I've made a small 6V DC motor, hand would field and armature coils, and a 1/2" copper pipe commutator with copper wire 'brushes' The design came from an old book. 'Electrical Things Boys Like to Make'. Problem is arcing between the brushes and commutator is erroding the commutator and the friction slows the motor down and increases current draw. At higher speeds the ammeter reads only 0.5 amps but as it slows, it increases to about 1.6 amps. Seemes a lot for a 6V lantern battery.

Any thoughts? This is a great hobby but I'm not a designer...

  • 3
    \$\begingroup\$ A picture would be useful. You might get better results by using graphite bushes made from pencil lead, or at least graphite lubricating the commutator. Otherwise the whole thing (commutator and bushes) have to be very clean. \$\endgroup\$
    – pjc50
    Mar 1, 2013 at 16:35
  • \$\begingroup\$ @pjc50 How would I insert a picture on the forum? I'm new. \$\endgroup\$
    – JIm
    Mar 1, 2013 at 17:00
  • 1
    \$\begingroup\$ Add a URL in a comment and someone with more rep can inline it. \$\endgroup\$
    – pjc50
    Mar 1, 2013 at 17:15
  • \$\begingroup\$ @JIm I suggest uploading to imgur. Once you get 10 rep it is possible to just press an image button and select it. \$\endgroup\$
    – Kortuk
    Mar 1, 2013 at 18:17
  • \$\begingroup\$ If you don't want to go for carbon brushes, perhaps tinning the contacts with solder would prevent the erosion, and it's something you could repeat regularly as a form of maintenance. \$\endgroup\$
    – Kaz
    Mar 2, 2013 at 0:52

2 Answers 2


There have been many improvements in motor construction over the last hundred years. I'd guess you're probably using a two-pole motor, since many project books describe such motors; a three-pole motor will likely work much better. In a two-pole motor, you must make sure there's a big enough gap between poles to ensure that the commutator never poses a short across the supply. Assuming that's the case, you may be able to reduce arcing by installing a pair of back-to-back Zener diodes across the coil (on the rotor). Something like 12-15 volt diodes should probably be pretty helpful. Make sure to keep an eye on their temperature, though; depending upon various factors, they may end up absorbing a fair amount of energy. Zener diodes can tolerate getting pretty hot, but beyond a certain point they may fail shorted, and the current from a lantern battery may be sufficient to make them fail dramatically. Still, I would expect that 12-15 volt diodes may help with your arcing problem.

Another thing I would suggest is that you avoid metal-on-metal contact; carbon brushes are apt to be far less problematical. Pure copper tends to oxidize quickly, and copper oxide is an insulator. If you can put a reasonably smooth layer of solder on your commutator that may improve things considerably.

An alternative approach (since this is an electronics forum after all) might be to add to the motor a position sensor, and then use electronics to switch the current to it. Two approaches may be useful here: (1) keep the mechanical commutator to reverse polarity, but use electronics to ramp down the current flowing to the motor before the commutator switches, so that the commutator never has to switch under load; (2) replace the commutator with side-by-side solid rings, and then use a circuit called an "H-bridge" to control the polarity. Both approaches would enable some explorations which would not be possible with a purely-mechanically-commutated motor.

  • \$\begingroup\$ Layer of solder - could that be done with electro- or chemical plating at home? \$\endgroup\$
    – pjc50
    Mar 1, 2013 at 16:44
  • \$\begingroup\$ @pjc50: It could, though I'd been thinking in terms of a soldering torch (actually, I'd been thinking in terms of a soldering iron, but then realized that heating up half a 1/2" pipe with a soldering iron isn't apt to work). \$\endgroup\$
    – supercat
    Mar 1, 2013 at 16:49
  • \$\begingroup\$ @pjc50: yeah sweat soldering is fun too :) you could also use a disposable (sacrificial) sleeve on your commutator. \$\endgroup\$ Mar 1, 2013 at 18:09
  • \$\begingroup\$ @supercat Brazing time! \$\endgroup\$
    – Kortuk
    Mar 1, 2013 at 19:08

The fact that the motor draws less current when running fast is not because of the brushes doing something strange. This is how motors work. Think of a motor like this as the motor coils in series with a generator. As the motor spins, the generator produces more voltage that offsets the voltage applied by your battery. This reduces the apparent voltage to the coils, which reduces their current.

That also reduces torque, so the system finds a equillibrium speed where the generator offsets the power voltage so there is just enough left to drive the motor at that speed. If the motor were to go faster, the generator would offset more drive voltage, slowing it down. If the motor were to go slower, the generator would offset less, the torque would increase, and the motor speed up again.


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