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Why don't regen brakes stop the car completely?

Discussion in 'Model S: Driving Dynamics' started by MrJones390, Jul 26, 2015.

  1. MrJones390

    MrJones390 Space Boat

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    I haven't been able to find the answer to this. Would be nice to roll to a complete stop for true one-pedal driving.

    Does anyone know why we still need to use the brakes for complete stops?
     
  2. efusco

    efusco Moderator - Model S & X forums

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    I've come to complete stops with regen only, but it takes a bit longer to do so. The slower you go the less ability there is to regen.
     
  3. jerry33

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

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    The reason regen cuts out at around 10 km/h is because there is not enough kinetic energy left to provide useful stopping power. It would be possible for the motors to simulate this and give the feel of regeneration to stopping, but it would actually reduce range to do so. You will eventually come to a stop due to tire and bearing friction, but it will take a long time to do so.

    In addition, if the rotors aren't used frequently, rust will build up and they won't work properly when you need the friction brakes.
     
  4. brucet999

    brucet999 Active Member

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    BMW i3 stops completely with one pedal, but not entirely with regen. It automatically activates the disc brakes at very low speed as regen fades away. I prefer Tesla's choice, enabling me to control the use of friction braking.
     
  5. E39_S62

    E39_S62 Member

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    Unfortunately I own an i3 until I get a Model S, hopefully later this year. The i3 stops completely with regen and no friction brakes.
    It does simulate regen using the frictrion brakes, but only when the battery is too full to do regen, but after it falls to approx. 90% then it is full regen 100%. I have verified this with a temp gun and then a finger, the discs are cool to the touch right after a bunch of full stops using regen.

    There could be some electronic motor wizardry faking the regen at the very low speeds, but if so there is no way to tell as the energy screens etc don't indicate anything changing.
     
  6. Bangor Bob

    Bangor Bob Member

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    Yep, an option is to have the motor apply reverse torque below some threshold speed (7-10mph?) to keep deceleration constant until stopped (assuming you have creep turned off) when the accelerator is not depressed and the brake pedal hasn't been touched. Yes, that would use some small amount of energy, but not much. Likely just a few watt-hours per stop.
     
  7. brucet999

    brucet999 Active Member

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    My mistake. So then it uses reverse power from the battery to come to a full stop.
     
  8. mspohr

    mspohr Active Member

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    When you're using TACC, it will come to a complete stop when the car in front of you stops. However, it uses the brakes for the last part of the stop. I don't think regen will stop the car at very low speeds... probably not any energy generation then.
     
  9. CHG-ON

    CHG-ON Still in love after all these miles

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    Turning off Creep will help. But have patience as it will take some time to roll to a stop.
     
  10. pmadflyer

    pmadflyer Member

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    Okay, lets talk about E and M.

    Imagine you are looking at the cross section of an conductor such as a wire, and have the ability to see the magnetic field lines that are perpendicular to the surface of this conductor, or flux as it is called.

    If the current is constant (non transient DC), the flux is not changing and everything is good in the world. Now, if you were to increase or decrease the current, or change the properties of the conductor, the flux would change. This change in flux is responsible for the creation of electromagnetic force, or EMF. The EMF is more of a potential than a force, but it's a non issue here.

    Now, I will take you to a relatable example. You may have heard a parent tell you as a child not to play with a light switch, because it will burn out the bulb. You may even have noticed that bulbs tend to pop when they are switched on, more often than other moments. The reason for this involves the conductor being at a steady state with a steady magnetic field, and therefore flux, until the switch is flipped. Once the switch is closed and the EMF stats sending current through the wires, the flux in the wires starts changing. A change in flux results in an electrical current being produced that opposes the change. This current flowing in the opposite direction has a voltage that increases with the rate of the change that is being resisted. This is called self-inductance and is ever constant in AC circuits, including the Model S drive train.

    When you are sitting at a red traffic signal, and the light turns green, you press the right pedal. This pressure is sensed by an elaborate combination of hardware and software that ultimately sends AC current to the motor. The current causes a change in flux through the rotor that can be resisted if the rotor changes position in relation to the EMF source. As said previously, the rate of change in flux is what affects the magnitude of the induced EMF. The rotor begins rotating in an attempt to minimize change in flux and the controller keeps sending pulses of AC current at just the right interval to keep the rotor "chasing" equilibrium. By increasing the current of the bursts, the rotor will accelerate more dramatically to escape the change. As this occurs, the back EMF increases in magnitude, reducing the ability of the original EMF to further accelerate the rotor to a higher velocity. This is overly simplified, and I'm the wrong kind of engineer to explain in more detail, and this may even be inaccurate in some places.

    When you are traveling at speed, and you begin to use regenerative breaking, an EMF is produced by the change in flux produced by the changing orientation of the rotor with respect to the stationary components of the motor. The faster you are traveling in the car, the more quickly the rotor rotates, the quicker the change in flux, and the stronger the induced EMF is. This induced EMF is sent to the inverter and then the battery as DC current. since the car is slowed down by this conversion of energy, the EMF decreases, and the effective breaking force is reduced. Once the vehicle speed is low enough, the rate of change of the flux and therefore the induced EMF becomes to low to effectively slow the rotation of the rotor, or the forward movement of the car.

    That is why regenerative breaking will not stop the car completely.

    if you are on a hill or have enough friction you will get the sensation of coming to a complete stop under regen though.

    I spent more time on this explanation than i hoped for, so I'm not proofreading.
     
  11. MrJones390

    MrJones390 Space Boat

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    Where is this option? Hypothetical?
     
  12. Bangor Bob

    Bangor Bob Member

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    Yes. It's a possibility the Tesla designers could have chosen (but didn't, for some reason). I guess I could have said, "one strategy..."
     
  13. jerry33

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

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    Besides the reduced range, not exercising the brakes frequently is a bad idea. There are a couple of threads about brake rotor rust, there would be many more if they used the reverse torque to zero strategy. The way it's done now is a good compromise between efficiency and friction brake life.
     
  14. DoctorV8

    DoctorV8 Member

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    Thanks for the explanation.
     
  15. MrJones390

    MrJones390 Space Boat

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    Why can't Tesla program the brakes to engage the same way your foot would for a smooth stop?
     
  16. LetsGoFast

    LetsGoFast Active Member

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    They could, but I wouldn't want that. Sometimes the regen stops when I'm fairly close to the stopped cars, but sometimes it stops when I'm four or five car lengths. By letting it roll slowly thanks to inertia, I can apply the pedal to stop exactly where I want to be. If they used the same logic as TACC to always stop a reasonable distance from the car ahead, I'd be fine with that. But just applying the brake as soon as the regen finished would be no bueno.
     
  17. Tyl

    Tyl Member

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    #17 Tyl, Jul 27, 2015
    Last edited: Jul 27, 2015
    Elementary info to understand. Regenerative braking is a function of the motor/transmission and has nothing to do with brakes. Brakes are hydraulically actuated calipers which, when you step on the brake pedal, apply pressure pinching together the brake pads against the spinning disc attached to each wheel. i.e. Disc brakes.
     
  18. LetsGoFast

    LetsGoFast Active Member

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    Very cool post. Thanks for the lesson.
     

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