Momentary open-circuit issues in brushes/slipring contact 8

[Shahriar Kamrani]

I have two phosphor bronze PB1 sliprings of 60 mm in diameter and 10 mm in thickness and two brushes of carbon graphite with 8 mm in thickness. I am interested in applying a voltage from one of the slipring/brushes contacts, through the shaft (and load) and then collect it from the other slipring/brushes. As I am interested in monitoring this "contact" voltage at high frequency for data acquisition purposes, I have realised that I have an open-circuit at every 1-10 points per slipring cycle.
I have tried multiple different approaches to eliminate this by using different brush spring force, slipring material and conductive grease for the brushes/slipring, with no success. I think one problem that might also influence this is the oil/lubricant vapour reaching the surfaces of the slipring/brushes and act as a dielectric material.
Is there any suggestions/tips of how I can improve the slipring/brushes contact and also keep the lubricant away from the surfaces?

Thanks,

 

[waross]

I have had an extreme case of oil on sliprings. Used diesel lube oil was being deposited on the slip rings of a 60 KVA generator. When the brushes were lifted by the oil spots the field current would cause an arc. The arcs were eroding the slipring surface. One area of erosion was 0.030 inches in depth.
Yes, any oil on the surface may easily interrupt your current.
I suggest multiple brushes as a first resort.
If that does not produce satisfactory results, you may have to find a non-contact solution.
For a non-contact solution there are others here more better able to suggest hardware than I.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Strong]

Perhaps the spring-loaded brushes are bouncing from vibrations. Measurements with a tiny accelerometer may confirm this or not.

Walt

 

[Skogsgurra]

So this is not for excitation of the rotor? Right? Just for measuring "something"?

I would, first of all, make sure that the brushes are "run in". Not in the commutator/slipring sense but more so that they have a curvature that suits the sliprings. Use sandpaper between ring and brush and see to it that the brushes have the same radii as the sliprings. I would also use sandpaper on the slipring surface to get rid of those unimaginable difficult thin layers that form on sliprings and make lo-level measurements difficult.

Second, I would make the springs shorter (equals more force/pressure). If this is for weeks or months, the high pressure will not hurt. For years, you had better look up what the manufacturer recommends.

Third, I would put slingers on each side of the sliprings. They prevent oil and such from getting at the ring's surface.

Fourth, there are slipring/brush combinations that can be bought from science materials suppliers. It depeds on your application if you really need them. What are the voltages and currents involved in your test?

Fifth, if it is about excitation of the rotor winding - forget most of what I said.


Gunnar Englund

http://www.gke.org

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

 

[edison123]

Are trying to do RSO test of a rotor winding of ac generator?

Muthu

http://www.edison.co.in

 

[Shahriar Kamrani]

@waross - Can you give me some ideas with regards to the non-contact solutions?
@Skogsgurra - Those are applicable and thanks for the suggestions, the current is set below 200 uA and voltage also doesnt exceed 20V.
@edison123 - Just simple voltage measurement nothing really related to the rotor/other electrical motor components.

 

[edison123]

Measuring such low current needs a special meter. What meter do you use?

Muthu

http://www.edison.co.in

 

[Shahriar Kamrani]

@edison123 I am measuring the voltages using the differential input of DAQ and a current measuring resistor for the current.

 

[BrianE22]

They used to make mercury wetted slip rings for low resistance measurements. I don't know if they are still made (mercury = bad).

 

[waross]

You may be getting similar to a junction barrier voltage as your current passes from the slip rings to the carbon brushes.
I have seen non-linear readings when checking field resistance on different scales of a meter.
I wouldn't attempt to measure that through carbon brushes. Copper impregnated brushes such as are used on some automotive starters may work better.
For non-contact specifics I would ask for help finding suitable hardware. This forum may be a good place to start.
Does anyone have any suggestion?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

200 uA - thats a dry circuit. No wonder you have a problem. Try silver brushes. Available from that company in Milwaukee. Forgot its name, google Jeff Koenitzer. He knows all about it.

Gunnar Englund

http://www.gke.org

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

 

[edison123]

What is the purpose of this test or process? At such low currents, you need a measurement system that does not add to the resistance or impedance.

Muthu

http://www.edison.co.in

 

[BrianE22]

Jeff Koenitzer works at Helwig Carbon.

 

[Shahriar Kamrani]

Attached is the electrical representation of my system including the limiting-current resistor and the current measuring resistor, and the "contact" which includes the slipring/brushes 2x and two electrode contact which I am interested in measuring. I am interested in the voltage across the electrodes.

@edison123 I am interested in finding out how the voltage across the electrodes contact changes and it is for rolling element bearing application and a failure phenomenon that I am investigating.
@Skogsgurra & @waross you have suggested using silver and copper impregnated brushes respectively. For these brushes, do I need to replace the sliprings of PB1 phosphor bronze material or the material is compatible? I have found a document from Jeff Koenitzer which looks interesting.
@BrianE22 Thanks for the suggestion, I might look into finding these or similar designs.

 

[Skogsgurra]

YES!! That's the company. I have been to their factory and saw some very special materials.


Gunnar Englund

http://www.gke.org

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

 

[Shahriar Kamrani]

To clarify, the "contact" defined as a capacitor with a resistor is the simplified electrical representation of my lubricant.

 

[edison123]

You are studying bearing currents? If yes, Gunnar is your guy.

Muthu

http://www.edison.co.in

 

[dArsonval]

Interesting project.

As Bill (waross) suggests, "...Copper impregnated brushes such as are used on some automotive starters may work better."

Brushes used in high current/low voltage applications tend to be more copper color in appearance.
For example, brushes used in fork truck motors, automotive starters etc.

A brush found in a common tool motor that operates at 120 volts will be much darker in color
with no hint of copper appearance. (The above translating to a brush's grade of hardness or softness.)

It appears further experimentation with the brush/slip ring arrangement will lead to
further or better results. Including the selection of different material choices used for the rotating member.

Considering the very low current involved, I wonder if a straight copper-to-copper arrangement
would yield better results with what you are attempting to achieve with your measuring project.

Again, intriguing experiment.

John

 

[Skogsgurra]

OK? That kind of measurements?

We are using a so-called KC Attachment when doing EDM tests over hours. Use any wire directly on shaft or slip ring. But make sure that you do not use similar materials. Steel wire and steel will cause fretting. So does Cu. For a bronze slip ring, I would use a stranded steel wire. Use any insulating material to stretch the wire. Rubber bands are simple and work well if you do not need longer test runs than a few days. After that, it is a simple matter to change them for new ones. You can also use hooks and magnet feet. And springs. Like picture below shows.

Gunnar Englund

http://www.gke.org

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

 

[Shahriar Kamrani]

@dArsonval thank you for the interest and the suggestions, I have used the copper brushes and copper discs before and that led to the sliprings losing 2 mm in diameter due to wear in a period of 20 days of and hence the need for the replacement. Phosphor bronze so far they are the best in terms of wear and electrical performance.

@Skogsgurra great suggestion and thank you for the comments. I will look into modifying my test rig to make a KC attachment. My tests run for a maximum of 10-15 days so based on my experience (I used stranded wires sometimes to improve the electrical performance beside the brushes), wires seemed to improve the electrical performance. I will implement this KC attachment using stranded steel wire, fingers crossed hopefully it works.