Motor RPM at 155 mph?

Motor RPM at 155 mph?

From Elon's press conference, the software limiter on the P85D will now allow a top speed 155 mph. All things being standard, and given that the motor RPM is proportional to speed, is it possible to arrive at the motor RPM at this top speed, for each motor?

In the RWD it was easier, given a gear reduction 9.7:1 and tire rotation at a given speed to arrive at the RPM. How to approach the equation with two motors? Possible?

Bighorn | March 20, 2015

Now there are two different gears, the taller one presumably up front. The S85D will also have the same top speed, IIRC. Gear ratios haven't been publicized yet.

EVino | March 20, 2015

Given a 245/45R19 tire with a diameter of 27.7", and assuming the same gear ratio on the rear motor:

155 MPH translates to wheel RPM of 1881.

1881 wheel RPM on a 9.73:1 gear multiplies to 18302 RPM at the rear motor.

More or less, the front motor will be turning at roughly this RPM, but with the torque divided between the motors.

Sound right?

Anyone know what the max operating RPM of the motor is? 18000 is amazing.

sorka95032 | March 20, 2015

Not when you consider some motorcycle engines get that close and they have hundreds of complicated moving parts.

EVino | March 20, 2015

sorka, yes, that's impressive as well. I wonder how long an EV motor can sustain 18k RPM. That must be redline.

Bighorn | March 20, 2015

The motors would have different ratios so would be turning at different RPMs. My guess long ago was the rear ratio would be closer to 10 to increase torque and the front would be around 8 to take over for highway cruising. It's not clear to me whether torque idle can decouple the rear wheels from the motor so that it doesn't have to spin so fast.

Iowa92x | March 20, 2015

EVino, we don't know the gear ratio of the front motor, and can't assume same as rear.

EVino | March 20, 2015

Can the front motor handle 155 mph alone?

freemarket | March 20, 2015

I've got to start looking for a road in NM that doesn't have any traffic and is in NM. Anyone? :-)

EVino | March 20, 2015

Haha. Ok. Get a GoPro. Vid or didn't happen.

EVino | March 20, 2015

I take it back. That wouldn't be safe. Just organize a Salt Lake trip.

Bighorn | March 20, 2015

200+ HP should be able to overcome the wind resistance of 155 MPH.

Brian H | March 21, 2015

Not sure about the no traffic, but most roads in NM should indeed be in NM.

bonaire | March 22, 2015

Bighorn - it takes a lot of horsepower to go 155mph. I highly doubt 188hp front motor of the S85D will handle anything more than 130. But if it does, that shows the capability of smaller electric motors over the ICE counterparts.

Out4aDuck | March 22, 2015

The motors are never decoupled from the drive wheels. If they run two different gear ratios, the faster spinning motor will simply drop off in torque sooner. But they will both contribute. The biggest challenge is to transfer heat from the motor. By having two motors, the challenge is significantly reduced. Has anyone calculated what the cruising range is at 155 MPH?

GreenP85D | March 22, 2015

As a data point, my '89 Taurus SHO topped out at 140+ with 220 hp and claimed drag coefficient of .26 or .29, cannot remember exactly. Naturally aspirated 3.0 liter. People put turbos and better exhaust on them to get 160 mph or better.

Bighorn | March 22, 2015

Getting to 155 requires quite a bit of HP, but maintaining is a different story. Looking at the formulae, given the low coefficient of drag and frontal area, I think both versions may be able to use just the front motor.

Bighorn | March 22, 2015

Thanks for clarifying the coupling question. What are the upper limits of a free spinning motor not supplying motive force?

Out4aDuck | March 22, 2015

You can get a good idea by searching on Tesla Motor Torque Curve and click on images.

logicalthinker | March 22, 2015

18K rpm isn't really that crazy for an electric motor. Consider Dremel tools. example of 35K rpm Dremel:

Jolinar | March 22, 2015

good point, we have such Dremel at home and it really makes some crazy speed noise... Never tried it to spin whipping-top (is it the good word?) though.

milesbb | March 22, 2015

Worked on a 10,000 hp induction motor that turned 15,000 rpm. Drove a gas compressor in an oil processing plant. The motor had magnetic levitating bearings. The motor was fairly small, about the size of a 500 hp 1800 rpm motor. The inverter for the motor filled a medium size building. A few screws came loose inside the motor after a few months of operation. Took 6 months to rebuild the motor.

Bighorn | March 22, 2015

Frequency isn't always scalable. Think hummingbird:goose WRT wingbeat rate.

TeoTeslaFan | March 22, 2015

Elon mentioned 18,000 rpm in the following video at 43:22:

By the way, I highly recommend watching the entire video if you haven't watched it. It is Elon's best interview ever. Especially the Q&A session was perfect. When a student asked Elon a question about comic books, Elon asked him back and the audience was in awe of his modesty. It was a rare moment. Elon was like a hero. The audience stood up to applaud him as he left. I watched this video a few times.

Bob.Calvo | March 22, 2015

I thought that i remembered reading that the motor maxed out at 16,000 rpm but that was likely for the old top end of about 125 or 130 on the P85 so that 18000 rpm is in the ball park approaching 155 mph (but shy) if that rpm is possible. (18K/16K) x 130 = 146.25 mph. The reduction ratio that we're all familiar with is for the rear motor. I haven't seen what the reduction ratio for the front more is. It is likely the one whose gearing allows the higher top spead while maxed out to 18000 rpm (per Teo). The rear motor HP and gearing hasn't changed any, so that it could be the front motor's different gearing that predominantly adds to the top end beyond the increased rpm calculation shown here.

Tropopause | March 22, 2015

@Bob, you are correct. Page 132 of the Model S85 Owners Manual states:

Type AC induction motor, liquid-cooled, with
variable frequency drive
Rating 375 Volts
Maximum Speed 16000 rpm

I don't know what limits are published in the P85 or D manuals. Maybe someone can check?

Bighorn | March 22, 2015

How do you know the rear gearing wasn't changed for the D? That contradicts expectations.

Bob.Calvo | March 23, 2015

@ Bighorn, I'm not sure why you say that it contradicts expectations. I could be mistaken, but my experience with the engineering philosophy used at Tesla, Solar City, and SPACEX is to standardize as much as possible so as to minimize variation on the production line and to maximize quality while reducing costs. Examples include using four of the 17" touchscreens in the Dragon capsule and one in the Tesla as well as using the same basic lithium ion cells (packaging is likely different). Why do I bring this up? Because it does not conform with the design philosophy to change the electromechanical RWD portion of the design if you don't have to. As far as I know, unless someone else has additional information, that portion of the design is the old P85 RWD set up and it uses the same motor that is still sold in the conventional S85 (but without the P's higher capacity inverter).

The new portion of the P85D is the 221hp front motor and the additional software needed to seemlessly marry the two systems together. The fact that Teo's video has Elon now stating that the rpm has been increased from 16,000 to 18,000 rpm is essentially a software change that they are now comfortable in allowing the motor to be spun more quickly. Besides bragging rights in the US, the need to increase the top speed was likely necessitated by the desire to be competitive in Germany since the Autobahn still has portions that allow this extralegal speeds (by US standards.)

Bob.Calvo | March 23, 2015

Typo: should have been "these" extralegal speeds vs "this"

acegreat1 | July 24, 2016