ajdelange

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I have no idea what the motor selections are going to be. The PM motors are more efficient for sure so I would think they would want to use them to the fullest extent possible. The IM's have more low speed torque though so as you mention for the 3 and in my X there is a IM in the back and a PM in the front. The TriMotor will have 3 motors and thus three places to get torque from. Will adding that third source of torque be enough to overcome the torque limitations. I really don't know.

My comment on efficiency was based on the assumption that if power or torque is being derived from 3 motors instead of 2 each would draw 2/3 the current drawn by 2 for the same level of power or torque. The I^2R losses would thus be proportional to 3*4/9 = 4/3 for the tri motor and to 2 for the dual motor.





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HaulingAss

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I'm curious if Tesla will do what Ford did. Range is estimated with with a 1 thousand pound payload. If so the 500+ should definitely easily got beyond 610+ when not loaded.

Ford has done a very sly thing here by setting in people's mind that their 300 miles is not comparable to their competitions 300 miles. It gives their influencers a handy comeback whenever someone is critical of the short-range of their extended range battery option. But I don't believe Ford has actually said this in any official release. Because their 300 miles of targeted range is clearly labeled "EPA combined" and the EPA does specify driver weight but not any cargo weight.

But to see why this is important, consider the following:

My 2010 gas powered XLT F-150 4x4 clearly gets better highway mileage with 1000 lbs. in the bed. That's primarily because it doesn't have an adjustable suspension and it rides bed high without that weight back there. The highway MPG of a long vehicle like a truck is very sensitive to how close the undercarriage is to being parallel to the road. If Ford has added an under-fairing to the Lightning, and I believe they have, efficiency becomes even more sensitive to fore/aft levelness. Without an adjustable suspension, the highway range of the Lightning is going to be a lot less than 300 miles, especially when the truck is unladen. Yet Ford has slyly disseminated the idea that the 300 miles is with 1000 lbs. of cargo to imply the "real" range while unladen will be even higher. I do not believe customers are going to be happy with their highway range.

This is one important reason why a fully adjustable suspension comes standard on the Cybertruck, even on the lowest cost, RWD version. Because Tesla understands the customer would not be happy with the highway range when the truck was unladen due to the excessive drag that is created when the rear of the vehicle rides higher than the front. A Cybertruck will be automatically self-leveling to ensure good range regardless of how the truck is loaded (or if it's empty). And it will lower the front of the truck too at freeway speeds which increases range by limiting the airflow under the vehicle. The Lightning has a "dumb" suspension (non-adjustable) which will cause the range to vary unexpectedly depending upon how it's loaded.

It wouldn't surprise me to see Ford reverse course on this (and supply adjustable suspension or some type of load-leveling) once they realize how important highway range will be to their customers, and even more so, the predictability of that range. Because when your range is difficult to predict, you cannot comfortably use as much of it. When you only have 230 miles of it (or even 300), this matters a lot.
 

JBee

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My comment on efficiency was based on the assumption that if power or torque is being derived from 3 motors instead of 2 each would draw 2/3 the current drawn by 2 for the same level of power or torque. The I^2R losses would thus be proportional to 3*4/9 = 4/3 for the tri motor and to 2 for the dual motor.
I'm pretty certain that in partial load situations, like in normal on road cruise, the PM efficiency will always be better than the SIMs, meaning the SIMs will be mostly off in cruise.
 

HaulingAss

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I have no idea what the motor selections are going to be. The PM motors are more efficient for sure so I would think they would want to use them to the fullest extent possible. The SIM's have more low speed torque though so as you mention for the 3 and in my X there is a SIM in the back and a PM in the front. The TriMotor will have 3 motors and thus three places to get torque from. Will adding that third source of torque be enough to overcome the torque limitations. I really don't know.

