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Max motor RPM

Noticed that there is a max RPM that the motor spins at (16000.)
Since there are no reciporcating parts in the motor, why cant it go faster?

Here's the Roadster Sport's torque and power curve (I could not find a similar graph for the Model S, but the gist of it is certainly the same). I think the picture explains it quite well:
http://webarchive.teslamotors.com/performance/acceleration_and_torque.php

In any event, it is worth browsing to http://webarchive.teslamotors.com/ -- Tesla's former website frozen in time! It's like a museum, showing exhibits of ancient times, when the Roadster was new and the Model S was only just announced...

I am already aware of the power RPM charts.
I guess my question should be: why does the power drop off at high RPM.

Hmmm... That is a fascinating question, actualy. I'm not quite sure, but I can throw out a couple ideas. The Tesla motor is a variable frequency AC motor. To speed up the motor, you need to increace the frequency of the DC to AC inverters. They may have an inherent limitation on the frequency and power they can deliver. The motor might be subject to heat build up and cooling limitations that requre the motor to be power limited. The rotor is very heavy, and has a huge amount of kinetic energy at 16,000 rpm trying to pull the soft copper rotor appart. Basicaly, it's trying to fly appart. Consider it's under huge magnetic stresses as it spins. Next don't know if this applies to the Tesla motor, but motor cores are subject to magnetic saturation. At some point, you can't efficiently increace the magnetism of the material no matter how much power you dump into it. Finaly, you might have some insulation breakdown with high power at high frequency being dumped into the motor. Many of these things might be electricaly limited to preserve the motor, inverter, and battery systems.

Just thoughts for whatever it's worth.

@jbunn: This exlaination seems plausible...thanks!

You also have back EMF increasing with motor rpm, so to hold power steady you'd need to increase voltage. Assuming the drive inverter doesn't do any boost converting, you'd be limited by traction battery voltage. When this is reached, power will go down with increasing rpm. I see this very clearly on my e-bike, at low speed it will pull up to 1.3kW and at terminal speed (on level ground, no wind) of 26mph it will pull only ~200 watts.

I might be reverse EMF which caps the motors maximum RPM for a given voltage.

ooops... logged on and saw the response above.

Just an observation about efficiency choices:

when EPA came out with the ratings saying city driving got lower MPGe than highway, I was very dubious. Didn't fit the BEV profile at all, or the early beta reports.

But late in the game, it seems Elon ordained a retuning to give more oomph in the passing ranges. It now occurs to me that since you don't get something for nothing, that this moved the power band "north" at the expense of low RPM efficiency. Don't know how this was achieved, though.

I guess my question should be: why does the power drop off at high RPM.

Power is result of torque and RPM, and torque drops. Better question is why does torque drop? Beats me, but I recall asking that very question in some forum (might have been here, can't remember) and I got answer from someone who actually knows what he is talking about, IIRC it had something to do with motor slip at high RPM and magnetic saturation. Some sort of physical limitation anyway that can't be avoided.

The 16,000 RPM limit is more likely a mechanical limit based on the size of the motor. Think about a small motor, say a hand-held drill or dremel tool, the motors are small and light weight. These can achieve RPM's in the 18,000 to 25,000 range because they are smaller and lighter and thus won't tear themselves apart at those ranges. The larger motors, being larger and heavier, have to have a lower RPM setting, otherwise they would literally throw themselves apart at higher RPM's.

Think of it at the flywheel principle: 2πr(2) x mass (center of rotational gravity) x Max RPM = Limit that the flywheel can handle before coming apart. The upper RPM limit usually includes a 1.2 safety factor, so the motor/flywheel can handle the Max RPM x 1.2. Or, in this case, 16,000 x 1.2 = 19,200 RPM before failure.

The torque drop-off after reaching top RPM is due to the magnetic saturation of the motor and limits the max speed of the motor. Any additional voltage dumped into the motor would cause the windings to burn out, thus killing the motor.

TM controlls the upper limits with their software so as not to let these things happen. Although they could possibly tweak the software/motor pairing to eek out a little more power, the trade-off isn't worth sacrificing the power train after say, a year or two. This way, they maximise the power/torque curve for the life of the auto to 8-10 years or more, before the motor needs upgrading.

Hope this helps,

Dave

@Dave,

That's fascinating. Your formula for the "flywheel principle" seems to imply that the rotational limit is independent of the tensile strength of the flywheel material. Is that what you are saying?

