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Myth: AWD able to drive if one motor fails?

camalaio

Active Member
May 28, 2019
1,483
2,101
Vernon, BC, Canada
Hey folks,

So some have pointed out that when the rear motor fails, Tesla's statement of being able to continue with the remaining motor are false because of the design of the rear motor. The general statement is usually something like "because the permanent magnet motor can't coast because of <reasons>, if the rear motor fails then it can't drive". This explanation has appeared when rear drive units have failed, causing the car to be completely inoperable even though a functioning front motor is available.

But then... how does neutral coast so well in my LR AWD? Surely neutral isn't actively driving the rear motor??

Any insights?
 

GHammer

What a long strange trip its been.
Feb 1, 2016
922
2,164
Wren, Oregon
The general statement is usually something like "because the permanent magnet motor can't coast because of <reasons>, if the rear motor fails then it can't drive".
If this were true, how would a single motor rear wheel drive work?

NM-I see what this statement is trying to say.
 
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Tam

Well-Known Member
Nov 25, 2012
8,944
7,728
Visalia, CA
...Tesla's statement of being able to continue with the remaining motor are false...

Are you referring to the statement from Tesla CEO as falsehood?:

"Tesla dual motor means there is a motor in front & a motor in rear. One is optimized for power & one for range. Car drives fine even if a motor breaks down. Helps ensure you make it to your destination & don’t get stuck on side of road in potentially unsafe conditions.

11:31 PM - 19 May 2018"
 

camalaio

Active Member
May 28, 2019
1,483
2,101
Vernon, BC, Canada
If what your saying is true, how would a single motor rear wheel drive work?

Well, when in drive, the RWD models would of course be actively dealing with the motor (whether applying power or performing regen). Though perhaps that is a simpler question for what happens in neutral.

Are you referring to the statement from Tesla CEO as falsehood?:

"Tesla dual motor means there is a motor in front & a motor in rear. One is optimized for power & one for range. Car drives fine even if a motor breaks down. Helps ensure you make it to your destination & don’t get stuck on side of road in potentially unsafe conditions.

11:31 PM - 19 May 2018"

I'm referring to this from the Model 3 configurator:

"""
Tesla All-Wheel Drive has two independent motors that digitally control torque to the front and rear wheels—for far better handling and traction control. Your car can drive on either motor, so you never need to worry about getting stuck on the road.
"""

The tweet is along the same lines, but the above is from the Model 3 configuration pages.

EDIT: It's the "you never need to worry about getting stuck on the road" that appears to not hold true, for at least two threads I've seen on this forum in the last few months, specifically with rear motor failures.
 

Tam

Well-Known Member
Nov 25, 2012
8,944
7,728
Visalia, CA
...It's the "you never need to worry about getting stuck on the road" that appears to not hold true, for at least two threads I've seen on this forum in the last few months, specifically with rear motor failures.

Since you've read that it's false in reality because people do get stranded if one of two motors fails then I think Tesla needs to revise its statement because "never" is quite misleading.

I could think of a reason why that could happen and it has nothing to do with the ability to coast or not:

.If one is already damaged, it might automatically disable the good one so that the good one won't drag the car further and cause more damage.
 
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ZOMGVTEK

Member
May 19, 2015
559
434
'Merica
The trouble is there’s no metric for the vehicle to mechanically disable an axle. If the failure is a mechanical one, it’s quite possible the axle will lock up or debris will be circulating around causing serious drag and internal damage. Electrically, there’s effectively no difference to the drag on the motor between park, neutral, and the motor totally disconnected from the car. The car doesn’t need to actively do anything to ‘disable’ the axle aside from no longer send it power. It’s very much possible it could drive on the front axle. The rear will act as a generator, but this isn’t really an issue so long as the output devices in the inverter didn’t go short.

