Ball bearing motors – a mystery?

bearingmotor-250.jpgI had my nose in a book about motors (again), and came across something I don’t remember seeing before.

It consists of a couple of ball bearings and a shaft.

You put heaps of current through the shaft via the bearing outers, give the shaft a flick, and off it spins – or so the book says.

It claims thermal effects are responsible for the continuous rotation, but I feel an electromagnetic explanation is more likely.

‘Alice’

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3 Comments

  1. 'Alice'
    January 02, 2013 10:56

    Hi Ian and Gary.
    I had a look at the wikepedia entry and saw the reference to an Institute of Physics article by a couple of Greek scientists: P Moyssides and P Hatzikonstantinou.
    http://iopscience.iop.org/0305-4470/23/14/017
    In the abstract to this, it says :” The various current densities and fields are determined by solving exactly the corresponding Poisson equations, which result from the Maxwell equations. The predicted values for the total power, the efficiency and the various required constants are in excellent agreement with the experimental results.”
    Unfortunately, Electronics Weekly’s budget does not stretch to access the full paper.
    Fingers crossed they have a solution with currents setting up magnetic fields that interrect with the currents and make movement without permanent magnetism.
    The paper points to several other papers on the subject, which I am guessing have alternative explainations.
    This puzzle is starting to look like the pop pop boat puzzle – where research papers get written that don’t actually get to the root of the mechanism.
    ‘Alice’

  2. Gary
    January 02, 2013 10:08

    “Thermal”.. oh please, HOW?
    Dear Alice,
    Yes, the effect is almost certainly electromagnetic and to my impression, the same as a homopolar motor. The inner races form the disk and the balls feed the current in and out of the edges. The two races have currents in opposite directions so to turn in the same sense, they need fields in opposite directions, maybe a magnetised shaft made it work?
    However, it wouldn’t work for very long because the current flowing through the balls would cause pitting, rapid wear and ultimately bearing failure which is a well known phenomenon, and where I came in because (and this was many years ago), my Tutor, Prof. Jennisson had a student project available investigating the phenomenon with something very similar to this.
    Prof. had a very nice demonstration piece with two meshed gears, a crank handle and a galvo. The galvo was connected between the shafts and when the handle was turned, a small current flowed from one shaft, through the meshed gears and out of the other shaft. There was no external static field and the earth’s field had little effect- because the current was proportional to the much higher field of the two gears, which were heavily magnetised.
    Schoolbook homopolar theory states the voltage/torque is generated because current carrying paths cut through the stationary external field when the disc rotates, however the field here was rotating with the disk so apparently nothing was cutting field lines.
    I don’t think anyone took the project (scared the life out of me at the time- too simple to waffle about and too far out to be sure of solving) but it has always interested me. Prof.’s concern was that any rotating machine could become magnetised and damage it’s bearings with the induced current.
    I would be pleased to hear of any explanation of this affect. Remember, the rotating item carries it’s own field which of course rotates with it, or does it?
    Gary

  3. Ian
    January 02, 2013 10:06

    Hi.
    I saw the link to “Bearing Motor” on Wikipedia, but find the “thermal expansion of the balls” explanation implausible.
    A little more digging around and I found this:
    http://www.physics.princeton.edu/~mcdonald/examples/motor.pdf
    The explanation shows roller bearings, but it makes sense:
    The big current Ia along the rotating shaft sets up a magnetic field Ba around it.
    The action of the roller rotating in the opposite direction cuts perpendicularly through this field, the leading and trailing sides in opposite directions, causing a circulating current Ib to flow – which would be around the “equator” of a ball bearing if the contact points are the poles.
    This current Ib creates field Bb which is radial to the shaft. The action of the current Ia in the presence of the field Bb generates torque.
    Reversing the current reverses all the fields, leaving the torque direction unchanged. Reversing the rotation also reverses the fields, changing the direction of torque. This is why it works happily on a.c. or d.c. in either direction!
    Ian

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