# Roadster 2 & new S/X motor configuration



## RocketRay (Jun 6, 2018)

So I just read on Telsarati that the S/X refresh coming will have a motor configuration like that of the new Roadster: two motors in the rear, one motor for the front.

What is the engineering advantage for this configuration? I'd think it'd be the opposite: two small motors for the front wheels, one monster for the rear. Then on turns the front motors adjust their power accordingly for maximum traction. With the other configuration you don't get that advantage for the front, and I don't see what the benefit is for the rear.

Inquiring minds want to know!


----------



## garsh (Apr 4, 2016)

I believe that Like Tesla Kim is the one who actually broke this story, so I'm including her video here.


----------



## garsh (Apr 4, 2016)

RocketRay said:


> So I just read on Telsarati that the S/X refresh coming will have a motor configuration like that of the new Roadster: two motors in the rear, one motor for the front.
> 
> What is the engineering advantage for this configuration?


During acceleration, most of the car's weight shifts to the rear wheels. That means that the front wheels lose a lot of available traction. Therefore, you don't need a very powerful motor up front for acceleration. A single motor can easily break the front tires loose during hard acceleration.

As for having two motors in the rear, it may mostly be a cost issue. Rather than designing two different size motors for the rear of the car (like the current Model S AWD vs Model S Performance), maybe they just designed a single smaller motor. The regular Model S gets one in the rear, and the performance version gets two of them. But now you cut your Model S rear motor assembly lines down to one instead of two. But that's just a guess.


----------



## garsh (Apr 4, 2016)

Also consider that Tesla currently uses one induction motor and one PM motor in each car. The PM motor is more efficient, and thus is used under all conditions (rear motor in the 3, front motor in the S and X). An induction motor can actually "free wheel" and not cause additional drag when unpowered like a PM motor would. So the induction motor is generally not powered during highway cruising, but can be employed when accelerating or decelerating.

So, if Tesla is going with 3 motors, then one intriguing possibility is that they continue to use the Model 3 PM motor as the front motor, and now they're now going to employ two Model 3 induction motors (which is the front motor on the Model 3) as rear motors in the S and X. That would really help consolidate their motor production lines and help bring down the costs of producing the S and X.


----------



## TrevP (Oct 20, 2015)

Not sure how accurate the info is given it's a leak and having been on the receiving end of such information in the past it's prone to errors or omissions from the party in order to cover their tracks. 

However some of it makes sense. The 3-motor setup is a direct derivative from what was developed for the Roadster so that makes total sense to me if they're going to take the car to the next level. Goes without saying the interior will be very Model 3-like albeit with better materials.

The battery is the big one. I've always said it's just a matter of time before Tesla switches to the 2170, not IF. Model 3 has proven the cooling, Model S needs better track cred and of course, the range needs to be industry-leading.

The big question is just how much of an exterior refresh is the car going to get? I think it depends on how much they changed the pack for the car. If significant changes were required to the pack for new cells then it stands that the rest of the car *could* get a significant refresh as the car is really designed around the pack.

Lastly, Tesla just revamped the car recently with new motors, power electronics and suspension. Can they really update the car again in such a short amount of time after doing it already? Sure they can, but how much backlash will they suffer from customers who just bought?


----------



## AO - Pete (Jul 26, 2017)

I agree with @TrevP The new (Raven) drivetrain will be the one carried forward to the refreshed Model S/X. I'm guessing the order of change for S/X will be:

1. Drivetrain (done)
2. Interior refresh, basic external tweaks (limited or no metal changes). Upgrade to Supercharger v2 charge rates CYQ3/4
3. Exterior refresh. Switch to 2170 (6 months after full Y launch).

It's all guesswork though, I think all we can say with reasonable certainty is that there's a refresh of some sort coming in the next 3-6 months.


----------



## ajdelange (Jun 26, 2019)

garsh said:


> An induction motor can actually "free wheel" and not cause additional drag when unpowered like a PM motor would.


Try turning the shaft on an unpowered PM motor. At rest the magnets are aligned with the teeth on the stator as this minimizes the reluctance. To rotate the shaft you have to apply torque and that means you are putting energy into the gap magnetic field. Once you have passed the midpoint between two teeth, however, the magnets want to align with the next tooth, the rotor supplies torque and thus releases the energy stored in the magnetic field. Thus the drag pulse is followed by a thrust pulse and the net drag is 0.

Looked at from a different perspective: I'll wager 11 beers that these cars use FOC which means that to the controller, the motors look like DC machines with independent control of torque and flux. The controller can thus command the motor to 0 torque. The stator magnetic field would be rotating at the same rate as the rotor with it's phase exactly opposite to that of the permanent magnet field. At least that's how it seems to me but I am not speaking from knowledge or experience here.


----------



## garsh (Apr 4, 2016)

ajdelange said:


> Thus the drag pulse is followed by a thrust pulse and the net drag is 0.


That makes sense. But that doesn't make for a very "smooth" coasting, so you would want to control the stator to come as close to cancelling out those forces at all points of rotation to smooth things out. So I guess rather than drag, you'd have inverter losses.


----------



## ajdelange (Jun 26, 2019)

The "experiment" was what you would expect with a traditional PM synchronous motor. That's not quite what's going in the Tesla's. They are PMSRM (Permanent Magnet Switched Reluctance Motors) with permanent magnets in the stator. In a typical SRM you would develop torque (for traction) by exciting the stator tooth when the rotor tooth is approaching it, 0 torque by not exciting the stator and absorb torque (regeneration) by exciting the stator when the rotor and stator teeth are aligned. How the addition of the permanent magnets (they are not ordinary magnets but rather Halbach arrays - field on one side only) modifies this I don't know but I suspect that the Halbach array prevents the rotor pole from grabbing a stator tooth as there is no magnetic circuit given that the array doesn't produce a radial field on one side of the magnet. Pure speculation there. But in any case you have a four quadrant machine which should be able to coast without inducing drag. As the stator would not be excited in this case there wouldn't be any loss in the inverter (other than the overhead).


----------



## carlk (Jun 29, 2019)

Like others said plus torque vectoring from independent motors. Torque vectoring is less needed or even desired on two wheels of the axle that performs the steering function.


----------

