Max power at high speeds

[teasla]

I have a factory uncorked 75. While the max power below 60 mph is close to 320KW, at higher speeds the power is significantly lesser. For e.g. max power at 75mph starts at the moment when the pedal is floored and in about a second, the power has reduced from about 300KW to 275KW. And it will continue to reduce if the pedal stays depressed.
Wonder why would Tesla design the car to reduce the power when it needs it the most( at high speeds to combat high speed wind drag)

 

[busaman]

i would guess its to stop the battery from overheating from sustained high load after all it doesnt take 275kw to sustain 100mph

 

[teasla]

i would guess its to stop the battery from overheating from sustained high load after all it doesnt take 275kw to sustain 100mph

I was referring to max acceleration. I suspect you would need much lesser than 275kw to travel at 100mph. However, with more max power you have faster acceleration, e.g. faster 65 to 85mph.

 

[Krash]

Astute observation. This is due to back EMF of the induction motors. I also believe that the larger motor has better performance in that regard as do higher voltage and current batteries. The model 3 permanent magnet motor should not have that drop and the 2020 roadster, if it has rear induction motors, should be geared differently so as to not have that drop at as low an absolute speed.

 

[AmpedRealtor]

Teslas have poor passing power due to the single gear. 0-60 is great, but above that you'll get your ass kicked by a Kia.

 

[Zetopan]

Teslas have poor passing power due to the single gear. 0-60 is great, but above that you'll get your ass kicked by a Kia.

Obvious BS is obvious. Show some actual evidence.

 

[Zetopan]

This is due to back EMF of the induction motors.

That is correct. As the RPM increases the back EMP increases proportionally and that reduces the voltage across the motor's windings, thus reducing the possible output power. As the RPM increases that causes the back EMF to approach the battery voltage and when the back EMF and battery voltage are equal the torque has to be zero since no current flows in the windings.

The model 3 permanent magnet motor should not have that drop ...

That is incorrect. *ALL* magnetic motors exhibit a back EMF or else they would not be able to produce any output torque at all and would not deserve the title of being called a "motor".


Note that a 155 MPH top speed in a P100D the larger rear motor is turning at about 18,000 RPM (9.73:1 gear ratio) and the smaller front motor is turning at about 16,000 RPM (9.34:1 gear ratio, I think). By knowing the torque (or voltage) constants of the motors it is possible to compute the back EMF at any given RPM and by knowing a few other parameters as well as plot we could plot the motor performance vs RPM.

 

[Krash]

...*ALL* magnetic motors exhibit a back EMF or else they would not be able to produce any output torque at all and would not deserve the title of being called a "motor"

...By knowing the torque (or voltage) constants of the motors it is possible to compute the back EMF at any given RPM and by knowing a few other parameters as well as plot we could plot the motor performance vs RPM.

My mistake. Always happy to be schooled. So permanent motors have no better back EMF performance?

Could you derive those constants from observed data? I have power vs RPM data. If so, could you use S RWD data to isolate the large motor constants? Then use S P data to isolate the front motor constants? Then use non P AWD data to isolate the small rear motor constants?

I don’t have actual voltages across RPM in all three cases but might be able to get it if needed.

 

[Zetopan]

So permanent motors have no better back EMF performance?

Correct, and that is true of all magnetic (wound coil) motors. The only potential advantage to a permanent magnet motor is slightly higher efficiency since the permanent magnets do not require any energy to operate. Three phase induction motors are asynchronous while three phase permanent magnet motors are synchronous.

A well designed three phase induction motor loses some energy in the rotor since it does not have zero resistance. The rotor acts like a shorted secondary of a transformer (the field windings act like a primary) but the short is imperfect due to finite rotor winding resistance. The Tesla induction motors are actually quite efficient since they have copper rotors rather then the more common aluminum (copper being the better conductor of the two). Silver rotors would have even lower losses, but the cost would be prohibitive.

On the other hand nearly all neodymium (plus iron and boron, etc) magnets come from China since they are by far the largest miners of that element. Neodymium also has a curie point that copper and aluminum do not have; a critical upper temperature where remnant magnetism gets completely and permanently lost. Remagnetization is required for any permanent magnets that have hit their curie temperature since otherwise they are not unlike stone doorstops.

Could you derive those constants from observed data? I have power vs RPM data. If so, could you use S RWD data to isolate the large motor constants? Then use S P data to isolate the front motor constants? Then use non P AWD data to isolate the small rear motor constants?

It is a bit more complicated than that but if you send me what data you have I can try to see what I can derive from it. The complication is that permanent magnetic motors have a single voltage/torque constant, while non-permanent magnet motors generally have variable voltage/torque constants, depending upon their construction and modes of operation.

The minimum data that I would need is both the power and field current vs motor RPM (or vs road speed, gearbox ratio [which we know] and the wheel circumference [depends on your wheels and tires]). Having this data sampled at least at every 10 milliseconds (at a 100 Hz or faster sample rate) would be ideal.

 

[AmpedRealtor]

Obvious BS is obvious. Show some actual evidence.

It's well known that passing power is weak compared to similar HP vehicles with gearing, I don't need to provide evidence when there are plenty of threads that you can look up on the subject. As for the rest, you need a sense of humor to appreciate.

 

[Zetopan]

I don't need to provide evidence when there are plenty of threads that you can look up on the subject.

