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How does Regenerative Braking slow you down??

Discussion in 'Model 3: Driving Dynamics' started by M falcon, Jul 6, 2018.

  1. M falcon

    M falcon Member

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    Ok, so I'm hoping someone can educate me (for my own interest and in case friends or family ask) about how regenerative braking slows down a car. I've tried using google and have watched videos on youtube that try to explain the mechanics. But i still don't get it. Now, I understand that when you lift your foot off the accelerator the car's wheels act as a generator that feed electricity back into the battery. What i still don't understand is how any of that slows down the car. How is it exactly that the car slows down without me having to use the brakes??
     
  2. Daniel in SD

    Daniel in SD Member

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    Seems like you've got it. The motor acts as a generator to put energy back into the battery. It's just the reverse of what's happening when you're accelerating.
     
  3. WarpedOne

    WarpedOne Supreme Premier

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    I suspect you should first try to understand why brakes slow down the car.
    After that it will be a walk in the park.
     
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  4. arcus

    arcus Active Member

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  5. arcus

    arcus Active Member

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    Or, just watch this video:

     
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  6. animorph

    animorph Member

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    Can't fill up your battery for free! That energy going into your battery comes from the "work" of slowing down the car. Think of using a hand cranked generator to power a light The more you crank, the brighter you can make the light (and the harder it gets to crank). Same for the car, the harder the regen braking the more energy that goes into the battery and the faster the car slows down. The car can control the amount of power from regen in order to modulate the braking force.

    We used to use a resistor across the motor of our slot cars (if you remember those) to provide braking when not accelerating. Kind of crude with a fixed resistor, but a dramatic braking effect as the motor actually had to work against the resistor instead of just an open circuit. Very simple regen braking.
     
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  7. M falcon

    M falcon Member

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    Ok, thanks this is making some sense. but still hard to visualize how the process of generating electricity physically slows down the wheels.
     
  8. Perry

    Perry Member

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    The momentum of the car will rotate a magnetic field (rotor) through a stationary armature (stator) and through induction electricity is created. The magnetic field will fight the force of rotation thus slowing the rotor rotation down. So you can think of the force of magnetism that's slowing the car down.

    In reverse, when you accelerate you convert the electricity to magnetic force so that force will speed the car up.

    It's all about controlling magnetic fields for both acceleration and regen braking.

    An alternator on a regular car works the same way to charge your 12v battery. See here for detailed explanation:


    Now look at this explanation of how a motor works and you'll notice that they have the same parts except it works in reverse:


    A Tesla motor is both an alternator and a motor switching its function when you press the brake.
     
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  9. M falcon

    M falcon Member

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    Umm...thanks I guess but couldn’t follow this.
     
  10. ohmman

    ohmman Maximum Plaid Member

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    @Perry gave you the right and concise answer.

    You know the resistance you feel pulling magnets apart? It's the same idea but in a rotary form. So really what you want to understand is how electromagnetism works with electric motors. Introduce electricity and get propulsion. Remove electricity, and turn that resistive energy into a reverse flow of electricity.

    I'd suggest if @Perry's answer is too difficult to digest at first, start simpler and learn how electricity and magnetism are related. It'll come together quickly after that. Some physics memories from school might start surfacing as well. I do a good job of burying a lot of that stuff, so I understand. :)
     
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  11. Ejl80

    Ejl80 Supporting Member

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    Ok. Think about it this way. Have you ever run on a powered treadmill?

    On a powered treadmill it is moving when power is input from electricity. It slows down and stops when power is cut, right? Well, that is because treadmill motors aren't set to "coast". It gradually slows down while you slow down. It isn't a perfect analogy... but hopefully it gives you an idea. Just like a manual transmission car slows you down when you let off the gas if you don't put it in neutral.

    Regen braking works because the motor basically works in reverse similar to how electricity is produced at power plants. When you go with a Tesla, electricity goes in to make a magnetic field to cause the "driveshaft" to spin. When you let off the go pedal, electricity is no longer going into the motor to make a magnetic field. The momentum that the "driveshaft" got now creates a magnetic field in the motor when the "driveshaft" coasts and slows down, and returns power to the battery. This is a complete oversimplification and not completely accurate description of what is happening, but does that make more sense?

    These two parts together are why regen braking slows you down AND returns some power to the battery.
     
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  12. brendon1970

    brendon1970 Member

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    #12 brendon1970, Jul 7, 2018
    Last edited: Jul 7, 2018
    So you need to first accept that your electric motor when rotated by a force will act as a generator. This charges your battery.

    Then you need to think about normal friction brakes - how do they slow a car down? They use friction between pad and rotor to dissapate the cars energy. Where does the energy come from? Kinetic energy in the car. Where does it go? Heat and noise. Kinetic energy in the car is converted to heat and noise thereby slowing down the car.

