I need to reverse the rotation of the universal motor in question so I swapped the wires leading to the brushes and that successfully reversed the rotation, but now the commutator is arcing and is producing a significant amount of sparks. It's worth noting that the arcing completes half a circle almost touching both brushes simultaneously.

I tried rotating the brushes 180° because I suspected they were worn out "asymmetrically" despite looking straight (still concave but not unevenly worn) and without a slope but the sparking continued.

Then I sanded carefully the brushes so that they were squared again to sort of resetting them to their factory position so to speak but the arcing still persisted.

The only variable left is that the commutator is not perfectly worn and is making an undesired contact with the brushes.

I want to make clear that the housing of the brushes are not angled in any way to favour any direction of spin and to my understanding, it should be very feasible to reverse the direction.

I also tried sanding the commutator to clean it with fine-grit sandpaper but didn't want to go any further without consulting.

This is the stock configuration:

Stock configuration with commutator more visible:

Brushes view:

"Custom" cables configuration:

Windings closeup:

  • \$\begingroup\$ can you show us a photo of the commutator and armature, including where the winding wires go from one to the other? \$\endgroup\$ – Bruce Abbott Dec 22 '20 at 4:40
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    \$\begingroup\$ Brushes are usually advanced from the nominally neutral position to minimise sparking. If you reversed the rotation, they are now retarded and will spark badly. Hopefully your brush assembly can be rotated to advance them in the new direction. \$\endgroup\$ – user_1818839 Dec 22 '20 at 12:09
  • \$\begingroup\$ @BrianDrummond as you can see the brushes are not tilted in any way or am I wrong? How could I know? \$\endgroup\$ – Juani Elias Dec 22 '20 at 18:41
  • \$\begingroup\$ @BruceAbbott I've added some pictures, please tell me if you need more \$\endgroup\$ – Juani Elias Dec 22 '20 at 18:42
  • \$\begingroup\$ Show us a close-up photo of the wires going from the armature windings to the commutator segments (like your first photo but closer with armature removed from motor). \$\endgroup\$ – Bruce Abbott Dec 22 '20 at 19:44

In an ideal motor, the ideal position for the brushes, the commutation plane, is perpendicular to the magnetic field.

In a real motor, with finite inductance, it is rotated a little with respect to the field.

This rotation may be ignored (especially in small motors), or be achieved either mechanically, by moving the brushes, or electrically, by running the rotor current through small auxiliary windings near the brushes to 'tilt' the stator field.

The surface condition of the commutator controls a surprising amount of voltage drop at the brush/copper interface. Deposits may have built up favouring one direction of rotation. If all else fails, try cleaning the commutator surface with very fine grit paper, remembering to clean all the copper dust from the slots before trying to use it again.

  • \$\begingroup\$ I have added images for better understanding. I sanded the commutator carefully a small amount but it still arcing, another thing that I'm worried about is that the new position of the wires leading to the brushes will affect the field and tilt it in any way. Should I be worried about that? \$\endgroup\$ – Juani Elias Dec 22 '20 at 18:56

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