Low Corona Slip Ring

You didn't say how fast the rotation is?

How many continuous degrees of rotation are necessary?

Can it then reverse and "unwind" ?

Are there physical limitations ?

Is this for a contamination test fixture ?

Some slip rings use mercury wetted contacts, no sparks or noise.

They may be used for thermocouples, but these will go to 240V

A 150KV slip ring could be fun to design!

But 'safest' would be to use good flexible cabling, especially if you mean 60° rotation.

There are a number of gas-filled (HF₆ probably) slip rings available for very-high-voltage service, but these would be overkill for your application. The fact that your whole assembly runs at line voltage simplifies the problem considerably.

For 150+ kV service all your convex surfaces will have generous radii where they turn or curve. Smaller radii and you get corona from the higher field density at those points - but not if you put a Faraday cage around them!

Assume for a moment that I have a rotating shaft which must be connected somehow a stationary conductor. The conductor and the shaft are conveniently perpendicular at the point where they meet.

I should probably warn you about the little-known physical law (not just a mere theory, like Einstein's) which states: "One component of every slip-ring assembly shall have sharp edges and square corners that maximize corona discharge in high-voltage applications."

(This law is nearly as fundamental as the one about forum threads: "The probability that some idiot will randomly use the word "cat" in a post approaches unity as the number of posts to that thread increases.")

Our slip-ring's 'brush' consists of one or more small metal wheels or rollers which ride directly on the shaft. The axles are hooked to the conductor. We'd prefer to use multiple wheels because, as everyone knows, electrical contacts - even rolling ones - get dirty over time and become intermittent. Multiple wheels would help to ensure that at least one wheel was in electrical contact with the shaft at any given time. Oh, and I'd spring-load the wheels.

The Faraday cage is built out of two spun-metal hemispheres which form the 'end caps' of a squat, sheet-metal cylinder. At the "pole" of each hemisphere is a hole large enough to clear the shaft which passes through the assembly, right down the center. The whole thing has the shape of a 'stubby' Tylenol capsule.

For its part the conductor leads out through a hole in the side of the cylinder, or it is attached to the cylinder wall directly. Doesn't matter which, really, so long as the connection point doesn't introduce any sharp points or edges (like the head of a bolt).

If care is taken in its design and assembly, the exterior of the Faraday cage will be smooth, curvaceous and have no sharp edges. As far as the electric field is concerned, the cage is just another run-of-the-mill HV terminal. The field need never (and won't) know the faux terminal is a Faraday cage with a home-grown slip-ring inside, Texas-style.

Inside the cage it makes no difference at all whether there are pointy, sharp things or not; because inside a Faraday cage there is no electric field. And where there's no electric field, there'll no corona.

But I will settle for a Dos Equis!

<Waiter!!>

Hi,

first question: why slip-ring, above mentioned mercury wetted contact can be a mercury pool as wetting most often is done with very small contact areas.

Mercury will readily evaporate to toxic levels at room temperature, so think about encapsulation or cryo-traps.

second question: which current?

third question: (as asked above) why not flexible cabling (or flexible joints) for limited movement. If mechanically engaging and disengaging this can be upgraded to a slowly moving slipring without any slip.

4th question: application for this slip-ring? , let us learn something, not only ask for solutions. The interest in answering questions in this beautiful forum of a wide variety of topics is fueled by the interest to learn something new and unexpected.

The biggest slipring I ever have seen was in Tucson, Optical Science Center feeding the rotational glass furnace intended to cast 14m astro-mirrors. This transmitted some MW of power but high current medium voltage.

RHABE

RHABE mentions: "The biggest slipring I ever have seen was in Tucson, Optical Science Center feeding the rotational glass furnace intended to cast 14m astro-mirrors."

And was the business end of the robot arm that grinds those mirrors still bonkers when you were there, or had they fixed it by then?

Hi Europium,

I did not see a polishing machine suited for these 14m mirrors, but the machine for grinding and polishing the 8m mirrors was running on tracks like railroad but 10 to 12m apart, on the traversing bridge a carriage with horizontal and vertical movement to carry the grinding and polishing tools.

In total you are right: a lot of nonsense and waisted money at a time when planning for the first big multi-mirror telescopes was discussed. (1986..1990)

(Lufthansa has a robot much longer that is designed to clean the airplanes from the outside. That is equipped with a big rotating soft wheel that can be oriented flat to the surface.)

RHABE

Hi Guest (please log on...),

why do you need a slip-ring? They're pretty painful, particularly if you want a PD-less connection. probably not possible, even with mercury etc. etc. Having measured a LOT of LARGE PD signals from shields & stuff that is BOLTED (just not...tightly bolted) down, if a HV connection is intentionally allowed to move ,it'll produce PD.

