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Slip rings, commutator, dc motor

  1. Oct 24, 2015 #1
    If i'm not mistaking , all DC and also universal motors have a brushed commutator not a slip ring commutator because the rotor wires need to keep the same current direction as they rotate pass the same magnetic stator pole.
    so the only DC motor which could operate on slip rings is a homopolar motor since the current doesn't need to change direction ?

    now in terms of wear and robustness , do slip rings have a longer life and can work at higher speeds than a commuttator , is it because a slip ring is made out of one piece with no slots and so the brush ravels smoother along its surface with less friction?
    also i suppose in slip rings there is much less arcing than in commutators beacuse the current isnt switching ?
     
  2. jcsd
  3. Oct 26, 2015 #2
    In AC the current switches so the brushes maintain contact, in DC the brushes disconnect from one polarity and reconnect to the other polarity repeatedly which does cause more wear from arcing.
     
  4. Oct 26, 2015 #3
    I can't quite agree , even with AC fed universal motor the commutator still switches the current , because many universal motors running on AC spin up to very high rpm much faster than an induction motor could on the AC frequency due to the fact that an induction motor is frequency locked but in the universal motor the rotor at very high rpm , a single half period of an 50hz ac wave can be long enough so that more than one commutator slot has passed the brush and that means that say for example 2 or more commutator slots pass the same brush before the AC current has changed direction into the other half period which again at very high rpm can last long enough for multiple slots to pass a single brush which would indicate that the commutator still does the switching even with AC in a universal motor, I would be happy if someone here could elaborate or confirm.

    thanks.
     
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