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Magnetic Levitation : Confusion with Lenz's Law

  1. May 6, 2015 #1
    Considering the above diagram, I am failing to see why the aluminium ring floats upwards rather than oscillates. I understand Faraday's law, but I am struggling with Lenz's law - as there is an alternating current, an alternating magnetic field is produced, similar to a sine wave. Therefore, I assumed that the flux through the iron core can also be modeled as a sine wave, with the flux increasing and decreasing with the magnetic field.

    As the flux increases through the ring, I understand that a current will be induced which opposes the increase in flux, i.e. a current which would produce a force upwards. However, in the case where the flux is decreasing i.e. from 90 to 180 degrees on a sine wave, it is my understanding that the ring will want to "increase" its flux, opposing this change and this would be a force downwards, rather than upwards so hence producing an oscillation.

    However, I am given that "When the alternating current supply to the coil is switched on, the aluminium ring moves up the rod until it reaches a stable position ‘floating’ above the coil".

    Can anybody shed any light on the situation?

  2. jcsd
  3. May 6, 2015 #2


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    Gold Member

    The ac-supply is a 1-phase connection, that is inducing a sine-waved magnetic field in the iron rod. This field has no direction ( like a pendulum swinging back and forth, but is getting nowhere.

    When you are switching on the power, I assume that the ring is placed upon the coil, so the ring cannot accelerate downwards. So when the ring is oscillating in field, the only direction it can take is upward. Having this initial speed-direction upward, the frequency of the current induced in the ring will fall if the ring continues accelerating upward. Contrary if the ring turns downward the frequency of the induced current will rise. Due to the self-inductance in the ring, the impedance of the ring will be proportional to the frequency, so the current induced by moving upward (not turning) will be greater than the current induced when turning downwards. Actually both currents are induced, but the "upward-current" will win the fight as it has the lowest frequency

    If you have an asynchronous motor driven by a 1-phase system, it does not know which rotational direction to choose, when power is switched on.Therefore such a motor is made with an extra start-coil with a start-capacitor attacked in series. The current in the start-coil will be phase-shifted due to the capacitor, thereby making a rotational magnetic field in the motor. When the motor has accelerated to some speed, you can disconnect the starting coil, and the motor will continue in the same rotational direction.
    Last edited: May 6, 2015
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