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Subtle stutter/pulsing at slow speed

Discussion in 'Model S: Driving Dynamics' started by Andrew, Jul 7, 2013.

  1. Andrew

    Andrew Model S #6151

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    Tonight as we turned into our driveway from the alley, going about 2 or 3 mph, we noticed a slight pulsing in the motor. As I then maneuvered into the garage, I noticed it wasn't doing it in reverse, but when I went back to drive and drove forward, it did it again. It was subtle, but unmistakable (my passenger felt it as well).

    We're on v4.5 (.55, if I remember correctly). Regen is set to Standard, Creep is off, Steering mode in Standard.

    I searched the forums but didn't find anyone else reporting this. Anyone have any ideas of what might be going on? Thanks!
     
  2. EMDoc

    EMDoc Member

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    I've got this too and noticed it a while ago. I think its normal and just a low power state for the motor...
     
  3. Todd Burch

    Todd Burch Electron Pilot

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    Yes, it seems normal to me.
     
  4. Andrew

    Andrew Model S #6151

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    Thanks EMDoc and Todd. Glad to know it's not something really wrong, but it's so weird that we've never had it happen before. Getting into our garage requires some careful maneuvering, and we always go very slowly as we make our back-and-forth turns to get into the spot... Pretty sure I wasn't going any slower this time or doing anything differently.

    Huh.
     
  5. Andrew

    Andrew Model S #6151

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    Just ran a couple of errands and when I came home, I couldn't get the car to reproduce the shuddering. Strange.
     
  6. cinergi

    cinergi Active Member

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    If I'm understanding the description correctly, I've had this since day one (October 2012). I can reproduce it if I let the car roll slightly backward and then apply a little power. I never experienced this in the Roadster. I did report it multiple times when I first got the car.
     
  7. TurboFroggy

    TurboFroggy Member

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    #7 TurboFroggy, Jul 7, 2013
    Last edited: Jul 7, 2013
    It is called motor cogging: http://en.wikipedia.org/wiki/Cogging_torque

    Every AC drive EV I have ever owned did this to some degree or another. It is because an AC motor has multiple poles each one is a little bump. At higher speeds they blend but at very low speed given enough torque you can experience the thump thump as the rotor passes each of the 4 poles of the stator in the motor.
     
  8. Andrew

    Andrew Model S #6151

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    Ah, interesting. Thanks!

    I tried again this afternoon to reproduce it, and couldn't. At least now I won't worry that something's wrong if it does happen again. :)
     
  9. cinergi

    cinergi Active Member

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    Except that this is an AC Induction motor -- there are no permanent magnets creating static poles in the rotor. Likewise, this didn't happen in the Roadster.
     
  10. nrcooled

    nrcooled P#8946 VIN 03225

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    I dont know if this helps but I have spoken to Tesla engineering about this and I was told that it is normal.
     
  11. Jason S

    Jason S Model S Sig Perf (P85)

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    It is very subtle. I find it only when using the power as 'hill hold'. It gives a slightly 'floaty' feeling for me, but I've asked my wife and she doesn't notice it.
     
  12. qwk

    qwk Model S P2681

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    I have noticed this too. Very subtle, and very difficult to reproduce. I figured it was a characteristic of the 3 phase AC motor at very low speed.
     
  13. Todd Burch

    Todd Burch Electron Pilot

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    Yes--I think it's more recognizable when you're on a very slight uphill incline, as more torque is needed to creep forward. If on very level ground, it might not be noticeable.
     
  14. Duckjybe

    Duckjybe S P232

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    Good that others are reporting this as I thought it was something potentially wrong with my car. I was wondering if there is a difference at all between the performance and non-performance drive motors? Since Andrew has the non-perf it appears that is not the case.
     
  15. mknox

    mknox Well-Known Member

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    I have noticed this too, but not all the time. It seems to start to happen for me if I've been in stop-and-go traffic for a while. I'll give it a (very) little "gas" and feel the shudder as I creep forward in traffic. Other times it will be as smooth as silk.
     
  16. Andrew

    Andrew Model S #6151

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    Hmm... I wonder if this occurs only [or more readily?] at a slightly lower state of charge? I had about 105 miles left when it occurred the other night; when I tried to reproduce yesterday my range was around 150.
     
  17. jcadman22

    jcadman22 MD: DRK NRG

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    I have felt this too. I believe it is normal and not related to charge level. It is most noticeable on smooth pavement going slightly uphill such as sitting in stop-go traffic.
     
  18. TurboFroggy

    TurboFroggy Member

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    It doesn't matter if it is an AC motor or a PM motor. When the motor is energized there are 4 distinct poles of electro-magnets in the motor that cause this. Here is a better link: http://www.lmphotonics.com/InductionMotor/MCog.php
    An AC induction motor makes it's own magnets via coils of wire in the stator and induces the current in the rotor which causes the rotation. Since there isn't a infinite number of poles in the motor it can cause a pulsing at very low speed since you can feel the rotor as it passes between the 4 poles. The torque curve will look wavy at very low single digit RPMs and will quickly smooth out due to angular momentum of the rotor. If you had a motor with 1000 poles on it you wouldn't notice it as the ripples of torque at very low speed would be very tiny, however on a 4 pole motor it happens 4 times per revolution, every 90 degrees or so.
     

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