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Electromagnetic induction torque-transferring slipper

  1. Aug 31, 2011 #1
    consider first , an induction motor... a rotating magnetic field is applied to an electrically conductive rotor, and the eddy currents induced will oppose the change in magnetic field experienced by the rotor, hence the rotor gets torque

    in an induction motor the rotating magnetic field is caused by coils of wire having electricity passed through them in sequence...

    but what if the rotating magnetic field is provided by actual permanent magnets placed on a rotating disc?


    this will be useful in applications where the controlled transfer of torque is more important than having the output RPM be equal to the input RPM ... the latter can never happen because induction depends on slipping between the input and output, so it is something like a wet clutch which is supposed to always slip(see the powertrain to the front wheels of the Ferrari FF)

    if you vary the distance between the magnets and the rotor, you can control how much torque will pass to the output shaft

    furthermore, if the magnets and rotor are arranged as concentric cylinders and torque control is achieved by moving the smaller cylinder into larger cylinder, and if the rotor is a squirrel cage design with a slight twist to it so that there is a nett attraction/repulsion differential along the axis of the cylinder (like a screw under torque) which would push the smaller cylinder out of the larger cylinder, it could behave similar to a traditional slipper i.e. the harder you press on it, the more torque goes through... the difference being that it is literally like a screw, the amount of pressure you need to provide will depend on the amount of torque going through, so where input torque and RPM varies this could be a complicating factor
     
    Last edited: Aug 31, 2011
  2. jcsd
  3. Sep 1, 2011 #2
    I think that an advantage of this design over a purely mechanical slipper or clutch is that it is possible to change the ratio of rate of rotation between input and output , in a non mechanical way

    suppose that the squirrel cage rotor is actually made of coils of transformer wire, and that these wires are interconnected to give rise to a number of phases, somewhat analogously to an alternator...
    for example, 15 coils = 5 connected coils x 3 phases

    the difference from an alternator is that terminals of the phases, which would have been the output on an alternator, is instead connected to the collector and emitter legs of a pair of transistors, connected in parallel and in opposite directions

    on the outer perimeter of the squirrel cage rotor, there are additional 'sense' coils , one for each phase and arranged regularly in the circle, whose purpose is to generate a voltage via a passing magnetic field, and the ends of each coil are connected to the base legs of the pair of transistors in each phase

    there are magnetic flux sources on the housing around the clutch, and as the squirrel cage rotor spins and the 'sense' coils pass over these magnetic flux sources in turn, each phase of the rotor will begin to conduct in a sequence, effectively creating a 'rotating conductor' within the rotor itself, analogous to the rotating magnetic field of a motor

    by varying the number of active magnetic flux sources on the casing, the rate of phase switching relative to the rate of rotor rotation can be varied, and the ratio between the input shaft and the output shaft is a simple function of the number of coils and the rate of phase switching

    so, using the example of 15 coils and 3 phases :
    rate of induction rotation = rate of rotor rotation - (rate of phase switching / number of coils)
    = rate of rotor rotation - (rate of phase switching / 15)

    rate of phase switching = number of phases * number of magnetic flux sources
    = 3 * number of magnetic flux sources

    ratio = rate of induction rotation / rate of rotor rotation
    = (rate of rotor rotation - (3* number of magnetic flux sources / 15)) / rate of rotor rotation

    also, there should be a non-interger ratio of number of stationary magnetic flux sources to the number of phases on the rotor
    furthermore, one of the transistors should be NPN and the other transistor should be PNP
     
    Last edited: Sep 2, 2011
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