# How do the AWD different motor types complement each other?



## Scott Wilcox (Dec 8, 2016)

I am sure it has been talked about somewhere but I can’t find it. There has to be a reason for the induction motor in the front of the AWD. It is not efficiency. I assume the two types complement each other. 

Does anyone know at what speeds each motor is most efficient? Will the induction motor warm the battery and the cabin faster?

I have heard it both ways that the switched reluctance motor jumps off the line then others have said it lacks the punch of the Model S induction motor. I have also heard that the switched reluctance does not over heat like the induction motor.


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## garsh (Apr 4, 2016)

Some previous discussions on Tesla motor technology:

Tesla Electric Motor
dc motor vs ac motor

An article on the Tesla blog:
Induction Versus DC Brushless Motors

I'm curious to find out why Tesla is going with the two different technologies. There's not a *huge* difference between the two types of motors. Induction motors tend to be a little larger & heavier for the same power, but are less expensive to manufacture.


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## Guest (May 25, 2018)

I'm also surprised induction motor is at the front.
I thought more powerful one would be at the back in case of spirited driving.
Induction motor is more about power less about efficiency.

Anyway, I do no consider dual motor version. 
Also it appears induction motors are not as reliable as PM motors.


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## garsh (Apr 4, 2016)

arnis said:


> Also it appears induction motors are not as reliable as PM motors.


Citation needed.

Induction motors are about the simplest type of motor there is. There's less to go wrong.


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## Guest (May 26, 2018)

garsh said:


> There's less to go wrong.


That is not enough for the big picture. ICE engines have hundreds of moving parts with
extreme tolerances. And most engines models are 10x+ more reliable than Model S drivetrain
(not just rotor and stator).


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## garsh (Apr 4, 2016)

arnis said:


> And most engines models are 10x+ more reliable than Model S drivetrain.


You made a general comment about induction motors (not Model S drivetrains) being less reliable. Now you're changing the argument.


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## John (Apr 16, 2016)

I was under the impression that:

1. The base Model 3 switched reluctance motor is optimized for efficiency and cost and fairly good performance
2. The traditional induction motors favor performance and offer a side efficiency benefit of field shaping at high speed/low load, when other types of motors tend to have a back-EMF efficiency drop off.

So if someone is going to spring $5000 for an extra motor, the induction motor is a good choice for performance and as a highway-speed cruising efficiency aid.

Well, that's my impressions anyway.


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## Guest (May 26, 2018)

It's a Tesla forum. By default, everything told here is about Tesla if not mentioned otherwise. 
Be it induction motors made by Tesla or something else.
I definitely didn't mean induction motors used for water pumps.
Tesla's induction motors are not reliable, compared to, let's say Nissan's PM motors (no failures at all AFAIK).
Though often something else fails in the drivetrain, not specifically motor.


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## Scuffers (Jun 8, 2017)

arnis said:


> It's a Tesla forum. By default, everything told here is about Tesla if not mentioned otherwise.
> Be it induction motors made by Tesla or something else.
> I definitely didn't mean induction motors used for water pumps.
> Tesla's induction motors are not reliable, compared to, let's say Nissan's PM motors (no failures at all AFAIK).
> Though often something else fails in the drivetrain, not specifically motor.


that's a reasonable point to make, although reliability is not down to be being induction or PM, but down to quality of manufacture/design.


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## Guest (May 26, 2018)

In case of motor problems, I've read years ago, that Model S did suffer
induction motor specific flaw of current passing through motor bearings.
Shaft voltage is the reason.
And we can categorize that as "design mistake". Not the same as quality.


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## garsh (Apr 4, 2016)

arnis said:


> It's a Tesla forum. By default, everything told here is about Tesla if not mentioned otherwise.


arnis, stop changing your argument. By that logic, your original statement:


arnis said:


> Also it appears induction motors are not as reliable as PM motors.


