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Let me try to get this straight.

  1. Jan 30, 2008 #1
    The emf induced in a coil by a magnet is proportional to the velocity of that magnet. The induced emf cannot exceed the emf used to cause the magnet to spin in the first place. Right? So let's say this initial emf source was current from that same loop in which the induced emf will result. Let's say we know the input current to be 1 amp and we allow to the voltage going into the coil to change. We could use a transformer to do this right? Obviously, if that is set to be the case, then doubling the voltage would double the electrical input power. The limit to the motor's efficiency is the back emf relative to the input voltage is it not? Since voltage is magnetic flux change per unit time (i.e. webers per second), shouldn't we expect the magnet to be able to spin faster in rpms proportional to the coil input voltage, assuming that friction was negligible? The problem with this view is that the kinetic energy of the rotor would increase with the square of the rpm, assuming that the magnet rotor has a constant moment of inertia. I am missing something, but what is it?
     
  2. jcsd
  3. Feb 1, 2008 #2
    <The induced emf cannot exceed the emf used to cause the magnet to spin in the first place. Right? >
    That is not generally true. Can you be more specific about your design?

    <We could use a transformer to do this right? Obviously, if that is set to be the case, then doubling the voltage would double the electrical input power.>
    In the ideal case, transformers maintain the same power. As voltage goes up, current goes down and vice-versa. In the real case, a small% of the power is lost to heat.

    I don't really understand your overall design. What is this?
     
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