My comment on efficiency was based on the assumption that if power or torque is being derived from 3 motors instead of 2 each would draw 2/3 the current drawn by 2 for the same level of power or torque. The I^2R losses would thus be proportional to 3*4/9 = 4/3 for the tri motor and to 2 for the dual motor.
Probably a bigger factor is that three motors eliminate one of the energy robbing differentials. It also allows for slightly different power levels between the two rear motors depending upon steering angle which could slightly increase efficiency, mostly in tight turns.
 

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Stick in the mud: CT cannot achieve the projected mile range due to 2.9 seconds. 😈

When I first got my volt, I tried to get maximum battery range. It quickly got boring. My efficiency will be on the lower side.
 

ajdelange

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Good point on eliminating the differentials.

My gut says the IM's are history. Weird for a company named Tesla. But I could well be dead wrong in this. If we consider towing 14,000 pounds we are going to want torque at the rear end. Two PMMs are going to supply a lot of torque for sure but not as much as two IM's. Perhaps these motors have ammortisseurs. Maybe it's enough - maybe not. We'll just have to wait and see I'm afraid.
 
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JBee

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Probably a bigger factor is that three motors eliminate one of the energy robbing differentials. It also allows for slightly different power levels between the two rear motors depending upon steering angle which could slightly increase efficiency, mostly in tight turns.
Interesting idea. But I can't see the diff losses being higher than the efficiency difference to the PM and a single controller. You would have to be doing a lot of corners which is not typical highway range consumption. In town any cornering losses would be significantly less than the reduced aero drag from traveling at lower city speed limits. It would be interesting to know diff losses though to compare.
 

JBee

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Good point on eliminating the differentials.

My gut says the SIM's are history. Weid for a company named Tesla. But I could well be dead wrong in this. If we consider towing 14,000 pounds we are going to want torque at the rear end. Two PMMs are going to supply a lot of torque for sure but not as much as two IM's. Maybe it's enough - maybe not. We'll just have to wait and see I'm afraid.
I expect the IMs to stay because of cost/resource use, price/performance and torque profile. The PMs are there to extend range and provide front axle drive, the IMs are for peak output and rear axle drive. Each serves a purpose to cover the full dynamic range of driving. A hybrid motor drivetrain of sorts.

BTW Why do you think there is a torque problem? To reach the proposed performance figures?
 

ajdelange

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I think I figured that going from 0 - 60 in 2.9 seconds required 0.94g's if the acceleration is uniform. If the truck weighs 5000 lbs (2300 kg) that means a thrust of 21 kN. That's a lot of thrust that translates to a lot of torque implying that you could accelerate CT + 14000 lb trailer to 60 mph in about 11 seconds! But in any case you need a lot of torque to be able to get up to 60 in 2.9 sec. Synchronous motors are notoriously hard to start. These aren't pure synchronous motors but rather a sort of hybrid between a switched reluctance and PM motor. So perhaps their torque dropoff at slower speed isn't so dramatic.

What is clear is that if three PM motors can produce the 21 kN they will not use any IM. They are trying to hit 500 mi range. That requires saving every Wh they can.
 
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Challeco

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We all have seen the patent UI mock-ups - 610 miles of range! Shocking, surprising? Even unbelievable? Just a random number in a mockup? How does the CT get there in a package that weighs the same (or less) than a Ford ICE F-150 and cost $70k (or less)?

Elon had stated in the reveal that they didn't "cheat" with the Cybertruck - they stayed within the dimensions and weight of the F-150 which weighs 5700 lbs max. (8) If you take the F-150 body and frame as a reference, it weighs 5150 lbs (current F-150 weight + 700 lbs for steel and subtracting 1250 lbs of powertrain weight).(1) Elon has stated that the CT will be a stressed-skin design and use a three-piece casting process for the frame with a structural battery pack using the new 4680 cells. Switching a body and frame to a unibody build results in a typical weight savings of 25%.(2) I think we can expect a similar weight reduction for the stressed-skin design, which would bring the body/frame weight down to 3863 lbs. If we subtract that from the max weight allowed for the truck (5700 lbs) we get 1837 lbs left for batteries and drivetrain. Subtracting 70 lbs for the drivetrain leaves 1767 lbs for batteries.(3)