Is completely non-sensical, because Tesla didn't even make a PM motor until the Model 3, and that hasn't been in service long enough to gauge reliability.

Tesla had some design issues with the Model S drivetrain. They've since fixed those, save for overheating when racing. But that is not a valid basis for asserting that PM motors are more reliable.


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## Guest (May 26, 2018)

AFAIK 2017 Model S-X drivetrains are still not as reliable as they should be (on par with other EVs).

My argument is that using induction motor on Model3 might not be the best idea.


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## Scuffers (Jun 8, 2017)

arnis said:


> AFAIK 2017 Model S-X drivetrains are still not as reliable as they should be (on par with other EVs).
> 
> My argument is that using induction motor on Model3 might not be the best idea.


not sure that's a fair statement?

there are more Tesla motors out there (so bigger pool to fail) as well as their motors are at least double the output of every other maker's, so your not really comparing apples with apples etc etc.

be interesting to get the full stats on failures (in respect of the fleet numbers etc)


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## Guest (May 26, 2018)

There are more than 300 000 Leaf in the world.
And about the same number of Model S's.
Though at least half of them have two motors.

Anyways... Failure rate is not slightly more.
Like I said, I've not heard of Leaf motor/inverter failure, at all.
And there are tens of thousands of failures with Tesla.
Some have 3-4 replacements.

Stats would be awesome.

PS:
Leaf motor has higher nominal power output than single Model S motor.


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## John (Apr 16, 2016)

Quick search:
Leaf: 147 hp and 236 ft-lbs (weighs 3,433 to 3,508 lbs)
Model 3: 271 hp and 307 ft-lbs (weighs 3,549 to 3,838 lbs)
Model S: (weakest single motor version): 382 hp and 325 ft-lbs (weighs 4,469 to 4,941 lbs)


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## Guest (May 26, 2018)

382hp is peak power. Not nominal. Nominal is around 100-200hp, those values are hard to find.
Easiest way to estimate is to analyze top speed. If it is below 155mph/250kmh, then nominal power
is less than 170kW/230hp. 
147hp mentioned by Nissan is actually nominal power.


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## Scuffers (Jun 8, 2017)

arnis said:


> 382hp is peak power. Not nominal. Nominal is around 100-200hp, those values are hard to find.
> Easiest way to estimate is to analyze top speed. If it is below 155mph/250kmh, then nominal power
> is less than 170kW/230hp.
> 147hp mentioned by Nissan is actually nominal power.


in the context, that's a meaningless metric.

Unless you have the full datasheet for each motor with their load/temp/etc characteristics?

As for numbers, how many cumulative miles do you think all those Leaf's have covered compared to the Model S's?

I bet most never do more than short commutes etc, and are second/third cars.


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## Guest (May 27, 2018)

Scuffers said:


> As for numbers, how many cumulative miles do you think all those Leaf's have covered compared to the Model S's?
> 
> I bet most never do more than short commutes etc, and are second/third cars.


Leafs have travelled 3 billion km-s by the end of 2016. Teslas 8 billion km-s by the summer of 2017. 
Therefore annual mileage is really close. Long commutes is a tiny fraction of total miles travelled.

I do not have full specsheet for every version but I can browse registrational data for random Teslas that are on sale. And there I can see real nominal motor power.


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## Scuffers (Jun 8, 2017)

arnis said:


> Leafs have travelled 3 billion km-s by the end of 2016. Teslas 8 billion km-s by the summer of 2017.
> Therefore annual mileage is really close. Long commutes is a tiny fraction of total miles travelled.
> 
> I do not have full specsheet for every version but I can browse registrational data for random Teslas that are on sale. And there I can see real nominal motor power.


In what universe is that 'really close'?

Look, you go live in your delusional world, I'm out.


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## Guest (May 27, 2018)

Scuffers said:


> In what universe is that 'really close'?


