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First, a little bit of application background. Recently I came across a shipment of DC-powered pumps (like these, it says diaphragm but in reality they are more like plunger ones, i.e. they require a "slow" driving motor), all of which turned up dead after running for 6-72 hours.

Their DC-motors were to blame - all of them seized to start and pulled excessive current (even more than if you were to stall the rotor of a "still working" one). So, I figured either the commutator or the rotor itself shorted, and disassembled some of them to see what's up. And here's what I saw (please excuse the shaky hands): The brushes

See the radial surface of the brushes? Its axis is perpendicular to the rotation axis (don't mind the little "correctly" oriented grooves, the commutator grinded the brushes a bit)! That's how all of them came from the factory. Is it just me, or is the radial surface shape actually supposed to maximize brush-commutator contact area and slow down brush wear? And thus the aforementioned axes should coincide? And the reason why they all died is a shorted commutator due to brush material being grinded away too fast and clogging up grooves in the commutator?

At first I thought they messed up brush placement, but no, there's no way to rotate brushes 90 degrees, the springs are specifically designed to enforce this exact orientation.

Fine. Forget about pumps. I ordered a bunch of (very) different RUICHI DC-motor models, and all of them use the exact same brush housing design (at least the ones with a plastic back) and possess the same geometrical issue. However, this time, none of them died during my testing. Also I noticed that commutator sparks had different color (white instead of yellow), so I'm thinking different brush materials are used.

Well, I am starting to doubt myself. Is this counterintuitive brush configuration OK? Why is it used?

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  • \$\begingroup\$ To clarify the stalling aspect. The shaft is easy to rotate by hand, so I don't think the bearings are at fault. Same goes for the pump head, first thing I did was dismantle it and try running with no load. I do in fact undervoltage the motors to get required flowrate (though within manufacturer's limits of 6 to 12V). \$\endgroup\$ Jul 3, 2023 at 18:41

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Is this counterintuitive brush configuration OK? Why is it used?

Yes, it's OK.

The brushes are shaped that way to wear into the curve of the commutator faster. You might think it would work better if they were a closer fit to start with, but these are 'hard' brushes designed to last the lifetime of the motor, so they need to bed in fast and then will wear slowly after that.

Here's another example that has a 3 ridges to provide more contact area while bedding in:-

enter image description here

Your brushes appear to have very little wear, which suggests the motors are not running at high rpm or current. If they are seizing then perhaps they need a drop of oil on the bearings, or the pump mechanism may be seizing.

You should also examine the armature windings for signs of overheating (eg. bubbled or burnt enamel on the wires) which may indicate a shorted out winding. This can be caused by running the motor on a lower voltage which doesn't provide enough startup torque to get out of stall. Or they might just not have enough cooling and eventually overheat.

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  • \$\begingroup\$ Thanks, did not know that! I added a comment about mechanical issues. The thing is, at first they all worked at 6V. After 6-72h stalls appeared. Initially, I was able to "help" the motors out of stall by hand, but eventually they all died completely (consuming 0.6A compared to normal 0.2-0.3A stall current), staying still no matter what I do (spin the shaft, raise the voltage). And yes, they do run quite a bit hotter than the ones I ordered separately, even with no load, I can bearly hold them when running (and of course it gets worse in a stall). \$\endgroup\$ Jul 3, 2023 at 18:46
  • \$\begingroup\$ Makes me think, is it possible for the magnets to degrade due to overheating, leading to less torque and speed, leading to more overheating and so on? But then what's the reason for overheating in the first place? Is it possible for an undervolted (but not to the point of stalling) DC-motor to pull excessive current due to insufficient back-EMF or something? \$\endgroup\$ Jul 3, 2023 at 18:52
  • \$\begingroup\$ Yes, an under-volted motor may draw excessive current. However if it has Ferrite magnets the armature will burn out long before the magnets cook. 0.2-0.3 A is a very low stall current. I can't find any specs on your pumps, but a similar one I found had a 'working' current of 0.5-0.7 A. \$\endgroup\$ Jul 5, 2023 at 3:53

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