Unfortunately Tesla has not released weight info on the 4680 cell, but it has been estimated that 1416 2170 cells occupying a volume of 1.1*10^8 mm3 and weighing 1060 lbs can be replaced by 960 4680 cells with a similar volume of 1.28*10^8 mm3 and estimated to weigh as much as 40% less.(5) This is for the Model Y and the CT will be a wider and longer vehicle. So if we increase the battery volume by 100% we would end up with 1,920 4680 cells and still potentially be under 1767 lbs. The estimated kWh would go to 200 kWh. A 100 kWh in the Model X 100D provides a range of 325 miles with 2170 batteries.(6) Increasing the kWh to 200 (while still keeping the car weight the same 5700 lbs) could increase the range to 570 miles (200 kWh / 351 Wh per mile). Using 200 kWh of more efficient 4680 cells in a vehicle the same weight should increase the range by 16% or to 660 miles. (7)

The problem here seems not to be weight but the increased size of the battery. Can a battery volume of 2.56*10^8 mm3 be fit into the middle structural component of the CT? That volume would translate into dimensions that would be, for example, 60" L x 52" W x 5" H. It seems like that could be incorporated into the center casting of the CT but I can't say for sure.

The other problem seems like it would be the cost to produce as that is a lot of 4680 batteries. However, Elon and Sandy Munro have stated that switching to 4680 batteries can reduce production cost by 50% per kWh.(7) Sandy has also stated that switching to a three-piece casting process could cut production costs by 1/3.(9)

**UPDATE** I need to make a correction - in using the Model X as a surrogate for the CT I forgot to factor in the efficiency penalty that the CT would have over the Model X due to the difference in cd values. The Model X has a cd of .24 and the CT should have a cd of .30 (per Elon’s tweet). I don’t have a source, but as best as I can glean from different examples (10) this would equate to about a 15% penalty in efficiency. (This would not be the percentage difference between the two values but the measured effect.) So with a 200 kWh battery, all other things being the same, the range would be calculated as 495 miles (200 kWh / 404 Wh per mile) and with the 16% efficiency gain from the 4680 cells the calculated range would end up at 575 miles.

My math or interpretation of my source data could be wildly off here, so I am open to valid critical review. (No, I am not an engineer, but personal attacks based on my vocation I don’t believe are relevant. In fact, in these forums it would be appreciated if we just leave personal attacks and insults off the table.)

Sources:

(1) https://www.torquenews.com/106/ford-breaks-down-2015-f150-weight-savings
(2) https://link.springer.com/article/10.1007/s42452-019-0733-8
(3) https://chargedevs.com/newswire/elo...io-is-the-challenge-with-ac-induction-motors/
(4) https://enrg.io/tesla-battery-weight-overview-all-models/#:~:text=Model X – 1200 Pounds*
(5) https://techtricity.substack.com/p/the-4680-cell
(6) https://insideevs.com/news/355282/2019-tesla-model-x-epa-ratings-compared/
(7) https://www.teslarati.com/tesla-4680-cell-pack-breakdown-video/
(8) The Cybertruck has been registered as Class 2b (8501-10,000 lbs GVWR). This means the max weight of the truck has to be 10,000 minus it's max payload, which for the CT is 3500 lbs. So the Cybertruck cannot weigh more than 6500 lbs empty. https://electrek.co/2019/12/13/tesla-cybertruck-medium-duty-vehicle-carb/
(9) (10) https://aia.springeropen.com/articles/10.1186/s42774-020-00054-7/tables/2
Well done!
 

JBee

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@ajdelange

Having enough torque is one thing but having enough traction is another. A typical launch control method is to reduce torque to the wheels on launch and add torque/power with speed. Given that they also have a gear ratio to give it more motor rpm, and aren't direct drive, the IM's low start torque profile should be able to match available traction quite well. Likewise due to traction limits acceleration won't be linear.

