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Electromechanical considerations

  1. Jul 14, 2009 #1

    Pythagorean

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    (please skip straight to the question if you don't want to read the setup)

    Backstory:
    I'm playing with a motor, both as a motor and a generator. It's from a treadmill, 90 V, 6 Amp.

    Observation:
    I hook the motor up to a power supply. Apparently it's an independent power supply, because I have little control over the current (it seems to me like I can set the maximum current, but in order to keep the voltage constant, the current changes).

    If I put resistance on the rotor, the current goes up. To accelerate the rotor, the current goes up (like when you first turn it on, to overcome the inertia.

    I've since refreshed myself on the dependencies between angular velocity, torque, current, and voltage.

    Question:
    I rigged the motor up as a generator (it worked, so it must not be induction) and power a small light bulb, using bicycle pedals, but it's too easy to spin the rotor, so I'm all velocity and no torque.

    If I put a bigger load than a light on the motor, will it put back resistance on the rotor so that I have to apply more torque to get it going?

    And, as my intuition suggests, will this supply more current (i.e. my power will more in torque than in angular velocity)?
     
  2. jcsd
  3. Jul 14, 2009 #2
    If you want more power just increase the angular velocity of the generator by gearing it differently. Use a high gear ratio.
     
  4. Jul 14, 2009 #3

    Pythagorean

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    I framed the question under constant power. It's the velocity and torque that would be changing for the same power.
     
  5. Jul 14, 2009 #4
    ok ... sorry i dont know if i get what you are saying but the way i see it you want the same power output and different tourqe applied on the pedals right?
     
  6. Jul 14, 2009 #5

    Pythagorean

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    yeah.

    My inevitable question will be, "how would I make a break that breaks from the electrical side (i.e., by braking, I generate power)" but I'm building up to that.
     
  7. Jul 16, 2009 #6
    Oh cool...well i don't know much about generators but i don't think that applying more electrical load to the motor will effect the tourque required to turn the motor at all. I mean when you turn the motor by hand and then apply more load ... does it get harder to turn? I don't think it would.
     
  8. Jul 24, 2009 #7

    stewartcs

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    What you are describing is called dynamic braking. You basically dump the electrical energy into a resistor bank which causes what ever mechanical device you are using to slow down. Most motor/generator books will have a section on dynamic braking.

    CS
     
  9. Jul 24, 2009 #8
    The motor is a dc brush motor with a permanent magnet stator. To operate the motor as a generator/dynamic load just connect various resistances. I will guess that the optimum RPM is 2000 to 3000 RPM. You shouldn't exceed 6 A load current, or you will exceed the current rating of the brushes and rotor coils. You should be able to get several hundred (200-300) watts out of it. The simplest load(s) would be 120V incandescent lamps with a 100-watt rating. It would be useful to know how much RPM is required to get 90 volts into an incandescent lamp. A better dynamic load would be several parallel NPN bipolar transistors or N-channel FETs (on good heat sinks) connected to the incandescent lamps. This way you could adjust the load setting with a knob.
     
  10. Jul 24, 2009 #9

    Pythagorean

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    Excellent. Thank you. A lot can be done knowing a term (i.e. "dynamic breaking"), makes research a lot easier! The mechanical device will be human legs, but currently it's too easy to turn, so you don't get the full power out of your legs you could if (I assume) a bigger load is put on the generator and more torque is actually required. Your legs have a top speed they can go regardless of torque, so after a certain point you can only get more power from torque.

    Yeah, I've thought about designing a control for it, I just wanted to make sure my theoretical understanding was sound. Thank you for your input!
     
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