Front motors smaller (less powerful) than the rear?

Could it be that the rear inverter will feed the front? There are many advantages to have it controlled by just one. Looks like just the motor and gearbox.

Excerpt from

http://ieeexplore.ieee.org/xpl/logi...ore.ieee.org/xpls/abs_all.jsp?arnumber=373800

This type of drive topology is targeted towards electric vehicle applications where the expected benefits are: (1) a simple drive train design (no mechanical differential required); (2) traction control; and (3) lower cost and smaller size compared to a two inverter, two motor alternative.

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It's an interesting conversation on the video, and I suppose there's some good reason to think that a front drive could be smaller and maybe there's no good reason to make it as large. Though there doesn't seem to be a great reason to make it smaller that I can think of either, unless it's for efficiency/range.

efusco said:

It's an interesting conversation on the video, and I suppose there's some good reason to think that a front drive could be smaller and maybe there's no good reason to make it as large. Though there doesn't seem to be a great reason to make it smaller that I can think of either, unless it's for efficiency/range.

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Power is already limited by the capability of the battery pack over much of the speed range, so the amount of use you would get from another motor of the same size would be very limited - and then you are carrying that weight around the whole time. The asymmetric arrangement probably matches the amount of torque you want from the two motors in the ultimate good-grip acceleration from rest case anyhow (this being the only case that's not power limited).

Packing it into the available space in the frunk area is also a reason not to just pick up another of the existing motors.

Yes, I got the impression that the second motor will prioritize traction over performance (ie: it will only activate in situations where there is risk of traction loss). You don't need a full-size motor to achieve this.

Front wheels need far less torque as they can't really put it to the pavement well..
So you could easily go with less than half and still be behind what the front rubber
can plant onto tarmac..

At the event and the Fashion Island showing of the X, George Blankenship made it very clear that the front motor is smaller.

It would appear that the only given here is that regen should be only (mostly?) on the rear axle. If you can disregard that issue then you would build with two equal sized motors, meaning each would be half the size of the MS motor. The X is already too heavy so we've gotta be sensible.

All I really want is very limited 'front wheel assist' to go up icy, muddy hills at 15, 20 mph max. A 'performance' X would be a whole different beast.
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> Not that would make sense because the front wheels are used for "braking" more than the rears. It would seem like a lot of waste of energy (and front brake pads!). [scottf200]

Just to keep it as equivalent in function to the S. eBraking only in the rear seems to work just fine in the S, maintaining a straight ahead orientation. You are correct since I've changed to half sized motors, now some of the regen during max deceleration will have to be distributed to the front axle or be lost. But if you are getting into front-rear apportionment, then you just might change over to a front-centric model ala Subaru Forester, where you basically are 80% front and 20% rear with more torque applied to the rear as needed.
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For breaking with front wheels, the front motor does not need to be more powerful.
The rear motor is by factors underutilized at braking, since it would spin the car if
it was more aggressive. The front motor could be even a third of the rear one and
simply set for more aggressive braking without issue. And in general, your brake
pads will last a very looong time on an EV, if using regen properly.

Paul Carter said:

Could it be that the rear inverter will feed the front? There are many advantages to have it controlled by just one. Looks like just the motor and gearbox.

Excerpt from

http://ieeexplore.ieee.org/xpl/logi...ore.ieee.org/xpls/abs_all.jsp?arnumber=373800

Click to expand...

It's possible, but I don't really expect it. What I'm expecting to see is a smaller motor in front geared a lot lower (and thus needing its own inverter.) I suspect that it is set up just like the rear motor - motor on one side of the central gearing, PEM/inverter on the other.

All electric motors are less efficient at higher speeds - I think this is a part of why Tesla initially tried to develop a two speed transmission for the Roadster. GM says they get 2-4% better freeway efficiency by the Volt's 2 motor drive mode, which drops the rpm by 50% on the main motor.

With two motors, you could gear the smaller front motor for best efficiency on the highway (you can't really use that much torque on the front wheels to begin with, as others have noted.) By doing steady state freeway driving entirely on the front motor with the rear one idled, you'd get at least the same benefits GM got - but if you floored it, the car could instantly charge forward with the full torque/power of the rear motor. All the benefits of two transmission speeds plus AWD for bad weather or off-road.

Done correctly, I think the AWD car could have measurably better efficiency/more range than an equivalent RWD version (of course, the X's heavier weight and larger profile will make it worse than the S by some amount regardless.) I think the opportunities of asymmetric gearing is what the Tesla spokesman is hinting at right at the end of the video.
Walter

As I posted in another thread they might use a smaller motor for both the front and the rear. With two motors I don't see any good reason to have a larger rear motor.

...other than the added cost/complexity of having more than once motor design to deal with. Right now every Model S receives the same motor. You're talking about having two or three different ones. Barring that, you're right. Having two motors would mean each could theoretically be less powerful and achieve the same goal.

JRP3 said:

As I posted in another thread they might use a smaller motor for both the front and the rear. With two motors I don't see any good reason to have a larger rear motor.

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On any car, weight transfer means you can push harder on the rear wheels during acceleration, so having more torque from the rear motor makes sense. However, the data from Tesla suggests they are planning to get extra efficiency at speed by gearing the front motor a lot lower than the rear. At that point, I suppose there's no reason they couldn't be identical motor packages on different gears, with the front turning half as fast or less. Tesla can probably save some weight and cost by using a smaller motor - or they can keep a simple supply chain by using Model S motors and controllers on both ends.

I bet that Tesla will continue to use the Model S motor/gearbox/inverter in the rear, along with a smaller motor/gearbox in the front.

My thinking is that it's a waste to use the S motor in the rear. Why carry around a motor large enough to power the vehicle at full performance on it's own when there will always be another motor onboard? Since they have to build a smaller motor anyway for the front it might make sense to use the same motor front and rear. It might also be the same motor that goes into the Gen3 vehicle.

JRP3 said:

My thinking is that it's a waste to use the S motor in the rear. Why carry around a motor large enough to power the vehicle at full performance on it's own when there will always be another motor onboard? Since they have to build a smaller motor anyway for the front it might make sense to use the same motor front and rear. It might also be the same motor that goes into the Gen3 vehicle.

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The bigger inverter is integrated into the existing motor, which is integrated into the existing rear subframe. Integrating the big inverter into a smaller motor, or using dual inverters, would be a waste of engineering time.

Controlling two induction motors from one inverter is not easy so as far as I know it's not a given that they can do that. Plus they will have to engineer the smaller inverter for the Gen3 vehicle eventually anyway. You do have a point about the rear subframe changes, but I'm not sure if that alone is a show stopper.