Has Tesla Changed How It Controls Front & Rear Motors For Model 3?

Tesla Model 3 dual motor chassis

JUN 18 2018 BY GEORGE BOWER 42

The theory is that the rear motor is now the “cruise” motor which is opposite to Model S.

Consider this a bit of speculative forward thinking. Tesla has not said they have changed the way they control the front and rear motors. However, they have given us a few facts that may indicate they have.

  1. The LR AWD version of Model 3 gets the same range as the RWD version (310 miles) while Model S AWD got better range than RWD version.
  2. The rear motor in Model 3 is a permanent magnet motor while the front motor is an induction motor. Model S had induction both front and back.

Both facts listed above are contrary to the Model S. The Model S AWD version gets better range than the RWD version. The Model S uses induction motors both front and back.

In the Model S, the front motor has a gear ratio tuned for highway driving while the rear motor has a gear ration tuned for acceleration. The control theory in the Model S is to let the front motor do the work at cruise since its gear ratio is tuned for that and to let the rear motor freewheel (or close to it).

In general, PM motors like the rear motor in Model 3 get higher efficiency than induction motors. This is why Tesla chose a PM motor for Model 3 – to get good MPGe and range and a smaller battery required.

Therefore, it makes sense for Tesla to use the rear motor at cruise speed, rather than the front motor like in Model S. Here’s another reason: the PM motor doesn’t like to freewheel. So again, switching to the front motor and freewheeling the back does not make sense for Model 3.

My engineering partner Keith Ritter and I are currently working on computer models for all four versions of the Model 3: LR RWD, LR AWD “P”, LR AWD “non-P” and SR RWD. His modeling seems to bear out our theory as well. We should have an article soon describing the things we have learned in the process. The Model 3 model does indeed have a “cruise” gear ratio in the back and an “accel” gear ratio in front,  unlike the Model S.

Do you agree with our theory?

Let us know in the comment section.

This article was a joint effort between the author and Keith Ritter.

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42 Comments on "Has Tesla Changed How It Controls Front & Rear Motors For Model 3?"

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Viktor

I have to say that I don’t agree with you and that you just like me for long time have gott fooled by Teslas marketing. Then Tesla introduced Model S they had one motor for all there setup, both 60, 85 and P85, the only differens between then was battery voltage and inverters but then they introduced duel drive they had develop a new smaller more efficient motor. On 85D they use the new motor as both front and back and there they got better MPG and range but on the P85D there they just added a the new motor in the front and kept the old one in the back they got poorer MPG and less range. This means that only adding another motor have never let Tesla to better efficiency. For Model 3 they have just like P85D on Model S kept the motor in the back and added a less efficient motor in the front wish makes it really hard for me to see how that would give better range. If I’m wrong I am happy to see source that prove otherwise.

Terawatt

Sources are great. What’s yours..?

Pushmi-Pullyu

“On 85D they use the new motor as both front and back and there they got better MPG…

MPG? As in, miles per gallon of gasoline? News flash: Tesla cars don’t use any gasoline at all!

“This means that only adding another motor have never let Tesla to better efficiency.”

Since the dual-motor versions of the Model S have about 10 miles greater range than the equivalent single-motor versions, it’s rather strange that you would make this clearly and obviously false assertion.

https://www.greencarreports.com/news/1094911_tesla-model-s-dual-motor-is-quicker-has-higher-range-too-how-do-they-do-that

antrik

This directly contradicts statements made by Tesla, i.e. you are accusing them of flat out lying.

R.S

I personally do even expect the AWD to do slightly worse IRL and on tests. Mostly because of weight and additional drive train friction.

Probably one reason why the RWD is underrated with 310 miles EPA.

Chris Stork

“the PM motor doesn’t like to freewheel” Does that include these fancy new switched-reluctance PM motors, or just conventional PM motors? Also, perhaps they don’t freewheel, but maybe it takes significantly less juice to simulate freewheeling (IE keep up with the car’s speed but not apply any motive force)?

Perhaps some electromechanical engineer can answer these; all my magnet experience comes from the front of my refrigerator.

