h a l f b a k e r yExpensive, difficult, slightly dangerous, not particularly effective... I'm on a roll.
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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)
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]
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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. |
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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) |
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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) |
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//As far as I can tell, this would be the quietest
possible motor (for a given size/power)// |
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Ahem... Build prototype motor... Receive defense
funding. |
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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? |
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*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" |
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//Faraday's...// yes, Michael Faraday's motor, 1822, was a homopolar motor and used mercury. And was the first electric motor! [link] |
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//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. |
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But I still have to get the current across the gap - so liquid metal providing both suspension and conduction seems like a good answer. |
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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. |
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//Faraday's motor, 1822// |
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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. |
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Just remembered something else about this
design- only 1 moving part. |
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// I've done a fair bit of research into liquid metal brushes and
homopolar motors. // |
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// So far, I've found none that use the liquid metal both as a
hydrostatic bearing and a conductor. // |
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// the related ball-bearing motor // |
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I don't see how it's related. It's an electrothermal motor, not an
electromagnetic one. |
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//I don't see how it's related. It's an electrothermal
motor// |
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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. |
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Hey, I've created an electric motor with NO moving parts. |
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Mike Harrison has an explanation of the ball-bearing motor's
mechanism (as well as advice on building and running one):
[link] |
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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. |
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// Two-rotor homopolar motor // |
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I haven't read it in any detail yet, but it looks interesting. |
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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. |
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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). |
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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. |
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