Precious metal brush systems in small DC motors

[Clyde38]

Does anyone have a recommendation for information (literature, websites, whitepapers, etc.) about the design of precious metal brush systems? There is a lot of information about carbon brush systems, but I'm finding it difficult to find anything about precious metal brushes. I'm interested all aspects of precious metal brushes.

 

[itsmoked]

What makes you think they exist? Not saying they don't, I have just never heard of them.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Clyde38]

Thanks for the reply Keith

Blind optimistic hope . . .

 

[MikeHalloran]

I think at least some sizes of the Faulhaber motors sold by MicroMo have gold brushes.

Try to find a really old book on servomotors and associated products.


Mike Halloran
Pembroke Pines, FL, USA

 

[israelkk]

MASA used “Long Life” DC brush motor for use on the Mars Surveyor Program utilizing silver-graphite brushes. Do a google search for "“Long Life” DC brush motor for use on the Mars" and go from there.

 

[ScottyUK]

Portescap were certainly using precious metal brushes in their brushed types a few years ago. I expect that they still are.

 

[Skogsgurra]

I think that the question is more about literature than if such brushes are used.

I have one book "Electrical Contacts" by Ragnar Holm. It is considered a classic in the field and it has chapters on contact materials, sliding contacts, commutators and such things. Difficult to find, but some googling may help. That's how I found mine.

Then, there is "Modern Relay Technology" by Hans Sauer. Not so much about sliding contacts and commutators. But a good deal on contact materials.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

Interesting. I suspect most would just reach for a BLDC motor and avoid messy mechanical commutation all together.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

For servo applications, the threshold voltage before a BLDC works at all is a problem. Not to mention the single direction.

You can use PM AC servos. But for low-powered and precise positioning in cost-sensitive applications, I can't see a better solution than a PM DC motor, be it Faulhaber or any other similar brand.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Clyde38]

Thanks to all for your responses.

Mike, I'm aware of the brush configurations used by Faulhaber, Maxon, Portecap, Namiki, and several Asian motors. They don't publish much about their systems.

Gunnar, you are correct. My search is for literature about precious metal brush systems. There is so much published information about carbon brushes and virtually nothing about precious metal.

Keith, there are a lot of applications for precious metal brush commutated motors due to battery (low voltage) operation, simplicity of the drive (and direct from the battery), and cost.

I will follow up on the suggestions that all of you have given.

 

[mikekilroy]

For servo applications, the threshold voltage before a BLDC works at all is a problem. Not to mention the single direction.

Interesting comment. Would you mind expanding on it? What is the threshold voltage? What value too? And why is single direction an issue?

Keith, there are a lot of applications for precious metal brush commutated motors due to battery (low voltage) operation, simplicity of the drive (and direct from the battery), and cost.

Seems with todays super ICs where a $ 1.00 one handles all the brushless commutation requirements, the benefits of cost, simplicity, and even low voltage you mentioned may not be advantages at all.

http://www.KilroyWasHere<dot>com

 

[Skogsgurra]

Mike - I do not understand your question.

Are your BLDC motors very different from the ones described here?:

http://en.wikipedia.org/wiki/Brushless_DC_electric_motor

The OP asked about precious metal brushes. Such brushes are used in small DC motors that are used in positioning microscope stages, measuring machines and other low-power applications with 1 - 10 W output power. They have starting voltages in the 100 mV or lower region and I have not seen any electronically commutated BLDC that works at voltages as low as that.

Are there new BLDC developments that can be used as drop in replacements for such motors?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mikekilroy]

Thanks Gunnar for your added info.

Inland Motors, among all the others mentioned and more, used silver impregnated brushes and gold coated comms for year for precision applications. Hugo Unruh, who founded Inland motor in the early 1960's, used silver in brushes to make super precision motor commutation in US submarine parescope motors, then continued that design into brush motor upto 4' diameter for many many years. Inland could do things with those motors that no one else could at the time. Of course there was more to it than just commutation design.

Then brushless came out for precision and servo motors of course in the early 1970's or so. Our first motors were no way equivalent to these precision DC torque motors. We never messed with BLDC design, we went immediately to BLAC since our applications were 99% super precision. Of course BLDC that does 6 step commutation is in fact potentially crude and I would not argue your points about it compared to DC at all.

