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Planning a design that has a 12V voltage source connected to a physical part that rotates.

I need to conduct electricity from the rotating part to a fixed part which is connected (electrically isolated) to free-standing structure like a pole or a tower.

I'm imagining the matching 'rotating' and 'fixed' conductive parts to be anywhere from a few centimetres across to maybe 20-30cm and spinning at maybe 500rpm max.

Does a solution for this challenge - or some sort of conductive, [edit: reasonably] frictionless, easily machinable material - already exist in general population?

Thanks.

rotating conductor

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  • \$\begingroup\$ To transfer voltage and current two connections are needed. \$\endgroup\$
    – Andy aka
    May 16, 2023 at 13:58
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    \$\begingroup\$ While the principle is the same at any current level, the details will vary quite a bit. Transferring 0.5A for power a small electronic device is quite different from 20A to power a big motor. \$\endgroup\$ May 16, 2023 at 14:35
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    \$\begingroup\$ How much electricity? \$\endgroup\$ May 17, 2023 at 1:23
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    \$\begingroup\$ For non-critical uses, regular ball bearings work ok. They might accumulate more wear due to tiny sparks, but I've never run one long enough to see that. \$\endgroup\$
    – jpa
    May 17, 2023 at 6:06
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    \$\begingroup\$ Electrical connections to a rotating part comes in two varieties a) the transfer occupies the central axis and b) the central axis is clear. When the central axis is occupied by the transfer it tends to make the mechanics of the device awkward. there is nothing then to mount the bearings or axis of the rotating part. \$\endgroup\$
    – D Duck
    May 28, 2023 at 10:42

10 Answers 10

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The term you are searching for is Slip Ring Connectors.

A slip ring is an essential component and parts that can solve the 360 degrees of continuous rotating, and get electricity to a continuously rotating part of the assembly, rotating without limits.

Read more

slip disc conectors

See on Amazon (available for way less on other websites though)

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    \$\begingroup\$ This is what I was imagining and it looks like there's small hobbyist/prototype versions available at a reasonable cost. Thanks. \$\endgroup\$
    – formerlolz
    May 16, 2023 at 14:38
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    \$\begingroup\$ Yes, an amazon search gives a lot of relevant products. I forgot to state that. I also didn't include a link because products' availability depends on location and they might even discontinue selling :) \$\endgroup\$
    – 44yu5h
    May 16, 2023 at 18:18
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    \$\begingroup\$ @robobozo Yes, a slip ring is very easy to make. I once made one out of a hole saw blade and a socket from a socket set. Used the hole saw to cut a hole out of plywood and left the wood in the hole saw blade as in insulator then mounted the socket to the center of the wood. Then fashioned some springy bits of metal to contact the 2 circular surfaces and viola, I had a slip-ring for a rotating Christmas tree display. \$\endgroup\$
    – Glen Yates
    May 17, 2023 at 16:02
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    \$\begingroup\$ @robobozo Slip rings and (typically carbon) brushes are an essential part of simple DC motors, hence why other types are referred to as "brushless" \$\endgroup\$
    – MikeB
    May 17, 2023 at 19:06
  • \$\begingroup\$ @MikeB: Most simple DC motors use split commutators rather than slip rings. Commutators need to be able to switch currents, which will generally create arcing and cause substantially greater wear to brushes than slip rings, which are intended not to interrupt current. \$\endgroup\$
    – supercat
    May 18, 2023 at 20:32
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Frictionless is not a possibility if there is direct flow. There are brushes, slip rings and roll rings. Of the three, roll rings are generally the lowest friction, all other things being equal. From here is how they work:

enter image description here

It's possible to transfer power across a rotating joint by using a transformer with an air gap. There is no friction (other than what results from air drag) and there may be little in the way of torque with a high frequency ferrite core transformer.

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    \$\begingroup\$ I've seen inefficient but working frictionless systems that use light as the power transfer medium, light source (laser/whatever) >> solar cell etc. \$\endgroup\$ May 16, 2023 at 23:13
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    \$\begingroup\$ There are mercury slip rings for frictionless direct flow (well, with bearing friction which is typical of most rotating joints, and I guess mercury seals). \$\endgroup\$
    – DKNguyen
    May 17, 2023 at 0:08
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    \$\begingroup\$ Roll rings are essentially ball bearings right? So each ring is connected to a different phase (in 3phase) and the bearing is filled with a conductive lubricant, else the ball-bearing to Race contact area limits the current that can flow. \$\endgroup\$
    – Criggie
    May 17, 2023 at 4:58
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    \$\begingroup\$ Liquid Mercury would be awesome here - I've seen a mercury tilt switch in an old mechanical timeclock used to remote-control hundreds of clock faces. But the likelihood of putting something like that into a new product would be very low in today's OSH compliant world. \$\endgroup\$
    – Criggie
    May 17, 2023 at 5:00
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    \$\begingroup\$ @Criggie Actually they use a ring that deforms. I've added a photo above. \$\endgroup\$ May 17, 2023 at 5:28
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There's another way to do it that doesn't use simple slip rings.

