Home Made Motor Problem

Question

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...

Answer 1

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.

Answer 2

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.