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Speculation on Model ☰ Powertrain

Speculation on Model ☰ Powertrain

As has been widely reported, JB Straubel stated last October about the Model ☰: “We’re inventing a whole new platform: it’s a new battery architecture, it’s a new motor technology — brand new vehicle structure…so it’s a lot of work. This is something we started on even a few years ago. But it is a brand new vehicle — and it has to be.” -- http://evobsession.com/tesla-cto-jb-straubel-model-3-is-going-to-be-most...

Just as the Model S was a clean-sheet design, this implies that the Model ☰ may also be one. This makes sense because the design goals of the new platform are different than the design goals of the Model S platform. As many have opined over the past months, it will be difficult to cost-reduce the Model S enough to get the price down to $35,000. It would instead be better to design a new car from a clean sheet with the desired goals in mind.

In such a design, what would be some of the key elements? To answer that we need to look at the design goals that have been stated so far:
Base model price: $35,000
Size similar to Audi A4
Coeficient of Drag < 0.20
200+ mile "real world" range
Performance better than similarly-priced Audi and BMW competition

There are a lot of things outside of the powertrain which are implied by the above list (and many more goals that I've not mentioned), but let's look at these in terms of the battery and motor systems.

Tesla has spent the last 10 years producing cars using AC Induction motors. These motors have both pros and cons. Wally Rippel wrote on the Tesla Blog in 2007 (https://www.teslamotors.com/blog/induction-versus-dc-brushless-motors) that there's no one motor design that is "best". Recently, according to articles I've read, it seems Switched Reluctance motors are the new shiney item in motors. They tend to be slightly more efficient, have a broader range of speeds, can maintain high power at the higher speeds, and generate less rotor heat than induction motors. There are also hybrid permanent magnet/switched reluctance designs (I believe the BMW i3 uses one).

Within a motor, the power lost to internal resistance is given by I²R. When running two motors each at half the current, each motor wastes only 1/4 the amount that a single motor loses when running at the same overall power. Therefore, the system loses roughly half the power of the single-motor system. If you ran 4 motors, each at 1/4 the power, the loss-per-motor would be 1/16 for an overall system loss of 1/4. (Note, the power lost due to parasitic heat being talked about here is considering the DC resistance of the motor windings--not the AC reluctance of the system as a whole--that would be measuring the total power including the useful power output. The better formula for that is likely P=IE--current * voltage).

So...might we see:
quad motor design
switched reluctance and/or permanent magnet motors
true electronic traction control and torque vectoring
new battery chemistry

The Model S 60kWh base model was rated at 208 miles range. With a more efficient motor system, lower drag, smaller frontal cross section, and lighter weight, might we see a 50kWh battery on the base model and still reach 200+ miles per charge? Quad 100hp motors could be standard.

A performance version may increase the rear motors to 200hp and have a 70kWh battery.

steveg1701 | 29 mars 2016

If the M3 doesn't have an induction motor can they even validly call it Tesla?

Chunky Jr. | 29 mars 2016

Interesting speculation.

I assume the cost of 4 motors be more than the cost of one bigger one. If it is more efficient perhaps the battery savings offsets the extra cost?

I'd be surprised if there was a new battery chemistry but that would make an already exciting event even more so.

jordanrichard | 29 mars 2016

Ya, I have been wondering what JB meant by "new platform". How can they improve on the simple skateboard design. They certainly wouldn't put a motor at each wheel. It would be too much upsprang weight.

Chunky Jr. | 29 mars 2016

I suspect "new platform" means that they can build a family of cars that all derive from a common design.

JeffreyR | 29 mars 2016

I think part of the reason for the AC motor design is to avoid rare earth metals used in other designs. For me a "new platform" would need to be designed simply because you need to subtract 20% of the size and 50% of the cost. I would be shocked if the M≡ did not have a skateboard design like the MS and MX. Part of the inherent benefits of the skateboard design include:

- Low center of gravity
- Battery pack adds stiffness/structure to the frame
- Battery pack is easily removed
- Several body types can be added — we "know" about the Model Y (mid-size crossover) and Model R (next-gen Roadster), but there are several other types that could be done including a two-door coupe, wagon, and truck (@Chunky Jr's point).

I see having dual-motor AWD being an option, but I think four motors would be moving away from the easy to manufacture goal Elon pointed out during the last quarterly call.

johnse | 30 mars 2016

I don't see them moving from the skateboard. I also don't see them using the partial skateboard that one of the Seeking Alpha authors opined. The solid structure does wonders for crashworthiness.

However, what goes in the skateboard could have lots of differences. The clean sheet design I am positing would be about re-thinking the materials, hardware and software tradeoffs. Just because they made sense for the Model S/X platform doesn't mean that they are still correct. Also doesn't mean they aren't.

When you build a system, make certain design choices, and then iterate on them, you often end up with bits of cruft in the design. In software, this often gets referred to as technical debt. Unfortunately, it is very hard to ever find the time to go back and clean up the debt. When you include hardware into the mix, often the state of the art moves on and it is only by doing a complete new design that you can drop the legacy considerations.

Moving to 4 motors doesn't necessarily lead to unsprung weight. That would only happen if the motors were put in the wheels. That would also add a lot of vibration to the motors. However, using one motor per wheel, with connecting drive shafts would eliminate the mechanical differentials. I think (and I'm not a motor engineer, so I could be wrong) that the non-induction motors could be designed for direct drive--no reduction gearing needed.

It would require more sophisticated drive software--but they've had time to create that.

As for calling it a Tesla...we're not powering these cars via free broadcast energy...That would truly be Tesla's legacy. I don't think Nikola Tesla would begrudge them using more efficient designs.

dd.micsol | 30 mars 2016

I don't think 4 motors would make any sense-more labor involved-more motor movement needed and more storage needed for those parts and probably more weight. Now on the other hand if the motors can be made by robots entirely-then you might have a point due to speed of production. We can speculate all we want right now but why waste the energy-It's only 34hrs away.

Red Sage ca us | 30 mars 2016

~*sigh*~

Elon has already said the base car will be single motor rear wheel drive.

The name of the company is 'TESLA' -- they will continue to use alternating current induction motors.

Traditional automobile manufacturers like permanent magnet motors because they are not serious about making electric vehicles a mainstream product line option.