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The real top speed of the Model S

Discussion in 'Model S: Driving Dynamics' started by TheAustin, May 28, 2013.

  1. TheAustin

    TheAustin Model S: P2009 P85

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    I posted this in Tesla forum, but couldn't get any definitive answers, so I thought I would ask the enlightened folks over here too..

    So, we all know that the top speed of the Model S is software-limited to around 130mph (depending on model/options)...If there was no software limitation, what do you think the real top speed of the Model S would be. I would think pretty high, considering the 416hp/443ft-lb torque, even considering the weight of the car. But if you had to pick a number, what would you guess? (Anyone out there "jailbreak" their S yet?!?!)

    And, why do they limit the top speed in the first place? Is it vehicle/passenger safety? Is it to specifically protect the life of the batteries? Would it be too difficult to properly cool the battery pack while going 160, 180, 200mph? Not that you could even drive at that speed for a significant amount of time. But still...

    Just wondering. Any thoughts?
     
  2. dsm363

    dsm363 Roadster + Sig Model S

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    The Model S only has one 'gear' I thought so likely limited for that reason but I know others will have a more technical answer and a more accurate one.
     
  3. SCW-Greg

    SCW-Greg Active Member

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    Yes, this. Electric motors eventually hit their max RPM given the voltage in. Don't know what the gear box can handle too. It's possible that the car would tear itself apart with pack input it has... motor seize, or coils melt, etc. without these limits, but who knows at what point.
     
  4. stevezzzz

    stevezzzz R;SigS;P85D;SigX

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    I wonder if TM has done high-rpm testing to destruction of the motor/gearbox, or if they just designed in a safety factor and set the software limit accordingly. Anyone know the answer?
     
  5. ChadS

    ChadS Petroleum is for sissies

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    I haven't heard anything for the Model S. But for the Roadster, I heard they did a 24-hour test with the motor at 24,000rpm. It's limited to 14,000rpm.

    I didn't hear why they set the limit there, but I would imagine:
    - they want a wide safety margin, especially considering manufacturing tolerances
    - that takes a TON of power, and especially on the Roadster it would probably quickly overheat. I suspect heat is the limiting factor, at least on the Roadster
    - in fact it would probably run out of power soon; aerodynamic drag is a huge force at that speed and increases quickly
    - they'd have to put on more expensive tires that may be less capable in some other criteria
    - performance would be pretty sucky at that point given the gearing, so there's not much to gain from messing with the above items
     
  6. Johan

    Johan Took a TSLA bear test. Came back negative.

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    I concur that the limitation is all about the fixed gear togheter with the heating issue.

    Also, dont forget this graph, that basically says that with the current setup and gearing torque becomes mediocre at high RPMS and in fact if you sped up the motor even more perhaps torque would drop to a level where there simply wouldn't be enough torque to increase speed over a given treshold (remember air resistance increases exponentially with speed). This particular issue of course would be solved by multiple gears, the heating issue not so much.

    attachment.php?attachmentid=2755&d=1317483193.jpg
     
  7. brianman

    brianman Burrito Founder

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    The phrase "escape velocity" comes to mind. I know it's a mismatch, but the threshold concept is similar in my mind. It's morning though, so perhaps I'm not thinking straight.
     
  8. Ames

    Ames Member

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    http://www.youtube.com/watch?v=gsQdwACgoXY

    Check this out. Top speed appears to be limited to 133 mph in a P85. notice once the limiter kicks in the orange power bar goes down quite a bit so clearly the car can go quite a bit faster...

    Alex.
     
  9. Johan

    Johan Took a TSLA bear test. Came back negative.

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    Good analogy. Let's have SpaceX look in to that one on Tesla's behalf, I'm sure they will find a nice after-market solution :)
     
  10. drees

    drees Active Member

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    Amazing how quickly it hits 90 mph, though you can see acceleration start to ramp down quickly after that. It still hits 133 mph very quick. I imagine that with higher gearing it could easily hit 150-160 mph if not more.
     
  11. Jim B

    Jim B New Member

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    Although it could be mechanical limitations of the drive train that people have mentioned I tend to think the reason for limiting the speed is the same reason manufacturers limit many cars. That is, if you go over a certain speed you have to upgrade tires, brakes, suspension, etc. For instance H rated tires are good for up to 130 mph. I don't think it is an accident that Tesla picked 130 mph. At speeds over 130 mph tires become expensive and difficult to find. Plus, let's be honest, how many people are really going to drive that fast in the US? 130 mph on a public road isn't just a speeding ticket, it's reckless driving. Sure, you can take it to a track but I doubt many people are buying these cars for that.

