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Except that with lower gearing you shift the RPM to ground speed ratio which means the operating points don't match between semi and passenger car. Other than slight differences like 60 MPH on semi vs 80 on passenger car.
Along with power limits are the torque/ current limits. Semi needs sufficient gearing to allow starts ehhile fully loaded on a grade with decent acceleration.
Y top speed is 155 for performance version, could cut about that in half for semi, plaid does 200+ so that could get a near 3x torque bump.
Olle
Active Member
For sure they need to do something about the high speed efficiency for the Semi. Axially arranged rotors should work ok in a 3 to 4 motor truck and be more efficient at highway speed.
As you can see in Munro's Plaid teardown video
Since the highway operating speed of a Semi is closer to its top speed compared to a passenger car, operating RPM will be proportionally closer to max RPM too. That should make it unnecessary to carry over the carbon wrap, unless you expect operating RPM in the high teens and have a margin all the way to 20k. Neither efficient nor reliable.
The regular Tesla motors have rpm limits around 16-18k. Should be more than plenty. Maybe just the Plaid motors without carbon for the Semi?
I think non-Staggered magnets could also be compensated for via waveform shaping. It doesn't seem like three independent motors would cancel ripple, especially since phasing is not guaranteed.
From what I recall the plaid rotor has none to near none material on the outer circumference, the carbon wrap is highly structural.
High speed motor operation can be efficient if the gains from reduced current offset the mechanical losses of the increased movement.
Plaid is also dual rear motors, so torque may be less than a 3/Y performance.
Olle
Active Member
- If you could completely do away with pulsing via waveform shaping, wouldn't they have done it before Plaid? Good point phasing isn't guaranteed but also unlikely that all three are in phase. The single motor will cross phases with the dual motors every number of turns but that the dual motors happen to be in phase with each other at the same time shouldn't be often. Crazy idea btw; couldn't the dual motors have ever so slightly different ratios avoid that they end up in phase for any length of time at high seed?
- Yes the magnets were at the edge. Could also crimp on a metal sleeve for lower RPM application, or just stay with carbon for inventory flexibility if its not much more cost?
- Ok, but isn't 20k RPM a lot for such a big motor? Sensitivity to imperfections in balance and air gap. Plaid wouldn't stay in that high RPM for long periods. Thinking minutes for Plaid vs years for Semi.
- Yes torque may be less than Y but Plaid has taller gearing so not much less.
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Good questions.
- ? May be continuous improvement, may be due to the rotor structure change. Would out phase rear motors produce yaw waggle?
- A sleeve would act like a shorted turn in the AC magnetic field. Structure needs to be in the wrap or the stacked laminations.
- Agree, lower speed is probably preferable; but, if it works, it works. I think the main question is the overall power curves and zero RPM torque of the two options. I say two, but for all we know, Cybertruck may do something different (though the extra motor count may be sufficient).
- Yah. Plaid rear motors only drive one wheel each so peak torque is halfish (power is still crazy high though). This allows steeper gearing to raise top speed. Alternatively, they could use a differently stator winding to reduce back EMF and raise top speed. Looping back to first point, would steeper gearing reduce the impact of torque ripple?
Elon has said that the higher rpm is a secondary benefit:
Elon Musk
https://twitter.com/elonmusk?ref_sr...97872.ampproject.net/2208051912001/frame.html
@elonmusk
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Main advantage of this is a much stronger EM field compared with a rotor that is held together by metal (usually high strength steel). Other advantage is that rotor can go to higher RPM, as carbon sleeve (mostly) stops copper rotor from expanding due to radial acceleration.
<end of Elon quote>
Source:
Tesla’s carbon-wrapped motor to pave way for new Roadster improvements
Tesla CEO Elon Musk appears to have shared one of the key updates that would be rolled out to the next-generation Roadster, an all-electric supercar that’s expected to be the company’s undisputed halo car. In a series of posts on Twitter, Musk explained that some of the Model S Plaid’s key...
www.teslarati.com
The interior permanent magnets in most rotors are usually wrapped in steel. Carbon fiber does not reduce the magnetic field strength as much as steel.
