Model X

Dual motor - dual regen?

edited August 2014 in Model X
I was wondering wether tesla would use both motors for regen.
Would it actually increase or decrease the amount of energy recovered when say taking your foot of the pedal at 50 miles/hour and coming to a stop?
And assuming they would use both motors for regen, could it have an impact on handling when turning? I've seen threads that advise to turn regen to low on icy roads because of the possibility the car loses grip.




  • edited November -1
    It depends... How fast you are going... Weather conditions... Type of surface... Orientation of the vehicle... But basically, the answer is, <i>"Yes."</i> There will be dual regeneration, but it will vary based upon varied factors.
  • edited November -1
    All controlled by genius computer TC, etc.
  • edited November -1
    Front motor is better for regen, decelerate the front tires since weight shift forwards and reduces wear and losing grip on the back. I bet both motors will regen, with realtime fluctuation between f/b for optimal energy recovery and even tire wear.
  • edited November -1
    Thank you for you answers!
    Will it also increase the net energy recovery? Will it, under the exact same conditions, recharge the battery more or less compared to a single regen setup?
  • edited November -1
    The sales person I had to my Tesla S test drive in this weekend said, that there will bee more regen, due to the double engine system, and therefor it will be with more energy regenerated. And with this issue, Model X will get closer to same miles on each charge as model S
  • edited November -1
    I might be wrong - but I'm not sure dual regen will make a difference - at speed any object (Model X and Model S) has a certain amount of kinetic energy - regenerative braking converts Kinetic energy into Electrical energy with some amount of loss - having two motors/generators does not change the amount of Kinetic energy in the system - nor does it change the conversion ratio…so if I remember my high school physics well enough there will not be more 'power' regenerated due to two motors - since the amount of potential energy will not increase…

    now with both motors sucking power (converting kinetic energy) they could slow the car down faster - but the amount of energy returned to the battery should remain roughly the same…

    I'm sure Tesla is tuning the regeneration so that it's a controlled burn and not very abrupt due to the additional generator coming online…

    remember you can not get more energy out of system than you put into it…in fact you can't even get the amount you put in due to loss of conversion and overhead.
  • edited November -1
    in fact the more I think about it - dual motors should only make a minor difference in amount of regen returned to the battery…

    dual regen can return _MORE_ power to the battery over a unit of time - i.e. they can charge the batter at 2x the rate of one motor…

    but the presence of dual motors does not change the total amount of energy in the system…

    so dual motors could slow the car down at a faster rate of deceleration - but they do not in of themselves add more energy back to the battery - they can not because the potential amount of Kinetic energy in the system is no greater than a Model S

    so I would assert the Tesla sales representative is wrong - he has to be otherwise the conservation of energy laws of physics would be violated.

    dual motors will allow a greater rate of charge (if the tesla software lets them) - i.e. faster deceleration
    dual motors will _NOT_ be able to recover more energy other than some minor variation possibly due to regenerating the energy faster due to less loss to rolling resistance or aero-dynamics

    example slowing the car down from 60 mph in 2 seconds vs. 3 seconds (dual vs. single regeneration) there may be some gain in that less energy was lost to aerodynamics and rolling resistance in the 1 second difference in the 60 to 0 mph test - but I would be surprised if this was significant in any way…

    now the Model X being heavier than a Model S - it has MORE kinetic energy than a Model S at the same speed (60 mph) - but it also took more energy to get the Model X to 60 MPH - nature always balances the books - but you will still only recover X% (where X is less than 100% due to loss) of the energy due to regeneration. So technically you will recover more energy in a Model X - but that is because you used more energy to get it up to speed - and will use more energy again to get it back up to speed…

    I would not be surprised if the Model X could recover 80 kw of energy under full regen braking (vs. 60 kw indicated on my model S). But while that is more energy per-second under deceleration (80 vs. 60) - that simply means the energy is being drained from the system quicker -not that there is more energy in the system. i.e. the Model X could slow down faster - but it would hard to say it recovers more energy because it has two motors…
  • edited November -1
    or to put it more simply

    think of a pump moving water into and out of pool - this is the tesla converting electricity to motion - it's adding kinetic energy into the system…

    when you are accelerating the pumps/motors are pumping water into the pool to fill it up - the amount of water in the pool equals the amount kinetic energy in the system - this is kilowatts used minus the overhead of aerodynamic/weight/rolling resistances…

    when you are decelerating using regenerative braking the pumps are pumping water out of the pool and storing the water back into the battery - minus some conversion overhead - this conversion causes the car to slow down and stores some of the energy back into the battery…

    the pumps can not recover more water than they put in…two pumps can empty the pool faster (harder more aggressive deceleration) - this will empty the pool faster (car will slow down more quickly) but two pumps do not change the amount of water in the pool…

    having two pumps can empty the pool faster - but they can not change the amount of water in the pool - the amount of water is the pool is related to how much was pumped into the pool when the car was accelerating - minus the overhead of maintaining the speed due to external drag/friction.

    the two motors on the Model X could slow the car down faster (up to the limits of traction of the tires and tesla's software settings) but they will not "add" any water to the pool - so they can not recover "more" energy…because the simple fact is there is not more energy in the system.

    gotta balance the books…
  • edited November -1
    sorry for the redundant paragraphs - but you get the idea - I really want to be able to edit a forum posting…
  • edited November -1
    All irrelevant. The limitation on regen right now is the max current the battery can accept, 60 kW as I recall. That is subject to revision in the new car. If TM wants more regen, it will have it.
  • edited November -1
    I humbly disagree - ok the limit is 60 kW - that mean it empties the pool at a rate of 60 kW/sec while it slowing down - until the pool is empty and all the possible converted energy is back in to the battery..minus loss.

