Forums

Dual motor - dual regen?

Dual motor - dual regen?

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.

Thanks.

Arne

Red Sage ca us | 17 August, 2014

It depends... How fast you are going... Weather conditions... Type of surface... Orientation of the vehicle... But basically, the answer is, "Yes." There will be dual regeneration, but it will vary based upon varied factors.

Brian H | 17 August, 2014

All controlled by genius computer TC, etc.

Iowa92x | 17 August, 2014

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.

arneva | 17 August, 2014

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?

deaconwp | 17 August, 2014

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

dortor | 17 August, 2014

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.

dortor | 17 August, 2014

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…

dortor | 17 August, 2014

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…

dortor | 17 August, 2014

sorry for the redundant paragraphs - but you get the idea - I really want to be able to edit a forum posting…

Brian H | 17 August, 2014

dortor;
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.

dortor | 17 August, 2014

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.

arneva | 18 August, 2014

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?

Red Sage ca us | 18 August, 2014

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.

Brian H | 18 August, 2014

More efficient regen is quicker slowing. In situations, e.g. emergencies, where the brakes would also be required, this would reduce that waste. QED. ;)

dortor | 18 August, 2014

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…

Red Sage ca us | 18 August, 2014

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.

georgehawley.fl.us | 18 August, 2014

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.

ghillair | 18 August, 2014

georgehawley

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.

milesbb | 18 August, 2014

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.

hpjtv | 18 August, 2014

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.

georgehawley.fl.us | 18 August, 2014

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.

milesbb | 18 August, 2014

georgehawley,

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.

georgehawley.fl.us | 19 August, 2014

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...

milesbb | 19 August, 2014

georgehawley,
sorry my response was intended to be to hpjtv's post not your post. I am the one that got confused.

dortor | 19 August, 2014

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.

Heart of Gold | 19 August, 2014

I think the 60kW limit on re-gen is a safety issue.
If you re-gen harder the car would tend to spin around.
So With two motor re-gen it should be possible to raise the limit.

Red Sage ca us | 19 August, 2014

milesbb: If you multiply 42,000 by 9.73 -- the fixed gear ratio -- you may find your answer... Looks like ~64.56828 kWh...?

georgehawley.fl.us | 19 August, 2014

Nominal voltage of 85 kWh battery pack is about 16 x 6 x 3.6 volts ~ 346 volts.
To charge the battery you need to apply something in excess of that.
I think the SC operates at about 400 volts which works, of course.
If 120 kW is the applied power, the charging circuitry in the car is looking at 300 amps!
If regen is capable of up to 60 kW, that would correspond to 150 amps at 400 volts.
If source resistance of generator and intervening wire is .25 ohms, it dissipates an average of about 5.6 kW.

If car stops in about 10 seconds, about .015 kWh is lost to heat. With two motors you would get about .0075 kWh less resistive loss.

Don't know if a quarter of an ohm is the right number but I think this hypothetical example illustrates milesbb's point. Did we beat this thing to death yet?

dortor | 19 August, 2014

given that I do not yet have my model x, or a date for my Model X, or a date for a date for my Model X, or a date for date when I can configure my Model X and then get a date for delivery - I would like to engage on this topic some more…

if we aren't going to get more power from dual motors, maybe the trick lies in quad motors…yeah that's the ticket! Quad motors will most certainly yield more regen than the wimpy-ass crummy model S that I've owned for over a year and can no longer stand to look at it cause it doesn't have quad motors…

the horse is still alive!

Iowa92x | 19 August, 2014

This horse has been spanked!

jjs | 20 August, 2014

Quad motors is a horse of an entirely different color.

bevguy | 31 August, 2014

When the Model S is slowing from high speeds regen puts out more electricity than the inverter can handle. Since the motors are AC I assume the regen is AC too, thus with most Tesla you are limited by inverter capacity when slowing from high speeds.
The kinetic energy of the car is a limiting factor in regen. So assuming the same KE the regen should be the the same if you have 1 or 2 motors. If they can absorb all the KE. i don't know how rolling resistance fits in here.Or whether regen is better from bigger or smaller motors. Maybe cost will be the determining factor. Just as it is for battery pack size, the maximum 85 kWh is cost/benefit limited. They could have used a bigger pack if they wanted to. No doubt the X design will reflect similar engineering cost compromises.

georgehawley.fl.us | 1 September, 2014

bevguy: please correct me if I am wrong but you are right that the regenerative energy is AC that needs to be rectified in order to charge the batteries with DC. Using some crude numbers, here's what I come up with: The Model S requires about 300 watt-hours of energy per mile at say 50 mph. That's a power drain of about 15 kW. Regeneration is supposed to be approximately 80% efficient. Of so, that would be about 12 kW of AC power available for the rectifier to handle. The NEMA 15-40 outlet that Tesla suggests for AC recharging can supply 40 amps at 240 V RMS or about 9.6 kW which is in the same ballpark so, I think, it is likely that the rectifier circuitry in the Model S is designed to handle whatever the motor (s) can produce. There may be short term peaks that are valved down in some way, as you suggest, but it's likely that the car is engineered to recapture as much energy as is practical.

