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Dual Motors Provide Redundency

Discussion in 'Model S: Driving Dynamics' started by breser, Oct 28, 2014.

  1. breser

    breser AutoPilot Nostradamus

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    Saw a video today where Elon Musk does a test drive of a D with a Bloomberg reporter that I hadn't seen. This test drive is basically the same thing everyone else got other than being done personally by Elon.

    But there is one bit of info I hadn't seen anywhere else. Around 20 seconds into the video Elon says:

    I have no doubt that's true with a P85D given that the vehicle has the same motor as the RWD vehicles for the rear motor. But I wonder how true that is in the 60D and 85D with a smaller rear motor. I guess there could be a limp mode where you can drive but not very fast on just one of the motors in the other models.

    Any thoughts?
     
  2. BrianC

    BrianC Member

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    I remember seeing this video right after they announced the D. 188hp is more than enough power to drive your car down the road, a 2014 impala for example has a 195hp engine and the DM 60s + 85s are twin 188hp.
     
  3. Todd Burch

    Todd Burch Electron Pilot

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    Ironically, the motor itself is among the most reliable parts of the drivetrain. I haven't heard of any motor failures around here--they all seem to be the pack itself (or a critical electrical component). So while this is true, the tangible benefits to reliability at this point seem pretty minimal.
     
  4. freds

    freds Member

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    But hey all it takes is someone getting home in the middle of blizzard to make it positive marketing event....
     
  5. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Even if they would have also made it home with a 2WD model.
     
  6. Saghost

    Saghost Active Member

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    This.

    In principle the redundancy is certainly valid, and the horsepower of either motor is ample for most everyday driving - but outright failures of the drive motors and gearing I don't think I've read about.

    There were a number of drive units replaced for noise that I saw, but all of the side-of-the-road failures I've read of have been either pack failures or failures that caused the car to lock out the pack for safety.
    Walter
     
  7. Chipper

    Chipper Active Member

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    My drive unit was replaced for mechanical failure.
     
  8. Saghost

    Saghost Active Member

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    Outright failure that left you stranded?

    That would be a case where having a D would have helped, then.
     
  9. Chipper

    Chipper Active Member

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    Yes. Outright failure that left me stranded a couple thousand miles from home. Grinding, crunching, then failure. Tesla was FANTASTIC about getting it repaired. I am not sure if the extra motor would have been helpful if the other one was grinding and scraping and crunching. I don't think I would have wanted to continue.
     
  10. basvk

    basvk Active Member

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    There were a few cases of broken drive shafts. With a dual motor setup you would have been able to get yourself to an SC or at least off the road in such a case (with the other motor spinning freely)

    But the opposite of Elon's statement is also true: not having a second motor means it can't break either... :)
     
  11. Brass Guy

    Brass Guy Member

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    I think there may have been one or more drive gear failures.
    Someday in the distant future someone may loose a CV joint. Maybe a electrical connector or conductor will corrode, or an inverter short out.

    (edit - It took me too long to type this, I missed the previous few posts)

    In normal driving, software will determine how each motor should be utilized; so having the two motors operational is probably integral to the system. For it to perform as a redundant system, failures for different reasons will have to be anticipated and the appropriate reaction (limp mode) programmed. I see this as completely feasible, but in similar fashion to the AutoPilot software, such redundant software overrides may come with later OTA releases. Think about the car knowing your schedule, waking up, warming the cabin, opening the garage door and driving around to pick you up. Sure the new cars are physically capable of that now, but software development has a ways to go. Given the apparent low actual failure rate of drive units, I don't think Tesla would invest that much development time on this aspect just yet. I bet programming efficiency and traction control with the dual drive system is no piece of cake, and getting the two motors to work well together is probably the main factor why a D has not been released even earlier.

    Or I could be wrong and redundancy is already programmed in.
     
