I just read about commutators and their uses in motors and dynamos. While revising the topic I came across this question:enter image description here

According to me the answer should be (c). Because: enter image description here

In the above diagram of dc motor the direction of current in coil changes from ABCD to DCBA.

enter image description here

Similarly, in the dc generator shown the direction of current in coil changes from DCBA to ABCD.

But the answer according to the book is (a). This makes me wonder if I missed something in the concept.(or if the given answer is wrong, which is less likely)

Can somebody please clarify.


  • \$\begingroup\$ A DC generator can potentially use a (3 phase) rectifier rather than a commutator, so the commutator may not be necessary. A rectifier will be far longer lifetime and more reliable. Having said that, A DC brush motor will work fine as a generator, so some generators have mechanical commutators. (Changed my answer to a comment as Heath below has an excellent point, which I upvoted.) \$\endgroup\$ – John D Jan 26 at 20:02
  • \$\begingroup\$ @JohnD That would make the generator an AC generator. \$\endgroup\$ – StainlessSteelRat Jan 27 at 1:35
  • \$\begingroup\$ @StainlessSteelRat Yeah, true- Kind of a semantic distinction, but you're probably right. \$\endgroup\$ – John D Jan 27 at 3:12
  • \$\begingroup\$ @StainlessSteelRat If BLDC motors can be called DC, then an AC generator packaged with a rectifier can be called DC too. \$\endgroup\$ – Hearth Jan 27 at 13:24
  • \$\begingroup\$ @Hearth That is a stretch. A DC machine can work as motor or a generator. A BLDC motor working as a generator is closer to a three-phase ac generator than a DC generator since it generates three pure sinewaves. But ac generators powering DC buses may make a comeback on ships. Onboard DC Grid \$\endgroup\$ – StainlessSteelRat Jan 28 at 20:03

When using it as a generator, the commutator isn't responsible for changing the direction of the current in the coil itself. The coil current changes direction just because that's how it physically works; the job of the commutator in this case is to 'undo' that direction change so that the output current stays in the same direction.

That said, don't feel bad. I would have picked c) as well; it's only your mentioning that a) is the correct answer that got me thinking enough to make this realization. It's a tricky question.


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