In the order of magnitude "really close". 
All Leafs 3 billion km: <100 failures.
All Teslas 8 billion km: >100 000 failures (more than half of driveunits up to some point).
It's in the order of magnitude "really close".



Scuffers said:


> Look, you go live in *your *delusional world, I'm out.


You are not the first one who's out, #Brexit


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## CoastalCruiser (Sep 29, 2017)

I don't have a definitive answer to the OPs question, but I just posted an article about the Model 3's powertrain that has a couple of paragraphs on the question. It's hard for someone not familiar with the specifics of how Tesla engineered both motors to speak about the differences in the two designs beyond the general info we can read about. The specific "secret sauce" Tesla used may be a significant factor.

https://cleantechnica.com/2018/05/2...rs-to-carborundum-youve-come-a-long-way-baby/


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## KarenRei (Jul 27, 2017)

> > Look, you go live in *your* delusional world, I'm out.
> 
> 
> You are not the first one who's out, #Brexit


"Kumbaya, m'lord, kumbaya...." 

Yes, the early Model S motors suffered from a design flaw. That design flaw is not inherent to induction motors. I don't however think it's possible to generalize between one or the other being more inherently reliable. IMHO it's the bearings that are the main concern when it comes to reliability, and both need them.

As for why induction at the front, PM at the rear: they want it to be a RWD car, because of handling. So if there's only one motor (which will be PM), it needs to go at the back, and the car is designed for that motor to be in that location. Rather than redesigning the back, they just put the new motor at the front. Also, the front motor will spend most of its time inactive, since it's less efficient. So, if you want the car to be RWD, you need the PM motor at the back; otherwise, it'll spend most of its time as FWD.


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## Guest (May 28, 2018)

Why not two PM motors?

If all accelerations above 100kW would be with the help of rear induction motor
and all normal driving is happening with front PM motor, nobody would ever notice.

I thought that most powerful motor (induction) would go to the back for performance acceleration and
heavy cornering. And not so powerful PM motor would go to the front.


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## garsh (Apr 4, 2016)

arnis said:


> Why not two PM motors?


We don't really know yet. Hopefully at some point Elon or some other member of Tesla will answer that question.


> I thought that most powerful motor (induction) would go to the back for performance acceleration and
> heavy cornering. And not so powerful PM motor would go to the front.


I'm not sure where you got this idea that an induction motor is "more powerful". Either type of motor can be made pretty much arbitrarily powerful by design. Given equal power, an induction motor will generally be a little larger, a little heavier, and a little less expensive. So my guess would be that the PM motor is in the back because it allows them to put a *more* powerful motor in the back without it taking up a lot more room.


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## John (Apr 16, 2016)

Maybe the motor strategy had to shift with Model 3. 
In Model S, easy to coast one induction motor while relying on the second one which is geared for highway speed.
But with a PM-variety motor in Model 3, you can't really coast it without losses. So one guess I have is that they stuck a small induction motor in the front which they can idle. Had they added another PM, no efficient idling.


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## Guest (May 31, 2018)

John said:


> But with a PM-variety motor in Model 3, you can't really coast it without losses. So one guess I have is that they stuck a small induction motor in the front which they can idle. Had they added another PM, no efficient idling.


Pure coasting almost never happens. But it is true, that PM motor can't handle "nothing".
For example, if I switch to neutral on Leaf while travelling at high speed, motor is still
pulling 1-2kW of energy. Marginal, but not actually 0W.
In case of two motors, both could draw something or regen something (a drop, literally).


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## Scuffers (Jun 8, 2017)

arnis said:


> Pure coasting almost never happens. But it is true, that PM motor can't handle "nothing".
> For example, if I switch to neutral on Leaf while travelling at high speed, motor is still
> pulling 1-2kW of energy. Marginal, but not actually 0W.
> In case of two motors, both could draw something or regen something (a drop, literally).


Whilst that may be true, (although the 3's motor would be somewhere in-between), why would you ever want the motrot to 'free-wheel'?