How much? I don’t know. I am guessing 1-2% motor efficiency improvement since it is hard to improve much when baseline motors are 90-95% efficient.
Maybe the US DOE will investigate this tech and a release a public report someday. For now, most likely the only people who have hard data work for Tesla, or for competitors that have bought the new S or X for instrumented tests.
GSP
Olle
Active Member
Sounds plausible
Yes, but hopefully not enough to cause problems. If these are the specs for Semi[Plaid]: mass=9[2.5], wheel radius=.50[.35] track/wheelbase=.40[.57], gear ratio 20:1[7.5:1] and for simplicity assume equal mass distribution and total stiffness to road per wheelbase and same motors on the dual motor axle for both vehicles, then the motor induced yaw acceleration is proportional to: gear ratio*track/wheelbase/mass/wheel radius≈1.78[4.89]
So Semi would in that case be 2.7 times better and if you can't even feel it in the Plaid it should be fine. Edit: Trailer force under load could be thought of as halfish since the single motor axle is pulling too. A trailer of 2.7*2 times heavier than the tractor would then be needed to create the same yaw wiggle as a Plaid.
There are non magnetic metal sleeves
Hard to say where the optimum is without having the data. I'd still be surprised if Semi comes near 20k.
Will be fun to revisit this thread once its out.
CyberTruck maybe. Surprising Plaid X has carbon btw.
True, peak torque is halfish in Plaid, under driving conditions. Peak available may be more. I think steeper gearing (I assume that you by this mean lower ratio) would make torque ripple worse. Think of the problem in the limit; If gear ratio is infinitely large, distance travelled per torque wave would be infinitesimally short. This is why a V12 is smoother than a 4 at low speed.
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Conductivity is the critical factor, not permeability. Like copper in a transformer.
Although, with a synchronous motor, this may be less of an issue?
Olle
Active Member
You definitely have a point. The more I think about it the more likely it seems that they’ll just go with carbon rotors, at least for the lone motor. Besides max speed, there seem to be efficiency benefits too.
https://www.researchgate.net/public...shless_motors_for_high_speed_applications#pf5
Buckminster
Well-Known Member
My thoughts:-
1) There is some chance a slightly smaller pack is possible in the 900-950 kWh range rather than 1,000 kWh. The 3 motor set up may improve cruising efficiency, allow a slightly smaller pack to be used. While this is a little thing, it makes a bug difference.
2) The best economics and environmental outcomes come from using solar charging where possible, solar on the warehouse roof,, solar powered mega-charging.
3) The cab might be slightly light weight, The frame provides a lot of structural strength. Tesla may have engineered good crash protection with a light weight cab. Tesla will take every possible opportunity to save weight, with a long time to optimise the design, they may have found some good weight savings.
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VIN decoder updated for 2023:
VIN shows that they are being built in Reno, NV. (No surprise there.)
VIN shows that they are being built in Reno, NV. (No surprise there.)
Talking semi (not Cyber)
Three motors, one rear axle with two motors (one per side like reveal), one rear axle with one motor and a differential.
Efficiency + power (and torque vectoring).
Doggydogworld
Active Member
I think it just means both rear axles are powered. The VIN itself does not seem to specify the number of motors on any given axle.
Watts_Up
Well-Known Member
This is what was announced last August 2022, in this aricle:
Tesla Semi appears closer to production with release of new details
The Class 8 Tesla Semi electric truck created the market for battery-powered freight movement. Is production edging closer?
www.freightwaves.com
Modified specs of the Tesla Semi
Tesla modified some aspects of the prototype. Instead of four electric motors,
the Semi will have three independent motors on the rear axles.
The company promises instant torque and unmatched power at any speed
so drivers can safely merge and keep pace with traffic.
To extend:
Structural packs can be self supporting which makes packaging a stack of them easier.
This can also be done with high ribs.
Tesla's internal cell lines will be 4680, so it is not unthinkable that they would use those for semi in the future. Could also be semi staus 2170 and other vehicles shift to 4680.
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