Claims made without evidence can also be dismissed without evidence. Asking me to find evidence to support your claim is typical of obscurantists everywhere. Adding a smiley face to a counterfactual claim does not magically reduce the counterfactual content. Show where a stock Kia out accelerates a stock charged Tesla at any speed.

 

[Kristoffer Helle]

Are you serious? A old 90s Swedish turbo family wagon eats tesla above 60-80mph.
A old v6 Kia is probably faster!
You should try a Tesla, before you write stuff like this on a forum.

 

[Zetopan]

A old 90s Swedish turbo family wagon eats tesla above 60-80mph.

You forgot to show some actual evidence.

A old v6 Kia is probably faster!

Probably? You forgot to show some actual evidence, again.
Show me some actual evidence and I will believe you.

Tesla P100D: 0 to 100 MPH (about 161 KPH) in 6.0 seconds.
The 2017 Tesla Model S P100D and the 0-100-0 Test - Motor Trend
Also, the 60 MPH to 80 MPH time requires about 1.5 seconds and 80 to 100 MPH requires about 2.1 seconds.

You should try a Tesla, before you write stuff like this on a forum.

Having owned one for over a year I'm pretty sure that I do not actually match your uninformed assumption.

 

[Kristoffer Helle]

You should try to drive it. Don't need evidence for this.
Everybody knows

 

[Zetopan]

You should try to drive it. Don't need evidence for this.
Everybody knows

Evidenceless claims can be dismissed without evidence.

 

[AmpedRealtor]

Claims made without evidence can also be dismissed without evidence. Asking me to find evidence to support your claim is typical of obscurantists everywhere. Adding a smiley face to a counterfactual claim does not magically reduce the counterfactual content. Show where a stock Kia out accelerates a stock charged Tesla at any speed.

Keep pushing the issue, buddy, I'm sure you'll hear from even more owners on this.

 

[SSedan]

I agree that the Tesla is soft up top and I don't think I have hit 100mph yet in my P85
Look at the 1/4mile times ICE vehicles with the same 1/4 mile time usually have a pretty significant MPH advantage at the end.

The other cars were slower off the line but catching up 5-10mph difference in the 1/4 mile is HUGE.

Engines and motors have power CURVES, electric motors make peak torque from 0 to a point where as discussed above feedback begins to slowly reduce output.

That said once you figure in the downshifting an ICE needs to do, Teslas fair well, but once an ICE has downshifted an equal HP ICE will out accelerate a Tesla at speeds beyond what is legal on public roads because they can access peak HP rpm where it is past for an EV.

I traded a Chevy SS on my P85 that car was 415hp 415tq, compared to the 416hp 440tq of my P85, low speeds the Tesla is a lot quicker to accelerate, but stab the pedal at 70mph and the Chevy once downshifted pulled a fair bit better, with half a dozen gears to pick from you could get the engine between peak TQ and peak HP where the electric motor is past peak HP and torque the curve on the decline. At 70mph the ICE and P85 are making both right near peak power BUT the ICE is 700lbs lighter and has gearing options where the Tesla is passing peak HP and sliding down the hp curve.

Before I saw this thread I started another on the power delivery topic but my understanding is based on memory so consider numbers general not specific the P85 makes peak torque to 5100rpm which is 43mph then plateaus at peak HP to peak HP at something like 8000rpm and 73mph, after than power output is on the decline.

Compare that to my previous Chevy that could access all 415hp at 36/62/95/127as it went thru the gears.
Now if comparing a P100D there is so much power on tap few have access to a comparably powered ICE to compare to and likely have to be above 100mph to notice the acceleration falling off.

The billiard table flat torque curve at legal speeds is what sold me on the MS. I don't often drive over 75mph and when I do am very rarely asking for a burst of acceleration.

Edit:Went back and found the data I had seen in the past and reran the numbers.
Tesla - Model S - P85 PERFORMANCE (416 Hp) - Technical specifications, Fuel economy (consumption)

 

[The Duke]

I agree that the Tesla is soft up top and I don't think I have hit 100mph yet in my P85
Look at the 1/4mile times ICE vehicles with the same 1/4 mile time usually have a pretty significant MPH advantage at the end.

At 70mph the ICE and P85 are making both right near peak power BUT the ICE is 700lbs lighter and has gearing options where the Tesla is passing peak HP and sliding down the hp curve.

The Tesla should always be slower given the HP to weight ratio. So why be surprised the ICE finally gets it's act together and accelerates faster? At the strip a higher trap speed with equal time means you have a better hp to weight ratio and you need to work on the launch.

BTW I can verify the P100D is not "soft up top".

 

[n2mb_racing]

If we had another gear or two, the highway performance would be amazing . My guess is the roadster will have two or more gears .

 

[mongo]

If we had another gear or two, the highway performance would be amazing . My guess is the roadster will have two or more gears .

I'll give two rear ratios (front vs two rear motors), but I do not think they are adding dual rear transmissions.

Alternate line of thought: Currently, the P100Ls can spin the wheels with one rear motor. Roadster has two and twice as big a pack. That means twice the current/ torque available to twice the motor capacity. So, if they gear the rear motors twice as steep as current cars, they will still spin the tires, but the voltage/back EMF to ground speed will be half. On the hand waving side, the motor's V/RPM coefficient can be adjusted also. But say that stays the same. That means it can go twice the ground speed with the same total rear wheel torque as current cars (double the effective HP). Front motor needs a gear ratio to prevent over speed, it can also be tuned to the lower end for fast starts, or toward the high end for faster top end acceleration.