Or perhaps the term to the right of the equal sign already accounts for the tensile strength of the flywheel material.

I think they could make the motor turn at 100 000 rpm if they wanted to, but there is no practical use for that. They would need a higher reduction ratio gearbox to use that in a car. The torque drops off at high rpm due to back emf induced from the spinning rotor. In general, if a motor is long and with small diameter, it will have less torque and higher rpm than a motor with equivalent maximum power, but short and with large diameter. I think also the thickness of the winding wires has something to do with it, not sure though. I don't believe that the motor spins at 16 000 rpm when the car is traveling 130 mph, it is electronically limited to some point after the torque starts to drop off, but not at the very limit. The motor would have virtually no torque at 16 000 rpm. According to their specs, max power is around 6000 rpm, and the torque starts to decline a few hundred RPM before that. Judging by the reviews and videos, tesla has done a magnificent job in tuning the motor and controller for optimal performance.

Here is a company that is making high speed AC induction motors

http://reuland.com/product/high-speed.html

High-speed Partial motors are available up to 100,000 RPM.

so it is possible, but only in specialized applications. No mention of power or torque for a 100k rpm motors.

In thinking about Douglas's question, if you double the size of the rotor it's surface speed increaces pretty dramitacly. A 6 inch rotor only has a surface speed of 18.84 inches per revolution. Double the rotor and it's now moving at 37.68 inches per revolution. The larger rotor also needs more material, and weight increaces. Even though the circumfrence is larger, the mass and increaced surface speed add up to a great deal of centripital acceleration. We could do the math with better data, but I think that the tesla motor at full speed, if it ever came apart would present quite a restoration challenge to the body shop.

@Vall;
I notice they list machine tools as the usual uses. But one motor claims 500 HP! Trying to imagine a 500 HP router ....‼

@wdaze;
yeah, to "eke out" any more at those speeds would be pretty dicey. To grok the mechanical stress, compare a 2' diameter motor with a 20' diameter one! At 1600 rpm, the speed of the rotor outer edge would be πd x 1600 x 60 = 44/7 x 1600 x 60/5280 = 114 mph for 2' diam. At 20' it would be 1143 mph, well over the speed of sound! Unobtainium is definitely called for.

;)

@Vall
According to their specs, max power is around 6000 rpm, and the torque starts to decline a few hundred RPM before that.

Actually max power is gained at 6000RPM but it stays more or less constant quite a lot higher, according to options-page 6000-9500RPM for standard 85kWh and 6000-8600 RPM.

Torque starts to drop before that: peak torque 0-5800 for standard and 0-5100 for performance.

(looks like performance has shorter peak than standard).

@Brian H
Unobtainium is definitely called for

Graphene for motor windings instead of copper. Mass decreases dramatically and structural strength increases at the same time.

Carbon is d*mn miracle material, you can use it for nearly everything. Only problem is to make those advanced forms of carbon in mass-production scale.

All this looks like that there really is a limit to how much speed and acceleration we can have with a one speed EV.
SO the follow-on question /observation is:
Should we have a 2 speed...
I know that Early Roadsters were not successful with this idea. However, it seems that they were trying to achieve maximum acceleration through the gear change. Being a structural designer I know this is the sort of thing that can strip the gears when you can slap on massive and instantaneous torque the way you can with an EV.
If the motor is electronically "throttled" back during the change and gradually power applied back after so as to not damage gears with impact loads this may give the German friends of ours hope of doing the Autobahn justice.
However, the Super-charging stations would have to be so close together that this idea might still be silly.
Or is it better to:
Wait for a battery pack with more energy / power (like 50% to 100%) and use four wheel drive - 2 motors with higher gearing. This would have sufficient extra torque to keep the acceleration and have the higher speed along with the extra range too.
I have a sneaky suspicion the Tesla is going for the latter.
Can't wait for the 4 wheel drive version if and when it comes out.

All this looks like that there really is a limit to how much speed and acceleration we can have with a one speed EV.

Not really because power is not limited (or the limit is really really high). 14000 RPM translated directly to tires would be well past speed of sound. All you have to do is increase power and lower the reduction gear to gain more speed.

Here is is simple layman explanation. Without the technical electrical limitations stuff.

