usually explained in terms of Faraday's Law of Induction: The voltage induced in a coil of wire is proportional to the time rate of change of magnetic flux through the coil, multiplied by the number of turns in the coil.  This changing flux is caused by the relative motion of the rotor coils and the stator magnets. In this way, motion causes changing geometry, which causes changing magnetic flux through a coil, which causes a voltage to manifest across the coil.

V = N*(dΦ/dt)

A practical result of Faraday's law, one that can be directly applied to building homemade generators, is that the voltage across a unloaded generator (or motor) tends to be proportional to its speed.  The faster the generator turns, the greater (dΦ/dt), and the higher the voltage.

What this means for you, as a generator designer, is that you'd like your motor-as-generator to turn very quickly, at roughly the same as the speed it was running at when running as a motor.  Fortunately the cordless drill comes with a drive train which is geared favorably, to make the motor turn quickly at low torque, when the spindle is turned slowly at high torque. 

It seems fortunate that a cordless drill can be driven backwards this way.  It seems fortunate, but is a coincidence?  Or is it some sort of deeper law of nature? 

The reason I ask this question, is because it turns out the humble cordless drill is just one of many physical systems that don't seem to mind being "driven backwards".

For the sake of beating this topic to death, examples of these other physical systems are given in the next step.

Comments

[mtaylor22]

What an interesting idea. I'd like to have a bottle dyno for my motorized bicycle. It runs about 20-30mph constantly, with LED & battery lights for stop and low speeds. Bottle dynos sold for bicycles are generally marked with a warning not to install on motorized bicycles as they will wear out faster.

Do you think a cordless drill motor can be made into a tire-driven or chain driven generator, esp. a DC generator for LED lights?

[Jack A Lopez]

I think a DC motor, from a cordless drill, driven in this way, would wear out as fast or faster than the bottle dyno which you say is  not recommended for a motorized bicycle.

But then I also think that if you pedal your bicycle like Lance Armstrong, that would wear out the dynamo faster too.

Anyway, the main why reason I think a motor from a cordless drill might not last as long as the bottle dynamo, is brushes.  The DC drill motor has brushes, and the bottle dynamo probably does not have these.  The brushes are always rubbing against the, whatchacalit (commutator?), and eventually they wear down.  Guessing that in the bottle dynamo the parts that wear out are the bearings. 

Or maybe the copper windings burn out from too much voltage, as a result of the dynamo turning too fast.  If too much speed is the problem, the dyno could be made to turn more slowly by using a larger whatdoyoucallit, (wheel?) that rides on the bicycle tire. I.e. if that wheel that rides on tire were made larger it would turn more slowly.

The formula for this is just ω=v/r, where ω is angular speed, v is the speed of the edge of the tire (and also the speed of the road underneath the tire), and r is the radius of the little power absorbing wheel.

E.g. if that wheel thingy has a radius of 2 cm, and the bike is moving at 10 m/s (about 22 mph), then the angular speed of the little wheel is (10m/s)/(0.02m) = 500 rad/s, and that is 500/(2*pi) rev/s = (500/(2*pi))*60 = 4800 rev/min.  If instead you used a wheel with a radius of 3 cm then, at that speed, the little wheel connected to the dyno only turns at 330 rad/s = 3200 rpm

However, it may be the case that the drill motor is cheaper, especially if you already have an old cordless drill in your possession.  Also you have to build the thing that connects that motor to a wheel that rides on your bicycle tire. Presumably if you buy the bottle dynamo, it comes with whatever mounting brackets are necessary for it to ride on your tire.

[park47]

Good idea.... for 2012 Doomsday when all electronics fail..... I like it.. :)

[arnookie]

This would work for charging a battery if you added a diode to stop current running back to the drill. If you had a 18v drill you might be able to put some charge back into a 12v battery. Might be worth a try at least. I will test this out. I was going to add wind turbine fins to my old drill to see if it could charge batterys but never got around to it.

[johnny3h]

Going "backwards" through the drill's gearbox means that a wind turbine would have to produce a very large amount of torque, so...

That means you will have to have a very large wind turbine, but it will work. 

IF you removed the motor from the drill [I wouldn't do this with anything but a junk drill] then the torque requirement would be reduced greatly, BUT...

The turbine/motor would then have to be turned at very high speed to get any significant amount of output.

[Mr Sinster]

this could also work with a regular motor as well.

[tocsik]

I knew there was a reason I've hung on to that old cordless drill all these years. Thanks for the idea. I think I'll actually try to fit it into a box so it looks less like a drill, and Make it look like it's powering some old radio tubes or something. I'm actually pretty excited about this one.