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Electrical Engine - Something Doesn't Add Up

  1. Aug 4, 2004 #1
    We start with a state where the rotor is aligned with the electromagnets, such that attractive force is at maximum, and potential energy is at minimum. The electromagnets are switched to the next state, and now the force is at minimum, and the potential energy is at maximum. As the rotor approaches the next aligned position, potential energy is gradually decreased from the maximum back to the minimum. This repeats itself over and over again.

    Ok. During each such step, the amount of energy gained is exactly the same as the amount of energy that was lost when switching the electromagnets. But if that's the case, then how is work being produced at all? I know it comes from the external electrical energy source, but how does that work? What energy transitions am I missing here? :confused:
  2. jcsd
  3. Aug 4, 2004 #2
    What about kinetic (rotational) energy? Inertia? Not sure about this :

    "During each such step, the amount of energy gained is exactly the same as the amount of energy that was lost when switching the electromagnets."

    It is not "exactly", a bit more is gained than lost (kept as kinetic energy). This bit more does the work. Just enough to comensate for friction or the work you are doing.
  4. Aug 4, 2004 #3


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    Alpha, it looks like you haven't said anything about energy transfer. If potential energy goes from some maximum to some minimum, where did the energy go?

    [to answer the first question] Potential energy is energy that has the potential to do work. As the potential energy decreases, the kinetic energy of the rotor increases because that potential is being realized: the force is accelerating the rotor. When the electromagnets are switched, no energy is lost: its already been used up. If the rotors were not switched off, what would happen? Force would switch directions, decelerating the rotor, and potential energy would increase again until the rotor stopped. That's giving the energy back.
  5. Aug 5, 2004 #4
    Before the switch, the potential energy is 0. After the switch, the potential energy is U. That has to come from somewhere, so you can't say that no energy is lost here, although perhaps "lost" is the wrong word. If you're saying that this comes from the external electrical energy source, such that you get the transitions electric -> potential + heat -> kinetic + work + heat, then ok. But if not, then I'm still confused...
  6. Aug 5, 2004 #5


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    Yes, lost is the wrong word: the energy comes from the electricity and is transferred to the rotor.

    Think of a water-wheel with a tank of water above it. Fill up the tank and you have a lot of potential energy that gets converted to kinetic energy as it spins the wheel. Every so often though, you have to re-fill the tank.

    What about an electromagnet under a table: place a paperclip near it and there is potential energy between the paperclip and the magnet. The magnet pulls the paper clip towards it, converting that potential energy into kinetic energy. Turn off the magnet just before the paperclip get there and it slides past the magnet. If at the same time you turn on another magnet a little further away, the paper clip will then accelerate toward the next magnet. This is the principle behind the linear induction motor that many monorails and roller coasters use. Its the same as a regular motor, just instead of rotating magnets, they slide past each other.
    Get rid of the heat, it just confuses things: assume no friction and no resistance in the wires. You're left with electrical energy->potential energy->kinetic energy.

    I always had trouble with the idea that a coil of wire around a piece of metal cared what was going on around it, but it does. Use it to move something and electrical energy is consumed. Is that the part you're having trouble with?
    Last edited: Aug 5, 2004
  7. Aug 5, 2004 #6
    Ok. In that case, I'm much less confused. :rolleyes: The water wheel analogy was great. :smile:
    My trouble was understanding the energy transitions. I now understand the what, but I still don't quite understand the why: why is electrical energy being used up here at all? In other words, why doesn't the energy to switch-on the next electromagnet come from the rotor's kinetic energy instead, thus essentially breaking the engine? In your water wheel analogy, if you assume zero friction, you can use the wheel's kinetic energy to pump the water back into the tank, but then you wouldn't be able to produce any work. Why doesn't this happen in the electrical engine?

    Edit: I'm willing to make an educated guess that the answer is "because the rules of inertia and electromagnetism don't allow it"...
    Last edited: Aug 5, 2004
  8. Aug 7, 2004 #7


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    If I understand your question correctly, the answer is because of the boundary conditions. A motor can operate as a generator, a generator can operate as a motor.

    If you feed electrical power to the electrical side of the motor, and the rotor is connected to a passive load, the motor has to be in the "motor" boundary conditions. There's no way a passive load can generate energy.

    If you have the rotor connected to another motor or some other external source of power that turns the rotor, and connect the electrical part to a passive load, you have a generator.

    You can have confusing situations where both modes are possible. This happens for instance when a synchronus generator is connected to a power grid. The intent is for the generator to act as a generator and feed power to the grid. If the generator loses synchronization with the grid, it can act as a motor. This can be a bad thing, so there are some safety mechanisms that will disconnect the generator from the grid if it's operating improperly. For the case of the synchronus generator, the angualr position of the rotor with respect to the magnetic field (the "phase angle") determines whether or not its supplying power to the grid, or drawing power from the grid.
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