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I recently heard that in a universal motor (and I'm presuming regular DC motors too) they use "high resistance carbon brushes", and I can't seem to find anywhere that explains why they need to use "high resistance" brushes. Wouldn't it be the lower the resistance the better? The only thing I could find was this on Wikipedia.

For certain types of electric motors or generators to function, the coils of the rotor must be connected to complete an electrical circuit. Originally this was accomplished by affixing a copper or brass commutator or 'slip ring to the shaft, with springs pressing braided copper wire 'brushes' onto the rings which conduct the current. Such brushes provided poor commutation as they moved from one commutator segment to the next. The cure was the introduction of 'high resistance brushes' made from graphite (sometimes with added copper). Although the resistance was of the order of tens of milliohms, they were high resistance enough to provide a gradual shift of current from one commutator segment to the next. The term brush remains in use. Since the brushes wear out, they can be replaced in products intended to allow maintenance.

Based off that, my question would be: Why is it important that there be a gradual shift of current from one commutator segment to the next? How is the achieved by having slight resistance in the brush, and how does that solve the initial problem shown in italics?

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Imagine a brush that momentarily spans the gap between two commutator segments, connecting to both of them.

Each commutator segment connects to a different winding on the rotor, in a different part of the magnetic field generated by the stator. Thus each winding is generating a different voltage (back EMF in a motor) and they are shorted together by the brush.

This is similar to a shorted turn in a transformer, and allows large currents to flow momentarily - and as the other answer says, large dI/dt causes sparking (in addition to the waste of power).

As the adjacent windings generate different - but fairly similar - voltages, the resistance needed to limit these currents is not very high.

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Based off that, my question would be, why is it important that there be a gradual shift of current from one commutator segment to the next?

Because di/dT in an inductor is what causes a flyback voltage spike and causes sparking.

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  • \$\begingroup\$ Ok, that makes sense, but how does having such a slight resistance on the brushes reduced the rate at which the current changes? The only thing I would have assumed it achieves is slightly reducing the overall current at all times, and only by insignificant amount. \$\endgroup\$
    – DrMolo
    Apr 10, 2020 at 4:54
  • \$\begingroup\$ @DrMolo Well, there's an LR time constant and the R is still a hundred to a thousand times more than copper. \$\endgroup\$
    – DKNguyen
    Apr 10, 2020 at 5:45
  • \$\begingroup\$ And if you use copper brushes on a copper commutator (ignoring friction) the sparking causes tiny welded contact points and the life is very short. Add friction and it is even worse. \$\endgroup\$
    – Solar Mike
    Apr 10, 2020 at 6:26

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