Tesla Model S P85D Top Speed Monster?

MAR 1 2015 BY TDILLARD 18

Tesla: Top Speed Monster?

Tesla P85D: Top Speed Monster?

Our buddies over at Green Car Reports posted a story suggesting that there’s more to the Tesla top speed numbers than meets the eye and the site is calling for a top speed test.  Though we’re as thrilled as anyone to see someone get out on the salt and run ’em til they pop, the suggestion that simply removing the speed limits on the car would give you free speed points to some basic confusions that we all wrestle with on this new EV drivetrain thing.

Back in the old days, when we were all riding minibikes with Briggs and Stratton 5-horse lawnmower motors, if our fathers had any sense at all they were equipped with “governors.” These were simple devices that, depending on the RPM you were spinning them at, would control the throttle.  Their main intention was to feed the throttle more gas when the RPM dropped, so it would respond to the load automatically.  Your Dad figured out that he could also use it to control the top RPM of the motor, thus extending the probability of his child reaching his next birthday with relatively little brain damage.  But we digress.

Later in the history of automotive development and speed controls we saw Detroit muscle cars with speed-limiters which, legend had it, were installed for insurance and regulation purposes.  There’s the notorious German “gentlemen’s agreement“, and Japan’s agreement to limit horsepower.  If memory serves, it was the Buick Grand National that was a particularly loathsome example of that, from where we sat in the driver’s seat of the Mustang 5.0.  Are we digressing yet again?  No, point being, this isn’t that kind of “speed limiter”.

Electric motor

Electric motor

While Green Car Reports‘ math and physics looks okay, how they’re thinking of the speed limiter might be a little wrong.

Here’s the theory:

“Both the Dodge Charger Hellcat and the 552-hp Aston Martin Rapide max out at 203 or 204 mph.

With far lower drag, far more power, the P85D ought to be able to top 210 mph–if CEO Elon Musk would let his engineers remove the speed limiter.”

Here’s what’s wrong with that.  Remember about how you rate power in an electric drive system being far different than a gas drive.  The bottom line is, yes, if the power rating of 700 or whatever is good for the several minutes you need to spool up a speed-run to 200+, then you should be able to get there.  Chances are, though, it’s not.

First, let’s start with the basic principle of reaching your top speed in any vehicle.  It comes down to a simple equation of drag, which increases exponentially, and power – your machine’s ability to push against that exponential force.  Given identical methods of rating horsepower, you should be able to say that two cars with equal power and differing drag should show a top speed reflecting the drag coefficient.  Also, let’s say that two cars with equal drag, but differing power should show similar results.  It’s fair to say that the Tesla with more power and less drag should trump the contenders.

Now, when you horsepower-rate a gas motor, you plug it into a dynamometer, spin it up, and see what it produces for power.  Power is a function of RPM and torque, remember, and gas motors develop peak horsepower at a very narrow RPM range.  An electric motor, in theory, will deliver whatever it’s load is, given adequate Watts supplies, until it melts.  Rating an electric motor then must be a rating for a given time period.  Typically it’s rated at peak, which may be only ten seconds, as well as continuous, which is, well, continuous for days on end.  Remember, electric motor ratings were developed for industrial applications.

Which brings us to that pesky speed limiter.  In the case of an EV, you may be limiting speed for any number of reasons.  The most likely is the inverter power limits.  As you (exponentially) increase load on the drivetrain from air drag, you increase load on the inverter.  There are very hard-and-fast numbers as to what the inverters are rated at before they fail, and how long they car run at various outputs.  It’s not at all likely that Tesla limited the top speed to 155 mph (not coincidentally, that 155 mph limit is the German Gentleman’s Agreement figure too) out of safety, insurance or regulatory concerns.  It’s far more likely it’s limited because of the power limits of the inverters.  Now, you may be running a drive system that can push out 700hp to the wheels, but for how long?  That’s the crux of the issue…  and we’ve already seen the cars cut back after a few seconds on the drag strip.  To pull a 200mph run, we’d have to run that peak power for minutes.

Misspent youths

Misspent youths (via MotoMusings.com)

So yeah.  Maybe you could go in and reprogram the current limits on the controllers, do a 210 mph speed run or two, and pop a couple of inverters after a few minutes.  But what would that prove?

“So, Mr. Musk: Take off the speed governor completely, at least on the P85D, and let’s see what the car can do!”

We may as well ask, “Hay Mr Musk, run a few P85Ds until they blow, and let’s see what the crater looks like!”  This ain’t like your minibike where you can take the governor off and “go 30!”.  (No. Of course not. We never would have done such a thing.  Not at least without getting it back together before our Dad’s saw it.)

… and we haven’t even mentioned the fact that the tires are limited to a 149-mph top speed.  …and it appears Tesla would like to keep it that way, since there’s been no sign of a P85D tire specifically developed for speeds above 149.  But that’s another discussion.

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18 Comments on "Tesla Model S P85D Top Speed Monster?"

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German P85D has 156 mph guaranteed TopSpeed. Just FYI.

Thanks for this article. I’m a big fan of the Tesla Model S but have no delusions about its capabilities. However bringing up its limitations at higher speeds or sustained higher speeds seems to set fanbois off, such as Autobahn driving or issues with it limping on the Nuremberg after a couple minutes, similar with your example of cutting back power at the drag strip. Aside from the inverter load cap, there’s also an issue of gearing, or more accurately lack of gears. At high speeds it’s spinning way past its peak efficiency. RPM is increasing, but HP is not–flattening out or even decreasing near the top end–since torque is decreasing as the RPMs rise. A lot of juice from the batteries ends up being dissipated as heat at those RPMs. It would be awesome if you could write up another article about that too. I’m still trying to understand what goes on internally, like is it eddy currents? Anyways, the single low gear used (about 9.8:1 I believe in single motor P85) is great for 0-60 times but less so at highway speeds and even less so in Germany, or long stretches of desert road in the US. Passing… Read more »

Re: tranny, gears; aside from race cars, I just remembered that the i8 EV motor also uses a 3 speed transmission, and other exotic hybrids as well.

