"Due to armature reaction, the natural direction of the magnetic field of the poles is distorted. Thus the neutral plane is also altered. Therefore, the armature conductors are not at zero potential when they come in contact with the brushes. This leads to sparking across the brushes and loss of power."

I've seen that statement in various different videos regarding the subject. However I'm confused. I understand how the neutral plane shifts due to armature reaction, but I'm wondering why the armature needs to be at zero potential when it comes in contact with the brushes?

Aren't brushes supposed to transfer the current generated in the armature to the load? Why would the armature have zero potential, when in contact with the brush?

Any help would be appreciated, thanks!


1 Answer 1


I'm not familiar with the theory of DC generators, but what your sources are trying to explain sounds fairly straightforward just on general electrical principles.

Look at the brush/commutator connection as a switch. In any kind of switch, when contact is made or broken, there will be a short time period of very high resistance (whether this involves arcs or merely a small contact area). That resistance will dissipate power uselessly, and arcing will cause wear.

If you instead arrange the timing of the commutator so that the contact is only made or broken when there is no potential difference and therefore no current — or, realistically, just very little — and the majority of the current flows while the commutator is “fully on” (contact over the entire area of the brush) then there will be no/little dissipation or arcing, so the generator will be more efficient.

That point would be when the magnetic field and armature are aligned so the magnetic field does not induce any voltage (the “neutral plane”) — the same position as when, if it were an AC generator without a commutator, the output voltage would cross zero.

  • \$\begingroup\$ +1. Bigger DC machines (30kW and up) have additional commutating pole windings on the stator poles which are connected in series with the rotor winding. This re-adjusts the neutral zone. \$\endgroup\$
    – Janka
    Apr 1, 2017 at 16:09
  • \$\begingroup\$ So I understand now how it's preventing arcing, because if there was a relatively high potential difference, it'll cause the higher potential to jump to the lower which causes arcing. However when it's not in the neutral plane, and it's cutting through lines of flux, if the brushes only make contact with the armature only when it's in neutral, where is that induced current in the armature going? \$\endgroup\$ Apr 1, 2017 at 16:25
  • \$\begingroup\$ @MattJ I think it's just a terminology confusion — by "make" I mean the process of going from no contact between the commutator segment and the brush, to full contact. The current flows during the full-contact period after make and before break. \$\endgroup\$
    – Kevin Reid
    Apr 1, 2017 at 17:16

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