    My present Lexus has never been over 90 mph. No need for more and I really didn't need that. The Model S was not designed to be a race car. Yes, it performs well, better than most of us can even appreciate but it is not a Boy Racer.
     
  12. Jeroens

    Jeroens Member

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    If traffic allows and the driver can handle it...

    driving to/in Germany I regularly drove 155 mph and up, still do when I get the chance. (Which is perfectly legal on some roads - is unlimited)

    probably not with the MS though...
     
  13. gocken2

    gocken2 Model S: P6931

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    Using this graph and the gear ratio, I calculate the top speed to be about 140 mph if the software limit was removed. With a change in gear ratio it can go well above 150 mph (my spreadsheet is currently limited to 150).
     
  14. EarlyAdopter

    EarlyAdopter Active Member

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    Two words: Back EMF.

    Electric motors turn into generators at high RPM. The resistance of an electric motor increases proportionally to RPM. More current is required to get any power out of a motor at high RPM. Once a certain point is reached the amount of current in is equal to the resistance back out and you've reached maximum RPM for that motor. Power (torque) drops off long before that point. This graph is a good example of this phenomenon.

    Take all you know about power and peak RPM from gas motors and throw it out the window. Time to learn some new science.
     
  15. brianman

    brianman Burrito Founder

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    Can someone go into more detail on the 7K and 15K rpm inflection points? Also the shape of the curve (why "concave" vs. "convex", or whatever) is interesting. Thanks.
     
  16. SCW-Greg

    SCW-Greg Active Member

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    Yeah I'd like to know at what speed are you going when you hit 7k?
     
  17. EarlyAdopter

    EarlyAdopter Active Member

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    If you pulled the motor, put it on a bench, hooked up a much larger current source with your own drive controller and measured it you'd no doubt see that graph actually curve up and to the left below 7k all the way down to 0 rpm in one continuous arc. I'd wager Tesla's motor is capable of generating upwards of 1200 NM of force at 0 rpm. Real world considerations however require it to be software limited to keep drive shafts from snapping and reduction gears from burning out from that much force. Hence the plateau from 0 to 7k.

    The continual slope down from 7k to 14k is described by Lenz's law (back EMF). I don't know why there's a sharp inflection down from 15k to 16k. Could just be a software control decision to gradually back you off as you approach top speed.

    Top Speed 130mph / x = 16k / 7k
    Solving for x = 56.875 mph
     
  18. ZestyChicken

    ZestyChicken Member

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    I hit 101 once. Car was stable as hell. I think the energy usage was like 700 kW per mile. Scary thing was how in control I felt. Took what seemed like a quarter mile for the regen to slow me down much. LOL.
     
  19. ElSupreme

    ElSupreme Model S 03182

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    Sort of expanding a little on early adopter's response.

    When you run an Induction motor off of a variable frequency drive (VFD). The motor is limited by the amount of current the coils can handle. This current is turned into torque. So you get a direct flat line for torque. As the motor increases speed the inverter/VFD will increase voltage. At the 7k inflection point the motor is running at maximum current and maximum voltage (and thus peaking in power). From that point downward the motor starts creating back emf, this is the motor producing voltage to counter the voltage supplied by the VFD, thus lowering the net voltage across the motor, and the current with it.

    The second inflecting point is probably inherent in the ratings of the components in the inverter/VFD itslef. Or it could be programmed in the inverter/VFD to gracefully hit top speed.
     
  20. derekt75

    derekt75 Member

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    Below about 40 mph (varies by car type), our cars are constant torque.
    The car is limited by things like how much AC current the inverter and motor can handle, and how sticky your tires are.

    Above that threshold, our cars are constant power, and torque * RPM is relatively constant.
    It's mostly limited by how much power you can pull out of the battery.

    Above a certain point, the motor is turning too fast to be able to efficiently use the battery power, and you get another inflection point where output power drops at higher RPMs. This threshold is apparently around 15k RPM.


    The constant torque to constant power threshold should be below 7k RPM in actual cars. I suppose the graph is for the motor itself which would not be constrained by wanting to not shorten the lifetime of a battery. In that case, the threshold would be how much power the motor can handle, and apparently the motor can handle noticeably more power than our 85 kWh batteries can put out without degradation.

    - - - Updated - - -

    If you graphed power vs RPM instead of torque vs RPM,
    I imagine you'd see it ramp linearly to 7k RPM, where it would stay relatively flat until 15k RPM, and would then drop off from there.
     

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