Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Constant output voltage from a variable output voltage source

  1. Jul 27, 2009 #1
    What i got is a permanent magnet 2 1/4HP@130v DC treadmill motor and everyone know those work awesome as generators. I've also got a 1000Watt inverter which needs an input voltage of 11 to 15 volts DC. Now if the input voltage to the inverter drops below 11 volts it sounds a low voltage warning and will shut off at 10.5v, it will also shut off if the input voltage goes above 15v.

    Now with this motor its easy enough to get 12v out of it but say i connected up the motor with a belt to my bike is there some way i can get the voltage the motor puts out to stay around 12 volts no matter how fast the motor is turning?
  2. jcsd
  3. Jul 28, 2009 #2


    User Avatar
    Science Advisor

    No, not really. If you are almost stopped, you couldn't expect to get 12 volts out. However if you could accept a minimum speed then it might be possible. A switch-mode buck-boost converter might help over a reasonable range of voltages, but 1000 watts is serious power from a 12 volt source.

    The problem is that it would need a fair bit of experience to develop something like that.

    Also, generating 12 volts off load is very different to generating enough power to drive your inverter if it is fully loaded.

    That motor would possibly generate 60 volts or so as a generator if it was running unloaded at its normal speed. So, it is very likely to develop too much voltage off load to drive your 1000 watt inverter and it would become very hard to rotate if it had a heavy load on it.

    Maybe the motor could generate current to charge up a 12 volt battery and the battery could supply power for the inverter, perhaps with reduced load.
  4. Jul 29, 2009 #3
    I know i would have to at least reach a minimum rpm to make it generate the 12volts thats not a problem i just want it to stay around 12volts once it reaches it.

    I've messed around with the motor quite a bit and at 6500rpms it generates 130volts dc which is not surprising because the motors rated top speed at 130v dc IS 6500rpms.
    Also the inverter will never fully loaded with 1000watts it will at most be loaded with around 200 to 300watts. I hope this simplifies things somewhat.
  5. Jul 29, 2009 #4


    User Avatar
    Science Advisor

    There is a lot of information on Google if you type in "human power output".

    THe absolute upper limit is half a horsepower for just a few minutes. 1 HP is 746 watts so half of this is 373 watts.
    For a sustained output, you might be able to get 100 watts but you would soon get bored with it. And you are very likely to lose a lot of it converting it down to 12 volts.

    However, it does seem like the wrong motor for the job. I wonder how an alternator from a car would go if you could get the regulator as well. The regulator is sometimes built in, anyway. It would be used with a battery, of course.

    Maybe you could also have solar panels or a wind generator helping with the charging and you on a bike for those nights when there was no wind (or sun, of course).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook