regen capability

Discussion in 'Clarity' started by victor_2019, Jul 29, 2019.

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  1. victor_2019

    victor_2019 Active Member

    I don't understand what you mean by this.

    the way regeneration braking/electric braking works is that the electric motor applies torque in the opposite direction of travel.

    so the motor is turning clockwise but applies torque counter-clockwise which causes braking force that slows down the car.

    this torque can be generated at any speed even at zero speed.

    and as long as the motor is turning in the opposite direction of the torque, the motor works as a generator and the power is going to the batteries.

    It's true that the pwoer generated is directly proportional to the speed, so at some low speed the power will be lower than the losses in the system and the battery will no longer be charged, but

    #1 the motor is spinning faster than the wheels due to the gearbox, so depending on the ratio this means very low car speed, and
    #2 as long as the motor is spinning in the opposite direction of the torque, no matter the speed, it will never be motoring, it will always be generating power.

    the motor in generator mode will transform mechanical power into electrical power, and then you subtract losses in the motor and inverters and what is left goes in the battery.
     
    fotomoto likes this.
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  3. DucRider

    DucRider Well-Known Member

    At slow speeds, the power generated is indeed very low, and hence there is virtually no slowing of the vehicle. Basic physics - if there is little energy being generated, there is little loss of velocity. In order to bring the vehicle to a complete stop in a relatively linear fashion (keeping roughly the same deceleration rate), energy from the battery must be used to supplement the resistance (it becomes heat, just like using brake pads).
    The vehicle could be brought to a complete stop (eventually), but not in a manner that allows "one pedal" driving.
     
  4. Sandroad

    Sandroad Well-Known Member

    So here's an example from my simplistic mind...... When I'm running the vent fan on my RV in forward, and quickly switch it to reverse, it 1) slows, 2) stops, and 3) starts turning in the other direction very quickly. I assume that durning the whole sequence, it is using battery power. I further assume that if I simply turn it off and let it coast to a stop, it uses no power (and may actually add a tiny bit back to the battery. Are these assumptions correct?
     
    KentuckyKen likes this.
  5. KentuckyKen

    KentuckyKen Well-Known Member

    Great observation and a useful analogy. I vote yes for the first assumption and maybe/maybe not for the second.
     
  6. victor_2019

    victor_2019 Active Member

    that's not how things work... Perhaps I still don't understand what you're trying to say.


    it's not the power or energy generated/used that changes the speed of the vehicle, it's the torque or force.

    in linear terms F = m*a
    you apply a force F on the vehicle of mass m and that results in acceleration a. as long as force is constant acceleration is also constant.
    in the rotational frame the equivalent is T = J * α where T is torque, J is inertia and α is angular acceleration. you can play around and change between linear and rotational frames using the wheel diameter and gear ratios. (F = T * r where r is the radius, assuming 90 degree angle between torque and force vectors)

    Power P = F * v (force times velocity) or T * ω where T is torque and ω is rotational speed.

    Power depends on speed, but acceleration does not. The torque from a motor is constant even at zero speed therefore acceleration is constant regardless of speed or power used/regenerated.

    the electric powertrain is perfectly capable of slowing down the car to zero.

    and the powertrain doesn't really transform anything into heat just to slow down the car. there are always losses in the inverter (switching and conduction losses) and in the motor (magnetic and copper losses). The switching losses depend on switching frequency so let's say they are constant, and so are the magnetization losses in the motor. the conduction losses in the inverter and the copper losses in the motor are directly proportional to the current going through, which is directly proportional to the torque.

    these losses are always there whether the motor is accelerating or braking (and will be lower when going at constant speed but will always be there). the only difference is that when you are slowing down the mechanical power of the motor is negative (motor acts as a generator) and as long as the mechanical power generated exceeds the electrical powertrain losses, the batteries are getting charged.

    but in terms of what makes the car slow down, it is the fact that the torque of the motor is in the opposite direction of rotation, and this remains true regardless of the speed.
    the car doesn't start to "run the motor in reverse" at low speeds. the motor is directly coupled to the wheels through a fixed gear. when the car moves forward the motor turns in one direction, when the car moves backwards the motor turns in other direction. the electric motor in this car can't turn in the opposite direction of the car movement.


    EVs have one pedal driving where the vehicle slows down and stops without touching the brakes at all. Nissan Leafs do it by default, the kona does it when you hold the regen paddle etc.

    the clarity could do it too if the system designers had decided so. the electric motor is perfectly capable of holding the car still even against load (as in on a hill/incline). if the car is on an incline and the motor is holding it still, then the only power used is equal to the losses in the inverter and motor.
     
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  8. 2002

    2002 Well-Known Member

    I'm not sure but I think I remember from my Prius days that the issue is that regen can't smoothly bring the car to a final stop as well as friction brakes, so that could be part of the reason why it switches to friction brakes at the end. If you are saying other cars can smoothly come to a stop with absolutely no friction brakes then I suppose it's possible but might be more complicated to engineer so a decision has to be made if it's worth it. Again from memory but I think at those low levels regen is less efficient anyway so you aren't losing that much. And you will need the friction brakes at the end anyway because the last thing you want to do is hold the car using battery power, I suppose it can be done if it can determine the slope. It can transition to park to hold the car but that will not be very smooth either unless friction brakes are applied first.
     
