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AC to DC quick clarification, thanks

  1. Feb 25, 2009 #1
    In my algebra/triganometry-based physics class (on electricity and magnetism) and yesterday our professor went over how to convert and AC generator into a DC generator; it got kind of confusing so i tried to read it over in our book but there's only like 3 senteceson it... could anyone help explain it to me in a simpler way? thanks.
  2. jcsd
  3. Feb 25, 2009 #2


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    This seems to explain it pretty well:
    http://www.electronics-tutorials.ws/diode/diode_6.html [Broken]
    Last edited by a moderator: May 4, 2017
  4. Feb 25, 2009 #3
    that goes beyond the scope of my physics class (it's just a physics 2 level course)... it does help a bit though
  5. Feb 25, 2009 #4
    What does your book say, exactly? If it's only three sentences copy them over here; then people will know what needs explaining.
  6. Feb 25, 2009 #5
    The book is like... "AC generators have alternating current flow; it flows both CW (clockwise) and CCW (counterclockwise) so that there are both types of currents in the circuit. DC, on the other hand, only allows one of the two current flows within the generator; it is either CW or CCW with no alternation between." then he went on to say that AC generators rotate the circuit loop in the magnetic field (B) direction which causes the change in current... i think?... so im assuming to go to a DC generator you stop the loop rotation...?
  7. Feb 25, 2009 #6
    Last edited by a moderator: Mar 1, 2009
  8. Feb 25, 2009 #7
    That Wiki link seems to be a dead end.

    This one might help:


    The commutator replaces the slip rings in the AC alternator. The commutator is split: it is two halves that don't touch each other, and one end of the loop that rotates between the magnets is attached to each half. As the shaft rotates one side of the commutator breaks contact with one brush and makes contact with the other brush. This insures that each brush is always picking up current that is going in the same direction, despite the fact the loop as a whole is generating current in alternating directions as it rotates bewteen the magnetic poles. This may take some time to sort out if you're not familiar with it, but it's actually fairly simple. You can see the commutator in the diagrams as a disembodied split cylinder. One end of the (greatly simplified) loop is attached to each half of the commutator. On an actual generator the commutator is mounted on the rotating shaft. The brushes are sticking out on each side, one opposite the other, with lines coming off them leading to + and - signs. The brushes are fixed in place to the generator housing and the commutator rotates around between them but in pressure contact with them.
    Last edited by a moderator: May 4, 2017
  9. Feb 26, 2009 #8


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    Looks like map19 meant this:
    Last edited by a moderator: May 4, 2017
  10. Feb 26, 2009 #9
    thanks everyone... seriously that helps
  11. Feb 26, 2009 #10
  12. Mar 1, 2009 #11
    I think I have a simple explanation of the difference between AC and DC current and how an AC generator can produce DC current. In an AC circuit the flow of electrons go forward and backward. In a DC circuit the electrons move in one direction. A diode is an electronic device that is like a one-way valve. If you put a diode in an AC circuit from an AC generator this will allow the electrons to pass through one way but not go backward.
    After the diode: the current will be going forward only and this is DC current.
    Automobile charging systems work in this way. The alternator is an AC generator and inside the alternator are diodes that turn the current to DC, which can then charge the Battery.
  13. Mar 1, 2009 #12
    Generators can't produce pure DC as from a battery.

    It is actually rectified AC which has a ripple. It can be smoothed with chokes and capacitors.
  14. Mar 1, 2009 #13
    With all due respect, while your statement that “generators can’t produce pure DC as from a battery” is in essence correct, your statement that “it is actually rectified AC which has a ripple” is patently incorrect.

    DC generators require no form of rectification whatsoever nor do they produce a rectifiable AC voltage. While the rippling on top of the DC voltage is often referred to as an “AC component”, this is entirely different than an actual “AC generated voltage”. AC requires reversal of both voltage polarity and current direction, which clearly isn’t occurring per the output of a DC generator.

    Rather ingeniously, the DC generator, via its multiple commutator segments pairs and associated multiple armature windings (wound at various degrees apart on the armature’s shaft) completely eliminates any need to rectify any AC voltage induced in any of the non-conducting armature windings. This is because only ONE armature winding is ever selected per a given orientation in the armature’s rotation making it the ONLY conductive armature winding capable of delivering current. Since this selected armature winding is always physically oriented within the same magnetic polar region of the stationary magnetic fields, the polarity of the voltage that will be induced will always be the same polarity hence, a DC voltage is generated per a given direction of the armature’s rotation.

    It makes no difference that the non-conducting armature windings located prior and after this select magnetic polar region of the stationary magnetic fields will have some varying degree of an opposing polarity induced in them, as by the time these armature windings once again rotate around into the select magnetic polar region of the stationary magnetic fields, the voltage polarity induced in them will once again be of the same polarity hence, a DC voltage is generated whose peaks only ripple due to the variation in flux density of the stationary fields within the select and limited magnetic polar region in which voltage is to be induced.

    Some technical explanations will misleadingly state that this technique of using multiple commutator segments and multiple armature windings constitutes a form of “mechanical rectification”, but this is as misleading as it is incorrect and here’s why.

    In rectifying an AC voltage, the AC present allows current flow “in one direction only” through the load (via half wave rectification, current in both directions via full wave rectification, but in either case, a single polarity is established in the circuit) therefore, current from the AC source IS provided and energy dissipated by the load.

    The DC generator’s ingenious arrangement of multiple commutator segments and multiple armature windings prevents ANY current from flowing from the non-conductive armature windings regardless of their induced voltage polarity or voltage potential. No actual AC rectification ever occurs nor is any form of rectification required.
    Last edited: Mar 1, 2009
  15. Mar 3, 2009 #14
    I don't think all that makes much sense.
  16. Mar 3, 2009 #15
    No, it was all correct. I think you may be thinking the only kind of DC generator is like the alternator in a car which actually produces AC that is then rectified. However the kind of multiple coil DC generator Gnosis described also exists and he described it's workings very well. If you take apart any automobile starter motor (which can be run in reverse as a generator) you'll see exactly what he's talking about.
  17. Mar 3, 2009 #16
    I know about generators of various kinds I just dont agree with some of the analysis.

    Rotating coils produce AC and whether you use diodes or mechanical rectification the effect is the same.

    The DC will have ripple obviously.

    I see nothing wrong in the term 'mechanical rectification'. In the early days there were devices which used a commutator spun by a low power synchronous motor which were mechanical rectifiers- rotary converters.
  18. Mar 3, 2009 #17


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    Let me chime in. Yes a commutator acts like a mechanical rectifier so we might expect a rippled output waveform similar to that produced from conventionally rectified AC. The difference however is that a typical commutator has very many segments, probably more than 60 (30 pairs) making it something like a 30 phase rectifier (possibly even more). Since the tops of sinusoids are relatively flat having such a high pulse number makes the output of a typical DC machine really quite flat, with the ripple typically undetectable. If you look at the output of a (mechanical commutator) DC machine then much more noticeable then any "rectifier ripple" is a noise spike at each commutator switching. That is, it looks pretty much like pure DC with a bunch of periodic noise spikes.
    Last edited: Mar 3, 2009
  19. Mar 3, 2009 #18
    If you have a lot of windings and segments on the commutator then ripple will be small and yes switching spikes will be significant.. Some generators wont have many. Basically it is pretty dirty DC that you get from a DC generator.
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