Commutator on a DC machine

Discussion in 'Homework Help' started by nyasha, Dec 11, 2011.

  1. nyasha

    Thread Starter Active Member

    Mar 23, 2009
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    Can someone help me understand how does the commutator act as a mechanical rectifier of the AC voltage ?
     
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    your question doesn't make 100% sense to me but I think I know what you are talking about. A while back I was reading about universal motors, which are basically a series wound DC motor with low reluctance laminations so that it can efficiently run on AC. The material I was reading said that the commutator acts as a mechanical rectifier. I do not agree with this statement. if you take a series wound motor and apply DC to it, it will spin ,say clockwise. If you then reverse the polarity, it will still spin clockwise. it's not until you swap the ends of the field winding that the motor will reverse direction. the series wound motor does not care about polarity. so if you connect AC to it, it doesn't care if the polarity swaps 50 or 60 times per second, it spins always in the same direction. So there is no rectification going on there, mechanical or otherwise.
    please inform if that is what you were referring to.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    There is a mix-up of ideas here. The commutator of a DC generator is indeed a sort of rectifier (if the armature output came out via slip rings, it would be AC).

    This is separate from the explanation of why a universal motor can run on AC as well as DC, which is because armature and field reverse at the same time.
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Wikipedia has tons of references to Mechanical Rectifier being built into commutators and brushes.

    I just did a search for it, and it seems somebody added that in every article they could.
     
  5. BillB3857

    Senior Member

    Feb 28, 2009
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    If you look at the drawing here.... http://www.pc-control.co.uk/dc-motors.htm
    you will see that current is going in a clockwise direction. As the armature rotates, the commutator bars cause the current to still go in a clockwise as referenced to the magnets,direction, even though it has reversed in the wire itself.
     
  6. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I guess I was thinking of a motor instead of a generator, which doesn't really make sense when talking about a rectifier. Thats why I assumed he must be talking about the universal motor.
    Can you link to one of these articles? I couldn't find it. I looked in these:
    http://en.wikipedia.org/wiki/Brushed_DC_electric_motor
    http://en.wikipedia.org/wiki/Electric_machine#Brushed_machines
    http://en.wikipedia.org/wiki/Electric_motor
    http://en.wikipedia.org/wiki/Electric_generator
    http://en.wikipedia.org/wiki/Dynamo#Dynamo_as_commutated_DC_generator
    http://en.wikipedia.org/wiki/DC_motor#Brush
     
  7. thatoneguy

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    Feb 19, 2009
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    strantor likes this.
  8. amilton542

    Active Member

    Nov 13, 2010
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    In addition, to avoid confusion when reading reference sources, where so many fail to address the state of the machine, I hope this will help.

    When the machine functions as a motor the commutator is a mechanical switch in order to preserve opposing polaritys. Conversely, if the reciprocal function holds true and it's a "Jenny" then yes the commutator functions as a mechanical rectifier.

    Faradays law will always hold when a conductor "cuts" a magnetic field. Amid the pole shoes (pole pitch) no voltage is induced in the coils. In effect, the commutator input will have a tendency not to be a pure sinusoid but a rectangular waveform changing polarity over a period of time. Once the induced EMf is being furnished as an output it will alternate between it's maximum value and zero due to the insulating medium between segments.

    In a two segment commutator this is AC superimposed on the DC output and is not desirable. To compensate for this you increase the number of segments until the output tends to pure DC. If the opposite is true, and the state of the machine is a motor, the extra segments will provide a uniform "current sheet" and will furnish smoother torque that does not fluctuate.
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    While this is true, not many people wanting to rectify AC to DC want to run it through a motor to get the result they are looking for.

    That is why I am confused at the number of references to mechanical rectifier on the wiki page. I would think the mechanical rectifier would be a buzzing 60Hz relay, similar to the "buzz relay" used in early DC-AC inverters.
     
  10. amilton542

    Active Member

    Nov 13, 2010
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    Unfortunately, still being a "kid", I am not aware who would want to rectify AC to DC through a motor.

    I am only familiar with T&D relays. When I search for a mechanical rectifier on wikipedia it takes me straight to the definition of a commutator in its entirety.
     
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