R.S

I am and I can answer, at least partly. IMO Tesla doesn’t use switched reluctance, but doesn’t really matter in this case.

The problem is the permanent magnet. No matter where it is, stator, or rotor, the iron in the other part will see a changing magnetic field.

A changing magnetic field will create losse, increasing with frequency. So unless you have a clutch, it will never turn off, like a induction motor.

At very high speeds you also get overvoltage problems, which are solvable with the right motor design, but still not very nice.

Kbm3

Tesla has confirmed it is a switched reluctance motor.

antrik

Actually, Musk said “mixed switched reluctance / permanent magnet”. That seems to be just a fancy description for the IPM (internal permanent magnet) design used by most modern EV power trains. Tear-down photos seem to confirm this.

Bill Howland

Yeah RS that’s nonsense, If the motor wiring is totally disconnected there will be no retarding torque. What George Bower has stated is misleading at best: Perhaps with Tesla’s control the motor drags, but the permanent magnet motor in the BOLT ev does not and the car is one of the most freely coasting vehicles I’ve ever driven.

R.S
@Bill Howald Oh sure it does, they just let it run at 0 torque, but at higher speeds, that means actively pumping current into the windings. That’s possible, but it requires energy. And while it’s theoretically possible to disconnect the windings, most EVs don’t and especially not the Model 3 with it’s SiC MOSFETs. Every MOSFET has an intrinsic diode in reverse, so it’s virtually impossible to not not have current flowing back into the battery, if the induced voltage is higher, than the battery’s voltage. You need 0 torque control for that. And even if you could disconnect, then the demagnetizing losses would still drag the car down. Unless it would be an iron-less motor. @Kbm3 I think they have an IPM with high reluctance torque. I know Elon has said switched reluctance, but the EPA certificate says IPM and switched reluctance just isn’t a really good concept, when it comes to traction motors. In a traditional switched reluctance motor only one phase carries current, but especially with Tesla’s SiC MOSFETs, where they can go for very high switching speeds, modulating a sine wave would be much easier. I know that some call the IPM with high reluctance torque… Read more »
Bill Howland
Doesn’t make sense RS – if Magnetic Circuit losses were as significant as you claim – they’d be in effect when the motor is running also and the advantage of using a PM motor would be nullified. I think people forget the main reason why ALMOST all cars have PM motors is that it is worth it to use a more efficient motor where electricity is EXPENSIVE. And there is no place where electricity is more expensive than in an electric car. If the powertrain can be made 5% more efficient overall, that means the battery can also be 5% smaller with no detriment. Companies designing the controllers are going to come up with essentially lossless arrangements – and the voltages concerned are of no problem since if they are over the rating of any given device they’ll just stack the devices in ‘poles’ as they do in Medium Voltage Drives. Now MVD’s are quite a bit more expensive than run of the mill low-voltage (600 and under) stuff, but that’s mostly due to it being a specialty product and the concept doesn’t apply to cars since cars are mass market – they making thousands of cars at a time.… Read more »
antrik

Why would fast switches favour sine waves? Sure, they allow for smoother curves — but I don’t see how that would disadvantage smooth non-sine modulation as needed for advanced switched reluctance designs?…

Having said that, photos from Munroe’s tear-down pretty much confirm that Model 3 uses IPM.