  9. Sandroad

    Sandroad Well-Known Member

    I appreciate your technical details, but this statement is definitively not the case, unless I’m misunderstanding. If the electric motor were holding the car on a hill, it would be at stall torque using the necessary stall amps to keep the shaft from turning. If it was at full stall torque a tremendous amount of amps would be used and the battery would discharge relatively quickly. Another RV example: the A/C unit on my RV takes a momentary 57 amps to start the compressor motor (stall amps), but only 15 running amps. So, an EV on a hill could potentially use a lot of power if held only by the electric motor and no friction brakes and the steeper the hill the more the amps.
     
    KentuckyKen and insightman like this.
  10. KentuckyKen

    KentuckyKen Well-Known Member

    I think @Sandroad just hit the daily double on examples.
     
  11. DucRider

    DucRider Well-Known Member

    And also similar to what it takes to slow the car to a complete stop using "regen". With out using battery energy, the electric motor cannot bring the car to a relatively normal stop (like coming up to a red light). The amount of "braking" force from regen falls to near zero as the motor rpm falls towards zero and will to very little to slow a 3800 lb vehicle.
     
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  13. Sandroad

    Sandroad Well-Known Member

    Nailed it.
     
  14. MNSteve

    MNSteve Well-Known Member

    Yes, it's the torque/force that changes the velocity of the car. But it's energy that produces that torque/force.

    Yes, you could slow the car to zero using the electric powertrain. But this little concept of "asymptotically" gets in the way. The math suggests that you reach zero speed only after infinite time. As your speed slows, the amount of force produced by the regenerative system, commonly called "braking", decreases. At some point you get so little braking force that it becomes appropriate to introduce friction brakes. Without them we would crash into that car in front of us.

    And it does take energy to hold a car still on an incline if you want to do it with an engine. (Friction brakes work so much better in this scenario.) Gravity is creating a force on the car. Something must counteract that force if the net acceleration is to be zero, otherwise F=ma applies and the car will move. If you are counteracting this force using the propulsion mechanism (engine/motor) that device must generate a force equal to what gravity is creating so that the net force is zero. This force is not free. It has to come from somewhere. In the electric scenario, you have a motor that is not turning but is using energy to create the force that equals the pull of gravity.

    Off-topic comment for the engineers out there: This problem on a test decades ago made such an impression that I still remember it. A train is accelerating while on a curved track. Inside there is a roller skate that is tethered to an upright peg with a string. Given the rate of acceleration, the angle of bank, the radius of the curve, the coefficient of friction of the bearings in the skate, the velocity of the train (given in furlongs per fortnight), the mass of the roller skate and the relative humidity . . . what is the tension in the string?
     
  15. 2002

    2002 Well-Known Member

    In rocket launches they refer to that as gravity drag. Most of a rocket's power goes into acceleration, but during ascent part of the energy is also needed to "hold up" the rocket against the force of gravity trying to pull it back down. Easier to conceptualize if you think of a rocket hovering, clearly it is using massive power just to stay at one altitude.
     
  16. KentuckyKen

    KentuckyKen Well-Known Member

    Y’all have now given my peanut brain a headache, but I certainly appreciate the science in those explanations. I never fail to be educated on this forum.

    Here’s a question for you.
    The Clarity is set so that in D and with your foot off the accelerator AND brake pedals, the motor will drive the car a few mph. So when held stationary by either Brake Hold or brake pedal, are we still using power from the battery analogous to my old gasmobile? I sure hope not as that would be very inefficient. I observe that the Power Meter seems to say no, but when the brake is released, the car accelerates immediately with no lag that I can discern. So does brake application shut off motive battery power even at a complete stop? (I note that it does when in motion as in it won’t let the motor fight the brakes.) Inquiring minds want to know.
    I note that from the above posts, this scenario is different from being on an incline where the slope would be just right for motive battery power to keep the car stationary without using the brakes. Then of course you are using power since there is no free lunch with F=MA.
     
  17. 2002

    2002 Well-Known Member

    The "creep" was put in intentionally to mimic gassers. Only happens when foot is off the brake. As for stopped on an uphill slope in drive with foot off the brake, yes you would be using energy to stand still unless it automatically engages the brakes in that situation but I doubt if it does.
     
  18. DucRider

    DucRider Well-Known Member

    It does if "Brake Hold" is enabled. That is one of the few scenarios where I use it.
     
  19. 2002

    2002 Well-Known Member

    Well yes with Brake Hold enabled any time you come to a stop using the brake pedal it automatically applies the brakes so it becomes a moot point in the scenario that I was thinking of. To be more specific I was referring to not having Brake Hold on, if you come to a stop on an incline where it requires power to hold the car, will it continue to perpetually apply power for as long as you sit there talking on the phone, or will the computer figure out that you are in that situation and automatically engage the brake. I suspect it will continue to drain power.

    But you raise an interesting point I hadn't thought of, I assumed Brake Hold only applies the brakes when you come to a complete stop using the brake pedal. But if you have Brake Hold enabled and coast to a stop on an incline without using the brake pedal, are you saying Brake Hold automatically apply the brakes? If so I guess I just never experienced it even though I use Brake Hold all the time, I turn it on before I even leave the driveway.
     
  20. Is the string made of cotton?
     
  21. In this scenario, the car will roll backwards.
     
  22. The “quick switch” method will consume slightly more energy. The rotation of the fan is being slowed and stopped by electricity before being reversed. Think of it as stall current times 2, or 1.xxx.

    Turning it off and letting it coast to a stop uses no power and may prolong the life of the motor. However, you now have an open circuit, so any potential energy from the slowing fan has nowhere to go.
     

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