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What causes the back emf in motors

  1. May 27, 2009 #1
    What causes the back emf in motors (brushless DC motors to be exact).

    First thing is the rotating magnetic field of the rotor, which cuts the stator coils inducing an emf in the opposite direction. This would increase as speed increases. This is just a generator.
    Second, what about self inductance. You are switching the stator coils at a higher rate to gain more speed. Self inductance would limit the voltage.

    Do both together contribute to back emf?
  2. jcsd
  3. May 28, 2009 #2
  4. May 28, 2009 #3
    as for the generator aspect, yes. the motor becomes a generator, and the opposing currents fight against one another, causing the dc motor to max out it RPMs. as for self inductance, i think so, but am not sure.
  5. May 29, 2009 #4
    A conductor moving in a magnetic field will generate, by virtue of inductance, an opposing magnetic field.
    This is back EMF.
    If this were not the case, the conductor would rapidly accelerate and perpetual motion machines would be possible and the universe would simply not work as it does.
  6. May 29, 2009 #5
    I am rusty on this but I think there are self inductance effects in the rotor only when it is changing speed i.e. when the current changes.The current has its highest value when the motor is switched on but reduces as the speed and the back emf increases.When it reaches a top speed the input power is equal to the sum of the useful output power(work done per second against the back emf) and the joule heating
    (I^2R)power losses.
  7. May 29, 2009 #6
    The VALUE, or amount, of inductance does change with the changing speeds of the rotor.
    But inductance will occur regardless of speed changes.
    As long as there is ANY movement of a conductor relative to a magnetic field(or vice-versa), inductance will occur.
    Speed changes, as such, are not required. Movement is, even if it is a smooth constant speed.

    Hopes this makes any sense.
  8. May 30, 2009 #7
    I agree pallidin but I think we are at cross terms because of the terminology used.In addition to the inductance you refer to there is self inductance due to the changing current in the rotor itself and this sort of inductance does not need something else providing an external magnetic field.If we have say a d.c.circuit and switch it on, the current does not rise instantaneously to its final steady value but there is a delay due to the growing field around the circuit elements generating a back emf,this opposing the rising current.When you switch off the collapsing field tries to maintain the current.What you said is right but there will be self inductance effects when the rotor speed and current changes i.e. at switch on or off or when the motor loading changes.I think.
    Last edited: May 30, 2009
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