    Ok so the new car can empty the pool at say 100 kW - there still isn't more energy to be had - it will just capture that energy quicker - but it will not capture more energy...

    We're dealing with a fixed budget of kinetic energy..the only difference will be the rate of regen - it can not be the total amount.
  • edited November -1
    Thank you!
    So basically dual motor regen could slow down the model x faster compared to a model s, but it will not capture more energy. (Only capture that same energy faster)
    Just this, because you have losses in two places now, will it actually recover less energy?
  • edited November -1
    I believe that having regen from the front wheels will result in a better level of efficiency in recharging the battery pack during deceleration. Brian is correct though, the battery pack can only receives so much power at once as a maximum input from the regeneration system. The limit likely exists for a variety of reasons and most of those are likely centered around safety, reliability, and durability. It isn't really that you will get more power back, so much as you will get the power back more efficiently, with less waste, thereby refilling the battery pack sooner, and more often, using regeneration.
  • edited November -1
    More efficient regen is quicker slowing. In situations, e.g. emergencies, where the brakes would also be required, this would reduce that waste. QED. ;)
  • edited November -1
    Brian H - I agree more regen is more efficient but it's unclear to me that the efficiency loss right now is significant enough to notice a difference - certainly less braking means more of the energy was captured for regen (rather than lost to friction/heat of braking)…

    but for the most part I'm getting good enough with my Model S that I never use the brakes under normal circumstances…so if you are already getting the car down to zero using nothing but regen dual motors aren't going to recover more energy…

    but yeah I could see more regen capturing more energy as being perceived as improving regen because now even more people won't use the brakes.

    of course this is all in theory - right now the 60kw regen is pretty noticeable and I would question how much more you can ramp up the regen before people will prefer a softer impact - i.e. I might be able to regen at 200 kw due to two motors -but the drag/deceleration caused by that much load may be quite abrupt and unacceptable in daily use…so it's also a matter of tuning the software as to what amount of regen is allowed…

    also if the current limit is 60 kw due to the batteries ability to accept regen charge - then again two motors doesn't change that particular aspect of the system…
  • edited November -1
    I believe there will be an ongoing, continuous, refinement of power electronics systems and the software that controls them, for quite some time, leading up to the introduction of Tesla Generation III vehicles. Those cars will benefit greatly in terms of increased efficiency, range, drivability, and durability provided by those system enhancements. Chief among these will be the performance of AWD implementations on those cars, which I expect will be pivotal in gaining market share in multiple territories. The Tesla Model X will pioneer this realm for all mass market EVs to follow.
  • edited November -1
    Lots of speculation here but there is no data available to answer the question.
    I think dortor's thinking is basically right.
    Don't understand 60 kW "current limit", kW is a unit of power not current. Maybe power limit was what was meant. I have a question about that. It's my understanding that the MS circuitry can accept about 120 kW of power from the Superchargers. Is that right? If so, is that the power limit for regen or are there two power limits?
    Anyway, if you go from speed x mph to zero, you will have y kinetic energy loss available. Some goes to air and rolling friction. The rest goes to regen. If regen is 80 per cent efficient, that's what's left for the battery. Doesn't seem like 1 or 2 motors will make much of a difference.
  • edited November -1

    The supercharger is straight DC. I believe the motor there for the regen is AC so the 60kW limit maybe in the AC to DC conversion.
  • edited November -1
    The energy of a 5000 lb car moving 50 mpg is 42,000 ft-lb's=0.158 kWh. The rate of energy extraction at 50mph (foot off pedal) will be set by Tesla or may be selected by the driver, in any case the rate of energy extraction should be independent of using one or two generators. I believe the question is which method will be more efficient recapturing the 0.158 kWh of kinetic energy. I believe using both motor inverters will be slightly more efficient. By splitting the energy flow through both front and rear drive systems the current (amps) should be about half in each system. A main component of energy loss in the drive systems is resistance loss. Doubling the current increases the resistance losses by 4 times so you get the about twice the resistance loss by using only one motor. in total this will be a small amount but should be measurable.
  • edited November -1
    dortor is correct about the kinetic energy. As for the 60kW limit, do you honestly think regenerative breaking will generate that much power? I don't think so. The model S uses approx. 300W/mi. (85kW/300W per mi gives you about 283 miles) If you could regenerate 60kW, you'd be recovering way more than what you started with. Not to mention you'd be able to recharge your battery just a bit over an hour. I'm sure the regenerative braking cannot delivery anywhere the amount of power a supercharger can.
  • edited November -1
    Milesbb: you are right about ohmic losses in wiring being half as much when current is split between two sources making some difference. That's the answer then. Two motors are a little better than one.
  • edited November -1

    do not confuse kW with kWh. The car at 50 mph only has 0.158 Kwh of of kinetic energy. Regenerating at 60 kW will take all of the of kinetic energy (stop the car) in 10.8 seconds. You only get back the 0.158 Kwh minus some system efficiency losses. You get about 0.5 miles more range then if you had stopped using conventional brakes.
  • edited November -1
    Not feeling confused about kW vs kWh.
    As a collateral issue, I was just confused about where 60 kW came from.
    But, if you say so...
  • edited November -1
    sorry my response was intended to be to hpjtv's post not your post. I am the one that got confused.
  • edited November -1
    well for my part the 60 kw value comes from the on-dash display - as I have never seen it go over 60 kw while under full re-gen…i.e. slowing down from > 60 mph on a flat level surface.
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