I know that after couple of uses DC Motors we should replace carbon brush because the physical size of carbon

brush will be reduced and it can not do its job!

But let me ask my 3 main questions....

  • 1) Does reducing of physical size means that the resistance is also reducing of carbon brush?
    I know that new carbon brush which is unused has bigger physical size than used carbon brush? And according a theory that longer wire has high resistance than short wire. So I think that longer carbon brush might have a high resistance as well!
    Am I right?

  • 2) When resistance is reduced down to very small amount will it make a short circuit?
    Will it destroy another components of dc motors like rotor or stator or trigger?
    Which one could be damaged?
    If this is true than tell me how can we prevent this problem?
    Does modern drill drivers based on DC Motors or based on Universal Motors have any kind of protection like 5amp fuse?
    Is the fuse the best solution?
    When we are triggering a universal motor and little spark appears is it OK?
    Can you tell me what does it mean?
    Does it means that we should replace a carbon brush?
    Even when I replaced a carbon brush there was still sparks during triggering!
    At one time even little smoke appeared and the universal motor of drill driver was working but suddenly stopped when I was triggering!
    Does it mean that problem is inside a trigger?

  • 3) When Drill driver or Universal motor is working with AC 220v and its power is lower like 100watt does it mean that the resistance of carbon brush mist be much higher and higher compare to any another universal motors like more powerful motors which works 220V AC with 1500watt power?

please answer my question on a title!
Does resistance of carbon brushes in DC Motors decreases after couple of uses?
Thank you!

  • \$\begingroup\$ Way too many questions...... \$\endgroup\$ – Trevor_G Nov 27 '17 at 14:03
  • \$\begingroup\$ just answer the title! OK! \$\endgroup\$ – IremadzeArchil19910311 Nov 27 '17 at 14:04
  • \$\begingroup\$ Then just ask the title, OK? \$\endgroup\$ – Olin Lathrop Nov 27 '17 at 14:09
  • \$\begingroup\$ Maybe, just measure the resistance with a multimeter :) \$\endgroup\$ – Gomunkul Nov 27 '17 at 14:10
  • \$\begingroup\$ If I typed anything wrong then tell me in order to remove... \$\endgroup\$ – IremadzeArchil19910311 Nov 27 '17 at 14:10

Brushes do wear back and like all conductors are rated in ohms/unit length. However, the resistance of the brushes is trivial compared to the resistance of the coils.

Here is an interesting document that lists typical brushes in the milli-ohms per inch range. Mind you those are for large motors, but it's just a matter of scale. You can get the idea just the same. The document also talks about every other factor you may ever want to know.

A bigger issue with brushes is as they wear the graphite dust can accumulate around the commutator causing a significant amount of current leakage.

New brushes also need to wear in so the face of the brush matches the curvature of the commutator before they work optimally. Brush life is usually measured in months or years though, not a couple of uses.


In addition to the answer by Trevor, fundamentally it's not the resistance of the carbon brushes that limits the current. It's the resistance of the windings, and then the back EMF created once the motor is spinning.

Which means that most of the other questions don't apply.

A few small sparks from a motor is normal. Lots of sparks mean that something is worn out, perhaps the carbon brushes have worn down to nothing.


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