I’d suspect a fair amount of the time, a fault with the rear drive unit fails in a manner that that causes the axle to no longer be keen on rolling. If that happens, there’s nothing the car can do to mechanically or electrically isolate the axle. Probably about all it could tolerate is an external wiring fault, or logic level control issue. It would have to be mechanically sound, and electrically sound on the output side.
 
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Ulmo

Active Member
Jan 19, 2016
4,328
4,427
Vienna Woods, Aptos, California
Thoughts:

  • Some Tesla motors require intricate software programming to coast, since they are actively exactly matching the speed of the car while not pushing or pulling, and those motors don't have a neutral behavior at all, so the software has to apply the exact amount of spin on the motor to give it 0 torque or whatever the technical term is. Everyone was requesting this feature before the Teslas at the time had it. Elon had to promise it in a software upgrade and program the software upgrade for it. It took him a while! That was true for many S&X models. I forget which kind of motor and which kind is on the Dual 3 of which you speak.
  • I'm pretty sure it's that type of motor that if it's dragged along, it will fry the electronics, overheat, melt, cause physical damage, fire, etc. Don't do that.

So, that kind of motor failing would be an entirely different kind of event than just coasting. That applied to that S&X motor. I don't know if it applies to Model 3.
 
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eHaw

Member
Aug 16, 2018
34
29
Laramie, WY
neither
Thoughts:

  • Some Tesla motors require intricate software programming to coast, since they are actively exactly matching the speed of the car while not pushing or pulling, and those motors don't have a neutral behavior at all, so the software has to apply the exact amount of spin on the motor to give it 0 torque or whatever the technical term is. Everyone was requesting this feature before the Teslas at the time had it. Elon had to promise it in a software upgrade and program the software upgrade for it. It took him a while! That was true for many S&X models. I forget which kind of motor and which kind is on the Dual 3 of which you speak.
  • I'm pretty sure it's that type of motor that if it's dragged along, it will fry the electronics, overheat, melt, cause physical damage, fire, etc. Don't do that.

This is really interesting. Such a different mental model than an ICE car disengaging a transmission so that wheels can roll free.

As a former ICE hypermiler, I have wondered if Tesla's neutral mode would properly handle a coast down a long mountain road at highway speed. Ulmo explains that the motors would have to be modulated to match the vehicle's speed applying neither power or regeneratively braking. Neutral is usually discussed in the context of a car wash that rolls a vehicle through at slow speed. Is there any limit to the speed neutral mode will safely support? In other words, is it safe for the typical Tesla owner to treat neutral as they would neutral in an ICE car?

Then there is towing and the use of Transport Mode. I don't find any reference to TM putting the vehicle into neutral as well ask keeping the parking brake from engaging. If one were allowed to remain in the Tesla and ride the vehicle up on to the truck, would it suffice to put the Tesla in neutral and keep one's foot off the brake? Seems like all TM does is allow one to exit the vehicle without causing it to switch from neutral to park.

Which brings me back around to Elon's statement. If a dual-motor Tesla loses one motor the car is simply flipping that motor in to neutral and doing its modulation magic.

If the car senses that this modulation is working, you'd have reduced power but could continue driving to safety.

If the care senses an electronic or mechanical failure is such that the motor can not be placed into neutral, I presume the car would instruct us to stop immediately in order to prevent overheating and further damage to the ailing motor.
 

S4WRXTTCS

Well-Known Member
May 3, 2015
5,550
6,299
Snohomish, WA
Hopefully we'll get an answer to this question as it applies to the Model 3. I don't know enough about the electrical/mechanical drive characteristics of both the Motor types used on the Model 3 to answer definitively.

The Tesla answer is "Your car can drive on either motor, so you never need to worry about getting stuck on the road. If one motor stops working, you can safely continue to your destination with the second"

My pull it from my butt answer is it's a half truth. Where it only holds true if the front motor is the one that fails. If the rear motor fails then you're stranded.

