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Forum Index : Electronics : Carbon brushes

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irishron40
Senior Member

Joined: 22/09/2014
Location: Ireland
Posts: 216
Posted: 08:17pm 10 Nov 2021
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Hi everyone.

I just wonder the following.

I had to make a carbon brush and holder for
a project I'm building.

I have a carbon brush taken out of a lantern
Battery.



Carbon brush is round and  is about 3 inches long and about 1/4 inch thick

On a 24vdc supply how much amps could this brush handle

Thank everyone
 
phil99

Guru

Joined: 11/02/2018
Location: Australia
Posts: 341
Posted: 11:23pm 10 Nov 2021
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Battery carbon rods are pure carbon which is usually used for mains voltage  motors with a modest current draw. High current brushes usually incorporate powdered copper. I would guess at 5 amps or so before the carbon rod gets hot. The end needs to be a perfect fit with the moving surface to prevent hot spots burning the surface.
 
tinyt
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Joined: 12/11/2017
Location: United States
Posts: 393
Posted: 03:49am 11 Nov 2021
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If you can get starter motor from auto scrap yard, you probable can re-shape its brushes to fit your application.
 
irishron40
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Joined: 22/09/2014
Location: Ireland
Posts: 216
Posted: 04:43am 11 Nov 2021
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Thank you both, but isn't here a calculation that can be used,

Cheers
 
Godoh
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Joined: 26/09/2020
Location: Australia
Posts: 119
Posted: 05:58am 11 Nov 2021
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Hi Ron, I have never seen a calculation for brush currents.
I worked on electric motors when I was a working person. As phil says they are made from different materials.
Some seem to be just carbon others look like they are pure copper. And everything inbetween.
The materials vary for different voltages, so the resistance of the brushes also changes. More copper less resistance.
Someone may have seen a forumla for it but unfortunately all we did was replace the brushes with the right ones for that motor.
Pete
 
phil99

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Joined: 11/02/2018
Location: Australia
Posts: 341
Posted: 06:00am 11 Nov 2021
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https://www.repcoinc.com/articles/Carbon_Brushes__Oct_2011___Current_Density-ad.html
https://easa.com/resources/news-center/carbon-brushes-current-density-and-performance
https://www.electrical4u.net/basic-accessories/calculate-current-carrying-capacity-carbon-brushes/
And many more in your favorite search engine
 
irishron40
Senior Member

Joined: 22/09/2014
Location: Ireland
Posts: 216
Posted: 07:12am 11 Nov 2021
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Thank you all for your rsplies.

What if I supply negative though brush instead of positive. Should that solve the problem?

The negative only has unused current flowing so the current amount would be considerably less?

Thx
Ron
Edited 2021-11-11 17:18 by irishron40
 
irishron40
Senior Member

Joined: 22/09/2014
Location: Ireland
Posts: 216
Posted: 07:50am 12 Nov 2021
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Anyone agree with my last idea?
 
Murphy's friend

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Joined: 04/10/2019
Location: Australia
Posts: 230
Posted: 12:47pm 12 Nov 2021
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Well, your last idea sounds a bit "Irish" to me     .

FYI, the current in a loop from the positive to the negative terminal of a battery is the *same* everywhere along the loop. It depends on the total resistance and applied voltage.
 
irishron40
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Joined: 22/09/2014
Location: Ireland
Posts: 216
Posted: 03:04pm 12 Nov 2021
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I disagree, current amount on the negative return is always less
 
InPhase

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Joined: 15/12/2020
Location: United States
Posts: 136
Posted: 03:39pm 12 Nov 2021
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The current in any series circuit is the same throughout the circuit.
 
irishron40
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Joined: 22/09/2014
Location: Ireland
Posts: 216
Posted: 03:44pm 12 Nov 2021
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If that's the case my motor would never drain the battery  
 
phil99

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Joined: 11/02/2018
Location: Australia
Posts: 341
Posted: 11:11pm 12 Nov 2021
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The Voltage decreases as you go around the circuit, relative to the ground reference. (That is why the correct term is Potential Difference. Volts are just a unit of measurement.)
The current is the flow of electrons and has to be the same everywhere around the loop, or they would pile up. In a capacitor or battery they do accumulate on one plate but that is exactly balanced by a decrease on the other, so the circuit current remains the same everywhere.
The draining of the battery is the result of the electrons moving from one plate around the circuit to the other plate. The electrons are made to move by the potential difference (or electromotive force - another name for voltage).
 
irishron40
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Joined: 22/09/2014
Location: Ireland
Posts: 216
Posted: 04:51am 13 Nov 2021
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Well it seems I am wrong in what I thought to be correct.

Apologies to the people I doubted.

Ron
 
Murphy's friend

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Joined: 04/10/2019
Location: Australia
Posts: 230
Posted: 08:09am 13 Nov 2021
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  irishron40 said  If that's the case my motor would never drain the battery  


I'm assuming you posted here to learn so be not surprised when those who have been in the electrical/electronics game for a very long time tell you something you thought was right turned out not to be so .
It all comes down to Ohms law (google it). Its a surprisingly simple formulae but its also surprising how long it can take to *really* understand it.

Now, do this with the motor you quoted above: Attach a fixed load to it and connect it to your battery.
Monitor the battery voltage and the current (the Ampmeter is in series in one lead). Look at the amps drawn then swap meter to the other lead.
!Surprise! its the *same* current.

If you let the motor drive the same load for a while you'll notice the battery voltage dropping but the current going up to maintain the motor power.
Eventually the battery voltage drops low enough so it no longer can supply that current.
So the motor *does* drain the battery  after all
 
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