If you are driving the car, you are either 'driving' it forwards or braking, I cannot envisage a situation where you want to free-wheel as such?

think of this as the equivalent of selecting neutral with an ICE car whilst driving, why would you do it?


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## Guest (May 31, 2018)

There are moments when you want to conserve energy (use nothing) while getting as far as possible.
Ideally, air should slow down the vehicle, not regen. 

There are way too many excessively binary drivers who keep accelerator pressed (or cruise engaged)
up until last moment when they brake hard (regen hard). Some even touch brake pedal while 
following a vehicle closely. Every 5-15 seconds.


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## garsh (Apr 4, 2016)

arnis said:


> For example, if I switch to neutral on Leaf while travelling at high speed, motor is still
> pulling 1-2kW of energy. Marginal, but not actually 0W.


Off-topic for this thread, but along these lines, try this experiment:
While coasting down a hill in a Leaf, put it in neutral.
Now while coasting in neutral, stomp on the accelerator.

I would expect nothing at all to happen, but it actually causes the car to slow down (regen) a little bit. Very strange.


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## John (Apr 16, 2016)

Scuffers said:


> Whilst that may be true, (although the 3's motor would be somewhere in-between), why would you ever want the motrot to 'free-wheel'?
> 
> If you are driving the car, you are either 'driving' it forwards or braking, I cannot envisage a situation where you want to free-wheel as such?
> 
> think of this as the equivalent of selecting neutral with an ICE car whilst driving, why would you do it?


Sorry, I guess I didn't word my thoughts well, and I think both you and @arnis may have missed my point. We're talking about AWD, where the car's controller would intentionally disconnect one motor and just drive one motor (presumably a more efficient or more optimally geared one) at that speed. In Model S at constant highway speeds I believe it coasts (electrically disconnects) the rear induction motor, and just drives the front induction motor which is geared at a more optimal ratio for efficiency.


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## Scuffers (Jun 8, 2017)

Kind of missing the point.

even if it drags 1Kw (which I doubt it does) that's still almost nothing compared to aero-drag (at any reasonable speed).

As for free-wheeling a motor, why? there is literally zero point in decoupling the motor.

Back to the S, I very much doubt it goes FWD at any point during steady cruise, once again, zero point.


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## John (Apr 16, 2016)

Scuffers said:


> Kind of missing the point.
> 
> even if it drags 1Kw (which I doubt it does) that's still almost nothing compared to aero-drag (at any reasonable speed).
> 
> ...


This is old knowledge, friend:
https://forums.tesla.com/forum/forums/why-does-dual-motor-get-better-mileage


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## Scuffers (Jun 8, 2017)

Sorry, that's all talk and no empirical evidence/data.

Also, changing gear does not change efficiency (unless you are already operating a motor outside it's efficient window - which with an induction motor is pretty wide)


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## John (Apr 16, 2016)

Scuffers said:


> Sorry, that's all talk and no empirical evidence/data.
> 
> Also, changing gear does not change efficiency (unless you are already operating a motor outside it's efficient window - which with an induction motor is pretty wide)


Wikipedia and Elon Musk need your knowledge. Please help them!
https://en.wikipedia.org/wiki/Tesla_Model_S
https://www.greencarreports.com/new...s-higher-range-too-how-do-they-do-that/page-2


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## Scuffers (Jun 8, 2017)

Once again, no evidence of anything you have tried to say.

And no, I doubt Tesla need any help on this subject.

Before you post up some more links, please read them first and check they actually have the data/info you are trying to 'prove'.

PS. the higher range of the 4WD version is really just down to the format of the std drive test cycle and the use of regen from the front axle instead of just the rear, thus more energy recovery on said drive test.


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## Spiffywerks (Jul 30, 2017)

Not familiar with the size of S/X AWD front motors, but could Tesla be possibly using the same front induction motor from the S/X? In that sense it would cost less for them to add AWD to the Model 3 if it uses the same motor as all the other cars. Could it just be as simple as that? Then pair it with a slightly different transmission that fits the 3's space and provides a gearing to optimize higher speed or acceleration.

