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I understand that how armature reaction causes neutral plane to shift by some angle . If brushes are placed along Geometrical Neutral Axis, then there'd be sparking during commutation as voltage across coil being short-circuited wouldn't be zero. Then I read that brush shifting (technique which is obsolete since 1910 but still finds its way in tricky questions in kind of exams I'm preparing for) changes current distribution. Now, I have two doubts, please refer to following figures, fig. A (left) and fig. B(right):

enter image description here

  1. Referring to fig. a. If I keep brush at original position (i.e. along Geometrical Neutral Axis), then considering the fact that armature reaction causes neutral plane itself to shift, shouldn't there be current flowing through conductors which are along GNA (because now they're not along Magnetic Neutral Axis, since GNA and MNA do not coincide)? Why is there no current marked on those conductors? PS it doesn't seems that it is just printing error because I've refer several books and all had to say something like "current redistributes when brushes are shifted" (and I assumed "not otherwise").

  2. With ref. to fig. B, I don't exactly understand how current redistributes when brushes are shifted. I mean, isn't the current depended on voltage? And if voltage is depended upon flux density only (speed and other things remaining constant), how can changing brush position redistribute current? I thought effect of brush and commutator is just to rectify induced alternating current into direct, how it affects something inside the machine? (I know it does because so many sources said that, but I can't figure out how, please help me, even small hint would be enough). Also the dot and cross marks are shown for current and not for voltage, right?

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  • \$\begingroup\$ Look up "armature reaction". Voltage is dependent on the flux the conductors are passing through, but that flux isn't invariant. \$\endgroup\$ – Phil G Sep 26 at 21:13
  • \$\begingroup\$ I have got my answers. The marking is for emfs. Ans to Q1 is current in fact flows through those conductors & emf is induced which could've been marked. Ans. to Q 2 basically lies in ckt theory that if I connect 3 batteries of 5 V in series such that vltg across combination is 10V, then one would be charging, i. e. independent voltage can't alone decide current direction (duh, i admit it was me being dumb). BUT what is more interesting thing that I learned that even though polarity of that emf changes, it is compensated by othr conductor thus no net change in op vltg.I shall self-ans later. \$\endgroup\$ – Deep Sep 27 at 7:21
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ad 1) The current is distributed to the each half. The brushes are short circuiting the segments at GNA position, so the only current that flows to that section is a recirculating current, from one segment to the other through the brush contact.

enter image description here enter image description here

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  • \$\begingroup\$ ty sir, but suppose there was one more coil side in my fig. a, in between new neutral plane and GNA(i. e. in left half) , what would be the direction of current through it? Cross (because it's under 'new' S pole now) or Dot (because it's left to the short-circuited coil? If my understanding is correct, it must be Cross, am I right? \$\endgroup\$ – Deep Sep 27 at 2:47
  • \$\begingroup\$ it seems that in that case (mentioned in my prev comment) voltage and current will not be in same direction through that coil side..? \$\endgroup\$ – Deep Sep 27 at 4:31
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    \$\begingroup\$ @Deep If you mean a current as vector, then it's not the same, it is not at right angle anymore vs. excitation vector. \$\endgroup\$ – Marko Buršič Sep 28 at 13:31

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