You have an electric motor that has a specific amount of power. You have a car that this motor is in. Obviously this motor doesn't have infinite RPM climbing possibilities :). Being an electric though, means you have all that power available to you the second you hit the accelerator.

But now lets put this into context.

At rest, all that power gives you the torque that the model S has so far been praised for. "AT REST" and let me add "CLIMBING". This power gets the MS to say 100mph. Now think of it, if something was turning at 16,000rpm from rest to get you to 100mph as quickly as possible (considering weight..etc), do you realize that unless it starts turning at say 20,000rpm; it gets to a point where turning at 16,000rpm stops having as noticeable an affect on a car already doing 100mph as compared to one that was at rest. At that point, there is no need to even draw that much power anymore considering the car is already going "fast" all you have to do is draw enough power to make it go faster and at a more relaxed pace.

At 100mph, and being held there... the MS may actually be drawing less power cause it doesn't need to draw that much power to get itself up to speed and only needs to intermittently draw just enough juice to maintain your current speed. Think of it as using you hand to spin a fixed bicycle wheel, to get it up to speed you constantly apply force to the wheel, at say two turns every second. Once up to speed though, all you need to do is turn it once every 20 seconds to maintain that current speed. But this is where this analogy gets interesting... here, your arm is the electric motor. If someone were to say, make that wheel spin 10 times faster than the fastest your arm could physically get it to spin; it will be impossible. Simple cause you are limited by how fast you can spin that wheel, at some point, you can be sweeting through your ears trying as hard as you can to spin that wheel fast and exhausting yourself.... but it won't work, that wheel will go no faster than you can move your arm. Until of course you start using two arms...... but thats a different story :).

lorddeff07, I must say I find the more technical explanations easier to understand. Does this mean I'm not a layman...? ;-)

+1 @VB

@Volker.Berlin

Haha!!

Well yes my friend, it means you are not layman. Its really simple though, in summary i could just say that its the law of diminishing returns in economics.

Just that in relation to this, as long as maximum power remains constant, it gets to a point where that power seeming fails to yield any noticeable effect.

@DouglasR

Doug, the tensile strength of the material is the limiting factor, I was including that in the max RPM factor to keep it simple without going into any higher math.

Here in Washington, fundamental forces limit the performance you can obtain with a vehicle of this type. To name a few, they are the King County police force, Seattle police force, and Washington State police force.

@jbunn :D +1

Top speed though has to be (well) over 100mph in German autobahns: if you drive slower it is you that is hazard into road. Relative speeds.

Timo, well... not entirely true, fortunately. Actually I'm sure I noticed an impact of gas price on average Autobahn speed during the recent years. No hard data though, just subjective impression.

100mph is only 160km/h. I have been on motorway where 140km/h was too slow even that speed limit was 100km/h. Everyone was speeding. In a place where you can legally drive even faster it sounds odd that most would actually drive any slower than 160km/h.

OTOH this is all my personal impression of the thing, I haven't actually driven in German autobahns.

I'd like to put things into perspective a bit. Of course it's my subjective view and others would probably describe the situation differently.

First of all, only stretches with three lanes can have unlimited speed. This probably sounds odd to Americans, but in Germany the Autobahn usually has three lanes (per direction). Sometimes there are only two lanes (usually with a speed limit at 75 mph), and only at major interchanges there are some stretches with more than three lanes (and due to the interchanges, these also have speed limits, usually 50 mph).

If there is a stretch with unlimited speed, the relative speed differences can get quite extreme, and that's certainly the most dangerous part of it. I agree with Timo in that observation, and it's also the major argument of speed limit proponents (environmental issues aside). It is not unusual that there is a convoy of cars going in the left lane at 110 mph, with an occasional singular driver at speeds well above that. At the same time, in the right lane there are vehicles that cannot (legally) go faster than 50 mph to 70 mph -- everything from small motor cycles to coaches and big rigs. Any vehicle that cannot legally go at least 40 mph is disallowed on the Autobahn.

Depending on traffic, it is generally safe to go at speeds of 80 mph to 90 mph. At that speed, you will usually find yourself in the right or center lane. Going slower than that when there is non-negligible traffic will indeed make you an obstacle in the center lane (let alone the left lane) and will force you into the right line, which is not a convenient place to be -- between small motorcycles and big rigs that frequently dictate an even lower speed.

Just curious - but since there is only one gear, does the car go as fast in reverse as forward...or is it software limited?


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