Also, does an Induction motor regen better than a PM? Maybe that helps with the range too.


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## Scuffers (Jun 8, 2017)

Spiffywerks said:


> Not familiar with the size of S/X AWD front motors, but could Tesla be possibly using the same front induction motor from the S/X? In that sense it would cost less for them to add AWD to the Model 3 if it uses the same motor as all the other cars. Could it just be as simple as that? Then pair it with a slightly different transmission that fits the 3's space and provides a gearing to optimize higher speed or acceleration.
> 
> Also, does an Induction motor regen better than a PM? Maybe that helps with the range too.


Likely your right, makes more sence for them to use existing FWD setup than develop a new one.

and no, PM is marginally better for regen as it will effectively regen down to almost zero RPM, an induction motor can't do that, but remember the Tesla 3 Rear motor is not a traditional PM and we have not seen any data on it to compare with traditional PM's in this respect.


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## Spiffywerks (Jul 30, 2017)

Scuffers said:


> Likely your right, makes more sence for them to use existing FWD setup than develop a new one.
> 
> and no, PM is marginally better for regen as it will effectively regen down to almost zero RPM, an induction motor can't do that, but remember the Tesla 3 Rear motor is not a traditional PM and we have not seen any data on it to compare with traditional PM's in this respect.


K.I.S.S. ... it's the best way to live.


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## John (Apr 16, 2016)

Scuffers said:


> Once again, no evidence of anything you have tried to say.
> 
> And no, I doubt Tesla need any help on this subject.
> 
> ...


How does having a second motor to recover during regen help? Was the single motor "missing" some recapture?


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## Scuffers (Jun 8, 2017)

John said:


> How does having a second motor to recover during regen help? Was the single motor "missing" some recapture?


Err?

because more braking is available from the front axle than the rear, you cannot pull them much off the rear axle without balancing it out with the front, (think pulling the handbrake on hard!), it would be less than ideal (and pretty unsafe - imagine being at speed on a wet curved road and lifting!)



Spiffywerks said:


> K.I.S.S. ... it's the best way to live.


Very much so, something a surprising number of companies fail at.


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## John (Apr 16, 2016)

Scuffers said:


> Err?
> 
> because more braking is available from the front axle than the rear, you cannot pull them much off the rear axle without balancing it out with the front, (think pulling the handbrake on hard!), it would be less than ideal (and pretty unsafe - imagine being at speed on a wet curved road and lifting!)
> .


You're saying some regen occurs near the limits of slip of two wheels? As if, sometimes you'd optimally want to regen so much that one set of wheels would slip, whereas four wheels could handle it without slipping?

I'd say that essentially none of it needs to happen near the limit of two wheels slipping, not even close.

The 0-60 times are performed with just two wheels not slipping. That's really hard acceleration, and in the reverse process that would be really hard regen. You're saying a four wheel drive car would have better regen because some fraction of the time you're decelerating as hard as the car can accelerate when you floor it?


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## Scuffers (Jun 8, 2017)

Look, I'm sorry, your just talking c**p now, I'm out.


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## John (Apr 16, 2016)

Scuffers said:


> Look, I'm sorry, your just talking c**p now, I'm out.


This isn't acceptable behavior here to dismiss and talk down to other members, like it might be in other places like Reddit or Twitter. If you want to ignore or not reply to other members' comments, fine. If you want to provide links that help explain things, fine. Saying "You're talking crap" is hostile and doesn't add anything to the discussion.


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## Scuffers (Jun 8, 2017)

John said:


> This isn't acceptable behavior here to dismiss and talk down to other members, like it might be in other places like Reddit or Twitter. If you want to ignore or not reply to other members' comments, fine. If you want to provide links that help explain things, fine. Saying "You're talking crap" is hostile and doesn't add anything to the discussion.


