I'm guessing that has more to do with the differences in the power plant, but programming could be a factor as well.
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Top Speed. Governed or maxed out?
- Thread starter spatterso911
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- Driving Dynamics Model S
Todd Burch
14-Year Member
I don't have the numbers right now, but I did a back of the napkin calculation earlier.
At 130 mph drag force is the major contributor to the force required to keep the car at that speed. It is about 1.05 kN.
To accelerate from 0-60 mph in 5.6 sec, over 8kN of force is required....significantly more than what is required to keep the car at 130 mph.
So it is not strictly a limitation of the power output of the battery.
I suspect it's due to a combination of gearing, loss of torque in the motor at high RPMs, and electronic limitation.
But it doesn't at all seem to have to do with max C rates of the battery.
At 130 mph drag force is the major contributor to the force required to keep the car at that speed. It is about 1.05 kN.
To accelerate from 0-60 mph in 5.6 sec, over 8kN of force is required....significantly more than what is required to keep the car at 130 mph.
So it is not strictly a limitation of the power output of the battery.
I suspect it's due to a combination of gearing, loss of torque in the motor at high RPMs, and electronic limitation.
But it doesn't at all seem to have to do with max C rates of the battery.
Todd Burch
14-Year Member
TEG: Agree...and also admit that my calc is only rough...there are other things to consider, like drivetrain friction, motor losses , etc...but it seems like a factor of 8 difference would be pretty large, including other factors and knowing that short term C rates can be higher than long term C rates.
Could even be a cooling limitation on the drivetrain.
Could even be a cooling limitation on the drivetrain.
robar10 had grabbed the images from an older version of the features page ... go to his post to see the curves.
Checked it out. Thanks, as that really helps me to visualize conceptually what the dynamics of the motor are. I wonder if that curve is still accurate to the current iteration today or if there have been changes.
Thanks clea -- I was looking for that post and couldn't find it: TMC - Model S - Horsepower
This torque curve is still hosted on the teslamotors.com site, no indication of how accurate it still is though.
Kipernicus
Model S Res#P1440
What are the implications of being at the far right of the graph, when you have little to no torque but the motor is spinning like crazy?
Does it mean that you can't hit 16k rpm unless you are going downhill with a tailwind?
Does it mean that you can't hit 16k rpm unless you are going downhill with a tailwind?
Basically, yeah - I think.
It depends if the torque drop-off is steeper than the increase in horsepower based on the higher RPMs.
With many engines and motors there is less horsepower near redline so you could hit top speed before you get to redline based on other load factors such as hills, headwinds, etc.
With a fixed ratio gearbox you don't have a choice to try a lower gear.
With just the torque & RPM graph, one could easily graph horsepower too and then calculate (knowing Cd, frontal area, ...) if there is enough HP at say 15K RPMs to achieve a particular top speed.
It depends if the torque drop-off is steeper than the increase in horsepower based on the higher RPMs.
With many engines and motors there is less horsepower near redline so you could hit top speed before you get to redline based on other load factors such as hills, headwinds, etc.
With a fixed ratio gearbox you don't have a choice to try a lower gear.
With just the torque & RPM graph, one could easily graph horsepower too and then calculate (knowing Cd, frontal area, ...) if there is enough HP at say 15K RPMs to achieve a particular top speed.
What is interesting is that if you draw a line from the intersection point at 7K rpm (415NM) and the equivalent power point at 14K rpm (207 NM), the torque curve is below the line. If you extend the line to 16K rpm, the torque curve is still below the line.
That means that peak power is at 7K rpm. (If the graph is accurate)
(OK - now that I looked at the linked post - that's what it says there as well)
That means that peak power is at 7K rpm. (If the graph is accurate)
(OK - now that I looked at the linked post - that's what it says there as well)
KBF
Model X owner (formerly Cdn Signature Model S)
Assuming that the technology is somewhat similar in principle, this is how it works with my 2012 Zero motorcycle (in transit to Canada right now, woohoo!): Max speed is 80 mph, but max sustained speed is listed as 70 mph (with the disclaimer that YMMV). However, this is the same for both battery pack sizes (6 kWh and 9 kWh). I would suggest that unless Zero is limiting the higher pack's top speed, battery size doesn't give a "hardware limitation". Therefore it's probably "better" for Tesla to limit top speed, but not really mandatory. At the very least it's "better" as far as pack size differentiation goes... buy the bigger pack!
The Roadster Sport (supposedly) has a special hand-built motor to get slightly higher torque output. Perhaps the Model S Performance has a special motor built to allow slightly higher max rpm, to get a little more top speed with the same gear ratio.
GSP
GSP
jeffhre
Member
Doug G, Todd Burch, and Teg are correct IMO with respect to limits on speed. Although any engineered product that was constantly run at it's limit, would not be robust in the hands of some consumers. So to approach the question from another angle, I'd ask; What would happen to the car if the speed were not limited by Tesla? What if the speed was limited only by the physical limitations of the drive train?
Would we expect some failure, or would the car simply accelerate happily to some speed where the drive train is no longer capable of surpassing, and maintain that as terminal velocity. Or would some part of the drive train be exposed to unsustainable stresses? I have always believed that the Model S drive train has been engineered to allow much higher speeds safely. But only for short periods of time. Why? Excessive heat build up in drive train components at the higher speeds would either lead to dangerous failures, or to emergency limits to current draw. I believe the current factor contributing the most to limiting the Model S from faster speeds for extended periods -is due to the limits of the motor cooling system configuration.
Would we expect some failure, or would the car simply accelerate happily to some speed where the drive train is no longer capable of surpassing, and maintain that as terminal velocity. Or would some part of the drive train be exposed to unsustainable stresses? I have always believed that the Model S drive train has been engineered to allow much higher speeds safely. But only for short periods of time. Why? Excessive heat build up in drive train components at the higher speeds would either lead to dangerous failures, or to emergency limits to current draw. I believe the current factor contributing the most to limiting the Model S from faster speeds for extended periods -is due to the limits of the motor cooling system configuration.
The drivetrain becomes very inefficient at those higher rpm's and also loses torque. It would be possible to take it over 20.000 RPMs probably, with a higher voltage (say over 500v) but it
would need a redesigned inverter, better cooling and the motor wasn't designed for those
RPMs. So it may be able to spin to some 18.000 RPM with existing hardware but consumption would be terrible (then again, so is with ICEs at those speeds). But you should see 250km/h at some stage with existing hardware probably. Cooling might be an issue. The rotor has to stay below 180C. Below that and at RPMs below rupturing point of the motor, it is fair game.
would need a redesigned inverter, better cooling and the motor wasn't designed for those
RPMs. So it may be able to spin to some 18.000 RPM with existing hardware but consumption would be terrible (then again, so is with ICEs at those speeds). But you should see 250km/h at some stage with existing hardware probably. Cooling might be an issue. The rotor has to stay below 180C. Below that and at RPMs below rupturing point of the motor, it is fair game.
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