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Electric load: Some explanation?

  1. Dec 2, 2015 #1
    A bicycle powered generator uses mechanical energy to create electrical energy, yes? I pedal, and some form of coils and magnets move around and around and generate electricity. This I understand.

    Upon pedaling at "x" rpm's, I'll produce a certain amount of watts of electricity, correct?

    If I have a light bulb or other appliance attached to the electricity generated, if its enough power, the appliance will function as normal because i'm supplying the electricity.

    IN the following video, the man is pedaling and producing a certain amount of electricity, upon turning on the appliance, you can see him slow down, as if someone attached a weight to the pedals and it takes him a few seconds to get back up to speed.

    This is load? How does the initial start-up of an appliance "pull" against him, or add this effect of "load" to him?

    Aren't his pedals spinning the motor freely and electricity is being produced, if the machine uses all of the electricity it starts slow until more is produced, however it seems in my mind that it is "pulling" the opposite way inside the motor with the coils, magnets etc. Its like an "anti-generating" effect.

    I don't understand this?

    the video is below, see minute 6:17

     
  2. jcsd
  3. Dec 2, 2015 #2
    A few domino questions, would this load effect be present when hooking up the bicycle to rechargeable batteries? would it be like pedaling under a load?

    If I had a water turbine producing electricity at a constant rate, and I hook up an appliance (or batteries) to the electrical output, would it put a "load" on the turbine and make it spin slower?

    This might be answered in answering the first post. so I'll stop until someone posts :D btw, I searched all over google in a reasonable fashion and found out about load, but nothing answering my questions. thanks!
     
  4. Dec 2, 2015 #3

    davenn

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    but only when a load is attached and current flows

    with no load connected, there is a voltage being generated across the generator terminals but not current is flowing
    ONCE a load is connected , current flows, the amount of current is determined by the voltage across the load and the resistance of the load
    For a fixed voltage, if the resistance of the load is dropped to a lower value, the load is said to increase and more current flows and more power is dissipated in the load
    eg lets say 10V is generated across a 100 Ohm load that equals 0.1 A flowing
    0.1A x 10V = 1Watt of power used

    now we lower the resistance of the load from 100 Ohms to 10 Ohms ( a much heavier load)
    10V / 10 Ohms = 1A of current flowing
    10V x 1A = 10W of power being dissipated by the load

    that means its going to take him more effort in peddling to generate the extra power dissipated by the load


    Dave
     
  5. Dec 2, 2015 #4

    davenn

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    yes

    yes .... so in a power station the speed of the turbine/generator needs to be kept constant to maintain the output

    if that generator is unable to supply the required power, then additional generators will be brought online to supplement the output
    ( say the difference off-peak and peak power requirements for a city)


    Dave
     
  6. Dec 3, 2015 #5

    russ_watters

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    Load is load. No, the motor (you mean generator) is certainly not spinning freely: it is a load! Power is torque times rpm.

    How? A generator and motor are exact opposites. One uses electromagnets to produce a force and the other uses a force to produce electricity with magnets.
     
  7. Dec 3, 2015 #6

    CWatters

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    If you assume the bicycle and generator are ideal then... Without a load/appliance it takes zero torque to turn the generator. As Russ said... Power = torque * rpm. So without a load it takes zero power to turn the generator. At this stage the man is not generating any electricity.

    When a load is connected it takes more torque to turn the generator. The man pedalling may or may not slow down depending on his ability to deliver the combination of torque and rpm required. He might be able to generate the torque but not at the same rpm as before, in which case he goes slower.

    Real (eg non ideal) bicycles and generators do require some power to turn them even when not generating electricity (due to friction etc) and this can be misleading.
     
  8. Dec 3, 2015 #7

    CWatters

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    In effect that's exactly what is happening. Inside the generator there is a rotating magnetic field which makes electricity flow through the wires to the load. But this works both ways. The load current creates it's own magnetic field that opposes rotation. If the load current is increased by connecting another appliance the increased magnetic field slows the generator or rather it increases the torque required to spin it at the same rpm.
     
  9. Dec 8, 2015 #8
    Sorry for late reply, was out of town on work for a few days.

    Thanks everyone for the replies.

    That makes a lot of sense,
    Thanks again for the explanations, I'll now run my brain through this and see where I end up :)
     
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