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Why does tesla use an AC motor?

Discussion in 'Model S: Battery & Charging' started by doctorwho, Jan 8, 2014.

  1. doctorwho

    doctorwho Member

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    Hi there
    my second thread is a question, does anyone know why Tesla has chosen to use an AC motor rather than a DC motor? Given that Nikolai Tesla was a champion of DC and there would be inefficiencies converting the stored DC energy into AC, there must be a good reason for this. I won't lose any sleep if nobody knows, just Teslacurious.
     
  2. markwj

    markwj Moderator, Asia Pacific

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    Tesla (the man) was a proponent of AC (not DC). Wasn't it Eddison that was pushing DC?

    AC motors support regen.
     
  3. Yggdrasill

    Yggdrasill Active Member

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    Tesla was the biggest champion for AC, Edison was the biggest champion for DC. And an AC motor is more efficient, maintenance-free and easily allows for regenerative breaking, so there are good reasons for using them.
     
  4. gpetti

    gpetti Active Member

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    Not particularly an expert but doesn't a DC motor have more moving parts in order to swap polarity etc. (therefore wears out more quickly and more likely to break)?
     
  5. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Tesla invented AC electricity. You are thinking of Edison, who championed DC.
     
  6. spentan

    spentan Active Member

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    Strong thread backfire
     
  7. tom66

    tom66 Member

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    Brushed DC motors allow for regen, too. However, it's a lot harder for the regenerative mechanism can't recover all of the energy down to 0RPM, unlike with induction motors, which can be designed to allow this.

    There are two main types, brushless DC and brushed DC. Brushed DC is probably what you are thinking of.

    Advantages of DC motors:
    - control/power electronics are simpler, run cooler (more efficient operation)
    - flat torque band (AC motor torque rolls off after about half speed reached), near infinite torque from 0RPM

    Disadvantages of DC motors:
    - requires manual/automatic gearbox with 3-4 gears
    - brushes have to be regularly serviced - maintenance item
    - expensive magnets for output power required if using permanent magnets; if not, requires a field coil, creating losses at low speeds
    - lower overall efficiency
    - harder to cool motor
    - larger size for comparative power levels

    As for brushless vs induction, it's a lot less clear cut, but there's a Tesla blog on this:
    Induction Versus DC Brushless Motors | Blog | Tesla Motors

    Brushless DC are a lot like induction motors, in that they operate with 3 phase power, although the brushless DC motor can only operate from an inverter capable of tracking rotor position to sequence the motion of the rotor correctly.
     
  8. artsci

    artsci Sponsor

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    As Jerry33 pointed out, Tesla was not only a champion of AC, he invented it.
     
  9. tom66

    tom66 Member

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    He did not invent AC; the concept of AC had existed before. He designed a system of electrical generation, transmission, conversion and use around AC, which included designing practical transformers, motors and generators. He was the father of AC distribution, but he didn't invent AC.
     
  10. WarpedOne

    WarpedOne Supreme Premier

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    Actually both DC and AC motors are 'alternating current machines'.
    ALL electric motors operate/turn because of rotating magnetic fields and you can only generate rotating magnetic fields by alternating direction of the current through the coils.
    In a DC motor there is AC through its coils. It is just that DC motors have a mechanical 'inverters' that make AC out of DC (in a fixed way) and AC motors have electronic inverters that create AC out of DC (in variable way).

    It is not surprising that electronics offer more flexibility and adaptability to different circumstances than mechanical systems can.
    A simple mechanical system can be more efficient than a simple electronic system because of less conversions and hence smaller losses.

    DC motors are therefore best suited for systems with small variability/constant load. AC motors are better where there is more variability/changing load.
    Driving at 20mph is different than doing 100 mph, accelerating is different from decelerating, ... hence AC is better in a car. Better meaning better HP output, better average efficiency, better responsiveness, better control.

    A DC fan could make more sense than an AC fan.
     
  11. stevezzzz

    stevezzzz R;SigS;P85D;SigX

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    Good discussion and an excellent question that every newbie must ask. I remember, way back in early 2010, when as a new Roadster owner the question first occurred to me. I was standing in an enclosed vehicle trailer on the shore of a frozen lake (trying to stay out of the bitter wind between runs on the ice), with a bunch of other Roadster owners and Tesla gallery employees. I asked the question, and in that crowd nobody had a good answer.
     

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  12. ggies07

    ggies07 Active Member

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    Seems like everyone needs to brush up on this man. I just got this book for Christmas and it's quite good so far:

    512QmqhobhL._BO2,204,203,200_PIsitb-sticker-arrow-click,TopRight,35,-76_AA300_SH20_OU01_.jpg
     
  13. Shumdit

    Shumdit Member

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    Wait! These cars run on electricity?? How long is the extension cord and does it auto wind back into the car when you reach your destination?
     
  14. mnx

    mnx 2013 P85

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    Interesting Tesla blog post, thanks. I wonder how much higher sustained performance levels could be achieved if they had used a DC brushless motor...
    It seems like for racing applications DC brushless is the way to go.

    "One of the main differences is that much less rotor heat is generated with the DC brushless drive. Rotor cooling is easier and peak point efficiency is generally higher for this drive."
     
  15. bonnie

    bonnie Oil is for sissies.

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    I was thinking it was more like thread regen.
     
  16. scaesare

    scaesare Active Member

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    This list covers some good points. I'd like to point out that the two items I highlighted also lead to significantly increased weight, which is obviously a factor for an electric vehicle...
     
  17. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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  18. scaesare

    scaesare Active Member

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  19. neroden

    neroden Happy Model S Owner

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    Only one moving part, no meaningful wear parts, no permanent magnets, cheap materials.

    AC induction motors are everyone's motors of choice these days. They were unpopular in the past because they require extremely extensive control circuitry, but that became cheap in recent decades.
     

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