HVACman
Bill – RS has a very detailed response that is way over my pay-grade with detail on the MOSFETs in the inverter, but I’ll summarize a couple of things based on my experience with PM motors running backwards as generators for hydro turbine applications. You are right about the Bolt coasting OK, as well as I experience with my Volt. Put them in “N” (which just disconnects power to the motors but leaves the motors connected to the drive axles) and they coast just fine at speed – no auto-regen feel at all. But that said, I don’t think you want to run that way for hours and I think GM would have a cow if you towed one like that. There is likely a lot more to it. 1. “standard” PM-rotor synchronous 3 phase AC motors as used with the Bolt, the Volt and most other freewheel pretty smoothly when the field windings are open-circuit. Typically all I’ve seen with the industrial PM motors I’ve played with was bearing and windage energy losses. But these had relay-type switches that made a definitive open circuit. I did find the rotating magnetic field created an AC voltage within the windings, with… Read more »
Bill Howland
Where to start? First off, an induction motor disconnected for a non-trivial time will not provide any voltage at its terminals ( it needs to be excited to do anything, and a disconnected motor is not). Induction motors running substantially over their ‘base’ speed (funny that on EV Blogs no one ever talks about this) have decreasing efficiency due to ever lousier (read: lower) power factor. The double motor deal was used for a while by Baltimore Air Coil but I always thought such complication was silly, since dual speed motors were always commonplace, and prior to that wound rotor variable speed AC motors were used for over 100 years – the larger models even having ‘slip energy recovery’ with an ancillary motor generator set to push otherwise lost energy BACK into the power line. In HVAC fan or Trane Centrivac (i.e. variable torque) applications, since the horsepower drawn from the motor is proportional to the cube of the speed, there just a slight slowdown results in an incredible capacity decrease, and even ‘resistance’ controllers can be used with reasonable overall efficiency. I wasn’t there to witness the ‘cogging’, but any legitimate cogging being inherent in the design of the… Read more »
Bill Howland

Sorry Omega Squared R. I always state that wrong – thinking one thing but saying another.

antrik

Doesn’t the Bolt use a different motor design than the IPM design used by most others nowadays?

Bill Howland

One other thing – since the voltage measured under zero current conditions has no inductive reactance voltage drop, the voltage is going to be higher than the same motor ‘generating’ a significant amount of power back to the power line since the reactance drops the voltage seen by a voltmeter, especially if the motor is running over its base speed.

theflew

PM Motors don’t freewheel, because they generate electricity instead. That’s why cars like the Bolt have a hilltop reserve mode. If they didn’t and your vehicle started fully charged going down a large incline there isn’t anywhere to store the juice, so it gets converted to heat. Instead in hilltop reserve mode the battery only charges to 90% leaving a 10% buffer to charge as you go downhill.

jelloslug

They will only generate electricity if there is a load connected to it.

Bill Howland

More Nonesense from theflew. If you disconnect all the wiring the motor generates no electricity, although it will generate voltage proportional to the rotational speed – but then since the current is 0 the power generated is also a big fat zero.

If you start down an ‘incline’ and the battery can’t store the juice the amount of ‘regen’ backs off. There is no heat ‘generated’ since there is not much power to get rid of – which it does by trickle charging the battery – being less the more closely charged to 100% the car is. The regen amount gradually increases as the battery improves its ability to ‘take it’.

But there is nothing out of the ordinary converted to heat, dumped, or thrown out the window, or any other place you guys think the juice is thrown.

JakeY

I would not read too much into Tesla keeping the ratings the same. They are allowed to do that to reduce certification requirements even though if tested they might be different.

I believe they have done that before in previous cars (Model X? don’t remember exactly). I remember GM doing that for the Volt (not rerating the car).

Terawatt

Yes

Josh

George,

Great work.

I think you are spot on with the speculation on the drivetrain. And you probably cost me another $5k, cause that front acceleration geared induction motor is very tempting.

There is no way I can spluge for the P, but I would take dual motor over AutoPilot every day of the week.

I look forward to seeing the modeling results.

Pushmi-Pullyu

Hey, George, thanks for another great article on EV engineering!

“Do you agree with our theory?”

I don’t have an informed opinion, and I see no rational reason to doubt your analysis.

“In general, PM motors like the rear motor in Model 3 get higher efficiency than induction motors.”

I see this repeated in technical EV discussions all the time, but nobody ever cites the actual figures. Just how much more efficient is a PM motor than an induction motor? I could be wrong, but based on my readings of the subject, I get the impression that the actual difference is a very small percentage. Not that this should be ignored; even 1-2% is still a 1-2% improvement in efficiency and thus 1-2% greater range for the car.

But I wonder if it’s really as big a difference as many people seem to think.

