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How do motors work?

  1. Aug 12, 2011 #1
    The problem statement, all variables and given/known data
    How do motors work? Is what I have right so far?
    The attempt at a solution

    Generators: I understand that a coil moves and it cuts across the magnetic flux lines in a magnetic field, emf is induced. So mechanical energy (coil moving) changes into current from the emf (electrical energy).

    Motors: A current exists in the coil and it will experience a force in the magnetic field causing it to turn. When it turns, it will 'become a generator' and induce it's own emf. This emf will be in the opposite direction of the source emf... that's why it's called counter emf... So then what happens?
     
  2. jcsd
  3. Aug 12, 2011 #2

    PeterO

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    remember that induced currents/fields never win.
    There may be a "counter emf" induced, but it will never overwhelm the applied Emf that is causing the motor to operate.
     
  4. Aug 12, 2011 #3

    cepheid

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    Yeah, the whole point is that the charges being carried in the coil experience a Lorentz force due to the external magnetic field in which the coil has been placed. Therefore, electrical energy is converted into mechanical energy (of rotation of the coil). For DC motors, you need commutators to continually switch the direction of the current in the coil so that the rotation direction is always the same (even after the coil has undergone half a turn). But basically, a motor is just the exact opposite of a generator.

    Generator: supply the movement: produce the electricity

    Motor: supply the electricity: produce the movement
     
  5. Aug 12, 2011 #4
    Oh I get it!
    So basically, "A current exists in the coil (electrical energy) and it will experience a force in the magnetic field causing it to turn. (mechanical energy)"
    The counter emf thing is sort of what happens like a side effect. The counter emf would never overwhelm the original emf. It threw me off a bit.

    Thanks!
     
  6. Aug 13, 2011 #5

    cepheid

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    Well, never say never. I think that the back emf depends on the rotation speed of the motor. There is a certain speed above which the motor will saturate i.e. dumping more current into it does nothing. So the back emf sets an upper limit on how fast you can drive the motor. But this is usually pretty fast.
     
  7. Aug 13, 2011 #6
    One more question, what is the relationship between the speed and the back emf?
    I assume directly proportional? How would I be able to calculate the back emf, knowing the frequency?
     
  8. Aug 14, 2011 #7

    cepheid

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    The only experience I have with this is that the data sheets for some motors I've dealt with give the "torque constant" (constant of proportionality between motor current and motor torque) and the "back EMF constant" between motor speed and back EMF. So it would appear that they are directly proportional (or at least this is a good enough model for most purposes). I don't know how you'd calculate it theoretically: I imagine that it would be tough to do because it would depend on the specific geometry of the motor windings etc. Sorry I couldn't be of more help.

    Maybe you could drive the rotor mechanically and attempt to measure the back EMF?
     
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