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Generator question

  1. Dec 16, 2008 #1

    I am looking to build a generator that goes on a bike stand so when i ride my bike on the stand it will produce electricity.

    Im planning on having a copper coil with magnets inside spinning. This is a dc generator right? Well, anyways... i know if i spin the magnets faster it will produce more electricity. Can i increase the amount of copper in the coil in order to produce more electricity? Like if my coil is 500 ft long and i have 2 1" x 3" x 1" rectangle magnets spinning at 100 rpm i should produce x electricity. If i spin faster will i get a higher voltage or amperage? How much more electricity can i produce if my coil is 1000 ft long and i use the same magnets and same 100 rpm?
  2. jcsd
  3. Dec 16, 2008 #2


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    Science Advisor

    I would think that its a function of the inductance of the coils, which isn't necessarily a simple equation as a function of coils and length. A quick google gives for a circular loop:

    [tex] r\cdot \left(ln \frac{8r}{a} - 2 + Y \right)[/tex]

    Where r is the loop radius, a is the wire radius, and I have no idea what Y is.
  4. Dec 16, 2008 #3
    First: Don't ever attempt a project like this that could potential produce fatal voltages if you don't know what you are doing. You obviously do not have the skills or knowledge to attempt a project like this so I suggest you just go buy something that is commercially available.

    Second: What you described is not a DC generator, but a permanent magnet synchronous machine. In order to get DC you will need to rectify the output.

    Third: You can't model a generators power output with speed alone. Voltage is a function of speed, torque a function of current, and power a function of both. In the case of double-fed machines its also a function of the input electrical work.

    What do you plan to do with this power anyway? Charge a car battery?
  5. Dec 16, 2008 #4
    its like a simple generator but larger. I had made one before in class a long time ago where we wound some copper around a plastic bottle, then spun a magnet inside and had the two ends of the wire connected to a small light bulb and when you spun the magnet it would light up the bulb. Its called induction right? Well all i want to do is build a larger one that can charge batteries, make my tv turn on and other stuff like that.
  6. Dec 22, 2008 #5
    Unless you have a better reason than you have described, save some time and money and buy a commercially available unit. If this is a "relearning exercise" for instruction go for it!!!,.

    Even if you were able to build a generator which produced the voltage and current you require, it would be a miracle if it lasted a week. Without close tolerances and proper bearings it will wear out fast. A used alternator from a junkyard would be a far better source than anything you can make. Commercially available gens and alts are the result of tens of thousands of designs, manufacturing techniques, wire coatings, diode size tests,etc,fan size for cooling,etc,etc.
  7. Dec 22, 2008 #6
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