With all due, what's the point?

you have done nothing to further the subject but post irrelevant links and then made some wild assumptions/conjecture.

I'm all for informed discussion, but you don't come across as somebody that either listens or can understand basic engineering/electrical principles, yet have the air of an internet expert.

I come here to learn from what others have found and share experiences, your interaction in this thread is neither.

Sorry if this comes across as harsh, but somebody needed to tell you, and I'm not the sort with infinite patience.


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## GregRF (Sep 25, 2017)

Scuffers said:


> Kind of missing the point.
> 
> even if it drags 1Kw (which I doubt it does) that's still almost nothing compared to aero-drag (at any reasonable speed).
> 
> ...


It does indeed go FWD while cruising on the highway by putting the Rear motor into torque sleep. See graphs here.


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## GregRF (Sep 25, 2017)

Scuffers said:


> Kind of missing the point.
> 
> even if it drags 1Kw (which I doubt it does) that's still almost nothing compared to aero-drag (at any reasonable speed).


Also note 1kW = 1.34 hp

According to the EPA doc's the Model 3 only requires 9.95 hp to cruise at 50 mph so a 1kW increase in drag would cause a 13% reduction in efficiency.


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## Scuffers (Jun 8, 2017)

maybe so, but assuming your trying to stay are 50mph, you will be requiring 9.95hp from said motor and not dragging anything.

the point I was making was that the zero-torque point is pretty meaningless in the context, as your either asking the motor to provide positive or negative torque.

All that said, there is no reason it cannot provide zero torque..


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## Scuffers (Jun 8, 2017)

yes, but once again, why would you want to?


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## GregRF (Sep 25, 2017)

The front motors on the S/X are more efficient, why not turn off the rear motor when not needed?

On the Model 3 I think the rear motor will be more efficient at most speeds, so the front motor will only be needed for traction and increased power.


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## Mesprit87 (Oct 29, 2017)

I agree with @Scuffers on the regen on the fwd beeing more effective, I remember a comment from You You as he was coming into Canada with summer tires. He noted that he could feel the rear end getting loose with regen, something you wouldn't get with proper regen in the front.


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## Diamond.g (Jun 26, 2017)

garsh said:


> We don't really know yet. Hopefully at some point Elon or some other member of Tesla will answer that question.
> I'm not sure where you got this idea that an induction motor is "more powerful". Either type of motor can be made pretty much arbitrarily powerful by design. Given equal power, an induction motor will generally be a little larger, a little heavier, and a little less expensive. So my guess would be that the PM motor is in the back because it allows them to put a *more* powerful motor in the back without it taking up a lot more room.


Has anyone taken a look at the efficiency ratings of the I-Pace vs the Model X 90D? Then compare it to motor size? Based on first glance it appears Tesla has more powerful motors and longer range.


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## KarenRei (Jul 27, 2017)

Diamond.g said:


> Has anyone taken a look at the efficiency ratings of the I-Pace vs the Model X 90D? Then compare it to motor size? Based on first glance it appears Tesla has more powerful motors and longer range.


Why do people keep comparing the I-Pace to the much larger Model X? Because Jaguar (deceptively) did it?


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## garsh (Apr 4, 2016)

KarenRei said:


> Why do people keep comparing the I-Pace to the much larger Model X? Because Jaguar (deceptively) did it?


Because if the X is both larger and more efficient, then it makes the I-Pace look really bad.


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## Diamond.g (Jun 26, 2017)

KarenRei said:


> Why do people keep comparing the I-Pace to the much larger Model X? Because Jaguar (deceptively) did it?


Smaller lighter vehicle using dual permanent magnet motors isn't as efficient as larger vehicle using "inefficient" induction motors



garsh said:


> Because if the X is both larger and more efficient, then it makes the I-Pace look really bad.


Basically

I was drawing parallels as to potentially why Tesla isn't using PMSRM on both ends.