I am not referring to any new developments in brushless motors. I am just used to BLAC rather than crude BLDC. Even with BLAC, that same $ 1.00 chip can handle most of the commutation needs. We routinely have built and shipped BLAC motors that will run very precisely andrespond to 100mv or 10mv changes for decades. These motors used to cost 10-20% more than the cheaper performers; today there are no differences in cost between a precision BLAC motor and top of the line BLDC motor - or drive.

We used to demo back in 1980's an industrial 4" dia BLAC standard motor/drive by shooting a laser at at a mirror glued to the shaft. It bounced across the lab, thru the hall, another 50' to a blackboard in lab across the hall. It was programmed to simulate the sun movement (1rev/day). We would begin a shop tour with visitors at this site. They could watch the laser beam on the blackboard across the hall. It was pretty smooth. IIRC we did 1024 (4096 quadrature) counts per rev at the time. The laser would be slowly moving between counts - almost un-perceptibly. Then an hour later come back and see the laser moved up about 10 encoder counts. Motion was very smooth. Today we do 2^32 resolution so it is much much smoother.

Anyway, I do not believe it is fair to say an industrial BLAC motor/drive today would perform LESS than any DC motor with the best available (silver graphite?) brushes. I believe today's BLAC systems will perform 100 times smoother than yesterday's best DC motor system even at low speed.

http://www.KilroyWasHere<dot>com

 

[Skogsgurra]

You know that we are in the same business, Mike. So we do not need to educate each other about these things.

What I think that you missed is that we are not talking about machine-tool applications with pancake motors and such things in the 100+ W range. It is about little motors, typically half an inch to one inch diameter that use silver "whiskers" instead of carbon brushes with or without added silver and other additives.

Also, the commutator doesn't look like a commutator. It is more like a barrel made from "cordwood" with very few bars.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mikekilroy]

Sorry, I did miss that Gunnar!

http://www.KilroyWasHere<dot>com

 

[Clyde38]

Gunnar, thanks for the clarification on my behalf. This type of motor is typically less than 30 mm in diameter (usually half that) and will have output power capabilities in the Watts, not tens of Watts. There is so much literature available for carbon brushes (also copper graphite, silver graphite, etc.) and virtually nothing about these "whisker" type brushes that you mention (see attached example).

 

[mikekilroy]

but are these 'whisker type brushes' really any different than regular brushed motors? The whisker just has the silver graphite or carbon in a blob on the end of the spring metal arm rather than in a cartridge hole with a spring behind it. Isn't the concept just the same as a larger motor?

Perhaps any performance difference in these 1" dia motors compared to 10" dia motors is more a function of number of poles?

It would be very interesting to get a motor designer of this size motor to tell us if a brushless motor would indeed be lower performance than a brush version of same?


www.KilroyWasHere<dot>com

 

[Skogsgurra]

It depends. If you need a small motor that has two wires and that reacts to a simple DC voltage moving between, say, -12 V to +12 V and that has a speed and rotation proportional to applied voltage and polarity, then there is no BLDC motor available that will do that.

On the other hand, if there is a drive available and you supply that drive with separate power and a speed command, then you can have any characteristics you want. And a lot better performance than the brushed DC motor has.

On the third hand, if you make the drive an integer part of the motor and have only two wires going to that integrated drive-cum-motor, then you have a BLDC as it was once introduced and defined. I do not know about any developments that make such a BLDC react to voltage and polarity like a brushed DC motor does. It could be designed if it wasn't the need to operate down to a tenth of a volt or lower where the necessary electronics doesn't get enough "head-room" - digital electronics still need at least 1.2 V and I am not sure if the analogue parts can work satisfactorily at such low levels.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

Mike,

As far as I recall the Portescap design - and possibly others, but that's the type I've had in bits - didn't have a silver-graphite or carbon blob at all, and contact was metal to metal. If there was anything there it wasn't apparent as a normal brush material. They were an ironless rotor design which was relatively unusual back then.

 

[mikekilroy]

Thanks Gunnar. That clears up why you and I are seeming to come from different pages. I ASSUMED there is a good
DRIVE in series with the supply line, and your answers assume there is not. NOW we are the same page. Put a good drive in series with the power supply and you get potentially much better performance than the dc brush motor. No good drive and much lower performance than the dc brush motor.

http://www.KilroyWasHere<dot>com