With a single reduction gear, you must necessarily compromise, choosing between low speed and high speed, but not both. An interview with one the BMW heads for the i8 reaffirmed that, as well as an interview with one the EV motorcycle creators (I forget his name, but he was a gas bike racer who converted to EV enthusiastically).

You can get away with taller gears the higher the output or the more motors you have of course, but it still applies nonetheless

I would like to point out that the illustration above is of a DC motor. The Tesla uses an induction motor.
AC induction motors constant torque until the motor has reached its base speed. After this point the motor is in constant HP. Another words as speed increases output torque is reduced.
(Torque * RPM) / 5252 = HP
The limiting factors is the ability of the motor, or the inverter to remove heat. The ability of the battery to thermally manage the heat buildup in pack. Remember there is a lot of power being supplied by the battery. It could also be the motor or gear box has reached its maximum speed structurally.

There is another regime, too. After the constant hp regimes comes constant voltage, above which you can’t go because the wires aren’t spaced far enough to avoid breakdown of the insulation. At this point, torque decreases with the square of RPM.

Excellent summary.

I was left scratching my head after reading Greencarreports blog post. After years of EV coverage I would think they would understand this stuff.

GreenCarReports Trolls it’s readers with every other blog post.

I think you meant Buick Grand National.

While I was looking for an explanation of the rpm. limit’s for induction motors, Jeffand posted a point I was going to make about the illustration being that of a DC brushed motor when Tesla uses brushless AC induction motors. I think one thing the people who are asking for the Model S’ speed governor to be “removed completely” fail to acknowledge is that electric motors have limits as well. For the induction motors used by Tesla I believe there are two considerations. One is that as the rotor spins faster and faster the centrifugal force increases to the point that it threatens to rip the rotor apart. That is the same reason why tyres have speed ratings. The other limit is imposed by the limits of the power electrionics and magnetic materials in the stator to create a rotating magnetic field that is rotating fast enough. Ther is a discussion of this at: http://my.teslamotors.com/forum/forums/max-motor-rpm Since, according to the following story, the world record is one million rpm. (for a very small motor), I would hazard a guess that the speed limit is more likely a result of the limits of the physical forces on the rotor. http://www.sciencedaily.com/releases/2008/11/081114081213.htm It could… Read more »

To over simplify the basics there are several factors that standout.

1. The speed of an induction motor varies exactly as a ratio of the applied hz. less the slip rpm. The speed of a synchronous motor varies exactly as a ratio of the hz.

2. The AC motor wants to see constant volts/hz. so as speed is increased you will run out of voltage at some point.

3. Basically the motors produce constant torque up to base speed and then they produce constant hP as the volts/hz drops off.

4. Motors can be designed to hold together at higher speeds than required in the Tesla but other factors are so huge that increasing the rpm is not the answer to greater MPH. At some point a gear box or two winding motor is probably the best solution.

Heh, Drayson Racing with Lord Drayson-

850 Hp, 200 MPH+ Record Holding B12/69 EV-

1) Robert Llewellyn (@bobbyllew) Fully Charged Review-

https://www.youtube.com/watch?v=lS1fWUg2oDY

2) Drive’s Chris Harris (@HarrisMonkey) looses his lunch-

Video/ 6:10

https://www.youtube.com/watch?v=q_cRqcbXAs4

Drayson Racing-

http://www.draysonracingtechnologies.com/projects/B12/project_article_B12.html

Video Credit- YouTube

“Lordy, Lordy thats fast”
Chris Harris

Best-

Thomas J. Thias

517-749-0532

https://twitter.com/AmazingChevVolt

When I was a kid with a minibike disabling the governor was the first thing we learned how to do.

Drag doesn’t increase exponentially, it increases quadratically! A quick wikipedia search confirms this:
http://en.m.wikipedia.org/wiki/Drag_(physics)#Drag_at_high_velocity
It also tells us that while the drag force increases quadratically, the power needed to overcome drag increases cubicly.
(Because work = force*distance, and speed=distance/time, so power=work/time=force*speed)

I see two or three comments already touched on the problem of overheating at high speeds. We know from a few racetrack tests done by some auto review magazines/ websites that running the Model flat out for just a few minutes causes the motor to overheat, followed by the car automatically going into reduced power mode. (Not “turtle” mode, but — if I understand it correctly — into “econo” mode, limiting power, speed, and acceleration.) Removing the electronic speed governor would just make the overheating all that much worse. In fact, I wouldn’t be surprised if “hot rodding” the car would do some permanent damage to the motor and/or other parts.

Other than bragging rights, who cares. I tried to achieve top speed on a sporty car I had and on 4th gear at around 150 I realized how stupid it was to keep going and if caught it was a “straight to jail” infraction. Even the 101 mph top speed of my Volt is enough.

That’s what race tracks are for, a fact lost on street-racing Tesla owners.

I think gearing could do a lot to make EVs more fun. Being high in the RPM, on a track, can be slower, twitchier, but it’s better than being ‘torqued’ around in a low gear. Direct injection did the same thing EVs stand to do, in this regard. Chris Harris remarked how simply keeping it in a low gear was the fast way to drive the i8. Yeah, but is it fun? Is it as frenetic as stepping the tail out with your right foot?

The 500cc GP 2-stroke motor-cycles versus the electric ones, could some day turn the same lap times, but their feels would be radically different. HP vs. Torque