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No tow mode because of permanent magnet motor?

Discussion in 'Model 3: Driving Dynamics' started by brucet999, Aug 4, 2018.

  1. brucet999

    brucet999 Active Member

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    Can some of you engineer types help to clear this up for me?

    A Tesla mobile technician told me that M3 dual motor gets less range than single motor because torque sleep is impossible when one motor is permanent magnet type. According to him, the PM motor cannot free-wheel, so it must either drive or regen whenever the car is moving. For that reason, he said, the car cannot be towed. In "tow mode", software allows just 20 minutes to move the car (to the side of the road or onto a flat bed, for instance) before the parking brake automatically sets itself.

    It would seem to me that, even in permanent magnet motors, when there is an open circuit there should be no EMF. What am I missing?
     
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  2. chillaban

    chillaban Active Member

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    In a PM motor, the mere act of rotating the rotor will generate AC current at the machine's native frequency. If you have an "open circuit", this results in the motor being a capacitor, accumulating hundreds of volts until it either air-discharges (sparks) or the next moron attempts to touch it gets electrocuted.

    Needless to say, you NEVER open the circuit to a PM motor. Most safety protocols for HV motors require that the rotor be mechanically locked prior to removal from a ground.

    So what the mobile tech said is kind of true. A PM motor can't be turned "off" without a mechanical disconnect. If you are freewheeling it, it will passively generate electricity and increase the bus voltage on the car. If you want it to not generate, you basically have to command the car to zero torque or zero current and that's actually actively performing PWM-switching and motor position sensing in order to minimize the amount of current generated. (consuming energy to do so)
     
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  3. brucet999

    brucet999 Active Member

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    So what happens in a M3 when you back off the accelerator if the battery is at full SOC or too cold for regen?
     
  4. chillaban

    chillaban Active Member

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    The car simply won’t regen and will command for zero torque (no generation)

    The brakes will have to slow down the car.
     
  5. hacer

    hacer Member

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    Explaining my disagree here. The voltage is strictly limited by the motor back EMF which is proportional to speed, so there is no possibility of the voltage rising to levels where it could arc-over through air. It likely can't ever exceed the battery voltage even at highway speed although I could be wrong about this if the invert is capable of boost. Even so it surely won't go above 900V which will not arc through air (3kV/mm would require an air gap of just 0.3mm between un-insulated motor terminals which is impossible in a Tesla unless it was totally demolished in an accident). So it is perfectly fine to open the circuit to the PM motor, so long as that open can withstand the maximum back EMF.

    The most likely reason that there is no tow-mode is that if it is not possible to guarantee that there is no fault in the inverter, or that the inverter can present an open to the motor even with no HV battery power, then you run the risk of damaging the motor (and maybe the inverter too) if you tow the car and the motor terminals are, for example, shorted through a broken inverter. With an induction motor this won't matter because with the HV relay disabled, there can be no power to the inverter and thus no field in the motor so no back EMF is generated. You can't guarantee that if the motor is PM and something has failed (you are towing presumably because something is wrong).
     
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  6. DanCar

    DanCar Member

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    In the Model S the rear motor was optimized more for torque than for efficiency. Not true of the Model 3. The M3 rear motor was optimized for efficiency, so adding another motor isn't going to increase efficiency.
     
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  7. NathanielHrnblwr

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    So the short answer is Model 3 has to be towed via flatbed, right?
     
  8. noam

    noam Member

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    Yes
     
  9. okashira

    okashira Member

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    The model 3 motor BEMF is around 350V at 40mph.
    This would not be a problem as the inverter MOSFET's will simply block any current flow and 350V is no problem.
    If it were to be towed at 80mph, the BEMF would hit 700V, and the failure would be catastphic and perhaps even a little spectacular. The mosfets would immediate avalanche and shunt a good amount of current with a 650V drop and their failure will be guaranteed with a good bang.

    You could probably tow safely to 40mph, I would not go faster.
    You could tow faster with the car on and in drive mode though. As the inverter will induce a current in the stator that counteracts the rotor magnetic field reducing the effective back emf.
     
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  10. AWDtsla

    AWDtsla Active Member

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  11. AWDtsla

    AWDtsla Active Member

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    Immediate thought: What happens if there is a fault that causes loss of power while moving? Destroyed motor/inverter???? o_O
     
  12. BestHand

    BestHand Member

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    Tesla mobil technician just gave you complete BS. He does not know basic things. When circuit is open motor can spin absolutely freely, no problem. Also, if you spin PM motor with circuit open, voltage will not exceed some value depending of a lot of variables including RPM. It will not add up higher and higher. Theoretically it can be up to the voltage of your battery if you tow the car with 150 M/h speed (which is unlikely :) ), but in reality less than that.
     
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  13. SigNC

    SigNC Member

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    Seems like the front motor can can torque slept though correct? This for the most part seems like it would put it close to RWD.
     
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  14. brucet999

    brucet999 Active Member

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    The car can turn off power to the induction motor so it can spin freely, but that is not torque sleep. Torque sleep allows one or the other induction motor to be automatically used when its gear reduction ratio was more advantageous - front motor for its more efficient higher ratio on level roads at highway speeds; rear motor when greater torque was needed for climbing or quick acceleration
     
  15. SigNC

    SigNC Member

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    Ah thanks for the clarification. So not advantageous like S or X since RWD is larger motor but close to efficient as RWD 3 minus having to spin the drive axles.
     
  16. SDRick

    SDRick Member

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    Can the car be flat towed in neutral? In a related question, what would be the difference between tow mode and neutral?
     
  17. hacer

    hacer Member

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    This is because in the event of power loss the parking brake can't be released, so the wheels will not rotate in that case. I'm sure it is not because of BEMF. They don't want a tow trucker driver to discover the brakes are locked after dragging the car so they tell them not to even try it. Most tow trucks carry sufficient equipment that this is not a problem.
     
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  18. hacer

    hacer Member

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    I don't have any hard data, but I'd be very surprised if the inverter was designed such that it could be damaged by a loss of power (at any speed). It is theoretically possible because of the strange variable inductance of switched reluctance PM motors, but it would likely be a bad trade-off.

    However an already damaged inverter could damage other components if you tow the car at significant speed. There are a lot of possibilities as to why a car would need a tow. Tesla recommends not to spin the wheels because this will always prevent damage, regardless of the reason for towing.
     
  19. AWDtsla

    AWDtsla Active Member

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    Tow truck drivers expect this. They don't have much problem dragging a car in park up on the flatbed with 2 locked wheels. It's not even that much force required. This would not damage the car.
     
  20. okashira

    okashira Member

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    I assure you the model 3 motor will produce a back emf that exceeds the battery voltage by 50mph, not 150mph.
     

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