Building a Magnetic Generator
Introduction
A magnetic generator is, in essence, a device which converts mechanical energy into electrical energy. By taking advantage of the laws of electromagnetism, in which electric current is induced by a moving magnetic field, we are able to translate the motion of moving a coil of wires past magnets into usable electrical energy. In our particular device, the energy is used to power a series of LEDs.
As a side note on the POV LEDs: we're just using the SpokePOV kit from Adafruit Industries - works wonders!
photo credit: Andrew Williamson
How it works
The magnetic generator’s base consists of a stationary circular shell, within which is a freely rotating disc. On the inside of the stationary shell are 4 magnets, placed 90 degrees apart from each other. On opposite ends on the edge of the rotating disc, we have two coils of wire. The wires then connect to a board of LEDs. Note that in this image, for demonstration purposes, only one coil of wire and one magnet is shown.
A gear on the lower part of the generator controls the rotating disc. By turning the handle on the gear, the user rotates the disc inside the shell. As the wire coils pass by the magnets, current is induced in the wire, thereby powering the LEDs. The process in which electricity is generated by a moving magnetic field is known as Faraday’s law.
In the next section, we will show you how we used Faraday’s law to determine the number of turns of wire needed in the coil.
Faraday's Law
We used Faraday’s law in order to figure out how many turns of coils our generator required to power the LEDs. In common terms, Faraday’s law states that a change in the size and strength of a magnetic field can induce an electrical current in nearby wires. In mathematical terms, this is the equation:
Where
V is the voltage generated (measured in volts)
N is the number of turns of wire
B is the magnetic field (measured in tesla)
A is the area of the magnet (measured in meter squared)
t is the time (measured in second)
Using this equation...
We knew that in order to light up our LEDs, we needed 6V of voltage, therefore:
V = 6V
Additionally, our magnets were rated at 11,000 G (gauss) or 1.1 T (tesla). Because there is a small distance between the coil and the magnet, the strength of the magnetic field is slightly decreased. For our purposes, we will divide the rating by 2, therefore giving us:
B = 1.1 T / 2 = 0.55 T
Since there are a total of four 2” x 1” magnets, the total area is 8 square inches or 0.00516 square meter:
A = 0.00516 M2
We measured on our wheel that there were 10 revolutions per second on average, therefore the time it takes for 1 revolution is:
t = 1 s/ 10 = 0.1 s
Now that we have all (well, all but one) the values in the equation, we just have to rearrange Faraday’s law and plug in our values to find out how many turns of coil we need:
N = -1 x (-6 / ((.55x.00516)/.1))
N = -1 x (-6 / (0.002838/.1))
N = -1 x (-6 / 0.02838)
N = -1 x -211.41649
N = 211.42
So, this equation tells us that we will need ~212 turns of wire in our coil. Using two coils, each coil will need 106 turns.
Our Generator in Action
Resources
How to build a magnetic generator: http://www.6pie.comUsing Faraday's Law: http://www.6pie.com/faradayslaw.php
Understanding the minus sign in Faraday's Law equation: http://physik.uibk.ac.at/04-05/erde/spezial/aufgaben/FaradaysLaw-sign.pdf
Feel free to contact us if you have any questions about these!