Why do I have the pull it from my butt answer? Because the people that have had their rear motor fail in a dual motor 3 have reported that they can't continue on. Plus the rear motor in the Model 3 is a PMSRM type that has magnets in the motor that generate high voltage when turned. So unless Tesla implemented a way to deal with that then it shouldn't be turned or it could cause more damage to the motor or the drive electronics. It is possible that they implemented some kind of limp mode if the rear motor fails.

It should be noted that it's more of a curiosity question. The odds of a motor failing without any other failure is pretty low. So it could be that what's happened in the rear motor failures is more than the motor.

Has any Model 3 owner been able to continue on their journey with the rear motor disabled?
 
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DrComputer

TeslaClubLA President
Jan 29, 2009
1,145
385
Sherman Oaks, CA
I know this thread is speaking about the Model 3 dual motor version, but I will chime in with my Model X failure. When my rear motor failed because of an internal coolant leak in the drive unit, the car shut down and wouldn't move. Mechanically, there was nothing stopping the car from rolling and the front drive unit could have kept the car going, but (for safety purposes I presume) the firmware is designed to shut down the car when it senses a failure.
 

mike123abc

Member
Aug 20, 2018
406
808
Norman, OK
When you charge your car up to 100% it rolls pretty good when it does not apply any regenerative breaking. I have AWD and when I charge to 100% and take my foot off the accelerator it will roll a long time before it finally comes to a stop, almost like it is in neutral.

A coolant leak would probably shut down the car since they could not risk the battery pack or other motor. Same with an axial problem.
 

ceekz

Member
May 14, 2018
465
467
Sacramento, CA
My car was undrivable when I lost the rear drive unit. I just had parked in the garage and received an error message "Car May Not Restart" and it did not. Called the online service and the rep said it was the battery pack, but apparent the battery shares the same circuit as the rear drive unit. The service tech said when it was towed in to Rocklin that it could be a bad sensor in the circuit, or the actual drive unit, but they would replace the unit either way.

I don't suppose I'll ever know the true cause of the failure, but I can tell you with absolute certainty that the failure of the rear unit or its circuit made the car undriveable. The tow driver had to jump the 12V battery just to get it out of the garage.
 

Sma3

New Member
Jul 19, 2020
1
1
Denbigh
Had my rear motor fail on my model 3 dual motor. There was a clunk, a message popped up saying rear motor disengaged (or something along those lines) and we 'limped' to charge station to call for recovery using only the front motor...
 
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holmgang

Active Member
Sep 9, 2019
1,300
1,316
eu
Once my car threw an error about the front motor failing. Power dropped precipitously and within 30 seconds or so the entire vehicle limped and clunked to a stop.

(It later restarted after a reboot and has since been 100%)

So in the 1 instance that a motor *apparently* failed, it definitely was not driveable.
 

1.21GW

Member
Jul 23, 2018
753
1,200
Michigan
This thread needs some information about power electronics, IPMSM, and back EMF.

Basically with the rear motor in the model 3, it requires deliberate control of the power electronics to create zero torque and manage the voltage that is generated by the permanent magnets. If this is not managed (i.e. something is broken) then the voltage generated by a motor spinning will continue to do more damage and be a hazard. Hence the reason the car needs to be stopped.

This is not true with induction motors, those can be simply switched off and spin freely. Which is what our front motor does most of the time.
 

GtiMart

Active Member
Nov 13, 2019
1,007
855
Quebec City, Canada
A technician came to check my car when my front motor threw a resistance error. He explained that the rear motor is the one that "drives" the front. If the rear fails, you stop whereas if the front fails, you can still drive. That is if the failure is not hardware blocking as others said. Not sure about the tech's explanation but it seems reasonable. Teslas statement that you can drive on any motor is just saying it's awd and can send 100% power to either end IMO.
 
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SigNC

Active Member
Aug 23, 2017
1,519
1,358
NC
as i understand it you are fine if the induction motor dies but if they PM one dies you are screwed.
 

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