Instead, you induce a current in a coil by moving it through a magnetic field. If the magnet is stationary, and the coil is moving with the shaft then current will flow.

Downsides are that the current will be AC as each side of the coil passes through the magnet. So you put a rectifier on the moving assembly.
You probably need a series of coils in a circle, and a series of magnets which could be permanent or electromagnets.
Also this will provide resistance to rotation so you may need more motor power to turn the shaft.
Finally, this won't send power unless the assembly is rotating. If you stop it spinning, then no power will be transferred across.
And you can't earth via an inductive pickup. This makes the rotating assembly isolated unless it earths through bearings, which might be okay depending on load. Bearings don't have a lot of contact area for current.


For completeness only - DATA

If you wanted to send a digital signal across the rotating interface, then a series of optocouplers could do the task at a fairly low bit rate.

For higher speed connections, perhaps fit a wireless ethernet card to the rotating assembly and use radio to jump the gap. A Pi on a Maypole might be perfect.

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    \$\begingroup\$ all of these are more complicated than slip rings \$\endgroup\$ May 19, 2023 at 17:15
  • \$\begingroup\$ @user253751 yes. But there are the other benefits like "no brushes" so you gotta weigh the pros with the cons. \$\endgroup\$
    – Criggie
    May 20, 2023 at 6:17
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Slip rings, of course, but there's also inductive power transfer which is getting more and more robust and easy to use by the year. In theory, it shouldn't care about rotation as long as the coils are relatively coaxial. It also has the advantage of letting the transmitter and receiver be completely separated.

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Here's an existing concept that may match your requirement.

It's an alternator with slip rings and brushes.

The rotating part comprises the armature / slip rings and the fixed part the carbon brushes.

enter image description here

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  • \$\begingroup\$ (Downvoters please comment what in particular make a post not useful.) \$\endgroup\$
    – greybeard
    May 18, 2023 at 6:26
  • \$\begingroup\$ Someone commented on the request for downvote clarification. It's not required, but asking why is as allowed as downvoting is. MOST answers do not attract downvotes, so --> || It adds "noise" but not a lot on average given the many comment-fests, most of which are technically against the rules.|| In this specific case the answer is a useful one. The two downvoters \$\endgroup\$
    – Russell McMahon
    May 19, 2023 at 8:08
  • \$\begingroup\$ Thank you @greybeard and Russell! \$\endgroup\$
    – vu2nan
    May 19, 2023 at 11:55
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Use the rotating tower as the armature of a generator. The stator - maybe a permanent magnet - is in the non-rotating base structure. This, of course can be a little unhandy if the electricity is needed in the tower to make it rotate.

But you can use the same as what's used to charge phones with no wires.

Or maybe photoelectric generation is enough - I mean the same as a solar cell. The needed cell is not big if you need only the same amount or less than what's used in solar cell powered calculators which run well in normal room light conditions.

The rotation axis may have two mechanically supported ends. If the axis is not a single metal bar the axis end bearings can be the feeding contacts. 2 contacts hopefully is enough.

Slip rings and brushes would be my last resort, but that's already mentioned by others.

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    \$\begingroup\$ slip rings seem to be way simpler than putting solar panels and batteries on the rotating part!! \$\endgroup\$ May 16, 2023 at 14:17
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Conductive ball bearings:

Usually used to prevent static electrical charge accumulation and damage, but can also be used to deliver power as described in

https://patents.google.com/patent/US3271723A/en

I had never heard the term "slip ring", but not my field; however, I know people who used conductive ball bearings (and roller bearings).

Of course, you might need conductive lubricant. Not all bearings need lubrication, but typical sealing techniques use non-conductive materials.

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Slipring might be the answer for you. https://www.digikey.com/en/products/detail/sparkfun-electronics/ROB-13065/5762402

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Expanding on Criggie's answer with a good real-world example of inductive coupling between stationary and rotating components:

Synchronous motors can have "brushless exciter systems" that use AC at a frequency higher than the power line† to couple to a rotating rectifier assembly that provides DC to the field coil.

I have seen these in service on large (many MW) motors for infrastructure applications (e.g. water pumping stations).

They can also be used on synchronous generators.

†If the exciter were driven at power line frequency, once the motor accelerated to synchronous speed, the rotation would effectively reduce it to 0 and eliminate the coupling.

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One way to transfer power is using coupled magnetic fields. On the base platform have a coil around the shaft, then have a receiving coil on the rotating bit. (or 2 coils side to side). This does not result in any extra resistance or friction.

If you then feed an AC signal into the first coil, it can be picked up by the receiver, even during spinning.

If the load on the receiver side is variable, you might need a voltage regulator on this side, as the coupling coefficient between the 2 sides is not the highest.

In the case of DC power transfer (you did not specify if your 12V was AC or DC in your question), you need to first generate an AC signal, then on the receiving side capture it and convert it back to DC.

One of the places where this power transfer system is used in a synchroscope, which requires low friction to see the finer details, but also needs power in the rotating bit so it can "compare" the magnetic fields of both supplies. An teardown of this bit of ai synchroscope can be found here: https://youtu.be/SJ3pnyAvngg?si=nwAAILb8Esf77HcI&t=439

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