milesbb | 4 September, 2014

The inverter that drives the motor is reversible. it can take DC and convert it to AC to drive the motor. The same inverter can take AC and convert it to DC to charge the battery and slow the vehicle. This inverter rating needs to matched the motor. The motor on the basic model is rated at 225kW, the inverter rating needs to match or exceed this value. I believe the inverter should be rated the same whether in Drive mod or in regenerative mode, it should be capable of transferring the rated 225kW. The 225 kW is a rating at one motor RPM and would decline as you move away from the rated motor speed. I think you will find the Model S regenerative braking is set to 60kW for drivability reasons, not battery or inverter limitations. The number of times the car needs to break with more energy reduction then 60 kW, would be very rare, the energy recaptured would be insignificant over the life of the car. Modern friction brakes work very well in emergency stoping situations. The battery can take up to 125 kw charging for extended periods using the supercharger, I do not believe "short bursts" of regenerative charging over 60 kW would be harmful to the battery even if the battery was at near full charge.

barrykmd | 5 September, 2014

Can't the excess energy from regen be stored in the flux capacitor?
(sorry, couldn't resist)

Iowa92x | 5 September, 2014

Barry, that one is played out. Please no more flux.

carlk | 6 September, 2014

@Iowa92x got it right with his/her first post. All others with opposing opinions did not hit the real issue of what's the most effective way to slow down or stop a car. Remember when disc brake was new and expensive many cars put it on the front only and not the rear?

Brian H | 6 September, 2014

Yep, a simple force diagram makes that clear. Should be very effective.

Mark K | 6 September, 2014

From a physics POV, It doesn't matter if it's one motor or 10.

Other than de minimis non-ideal ohmic issues with the inverter and conductors, the efficiency is the same.

From a body-dynamics perspective, you can certainly leverage the different locations for regen torque to control squat, grip, etc.

But, energy is conserved.

Anemometer | 9 September, 2014

Something not yet mentioned about the advantage of two motors and regen... you now having regen braking force applied at 2 axles. It's not really a good idea to try and suck too much power off the back wheels when it's cold and wet, or a bit icy.

However split the regen across two wheels and you reduce the friction required by each tire to slow the car the same amount , and can therefore increase the amount of energy you can reclaim without cuasing skidding or excess tyre wear. Especially considering the weight transfer under braking is to the front wheels.

I'd also imagine in heavier braking situations at higher speeds (in twisty mountain sections) you then get the advantage of recpturing more energy.

Given good driving conditions, you could start to think about changing the brake pdeal (if it's brake by wire) to use regen to start with and then progressively bring in friction brakes as a last resort.

Not sure why the inverter was getting mentioned earlier.. I'm presuming the motor genereates an AC signal that needs to be voltage adjusted, rectified and smoothed before going back to the battery management system. An inverter turns DC into AC - not what you want in regen.

Brian H | 9 September, 2014

Yeah, the inverter is generally run DC-AC, but it runs in reverse as a rectifier when doing regen. Its "name" is kept as "the inverter" for convenience, though.

eXergy | 10 September, 2014

Not sure if the X will have it, but some discussions about electric vehicles and regeneration consider possibilities for capacitor storage and there are some lithium battery/capacitor combinations available off the shelf already. The capacitor is a way around limitations of battery chemistry, where rate of charge and heat are issues.

Brian H | 10 September, 2014

No, it's not. It holds charge only briefly, in smaller amounts, and takes up much more space. Elon gave up his capacitor enthusiasm when he discovered you get the same benefits and much more with larger capacity batteries.

Good for providing or accepting brief surges.

Red Sage ca us | 17 October, 2014

I wrote, "The Tesla Model X will pioneer this realm for all mass market EVs to follow."

Well, it seems that after the Tesla Motors 'D' Event last week, the trail has been blazed -- quite literally -- by the Tesla Model S P85D. This was my experience:

My Test Ride in P85D

I really enjoyed it. I expected it, prepared myself for it, and I was still shocked by the level of acceleration. And yes, we had reached the tented tunnel before my sphincter caught up with the rest of me.

alanemay | 17 October, 2014

I believe the regen with be more efficient the same way that the AWD will be more efficient in producing drive power. It will perform a similar operation in ensuring that the regen process is performing at the optimal efficiency, distributing regen load between the two motors. The model D has been extended by 3.7% from 265 to 275 miles, depending on how this is measured, the improved regen may already be included in this figure.

milesbb | 19 October, 2014

(Not sure why the inverter was getting mentioned earlier.)

Anemometer ,
The induction motor does not generate AC voltage. An induction motor requires an AC source. If the frequency of the source is above the motor synchronous speed the induction motor is a motor and provides torque. If the frequency of the source is below the motor synchronous speed the induction motor becomes a generator and provides regen braking. Without an external voltage source the induction motor is decoupled and just freewheels. A normal rectifier would be of no value for regen braking. Then the inverter frequency is regulated below the motor motor synchronous speed the induction motor charges the battery. Then the inverter frequency is regulated above the the motor motor synchronous speed the motor drives the car forward. The inverter is key to driving the car as well as breaking the car.

Brian H | 19 October, 2014

And braking it, too.

Red Sage ca us | 20 October, 2014

When vehicles are decelerating, there tends to be dive at the front axles. This will mean that regeneration at the front wheels will be more efficient than before. More of the car's mass is creating friction with the road surface at the front than rear, slowing the vehicle more and allowing the regeneration magic to work that much better.

Brian H | 20 October, 2014

"regeneration at the front wheels more efficient than before"?? Not hard, since there was no front wheel regeneration at all before. ;p

alanemay | 21 October, 2014

I believe that the development of the 4 wheel drive technology to the production stage now provides Tesla with the platform for a Teala wagon. Image a 600hp Ford F150 equivalent or a 600 hp truck/tractor unit. This is a huge milestone for the auto industry.

Pages