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## ADK46 (Aug 4, 2018)

I would have guessed that the control scheme does not entirely turn off either motor, that it's better to send enough current to maintain a non-negative torque at all wheels. It just seems a bit more elegant than having the "idle" drive train fighting the driven axle all the time. When traction becomes a little dodgy, like in heavy rain, it would be better to be driving all wheels at least enough to overcome their own rolling resistance, to maintain maximum lateral grip. 

But perhaps generating this small torque is so inefficient that it's better to completely shut down one motor and maybe its power controller, too. Perhaps the engineers considered low-torque efficiency when choosing the motor they wish to "idle" during cruise. It's a very interesting engineering subject - many factors to consider. Hope they got it right, sitting in their California offices.


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## garsh (Apr 4, 2016)

ADK46 said:


> I would have guessed that the control scheme does not entirely turn off either motor, that it's better to send enough current to maintain a non-negative torque at all wheels.


The other way to prevent the "unused" motor from fighting against you would be to remove the "generator load" from its circuit (i.e. the power converter & battery). Removing the load means "infinite" resistance. By ohm's law (V = I*R), current will be 0, regardless of any voltage induced by the motor spinning. Then Power = I*V, which will also be 0 regardless of any induced voltage.


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## KarenRei (Jul 27, 2017)

ADK46 said:


> I would have guessed that the control scheme does not entirely turn off either motor, that it's better to send enough current to maintain a non-negative torque at all wheels. It just seems a bit more elegant than having the "idle" drive train fighting the driven axle all the time


There's nothing to "fight" in a freewheeling induction motor. An unpowered induction motor exerts no torque, apart from friction.


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## ADK46 (Aug 4, 2018)

I am indeed referring only to friction (maybe electromagnetic hysteresis in a PM motor?). Bearings, windage - is there a gear reduction set? That would be a significant source. I'm imagining how difficult it would be to spin one axle on a lift to a 65mph equivalent rpm...


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## garsh (Apr 4, 2016)

ADK46 said:


> I am indeed referring only to friction (maybe electromagnetic hysteresis in a PM motor?). Bearings, windage - is there a gear reduction set?


Powering the motor doesn't make any of those disappear. They exist under all conditions.


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## KarenRei (Jul 27, 2017)

garsh said:


> Powering the motor doesn't make any of those disappear. They exist under all conditions.


Indeed. And you want to overcome any friction - regardless of where it is - with whichever motor is more efficient.


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## ADK46 (Aug 4, 2018)

garsh said:


> Powering the motor doesn't make any of those disappear. They exist under all conditions.


I understand. My comment is not related to efficiency, but to (literally) where the rubber meets the road. We've all experienced what happens when we hit a puddle and felt the car wiggle a bit. At that split second, I'd prefer that the "active" axle not lose traction due to the extra force required to drive the "idle" drivetrain, easily achieved by powering the "idle" axle a small amount - I want the tires to roll easily. This is a general benefit for conventional AWD cars. Letting an axle genuinely free-wheel, as for a 2WD car, is obviously OK, but standard AWD is better. If a dual motor Tesla is not this sort of AWD - by completely shutting down one motor - it becomes a bit worse than a 2WD.

So, if I were a Tesla engineer, I'd think about this when designing the controller software. It might not be significant, but I'd think about it. I was an engineer, and this was the best part of the job - thinking!

But I'm often accused of over-thinking - sorry.


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## ADK46 (Aug 4, 2018)

Really clever drive controller software would take into account the rain sensor. Temperature, too. Rain at 32°F can be trouble.

The early Nest thermostats, by the way, had software that was clearly programmed by people who lived in Sunny California, who did not understand the sort of heating systems used in cold climates. I've been alert to this sort of thing ever since....