Djoni

The BMW i3 motor claim about 97% efficiency and the Nissan Leaf is about 96% efficient.
They are PM type.
Tesla induction motors are in the 93-95% efficiency.
So it can be as much as 4% difference in efficiency, but there’s other variable at play.
PM are more compact and lighter for a given output and easier to control, but more difficult and costly to manufacture compare to induction type.
This is just my memory, but I’ll post the link when I have time.
But for a start, consider this: http://www.thedrive.com/tech/17505/the-secrets-of-electric-cars-and-their-motors-its-not-all-about-the-battery-folks

Bill Howland

Yeah people get hung up on this ‘Trivial’ efficiency improvement – but then those people don’t understand numbers. A 93% efficient induction motor has 7% loss. A 96% efficient pm motor has 4% loss – therefore the induction motor has 175% the loss that the PM motor has.

Bill Howland

The other thing is that all things being equal, a more efficient motor decreases the cooling requirement for it resulting in a much lower refrigeration requirement. In the voltec products this is realized by rarely even running the refrigeration and using free cooling when possible.

Pushmi-Pullyu

Well now, a lesser need for cooling could be an important consideration. So thanks for pointing that out, Bill!

I’m not so sure the difference between a 93% efficiency and a 96% efficiency in just one component of the powertrain, considered in isolation (and ignoring any need for cooling), is all that significant. After all, the mechanical powertrain in the (original) Tesla Roadster was only about 80% efficient (we can only say “about” since the efficiency is slightly different at different speeds). No doubt Tesla has improved mechanical efficiency since then, but I doubt it’s been improved all that much.

Bill Howland
You are welcome, although your response is either incorrect or is worded sloppily. The mechanical power train – meaning the gears, couldn’t possibly less than 92% efficient since then it would require plenty of additional cooling. Now overall, at HIGH LOADING (in other words, not all the time, but just at the track) the efficiency of the OVERALL POWER TRAIN (including battery, inverter, induction motor, and reduction and differential gearing) would drop to 80%. IN fact, in those circumstances it drops to under 50% since the car, while EPA rated at 244 miles COULD drop to around 58 miles if driven hard, confirmed by the TOP GEAR Show, and also informally by my friend Brian’s test drive of it. The “S” I wouldn’t think would even have the ‘Mechanical powertrain’ efficiency of even the roadster, since to these eyes the robustness of the gearing in the “S” is not as beefy as it was in the final roadster production models. They learned their lesson in the Roadster by not going excessively cheap on the gear box – and in that sense it may be considered a perfected design – after burning out the original 2 speed gearbox which was quickly… Read more »
antrik

The drop in range when “driven hard” is due to air drag etc., not dropping power train efficiency…

Pushmi-Pullyu

@Djoni:

Thanks for taking the time to reply!
🙂

HVACman

Tesla’s induction motors typically are about 90-91% efficient at their “sweet spot” RPM. PM motors are about 95% efficient and have a bigger sweet spot for both efficiency and torque, which gives you more flexibility for gearing the EV for both good acceleration and maximum highway- speed efficiency. Net result is that PM motors will probably give an EV about 5-7% more efficiency overall for the same kWh, which means either more range or less battery.

Keith

Pushmi-Pullyu

That’s a significantly larger difference than I expected. Thanks, Keith!

offib

I thought it was permanent magnets on the front!?

antrik

Nope, they explicitly said front is induction.

EaglesPDX

Or they made the 2 Wheel Drive more efficient so there is no measurable difference.

That would kind of make more sense as the less weight and gear needing to be wound up of the two wheel drive should have been more energy efficient than AWD.

antrik

Yes, they made the RWD more efficient than Model S/X RWD, by using an IPM motor. Not sure what your point is, though?…

Gasper

I 100% agree with this theory. As far as I know Toyota uses same principle with e-four, only that front and rear are motors are switched. Use the induction motor for starting, low speed and improved tow rating and disconnect it at higher speed and/or cruising. It’s the only possibility, you can’t over rev PM motor so you can’t have it low geared and disconnected at higher speeds.

Evz3

Quite sure, the model 3 uses a IPM motor, not a regular PM. IPM allows Field weakening, this allows the motor to reach about any rpm regardless the battery Voltage. Just like the induction motor, although it needs its rotor to Be magbetised by using an electrical frequency that is Higher or Lower than the mechanica frequency.