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DC v AC EV system ?

Discussion in 'Technical' started by Cobos, Jan 7, 2008.

  1. Cobos

    Cobos S60 Owner since 2013 - sold, S85D owner since 2017

    Jun 22, 2007
    Oslo, Norway
    I feel a bit stupid asking this, but could someone please explain to me what the rationale for a DC or a AC system is ? It seems all personal EV conversions use a DC system while Tesla and I believe Think both uses AC.
    Am I correct when I assume AC is more expensive, but why then doesn't every EV use DC ?

  2. tonybelding

    tonybelding Active Member

    Aug 17, 2006
    Hamilton, Texas
    I could be wrong about this. . . But I don't think you can do regen with DC? Also many DC motors have brushes or permanent magnets, or both, so there would be more maintenance issues in the long run.

    Somebody who knows more about motors will probably give a better answer.
  3. malcolm

    malcolm Active Member

    Nov 12, 2006
    #3 malcolm, Jan 7, 2008
    Last edited: Jan 7, 2008
  4. JRP3

    JRP3 Hyperactive Member

    Aug 20, 2007
    Central New York
    Brushed DC motors can do regen with the right controller, but it takes a more sophisticated, (expensive) controller and it can often be hard on the brushes, so it's not really practical.
    Brushed DC is used for most conversions because it's the cheapest way to get the most performance for your dollar.
  5. Kardax

    Kardax Member

    Jun 8, 2007
    Minnesota, USA
    I agree with JRP3, DC is easier to do.

    AC induction is more complicated technically, but in terms of parts, I think it may actually be cheaper (no need for permanent magnets). I think AC also gives a smoother experience at very low speeds--the magnets in a DC motor cause a noticeable lurch as they turn over.

  6. stopcrazypp

    stopcrazypp Well-Known Member

    Dec 8, 2007
    From what I see it seems AC conversion kits cost a little more than DC conversion kits by a couple thousand dollars. AC motors also seem to cost a little bit more than DC ones for one of around the same weight/power/efficiency. Probably has to do with avaliability of AC motors, as DC motors seem more widely avaliable for the EV conversion market. Though I suppose complication probably has the most to do with it.
  7. TEG

    TEG TMC Moderator

    Aug 20, 2006
    Yes, that is my understanding as well.

    I have also heard that you can have more control of an AC (induction) motor.
    With DC (permanent magnet) you just put current and voltage into it and it basically spins as fast as it wants. With AC you are controlling the magnetic fields directly. It is ***much*** more work for the controller but then you have much more control over the RPMs of the eMotor. To try something ambitious like clutchless computer controlled shifting (with exact rev matching) might be more possible with an AC motor (although it sounds like Tesla didn't pull off that magic trick just yet).

    There are also differences in the way the torque and efficiency curves map out over an RPM range so there are different trade-offs between the two technologies.

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