Tagged: design

Better electric generator design and my shake generator

Background on generators for beginners:

It turns out that if you move a magnet past a wire loop, an electrical current will be created around the loop.  This is how generators work.  Going the other way, if you have an electrical current in a loop, it cause a magnetic field which will exert a force on the magnet. So while the magnet is causing a current in the loop, the current is causing a magnetic field that exerts a force on the magnet in the opposite direction of its motion. What is happening is that while the magnet is getting slowed down, that energy is being transferred to the current in the wire. This partially explains why perpetual motion machines don’t happen in electromagnetic systems. Energy can only be converted from mechanical to electrical energy and back but never created.

Better generators:

What is the ideal generator? First you have to figure out what you’re trying to optimize for. What I want is a high power output given a constraint on the number of magnets that I have. I’m assuming that wire is unlimited.  If I were being more thorough, I would look at the cost of each material and optimize the power with respect to the cost.

Before I get to better generator design, let me tell you how I got started on this. I had this image in my head of little tiny units of energy generation. Little generators that would create voltages when they vibrated. They’re called shake generators and I posted about this previously. I imagined hanging them from trees or placing them in a stream or really anywhere where there is a lot of shaking going on. I tried this out by taking a tube, actually a pen, and wrapping a whole bunch of coils around it. The diameter of the pen was about 5mm.  By the time I finished wrapping 1000 times, my coil had a diameter of 20mm. Horrible.  Even then my neodymium magnets only produced a tiny voltage when I shook it really really hard. Fail. This was also happening at 2am which probably left my roommates wondering what the hell I was doing in my room.

Here are the problems with small generators:

1. If I want to keep the radius of the loops small, I have to use thin gauge wire which means more resistance.

2. The voltage around a loop is proportional to the area times the rate of change of the average magnetic field within your coil.   V_{loop}=-A\frac{dB_{avg}}{dt}. So small loops mean a small voltage.

3. Bigger loops with bigger magnets lead to less resistance for the larger generated voltage and hence to a higher power output.

To explain point three, that bigger loops with combined magnets is better, imagine that you had 2 square magnets and two square loops of the same size and that the loops are in series so that the voltages add.  I’m assuming the magnets are moving to the left over the top of the coils.


There are 2 squares, each square has 4 sides and so there are 8 sides. By combining the two square magnets and making a bigger loop, you get rid of one of the sides,  reducing the length of the wire which then reduces the resistance..  Plus the combined magnets produces a larger magnetic field and thus a higher voltage. Though I wonder how much each magnet demagnetizes the other when placed side to side?

So I’ve come up with some principles to work by in making my shake generator.

1. I need wider magnets with bigger coils and so a larger shake generator.

2. The magnet needs a weight attached to it so that it can carry more kinetic energy. Depending on cost,  I might just use a very large magnet.

3. I should try to convert slow mechanical motion, the shaking, to very fast motion, which would increasing the rate of change of the magnetic flux and thus increase the voltage. I might do this mechanically with gears and levers. I was also thinking of putting a magnet on a wire and using torsion or vibration to create a high frequency vibration..

Finally, if anyone sees any mistakes or improvements that can be made, please let me know.