Sort of along the lines Europium proposes (but I'm not going to write that much )
Besides, polished alu hemi/sphers COST A FORTUNE (because they're dang hard to make)

Since you're only rotating 60°, why not just use a piece of wire, securely attached to both condutors, and then shield it. QaD (Quick and Drity). For shielding, we usually use aluminium 'crinkle' flexible dryer tubing..works like a champ. I guess the 6"/15cm dia will be plenty for only 150kV. Rem to secure IT to both conductors too, or IT will produce PD

I've used lots of sort of 30cm dryer tubing, that's good to several hundred kV.

Please keep us posted and if you want, send me an email.

Best Regards,

RF_Guy

For the experts:

Since this is for a single conductor and the rotation angle is limited, I was thinking a flexure pivot, or pivots assembly could work.

Hey RF,

You're right about the cost of polished aluminum hemispheres. Hemispheres meant for HV terminal service, that is. They're so bloody expensive because it's a niche market, and a small one at that.

But aluminum hemispheres needn't be expensive, nor are they, nor are they hard to make at all. In fact, you see them all the time: colanders! (and yes, colanders have holes. They wouldn't be colanders without them). However, those holes needn't be a problem so long as they are de-burred and with have rounded, inward-sloping edges. Low-to-medium-speed orbital polishing does the trick when used with a soft pad and a fine (>600) abrasive paste or slurry.

Next we have S.S. and aluminum mixing bowls. Mixing bowls galore, in nearly every size you can imagine. Those exorbitantly-priced spun-aluminum terminals? Mixing bowls. Mixing bowls, sans flat bottom (we were gonna cut a hole there anyway, non?).

Mixing bowls and colanders (sans holes). Pretty much what you'd see in the housewares section at a modern department store.

---

[scene opens with H. V. Engr (Chapman) gingerly placing four aluminum colanders and ten aluminum mixing bowls on checkout conveyor while the Checkout Clerk (Cleveland) looks on.]

Checkout Clerk: "Plannin' to do some serious cookin' this weekend?"

H. V. Engr: "No, miss. I'm a physicist, not some bloody cook!"

Checkout Clerk: "Watcha gonna do with all those things, then?"

H. V. Engr: "Right. Look. If you simply MUST know, Ms 'Please-Mind-Your-Own-Business,' I am constructing my own, personal Cockcroft-Walton linear accelerator."

Checkout Clerk: "A wha..?! What's a croftcock 'celerator or whatever you said?"

H. V. Engr: "A device I will use to neuter my neighbor's cat! Damn! It's a bloody particle accelerator, you coffe-nosed, malodorous git!!! Why are you pelting me with all these silly questions? I wasn't expecting a kind of Spanish Inquisition!!!"

[JARRING CHORD]

Checkout Clerk [mutters under breath]: "Oh gawd, not again."

[the door flies open and Cardinal Ximinez of Spain [Palin] enters, flanked by two junior cardinals. Cardinal Biggles [Jones] is wearing a leather WW-I flight helmet and goggles. Cardinal Fang [Gilliam] is...um...just Cardinal Fang]

Ximinez: NOBODY expects the Spanish Inquisition! Our chief weapon is Surprise. Surprise and Fear. Fear and Surprise. Our two weapons are Fear and Surprise..... and Ruthless Efficiency. Our *three* weapons are Fear, Surprise and Ruthless Efficiency - and an Almost Fanatical Devotion to the Pope. Our *four*..... no..... Amongst our weapons..... Amongst our weaponry..... are such elements as Fear, Surprise.....

Ximinez: I'll come in again.

[the Inquisition party exits. H. V. Engr pays and quickly leaves store obviously joyful over his new acquisition. Checkout Clerk locks up for the evening, then walks to bus stop. Ximinez, Biggles and Fang are nowhere to be seen.]

Checkout Clerk: [with contempt] "Gawd how I HATE British comedy troupes!!"

** T H E * E N D ***

Sorry everyone... I meant 60 degrees, not inches.

This is for quoting purposes only to integrate an exisitng PD test system into an assembly line to measure PD in vacuum bottles. Since the PD levels of the samples are very low, then a low PD connection system is required. Unfortunately, the vacuum bottles vary in size, so a flexible busswork would simplify how we attach to each part as it is moved under the tester.

Thanks, Mark

Hi Mark,

Only 60 degrees of motion? Well, then! Forget slip rings altogether (not to mention my earlier post) and use a flexible conductor between the moving piece and your DUT.

