Half a croissant, on a plate, with a sign in front of it saying '50c'
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Super-quiet Turntable motor

Liquid metal homopolar motor HiFi turntable
  [vote for,

One of the main criteria for HiFi turntable equipment is quietness.

Many turntables suffer from low-frequency rumble from the main bearing.

In addition, electromagnetic noise (either endogenous or from the environment) can be significant.

This idea is a hi-if turntable where the armature floats on a liquid metal bearing. The vertical axis motor has annular permanent magnets, the rotor-armature has a laminar DC current throughout the full perimeter.

Current and magnetic field are invariant with time, so no EMI. Rotor bearing is liquid metal, so no rumble.

Mercury would be ideal, but given toxicity, GalInStan might be chosen. The liquid metal both supports (through buoyancy) and commutates (conductively). Problems with liquid metal embrittlement/solution might be overcome with graphite/carbon contacting materials, perhaps iron. Not gold, silver, aluminium, copper alloys.

Might also find applications in stealthy submarines (where zero noise, zero electromagnetic field variance would be beneficial)

Frankx, Sep 18 2019

Faraday's Motor https://www.rigb.or...list/faradays-motor
1822 - the first electric motor - which was a homopolar motor using mercury as a conductor. [Frankx, Sep 24 2019]

Mike's Electric Stuff: Ball-Bearing Motor http://electricstuff.co.uk/bbmotor.html
Mentioned in my anno [notexactly, Sep 25 2019]

Two-rotor homopolar motor https://pdfs.semant...30ad4a7cf394f86.pdf
Some detailed modelling in this paper [Frankx, Sep 26 2019]


       The prior art worth looking into would include liquid metal slip rings, which are reasonably well baked. They, to my knowledge, usually use precision radially-symmetric surfaces wetted with just enough gallium to make a good contact around the whole circumference. I don't know at what temperature they usually operate, because I didn't look at them in much depth. If you want, I can point you to some things I found recently, once it's more convenient for me to do so.
notexactly, Sep 24 2019

       Thanks [notexactly]. I've done a fair bit of research into liquid metal brushes and homopolar motors. So far, I've found none that use the liquid metal both as a hydrostatic bearing and a conductor. That is - the rotor floats on a pair of liquid metal-filled concentric channels, and the current flows via the liquid metal to/from the rotor. The reason for doing this is to eliminate "hard" brushes (eliminating mechanical/acoustic noise and wear), to eliminate a "hard" support bearing (again, noise and wear), to have an homogenous current sheet through the liquid metal and rotor, by having "continuous liquid brushes" around the whole circumference - invariant circumferentially and with time (to eliminate EMI), and to have an invariant magnetic field (again, to eliminate EMI)   

       The motivation being to eliminate all sources of mechanical/acoustic and electromagnetic noise within the machine. As far as I can tell, this would be the quietest possible motor (for a given size/power)
Frankx, Sep 24 2019

       //As far as I can tell, this would be the quietest possible motor (for a given size/power)//   

       Ahem... Build prototype motor... Receive defense funding.   

       edit: clearly I didn't read the last sentence... Anyhow, I had a think about the problem and came up with an idea of using a magnetic bearing. 2 minutes googling later, it turns out they're stabilized by active electronics doing noisy switching, nasty. Anyhow, it turns out there are passive magnetic bearings, and audiophilles were onto those in the 60's. Anyhow a couple of questions 1: is there a liquid metal that approaches being safe? b: after reaquainting myself with the elegance of the homopolar motor* I wonder how you would achieve speed control?   

       *I can't find any practical use of this a motor should do work, supply motive power. Therefore I propose it be renamed "Faraday's Mercuric Whirligig"
bs0u0155, Sep 24 2019

       //Faraday's...// yes, Michael Faraday's motor, 1822, was a homopolar motor and used mercury. And was the first electric motor! [link]   

       //magnetic bearings// I have looked at maglev for rotor support - either it's controlled by active electronics (which I want to avoid) or as you say, passive magnetic bearings.   

       But I still have to get the current across the gap - so liquid metal providing both suspension and conduction seems like a good answer.   

       Is there one that's safe - well, GalInStan is regarded as safe and is liquid at useful temperatures. There are problems though: it's lower density than mercury, so more would be needed to achieve sufficient buoyancy; it causes a solution/oxidation /embrittlement problem with some metals (as does mercury); and it oxidises over time. Speed control is a challenge. Active control (with electronic sensors and a negative feedback loop)... would probably give rise to some EMI, so not ideal. There might be an elegant solution where the swirling of the liquid metal reduces the current-carrying section, increasing resistance and reducing current. It may be that once the machine has spun up, it achieves a fairly stable speed, but as the machine warms up, resistance is likely to increase. So, still a problem to solve. Prototype - yes, good idea.
Frankx, Sep 24 2019

       //Faraday's motor, 1822//   

       I can get my head around that, but the related ball-bearing motor, which has no magnet, has been filed away under "witchcraft" to protect my ego.
bs0u0155, Sep 24 2019

       Just remembered something else about this design- only 1 moving part.
Frankx, Sep 24 2019

       // I've done a fair bit of research into liquid metal brushes and homopolar motors. //   

       Oh good.   

       // So far, I've found none that use the liquid metal both as a hydrostatic bearing and a conductor. //   

       Me neither.   

       // the related ball-bearing motor //   

       I don't see how it's related. It's an electrothermal motor, not an electromagnetic one.
notexactly, Sep 24 2019

       //I don't see how it's related. It's an electrothermal motor//   

       I have no idea how a thermal system would create rotation either. I can think of a couple of ways of testing the phenomenon though. 1. heat it up beyond the Curie point 2. Run the thing on high frequency AC. If it's thermal 1&2 will both still work.
bs0u0155, Sep 25 2019

       //to protect my ego//   

       Hey, I've created an electric motor with NO moving parts.
lurch, Sep 25 2019

       Mike Harrison has an explanation of the ball-bearing motor's mechanism (as well as advice on building and running one): [link]   

       It runs fine on AC and DC, but I think the bearings would seize at high temperatures—it seizes after a few seconds of normal operation just due to overheating. (Blowing air through the bearings may help it run for longer.) It runs in whichever direction it was started in, regardless of AC or DC power.
notexactly, Sep 25 2019

       // Two-rotor homopolar motor //   

       I haven't read it in any detail yet, but it looks interesting.   

       It reminds me of what got me interested in homopolar generators: I wanted to build a Crookes radiometer that would turn a homopolar generator to light an LED. Homopolar generators are generally better at current than at voltage; I was imagining that it would have enough voltage for a red LED at least, and then the current would be limited by the extremely low torque of the radiometer. This would of course need extremely low friction.   

       Anyway, what it reminded me of specifically was that Nikola Tesla came up with a low-friction design for a homopolar generator with two wheels side- by-side, their circumferences connected by a metal belt, with the slip contacts at the centers of the two axles (the magnetic fields being set up such that the two wheels were electrically in series).   

       But I wanted a single wheel, so I was thinking of using a scrollerwheel or similar, which hadn't been invented back when Tesla was alive. But that would seem to limit the generator wheel's diameter.
notexactly, Sep 26 2019


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