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Why isn't regen integrated with the brake pedal?

Discussion in 'Model S: Driving Dynamics' started by dennis, Jan 24, 2013.

  1. stevezzzz

    stevezzzz R;SigS;P85D;SigX;S90D;XP100D;3LR

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    OK, that makes sense.

    Dumb question #2: why doesn't wall power go through the inverter, obviating separate charger(s)? The chargers must include their own inverters, right? Must be a voltage thing (he muses to himself): PEM has to deal with only one voltage, at very high currents, while the chargers have to accept a range of lower voltages and convert to high-voltage DC.
     
  2. wycolo

    wycolo Active Member

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    > Does the AC motor become a DC generator? Or is it generating a higher voltage AC that makes up the difference? [stevezzzz]

    Back voltage > applied voltage. It is now an AC generator.

    At cruise with motor simply 'going along for the ride', i.e. zero KWs going into motor or out of motor, the 'motor' is neither a motor or a generator. Emf = back emf.

    If you press further on the pedal then motor wants to speed up sufficiently to return to balance (that point where, again, there is no IN or OUTflow). Teslas are so powerful that this usually happens very quickly.

    On the other hand if you let up on pedal, then motor has more OUT than IN and KWs flow back into battery. I.e., back emf > applied emf. As long as you maintain the minimum required AC voltage to the 'motor' it acts as a generator up to ~100% of its power rating, which being a Tesla is quite high. But regen is limited by the inverter on purpose to protect battery: battery safe outflow > safe inflow. Why, I have no clue.

    This is all ~400 cycle AC going into & out of the INVERTER (gatekeeper to battery).

    [hey, I took a shot . . .]
    --
     
  3. jerry33

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

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    400 cycle != 60 cycle.
     
  4. HFh

    HFh Member

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    Reading some of these comments and opinions, it almost sounds like the how much you like one take vs the other depends upon whether you prefer a car that acts like a manual or a car that acts like an automatic. I personally prefer the former (so much so that I promised to never buy an automatic, a promise I feel I have kept with the Model S), but I could see that if I were into automatics that much about the way the car behaved would seem weird and wrong.
     
  5. dsm363

    dsm363 Roadster + Sig Model S

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    I have never owned a manual car before and have grown to love the one pedal driving of a Tesla. I may not be as efficient as regen on brake or coasting but it's a much more pleasant driving experience in my opinion.
     
  6. stevezzzz

    stevezzzz R;SigS;P85D;SigX;S90D;XP100D;3LR

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    Dumb question #3: I thought the drive system employs variable frequency AC. Where is this '400 cycle' number coming from?

    Is there a decent primer on the way Tesla converts DC from the battery to variable frequency AC for the motor, and why TM chose to implement the drive system this way?
     
  7. jerry33

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

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    Poorly stated. My bad. The cycles from the wall output are not variable.
     
  8. spleen

    spleen Active Member

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    Just to add more fuel to the fire, Car and Driver (a magazine not known for its friendliness toward EVs) had this to say in its recent review of the new Ford Fusion Hybrid. I have to say that speaking from my personal experience driving the Leaf, I have difficulty at times with brake feel given the transitions from regen to friction brakes.

    "The one remaining drivability failing is the brake pedal, which, as in many a hybrid, is springy and disconcertingly nonlinear. The pedal is so spongy that the tape switch we use during braking tests at the track refused to trigger. Despite the terrible feel, once engaged, the brakes bring the hybrid to a halt from 70 mph in 177 feet, shorter by several feet than most of your lighter, conventional family sedans.

    No hybrid maker has fully untied that Gordian knot of brake feel (although VW has come closest). The issue as ever is the hand-off between the regenerative braking system that’s activated at the top of the pedal travel and the conventional friction brakes that come alive at some point deeper in the travel. It might be time for Ford and others to have a look at Tesla’s approach of having very aggressive regeneration once you ease off the accelerator, leaving the brake pedal solely in charge of  the friction brakes."
     
  9. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    Technically, chargers will have rectifiers (AC->DC) to charge the battery. Just a nit. :)

    - - - Updated - - -

    Any generator (or motor acting as one) will provide variable frequencies in output power, depending upon RPM's. This is why generators always run at 3600 RPM or 1800 RPM depending upon the number of poles/windings -- so they can generate 60 Hz.

    In theory, you could probably use the existing rectifiers used for regen to charge the battery pack, but there are some challenges associated with that. First, you'd have to include some type of isolation circuitry or devices to keep the wall power from going through the motor windings (since that's where the AC inputs for the regen charger will be connected). Second, you'd have to engineer the regen rectifiers to handle the complexities of charge monitoring, etc.

    It's likely less expensive from an engineering point of view to keep the two separate.
     
  10. dennis

    dennis P85D

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    I have to disagree with that. I have owned a number of high performance cars with excellent brakes, and I find the Karma's regen-friction transition seamless and its braking performance right up there with the best BMW performance sedans. PM me if you live in the Bay Area and would like to set up a get together with other Model S owners so you can experience it for yourselves!
     
  11. MikeK

    MikeK R#129, TSLA shareholder

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    AC Propulsion (an EV pioneer whose technology was used in the EV-1 prototype and also licensed by Tesla during Roadster development) developed a system called "Reductive Charging", which used both the inverter and the motor windings as part of the charging system.

    AC Propulsion | Creating electric vehicles that people want to drive
     
  12. spleen

    spleen Active Member

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    dennis - I would totally take you up on that if I lived anywhere close to your area! lol
     
  13. eledille

    eledille TMS 85 owner :)

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    #113 eledille, Feb 3, 2013
    Last edited: Feb 3, 2013
    Ok, yes, a somewhat lazy CC would definitely help.