    In the case of an EV. How does the car slow down? it uses kinetic energy to rotate the motor, using it as a generator. Where does the kinetic energy go? Electricity to the battery and some heat in the electronics and motor. Why does the car slow? Conversion of kinetic energy into heat and noise results in the car slowing.

    In both cases the reduction or dissipation of kinetic energy is what slows the car.
     
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  13. WarpedOne

    WarpedOne Supreme Premier

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    #13 WarpedOne, Jul 7, 2018
    Last edited: Jul 7, 2018
    It boils down to the term "generating electricity" being a nonsense. There is no electricity being generated, what is happening is just a same old boring transformation of energy in various ways.

    You take a stopped car with a full tank of gasoline. The car has 0 kinetic energy and a shitload of 'chemical energy' in that gasoline.
    You start driving. What is happening? That chemical energy is being transformed into shitload of heat that is flowing out the exhaust and some kinetic energy (i.e. the car is moving).
    You stomp on the brakes. What is happening? That kinetic energy is transformed into heating up the brakes. When the kintetic energy runs out, the car stops.

    Switch to a stopped EV with a charged battery. It again has 0 kinetic energy and shitload of 'chemical energy' in its battery.
    You start driving. What is happening? That chemical energy is being transformed into electric current through the motor, where it causes various magnetic effects that cause things to rotate. That rotation is 'converted' into moving the car - it gets some kinetic energy.
    Then you lift up the accelerator. What is happening? You ceased demanding transformation of chemical energy into electric current, but the car is still moving and causing that thing to rotate and this rotation is causing magnetic effects that in turn are causing electric current to flow.
    But it does not want to flow, it resists, meaning the kinetic energy of the car is being depleted, causing it to slow down and battery being charged up.
     
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  14. M falcon

    M falcon Member

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    Thank you everyone for your help in making me understand this!!
    Ok. I think I’m getting it now. I guess I was stuck on how I know brakes on a bicycle work. Squeeze the handle brakes moves the brake pads which clamp onto wheels which causes friction to stop the bike. I was searching for something concrete that was clamping onto the tires to slow it down. Your explanation makes sense to me. So tell me if I’ve got this right: If the car did not have regenerative brakes, when I lift off the accelerator the car would coast to a stop as all the kinetic energy is converted to heat (friction of tires on the road). But with regenerative brakes the car comes to a stop faster because the kinetic energy is used up sooner since some of it is fed into the battery. Did I get it right?
     
  15. DR61

    DR61 Member

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    Yes, but also kinetic energy converted by aerodynamic drag - the car body pushing air molecules out of the way as the car moves forward. The air molecules are speeded up and heated somewhat.
     
  16. jamnmon66

    jamnmon66 Member

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    It's technically not "regenerative brakes". At least not in the sense of traditional friction brakes that we're all used to on bikes, cars, etc. And, the car won't come to a stop "faster" with regenerative braking than with the regular friction brakes (i.e. pressing on the brake pedal). Friction brakes are much more powerful than regenerative braking which is why the Model 3 still has them, for safety. But all of the energy is lost/wasted with friction brakes. But, with regenerative braking, much of the energy is re-captured, converted to electricity, and stored back into the battery as described by everyone above.

    As for the way to describe it to friends, family, etc., I usually just tell people that the motor can also work like a generator to recharge the battery. That gets the idea across without getting too bogged down in the details. I think it's fun to go on a deep dive on these types of things and really figure out how stuff works. But most people just accept that it works without asking how and move on. So, a simple explanation is usually good enough.
     
  17. DanCar

    DanCar Member

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    If you search for magnetic induction you will get a better idea about how the magnetic force slows down a car. In this video a magnetic ball slowly falls through a copper tube because of the magnetic force:
     
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  18. ewoodrick

    ewoodrick Member

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    The higher the load on the generator, the harder it is to rotate. When enabling the regeneration, the higher the load that the computer puts on the generator, by hooking it up to the battery, the more anti-rotational force that is added to the wheels.

    When regeneration is on, it is not directly connecting the generator to the battery, that would be pretty much like slamming on the brakes. Only a portion of the generators energy is routed to the battery, in relation to how much regenerative breaking is needed.
     
  19. WarpedOne

    WarpedOne Supreme Premier

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    #19 WarpedOne, Jul 7, 2018
    Last edited: Jul 7, 2018
    Yes.
    "Regenerative braking" has nothing to do with mechanical brakes. They do not engage, the do not heat up, they cause no friction to slow down the car. The motor has 'fixed' connection with wheels, it just stops 'pushing' the wheels and let wheels 'push' the motor to turn in magnetic field. This causes drag and slows down the car.
     
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  20. SabrToothSqrl

    SabrToothSqrl Active Member

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