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Superconducting motor thought experiment/ questions

  1. Dec 29, 2016 #1
    I am sorry but this question I have described is a long one.
    Ok so before I start a quick background , I have read and understand the Meissner effect of the superconductor expelling it’s magnetic field , also I understand that superconductor has zero resistance so a once induced or applied current can run forever in the superconductor unless it’s cooling fails or the current becomes so strong that the magnetic field collapses right?

    Now here is the interesting part , I read that superconductors apart from the magnetic field and zero resistance still obey Faraday’s law so if a current is already running in a superconducting coil then changing that current would still get a back EMF reaction and the magnetic field would oppose any change in current right ?

    So here are my questions. If we could cool down an ac induction motor so that it’s rotor armature and stator coils reach zero resistance , what would change ? I assume the motor would become more efficient and more powerful. But what would happen with its power consumption if it’s driving a fixed load? Normally I understand that as the motor drives a load its rotor is slowed down which creates more slip and less back EMF in the stator windings and allows more current to pass through them creating a stronger magnetic field which in turn creates more current in the rotor creating a stronger B field and driving the motor harder so that it can turn the load , but if there is zero resistance in the windings what does it mean , a larger current drawn still has zero resistance so what is then the mechanism for power consumption from the electrical load from which the motor is running ?

    A similar question yet bit different. I know some stuff about homopolar motors/generators.
    They are the only true DC power machines since all other DC motors use either a mechanical commutator to switch currents or semiconductor devices.
    So what happens here , a superconducting homopolar motor is running basically a disc with an axial permanent magnet behind it and a piece of wire stationary with respect to the disc connected via brushes, let’s assume the brushes are lossless or have zero resistance as the disc. The whole system is in a superconducting state and represents a zero resistance DC current loop and experiences no EMF since there is no changing magnetic field present anywhere in this system and also no voltage as the current is DC and has zero resistance. What happens when a DC current is introduced in this loop? The disc starts rotating , but since there is zero resistance in the loop the current doesn’t dissipate so it just keeps flowing, now what happens when we introduce a load on the rotating shaft , does the current decreases in any way
    This puzzles me because I cannot see any mechanism by which the current could be dissipated or influenced by attaching a load on the shaft which basically means an overunity device except for the power needed for cooling to keep the superconducting state.
  2. jcsd
  3. Dec 29, 2016 #2


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    If the stator resistance is zero, the motor will be more efficient but if the rotor resistance is zero, motor wouldn't start at all.
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