Homemade DC generator(dynamo) with a spinning coil

[robbieb]

Hi guys I have been given a college project to "construct a DC current generator (dynamo) from household materials".
So basically It has to be connected to a light bulb and to show that the faster the relative motion between the magnet and the wire coil, the brighter the light will shine.

My original design consisted of a stationary wire coil with the ends connected to the light bulb and a hand cranked rotating magnet in the center, however I showed my design to my lecturer and he said that it wasn't a dynamo because the magnet spun not the coil, is this correct? I always thought it didn't matter which of the two spun just that there had to be relative motion between the two.

My question is doe's anyone have any advice on building it with the coil spinning?
The main issue is that if the coil is spinning the wires connecting to the battery will become twisted?

I thought of using a slip ring of some sort but didn't think it would fit the "household materials" requirements.

All help will be greatly appreciated,
Thanks in advance.

 

[hevans1944]

It makes no difference if the magnet is spinning or the coil is spinning, but you still have to do as your short-sighted lecturer says if you want a passing grade. For a real-world example of a "DC current generator (dynamo)" with a spinning magnet look no further than a common automobile alternator, commonly used to generate three-phase alternating current that is subsequently rectified to produce DC current that keeps the car battery charged. The rotor in an alternator produces the rotating magnetic field that "cuts" the winding turns on the three stator coils. And, yes, slip rings are used to provide a variable current to the rotor, and hence a variable magnetic field, to allow the amount of current that is produced to be varied, since automobile engine speed is not constant.

As for "household materials" I suppose that would depend on the household. A common hand-held mixer has a "universal motor" inside that can be rigged to produce DC when the motor shaft is spun. Perhaps something like that qualifies, since you have to take apart and trash the mixer?

To produce enough current to light an incandescent bulb (even a tiny one) will require more than one turn of wire and more magnetic field than is produced by a refrigerator magnet... the kind used to hold notes to the side of the refrigerator. OTOH, many households have a personal computer with some sort with a hard-disk drive. If you take one of those puppies apart you will find very strong rare-earth magnets inside.

There is no way around the need for either slip rings or a commutator. As you noted, rotating the wire continuously in one direction will cause the ends to get all twisted unless the ends are free to rotate, as in attached to slip rings or a commutator. An "outside the box" solution to this might be to alternate the rotation, half a turn clockwise alternating with a half turn counter-clockwise, using an overly complicated mechanical mechanism and very flexible wire. In any event, you will also need some way to rectify the output from the coil to produce DC. A commutator is fairly efficient, although difficult to home-fabricate and mount accurately. Slip rings and a bridge rectifier are probably more appropriate.

Good luck producing a sufficiently strong magnetic field and/or spinning the coil fast enough to illuminate a light bulb.

 

[Laplace]

If you spin the magnet, does that produce AC or DC current in the coil? What is an alternator?

If you have a stationary field and spin a coil/commutator, does that produce AC or DC? Does it depend on the design of the commutator?

For a commutator you could put insulating tape on the shaft, then tape metal foil strips over the shaft tape. For brushes, bend some paper clips so they brush against the bare commutator strips. It does not have to look pretty, nor does it need to last for very long - just a few spins to demonstrate that it works.

 

[Minder]

Yes a dynamo has usually been associated with a DC generator, what you described was an AC version (alternator). Similar to what you get when back-feeding a common BLDC motor etc.
Edisons original used a wound field and relied on the existence of residual magnetism, and then pulled itself up by its bootstraps when it started generating.
M.

 

[Minder]

You may have seen bicycle light generators that run against the wheel being refered to them as 'Dynamo's', but being AC, strictly speaking they are alternators.,
M.

 

[hevans1944]

It does not have to look pretty, nor does it need to last for very long - just a few spins to demonstrate that it works.

...and lights a light bulb!

 

[Laplace]

...and lights a light bulb!

LED bulb?

 

[Minder]

LED bulb?

For which you would need DC somehow!
M.

 

[GPG]

hard-disk drive. If you take one of those puppies apart you will find very strong rare-earth magnets inside.

First thing I thought when starting to read
If junk counts as a household item (it does here) then you're halfway there

 

[GPG]

For which you would need DC somehow!
M.

Or two in antiparrallel if the output current is ac, field limited which seems likely

 

[Colin Mitchell]

You will find it very difficult to light a globe with a filament.
The best you will be able to do is illuminate a super-bright LED.
When ever you spin a magnet you get AC.
Slip rings also produces AC.
You need a set of brushes and a commutator similar to a 3-pole motor to produce DC.
This is all very hard to do as the armature has to revolve very fast past the poles to produce enough voltage and also sufficient current to to do any sort of illumination.
You can get a "hand-cranked-generator" in the $2 shop in the form of a torch but it is AC.
You can get a "hand-cranked-generator" in other shops for $10 in the form of a torch and it drives a 3-pole motor.
This is not something you can make yourself.
You won't find a gearbox sitting next to the butter and eggs in the fridge, like your instructor implies.

 

[hevans1944]

You won't find a gearbox sitting next to the butter and eggs in the fridge, like your instructor implies.

I'm thinking a hand-mixer has a gear box, but probably a worm driving two spurs (one for each beater) so it won't work in reverse. You would have to drive the motor shaft after taking the mixer apart and separating the motor from the gears. And crank it really fast if you hope to light an incandescent lamp, and hope there is enough residual magnetic field to "kick-start" the generation, and feed some of the output back into the field coil to produce a sufficient magnetic field. All very Edison-like in finding out all the ways this won't work before (hopefully) finding a way that does work.

 

[Minder]

A typical hand-mixer has a universal motor, and would make a lousy generator, you would need to rewire the fields in parallel with the armature instead of series to get any output.
M.

 

[hevans1944]

A typical hand-mixer has a universal motor, and would make a lousy generator, you would need to rewire the fields in parallel with the armature instead of series to get any output.
M.

I never said it would be easy or practical. The only requirement was it has to be made from "household materials." Can you offer up some other "household materials" that might work better than a re-purposed hand-mixer?

 

[Minder]

I would tend to use a portable hand tool motor of some kind, these are generally P.M. DC motors.
Or any kids toy that has a fair size DC motor in it.
To wind your own from scratch a com can be made from some 1/2" or 3/8" copper pipe and a dowel that will fit the I.D. cut slots in the copper pipe to attach the coils to and use a couple of strong magnets or large Horse shoe magnet for the fields.
Google home made DC motor for examples.
M.