I thought I saw on the UI pictures that it showed 8000Nm per wheel. That would be 32kN? Which would indicate that peak torque delivery is a fair bit higher than the linear requirement, which would be expected because traction is reduced at low wheel rpm. Its only with speed (and motor rpm) you can start putting the power down to get moving and reach the 2.9sec. I'd expect the torque curve to start lagging traction around 40mph. (Depending on tyres of course)

As before I still think the IMs will stay and we won't get PMs in the back because the CT is already cost sensitive. Hence sharing drivetrains with other models.

I'm pretty certain the range will be achieved with the PM on only. The IMs will be mostly off in cruise. You only need 20-30kW for cruise at which point the PM is only at under 20% load, so not close to winding max loads anyway, making winding thermal loses irrelevant to achieve range.

Btw whats your guess on vehicle mass?
 
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HaulingAss

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I thought I saw on the UI pictures that it showed 8000Nm per wheel. That would be 32kN? Which would indicate that peak torque delivery is a fair bit higher than the linear requirement, which would be expected because traction is reduced at low wheel rpm. Its only with speed (and motor rpm) you can start putting the power down to get moving and reach the 2.9sec. I'd expect the torque curve to start lagging traction around 40mph. (Depending on tyres of course)
Traction is not reduced at low wheel rpm. It's dependent upon the coefficient of friction between the tires and the road surface and the amount of weight on the tire.
 

HaulingAss

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We don't know this to be true. No one has torn apart a Plaid powertrain yet. I would bet it's not true, either :/

-Crissa
It's pretty much a given that two motors on the rear wheels (or the front wheels) will eliminate that differential. It would serve no purpose and would be useless cost, weight and additional friction.
 

ajdelange

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I thought I saw on the UI pictures that it showed 8000Nm per wheel. That would be 32kN? Which would indicate that peak torque delivery is a fair bit higher than the linear requirement, which would be expected because traction (edit: tractive force) is reduced at low wheel rpm. Its only with speed (and motor rpm) you can start putting the power down to get moving and reach the 2.9sec. I'd expect the torque curve to start lagging traction around 40mph. (Depending on tyres of course)
If the accelerartion exceeds 0.94g for some part of the run then indeed more torque will be required to produce that.

As before I still think the IMs will stay and we won't get PMs in the back because the CT is already cost sensitive. Hence sharing drivetrains with other models.
I'm now more convinced than ever that they will go. I had quite forgotten that synchronous motors are hard to start and require ammortisseurs only when you have a fixed frequency to start them. These vehicles obviously use VFD. Torque is a function of PM field strength and stator current. As the rotor position is known to the controiller it can place the stator field at the angle it needs to be at for maximum torque and you can get lots of current from a 200 kWh battery. Thus a synchronous motor with a VFD is likely (I'm not a motor expert to say for sure) better at low speed torque. The following video is pretty good at explaining the Tesla IPMSRM design. The reluctance switching is actually for better high speed performance
https://insideevs.com/news/461811/video-tesla-model-3-electric-motor-explained/

I'm pretty certain the range will be achieved with the PM on only.
I am too as I am now pretty certain that this will be the only type of motor in the truck. If you can get better torque and more efficiency from IPMSRMs than IMs why use IMs at all? Apparently the IPSRMS are as much as three percent more efficient than IMs. But still you could be right. We have to wait for the CT but we might get a clue from the Plaid vehicles. I was told by a Tesla employee that the Plaid S is a testbed for the CT.

The IMs will be mostly off in cruise. You only need 20-30kW for cruise at which point the PM is only at under 20% load, so not close to winding max loads anyway, making winding thermal loses irrelevant to achieve range.
If the drive train is 3% more efficient you will get 3% more range at any load - actually more than that because 3% less motor current means 6% less I^2R in the inverters.

Btw whats your guess on vehicle mass?
2500 kg?
 

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