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DC motor brush material

  1. Feb 13, 2017 #1
    Hello there

    Building DC motors has sort of become a hobby lately and I'm about to build the best version yet. However, I have a few questions when it comes to the brushes.

    I salvaged a couple of brushes from an old electric saw that my dad was throwing away but the resistance of those are much too high for what I'm going to use the motor for. The biggest battery I have is at 11.1V so I can't have brushes with a resistance of almost 10 ohms each. I have read that high resistance brushes gives better commutation but still :eek:. I also read that carbon graphite brushes can handle a current density of 7 amps/cm^2, which seems low to me. What material are the brushes made of in start motors? (low voltage high current)

    What do I search for on fleabay if I want brushes with a maximum resistance of 1 ohms each and that can handle a higher current density?

    Your answers are appreciated as always :)

    Edit: I've been using copper brushes until now
    What I've found so far is that Electrographitic and Metal graphitic brushes are good for what I'm going to use them for but there are so many grades that it's hard to find the right ones on ebay (or any at all).
     
    Last edited: Feb 13, 2017
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  3. Feb 13, 2017 #2

    Averagesupernova

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    What kind of current do you think an automotive starter takes? I doubt the current you require will exceed that in a starter.
     
  4. Feb 13, 2017 #3
    No, I won't be close to those currents. But if my motor would draw more than 5A under load then I would need brushes that can handle a higher current density that 7A/cm^2. Is there a name for low resistance brushes?
     
  5. Feb 13, 2017 #4

    RonL

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    This might help you with a starting point :smile:

    http://www.astroflight.com/781
     
  6. Feb 14, 2017 #5
    Silver is somehow the key word here. I can't order from the US though because of the shipping cost to Sweden so I searched on ebay (for silver brushes) and found these:

    http://www.ebay.co.uk/itm/Tamiya-53...028429?hash=item3ad6092bcd:g:phYAAOSw2xRYYsjB
    http://www.ebay.co.uk/itm/Tamiya-Racing-Motor-Brush-Set-Standard-Type-53581/371827349600?_trksid=p2047675.c100005.m1851&_trkparms=aid=222007&algo=SIC.MBE&ao=2&asc=20140122125356&meid=7524c5f15f8c434cbcc58416556621a1&pid=100005&rk=1&rkt=6&sd=252699028429

    Since tamiya makes RC related stuff I'm assuming that these brushes must have a low resistance in order to work with low voltage battery-driven motors.
    Thanks!
     
  7. Feb 14, 2017 #6

    RonL

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  8. Feb 15, 2017 #7
    Yes that might actually come in handy.

    I have another question but it's on another topic. Maybe you can help me again.
    I disassembled and old electric scooter today to salvage parts when I got the idea to build a radio controlled car using that 250W motor. My receiver can be powered on channel 2, which also controls the motor. The motor controller puts out 4.9V for the throttle signal (not sure about the terms here) but my receiver won't turn on with any less than 5.5V it seems. However, the receiver also has a separate power input so I could make a 6V battery pack and power it from there. So my question is this, what happens when you power the receiver with 6V on the separate input while it also gets 4.9V on channel 2?

    I could do a test. If the receiver blows up I only lose about $4.

    The throttle used to be controlled with some kind of magnetic component based on what I could see. It was really small, smaller than any potentiometer I've ever seen. Any idea what this could be?

    Edit: Turns out it's a hall effect sensor. I wonder how different the signal output is compared to my receiver..
    Edit 2: The signals are quite different. What are my alternatives then? Radio controlling the pot with a servo?
    The 24V RC ESC's on ebay are very expensive.
     
    Last edited: Feb 15, 2017
  9. Feb 15, 2017 #8

    RonL

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    My background in electronic component circuit speed control is very limited, I do have a few units that have been burned because of improper current and amperage loads during operation, so IMO there is little room for error in regard to voltage input.
    Most of my setups involve larger (up to 5 hp) DC motors and router speed control 120 AC/volt units, converted to DC using full wave bridge rectifiers.
    Below are a couple of links to one of my favorite sources of info.

    https://en.wikipedia.org/wiki/AstroFlight

    http://astrobobb.com/electric_motor_handbook.pdf

    Bob Boucher's book gives information you need to understand what is involved in the speed/power relationship of any electric motor.

    Look for small electric motors ( DC/AC and universal types) and speed controllers in hand tools, household appliances too numerous to list.
    If your thread continues for a while, I might have an idea to pass on......
    Best wishes on your learning.

    Ron
     
  10. Feb 15, 2017 #9
    Thank you for linking astroflight again. I found this http://www.astroflight.com/rcpwm2v. Ingenious. These are very cheap on ebay aswell. A cheap solution would be a PWM to constant voltage signal converter and a motor controller that uses a pot. I will look into this tomorrow.
     
  11. Mar 1, 2017 #10
    I thought I'd give you an update. My Arduino UNO arrived shortly after my last post and I realised that I can use that to read the receiver's PPM signal and convert that into PWM and feed it into an H-bridge to power the motor. So this is where I am now: https://www.physicsforums.com/threads/making-an-arduino-controlled-h-bridge.906028/
     
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