    Does it? If so, I don't know why, but I do know that a single A123 18650 cell can discharge at more than 70 A (~200 W). They're even better at charging, relative to Panasonic. On the other hand, they can only store half as much or less energy, and I hear the failure rate is high. How much power the Karma battery can absorb obviously also depends on its size.

    jerry33 is right, on those (and currently most) cars, the regen and the charger are separate systems. But they look much the same inside. The difference is even smaller between a three phase charger and the regen, because all modern EV motors are three phase machines, so the regen operates on three phase power of variable frequency produced by the motor. The grid frequency is in the frequency range the regen must be able to handle, 50 Hz corresponds to 1500 rpm for a four-pole motor. This input is then rectified, smoothed and fed to a DC-DC voltage regulator which matches the voltage and current to the battery.

    I believe all the various three phase motors are capable of producing full power as generators. But you would have to beef up the regen circuitry, this is a separate circuit from the forward drive circuit. And the battery must be able to absorb the power, and very few battery chemistries can charge as quickly as they can discharge.

    So powerful regen will increase cost, and if you really want to stretch range, you don't want to use powerful regen anyway.

    And Renault has done something quite similar with their Chameleon charger which comes standard with the Zoe, it will charge with 400 V three phase up to 43 kW. Pretty impressive for such a low price. Seems they may have to improve its single phase performance, though.
     
  14. dashrb

    dashrb Member

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    I *do* understand what you've said, but I had a thought on it. (I will also try this on my own but it would be nice to have corroborating observations.)

    It is a known fact that the Model S will be regen'ing at a certain rate (let's say 40 kW for sake of consistency), and as the vehicle's speed decreases, the regen rate will diminish gradually, until it is near 0 kW as the car approaches 0 MPH.

    Your observation is that in your context, the Model S was regening at 40 kW until you applied the brakes. You hypothesize that the car has decided to reduce the regen rate because of the application of the brakes. Do you think it's possible that by applying the brakes, you slowed the car's speed down to the point that lesser regen was being achieved due to speed alone (and not a car's decision)?

    It would clarify your hypothesis that you were to take a video of the dash display on your same hill, performing two separate test runs (you start with the vehicle at the same speed for both runs). The video must show your regen rate alongside your speed, for comparison. Your runs differ thusly:
    a) remove your feet from both pedals so that regen rate is seen without braking.
    b) remove your foot from the go pedal, and apply the brake (perhaps gently).
    You can then say for sure that you achieve X kW of regen in test a at a given speed (e.g. 20 MPH), but Y kW of regen in test b at the same speed.

    It may be useful to have a test c) where you apply the brakes with greater force.

    The reason I suggest video is because it is difficult to look at the regen dial while at a certain speed, if your speed is changing (it takes your eyes too long to move and read the number).

    Does that make sense?


    Also, at one point you said:
    But in the Model S, your brake lights do illuminate without you touching the brake pedal, if the deceleration rate is high enough. You can "feather" the go pedal to keep the "needle in the green between 3:00 and 4:00" and avoid brake light illumination, but above a certain deceleration, the brake lights will illuminate. I do not know (maybe I should get that iphone app) what G-rate will trigger the brake lights.
     
  15. smorgasbord

    smorgasbord Active Member

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    That sounds similar to the Karma's system.

    I did get to drive Lloyd's Rav-4 for a few blocks around town (thanks Lloyd!), but didn't get a chance to explore the combined regen/friction brake pedal feel.

    - - - Updated - - -

    Do you think that's 70kW going into the battery, or just a way for them to tell you that there's 70kW of braking force - some regen, some friction - being applied?

    If you do a semi-panic stop on the Karma, does the kW gauge also jump to 70? If so, that would indicate that it's not just measuring regen.


    It's a question of ergonomics, so there will be differences of opinion. I really like one-pedal driving. The one skill to be learned is that you don't usually lift your foot off the pedal completely to coast - you feather it such that the gauge is at 0kW. In traffic, it's great not having to take your foot off the accelerator to slow down. On the track or hypermiling, it's not as good, but then most of us don't hypermile or track our cars most of the time.
     
  16. Clint Eastwood

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    This may be a dumb question, but how does regenerative braking slow the car down without using the brake pads?
     
  17. brianstorms

    brianstorms Member

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  18. Clint Eastwood

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    good read, thanks. That helps explain how he brake pads are spared as the electric motor actually reverses direction to slow the car down while generating kinetic energy funneled into the batteries. So my next question is, where does the power come from to reverse the engine? Is it generated by the friction of slowing down naturally instead of coasting? Because intuitive,y to me it would seem contradictory to use the battery power to reverse the motor to generate peer to send to the battery.
     
  19. brianman

    brianman Burrito Founder

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    Your phrasing doesn't sound right to me. It's not a matter of reversing the motor's direction of spin, but rather about slowing the rotational rate.

    Perhaps wildly or poorly oversimplifying, but I think of it like this. When spending power (pressing the accelerator pedal down), energy is used to force the motor and thus wheels to turn. When collecting power (releasing the accelerator pedal), the motor's momentum is decayed by offering resistance in the form of the battery absorbing energy.
     
  20. scaesare

    scaesare Well-Known Member

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    In essence, the motor is turned in to a generator. It takes energy to turn a generator, and that energy is the kinetic energy of the car, thus bleeding of momentum/speed.
     

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