A few ideas: Choose a moderately-stiff conductor that is less likely to kink somewhere along its length (the sharp radius at such points being much more likely to induce corona), and configure the conductor so that bending and torsional forces are distributed fairly evenly along its length. You don't want the thing breaking from fatigue - especially at 150 kV (whilst not forgetting the fact that the parts of your assembly at potential have a bit of capacitance between themselves and ground - a capacitance which can easily store enough charge to really pack a wallop <enough so, perhaps, to spot-weld metals>, and even if your HVPS can provide at most only a few microamps or less. It doesn't matter what the current rating is; your assembly will charge to full potential regardless. This is especially true for insulated, shielded HV cables where there is a relatively-thin dielectric between potential and ground. HV cables make great capacitors and store a considerable charge even when disconnected. Worse, dielectric relaxation can restore a partial charge even though the cable had been discharged a short time earlier, so beware!

Back to the problem at hand: If need be, you can support the conductor somewhere near its midsection using an HV insulator and a swivel mount. If it is convenient to bolt the earth side of the insulator to the ceiling, you can support your conductor midway using a short length of plastic 'rope' which runs between the insulator and conductor. This is lots cheaper than an insulated swivel cable-mount.

Should you try this approach, be careful that your rope is not made from nylon or other hygroscopic (water-vapor-absorbing) materials. And although Teflon is a fabulous insulator, it can stretch considerably under constant tension even when only lightly loaded (and "creep" or "flow" when compressed).

Just some thoughts.

-e

One more thing while we're on this subject:

Power-down and ground your assembly. Then thoroughly clean the insulators and related parts using pure, de-ionized water. Next, spray a thin, contiguous coating of silicone oil on the insulating parts - being careful meanwhile to protect any plastics that might degrade from contact with the oil.

Why bother with this? Silicone oil greatly reduces surface leakage from condensation and high levels of humidity, even where a light accumulation of dust may be present on the insulating surfaces. It causes the moisture to bead rather than disperse and form an atoms-thick layer of moisture which greatly lowers the insulator's surface resistance.

But above all, keep your insulating surfaces clean. Cleanliness is absolutely essential for minimizing surface leakage in high-voltage, PD-sensitive applications such as yours.

-e

Hi,

I love flexure mechanisms but for this application my thinking is demanding a corrugated tube made from polished stainless steel.

Small (10 to 200mm diameter) ones are made for vacuum technology, big ones (up to 2m diameter) for the chemical industry to serve as expansion compensators.

Also used as part of high power microwave flexible coaxial cabling.

RHABE

Yes, I've seen corrugated piping like that used on hi-vac equipment. I agree that it would make an excellent flexible conductor in most cases. But not this one, unfortunately.

Think about what takes place near sharp edges and tight bends when the voltage is very high: you get corona. Those corrugations that give the pipe its wonderful flexibility would bury us in corona at 150 kV.

An excellent idea, even so! All we need now is to find some sort of flexible, conductive sleeving we can put over the conductor to hide all those nasty, 'sharp' corrugations from the E-field. Sleeving that doesn't kink or crease as the conductor flexes. Maybe something like...

"Faraday Pants?"

Just try the dryer tubing...usually suffices.

It's easy to check, if you've already got the PD measurement gear integrated into the system..

cheers

RF_G

This stuff?

I hope you're just kidding!

Yep....that's the stuff. Works great.

You just have to use a large enough diameter for the voltage. 150kV isn't very high, so I figure the 6"/15cm dia will work. Maybe 8"/20cm. It's easy to just try it, esp if he's already measuring PD (and probably quite sensitively too, if he's in the vacuum bottle business).

But one poster also suggested mixing bowls etc as poor-man's subsitute for 'polished aluminum spheres', that's a good idea too.

Have a nice day

Regards

RF_G

"150kV isn't very high..."

As compared to, say, a million volts?

"...so I figure the 6"/15cm dia will work."

No doubt.

"Maybe 8"/20cm."

Even better.

"It's easy to just try it, esp if he's already measuring PD (and probably quite sensitively too..."

And all those sharp ridges and gozillion creases and wrinkles? What about the veritable firestorm of corona we'll get from that? Think it'll affect his sensitive PD measurements much?

"150kV isn't very high..."

As compared to, say, a million volts?

RF_G: Exactly! compared to a million volts...or even say, 200kV, or 400kV.

"...so I figure the 6"/15cm dia will work."

No doubt.

"Maybe 8"/20cm."

Even better.
RF_G: exactly. The larger diameter will help lower the field intensity (but might impede movement, depending on his design..)

"It's easy to just try it, esp if he's already measuring PD (and probably quite sensitively too..."

And all those sharp ridges and gozillion creases and wrinkles? What about the veritable firestorm of corona we'll get from that? Think it'll affect his sensitive PD measurements much?