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## garsh (Apr 4, 2016)

ADK46 said:


> My comment is not related to efficiency, but to (literally) where the rubber meets the road. We've all experienced what happens when we hit a puddle and felt the car wiggle a bit. At that split second, I'd prefer that the "active" axle not lose traction due to the extra force required to drive the "idle" drivetrain


That would be handled by the traction control. It would reduce power to the slipping wheels, and possibly "brake" them in that split-second if necessary to maintain static friction with the road surface.

It's conceivable that the traction control could also activate the front motor in such a scenario to compensate for the traction loss at the rear. I have no idea if Tesla actually implements such a thing though - my guess would be "no". But I don't think Tesla would want to keep that less-efficient front mower powered at all times. I think that would reduce the car's highway efficiency even further.



> But I'm often accused of over-thinking - sorry.


It can be fun to think about such scenarios.


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## ADK46 (Aug 4, 2018)

Yup, the traction (and stability) control system can save the day. It's probably not going to prevent the first bit of wiggle. Could it power up the "idle" axle within milliseconds? 

Let's say it takes 2000 watts for the rear motor to push the front wheels along, overcoming their rolling resistance and the friction of the drive unit. Alternatively, you could reduce that requirement to zero by sending 2100 watts to the front motor. The extra 100 watts is due to that motor's inherently lower efficiency, exacerbated by low efficiency at low power.

But maybe the delta is 1000 watts, enough to matter a lot. I have no idea. As you suggest, that's the crux of the problem. It might take our Tesla engineer only a minute to decide which way to go, since he/she would know.


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## garsh (Apr 4, 2016)

ADK46 said:


> Could it power up the "idle" axle within milliseconds?


Of course! Have you seen any of the new Model 3 Performance drag racing videos?


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## Scuffers (Jun 8, 2017)

ADK46 said:


> Yup, the traction (and stability) control system can save the day. It's probably not going to prevent the first bit of wiggle. Could it power up the "idle" axle within milliseconds?
> 
> Let's say it takes 2000 watts for the rear motor to push the front wheels along, overcoming their rolling resistance and the friction of the drive unit. Alternatively, you could reduce that requirement to zero by sending 2100 watts to the front motor. The extra 100 watts is due to that motor's inherently lower efficiency, exacerbated by low efficiency at low power.
> 
> But maybe the delta is 1000 watts, enough to matter a lot. I have no idea. As you suggest, that's the crux of the problem. It might take our Tesla engineer only a minute to decide which way to go, since he/she would know.


No way would the front powertrain take 2kW to turn, that's a ridiculous level of drag for an un-powered induction motor and reduction gear.

there may be some transient loses when accelerating (to overcome powertrain inertia), but steady state, 2kW is HUGE.


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## ADK46 (Aug 4, 2018)

Scuffers said:


> No way would the front powertrain take 2kW to turn, that's a ridiculous level of drag for an un-powered induction motor and reduction gear.
> 
> there may be some transient loses when accelerating (to overcome powertrain inertia), but steady state, 2kW is HUGE.


I agree, but I said "*rolling resistance* and the friction of the drive unit". Rolling resistance of the tires is the big contributor. If going 60 mph takes about 10kW, something like 2-4 kW would go to overcome RR, the rest to overcome aerodynamic resistance. 1-2 kW for each axle. Not difficult to calculate except for unit conversions: weight * coefficient of rolling resistance * velocity. But the focus should be on the difference between letting the road spin the front wheels and drive train using power supplied to the rear wheels, versus powering the front wheels with their own motor. 100 watts? 1000 watts? Again, I have no idea, but this would determine the reasonableness of actual full-time AWD like that provided in conventional AWD cars.

I think we'd all like to know if "dual motors" is the same as "AWD" as we're accustomed, or if different, better or worse or what.

Have we touched upon whether we get regen from both axles? I know in a Prius that there's a disconcerting wiggle when traction is lost in the front wheels during regen braking - there's a delay until the system can apply the rear brakes.


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## Scuffers (Jun 8, 2017)

Woha! - that's a whole different argument.


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