  12. stevezzzz

    stevezzzz R;SigS;P85D;SigX

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    Old aviation saying (OT, sorry): When you lose an engine in a twin-engine aircraft, the remaining engine has just enough power to get you to the scene of the accident.

    :biggrin:
     
  13. Saghost

    Saghost Active Member

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    Ouch! I must not have seen that thread. Glad they got it fixed for you quickly.

    With proper fault detection and/or manual lockout options, the car could disable the damaged motor - but there's no clutch system, no way to isolate the wheels from the motor, so everything would still have to turn.

    I think I agree with you - in that situation driving on is undesirable and potentially dangerous (if metal bits are floating around in the axle housing, one could suddenly lock up the differential, stopping the wheels completely.)

    Tesla could redesign to give them clutches or 'fuses' - designed in shear areas - but I think this failure mode should be rare enough to make it not worth the time, money, and weight.
    Walter
     
  14. DRoss

    DRoss Banned

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    As a pilot how years ago had a catastrophic engine failure in flight, having the second was not "enough to get you to the scene of the accident". It saved my life and I was able to land normally at closest airport many miles away.

    Important point that has some adjacency to this discussion. Have to be able to feather the prop in order for such a smooth outcome. Without that, drag can be a big issue. So too having one motor fail in a Tesla.. If it stays connected, grinding about, going home on one is unlikely.
     
  15. Thumper

    Thumper Member

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    I think each motor would also have its own inverter, so there is more redundancy than just the motor itself. This should offer true double systems. Getting there no matter what is the greatest luxury. The old BMW 850s with the V12 had complete isolated redundancy (FI and Ign) for each bank of 6 cylinders in an attempt to accomplish the same thing.
     
  16. Rodolfo Paiz

    Rodolfo Paiz P85 "Plug and Play"

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    Statistically true, but mostly related to the high accident rate which is due to pilot error, which is largely due to insufficient/inadequate training and preparation. I've had an engine fail in a twin, and although I could have made a safe off-airport landing, the second engine allowed me to make a nice, safe on-airport landing at my destination airport about 100 miles further on. (Mountainous terrain meant that no other airport was closer than my destination.)

    - - - Updated - - -

    Two thoughts:

    1. Each motor is almost 190 HP, which is more than enough to get you anywhere. Acceleration will be affected, but the car should still drive as well as a BMW 528... meaning, smoothly and nicely.

    2. Maybe each motor would need a clutch, so it can disengage from the axle if it freezes or something? No idea, just thinking out loud.
     
  17. Saghost

    Saghost Active Member

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    If I were trying to set up a safety system here, I think I'd put a dog clutch type coupling on either the ring gear (or equivalent, since it isn't a conventional differential - whatever gear carries the spider carrier) or one of the axle shafts.

    You're dealing with high torque levels, so a conventional clutch would have to be big and heavy - but you don't need to slip it at any point, just on or off. A dog clutch gives positive engagement and doesn't need much weight even at high torque levels because it transfers loads mechanically rather than through a friction surface.

    Given the operating conditions, it'd probably be a normally closed type that could be opened by an electromagnet when needed. By putting it at the spiders, it should isolate any problems in the gearset or motor from the road.

    Gear failures are rare enough that I'm still not sure it'd be worth the added cost and weight even with this relatively minimal approach. But it's doable. :)
    Walter
     
  18. AmpedRealtor

    AmpedRealtor Active Member

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    Double redundancy = double the number of points of failure
     
  19. dsm363

    dsm363 Roadster + Sig Model S

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    Glass half full?:)

    It also means that if there is one failure you aren't completely hosed. Kind of like having one hard drive versus a RAID.
     
  20. AmpedRealtor

    AmpedRealtor Active Member

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    LOL... perhaps a little cynical after two drive unit replacements. The new D model uses two completely new drive unit designs, only the P85D retains the P85 drive unit for the rear wheels. I'm sure Tesla has improved a lot of things with their new design, but with new designs come new problems. Just saying. :)
     

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