RF_G: the 'ridges & wrinkles' shield each other, if the field strength at the surface is low enough. Like I say, 150kV isn't that high. Some tubing is smoother on the outside than others. But you're right, at some combination of voltage and dia (too small) of tubing, the little ridges and crinkles will start to produce corona.

As I said: he needs to just try it..

RF_G

"the 'ridges & wrinkles' shield each other"

If this were true, RF, then ask yourself why manufacturers of HV gear go to all that trouble and added expense to make their HV surfaces smooth and polished.

Then ask yourself why these manufacturers' customers -- all of them -- are willing to pay small fortunes for this stuff when a visit to the local hardware store would do at 1/100 the cost.

The OP needn't bother testing aluminum dryer hose at 150 kV, because I already have. I used the stuff a few years ago to make the top terminal (a toroid) of a 10-300 kV Tesla coil.

Did it work?

That all depends: If the objective is to generate a buttload of corona, ozone and Bog knows what else, then I suppose the answer would be a resounding 'Yes!'

Otherwise, "No, it didn't."

Hi Europium,

"...why manufacturers of HV gear go to all that trouble and added expense to make their HV surfaces smooth and polished....why these manufacturers' customers -- all of them -- are willing to pay small fortunes for this stuff when a visit to the local hardware store would do at 1/100 the cost."

RF_G: The mfrs go thru the trouble & the cust pay more in order to run a given dimension of kit closer to the field limit at which air corona occurs.

Put another way: it's clear that for OP's problem, he(she?) can't connect the 2 conductors with a piece of thin wire, because it will *glow* with corona, obscurring all PD in his samples. But if you use a bit thicker wire, and reduce the voltage, at some point for a given dia of wire and a given voltage, corona will no longer be produced. With the dryer/AC ducting, for a given diameter, at some voltage, corona won't appear. All he has to do is try it...if it works, it's a cheap solution...

As for what happened with your Tesla toroid etc. I don't know, but from your writing style it doesn't surprise me that you had difficuties making it work

Nice day!

RF_G

Once more corrugated piping:

If taking a big one there won't be any sharp edges,

if thickness is adequate there won't be tight bends.

Construction of end-pieces would require considerable care.

So without doing any calculations I would estimate that a minimum radius (of any curvature) above 10 cm will be acceptable, a pipe diameter of near 1m ! will result and a length of 2 to 3 m. Thius is not so easy to handle but should be possible.

What are the requirements of sleeving? Conductive or nonconductive? How much of the field-strength will act on the sleeve?

RHABE

Along the lines of what I was thinking, I think your dimensions are a little big, but maybe not. For sure you have to be careful about how you fasten down ends, keeping bend radii small, and making *dang* sure the sleeving is well-connected to the HV conductors.

This is accomplished by pulling out a length of the spiral wire, bending an eye in it for the fastening screw, and then bending that bit back 'under' the tubing/shielding (so it doesn't produce any corona).

We had tons of this stuff around the lab, the standard stuff with a tough plastic coating on the outside (I suppose it's polyethelene, it can be a bear to cut, especially hanging in a sling working 1/2 inside a 1.5 meter aluminum sphere )

Works fine as long as you're careful with junctions, bend radii, and keeping it well-connected to the conductors. Once it gets a kink or gets 'folded' that piece is usually trash (because corona starts..)

Haven't heard back from OP maybe he(she?) is busy in the lab!

nice day

RF_G

Sorry Everyone, I'm back.

Thank you for the considerable discussion on the subject, I really appreciate it! Unfortunately, when your customer is forking out just under $1 million for a finished test system that is to be shipped for use in an oversees production facility, you no longer have the freedom to use Home Depot dryer duct in your designes. I have used it in the past on my first (very crude) tesla coil. I have also used stainless salad bowls (on my partially successful Van de Graph), but both approaches just lack the finished look and feel of production equipment.

I found some flexible conductors by a European company named Haefely (part of Hubbell, which we should all be familar with): http://www.haefely.com/pdf/KIT.pdf . If you scroll down to page 20, you can see a production version of the previous poster's dryer vents. Maybe, we can look into those, but I imagine that they cost $10K each.

When I figure out a price, I'll let you all know. And yes, I will get a login eventually.

Good luck with the cat. -Mark

Yep, that's the stuff...$10k dryer ducting

Shown in action on pps. 7 & 8 too.

But you never SAID you were building a million $ worth of kit for a Customer!
Heck, then go for the Haefely stuff - it's bullet-proof.

They're just down the street..

Good luck & let us know how it turns out!

cheers & TGIF

RF_G