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PMAC vs induction motor for model 3

Discussion in 'Model 3' started by No2DinosaurFuel, Oct 7, 2016.

  1. No2DinosaurFuel

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    there is a lot of reasons for tesla to keep the drive train the same or at least similar to the model S/X but from what I have read in my research on electric motors induction motors are not as efficient as the PMAC. Size wise the PMAC can also be smaller, provide more torque, and will run cooler. The only down side is you have to obtain the rare earth magnets from somewhere. And if you over heat them the magnets might be demagnetized.

    The Chevy Bolt uses a PMAC and it gets EPA 238 miles on the same 60KWh energy that can only propel the model s to 218 miles on the dual motor version or 210 on the single motor version. Countless other EV are also using PMAC or PMSM which is pretty much the same thing.

    Now some are arguing the highway mpge of the bolt will be less than the model 3 but we don't know that yet. All we know is there is 2 numbers and when you scale one to 55% and another to 45% and add them you get the final mpge and use that to get the expected range of 238 miles.

    I guess my point is if tesla started using PMAC instead of the induction motor to begin with maybe the original model S couldve gotten 300+ miles EPA range right off the bat. And closer to 400 miles with the new 100Kwh battery.
     
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  2. Red Sage

    Red Sage The Cybernetic Samurai

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    Notice: The company is named 'TESLA'. Better to use the electric motor design that was patented by the company's namesake. Further, GM used the type of motor they did because it was off-the-shelf from another manufacturer, would not produce power sufficient to entirely embarrass the ICE powerplants in either CRUZE or MALIBU, and it could be mated to an off-the-shelf CVT. GM will NEVER have a version of that motor capable of performance to rival LUDICROUS, INSANE, SPORT, or even NORMAL operation of a Model ☰.

    Those who presume that using front wheel drive and wimpy motors will result in exceptional range are woefully incorrect. It doesn't work that way at all. That's why none of the many 24 kWh battery pack compliance cars had a 150+ mile EPA rated range.
     
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  3. Booga

    Booga Member

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    The other factor is that the Bolt is 3,580 pounds while the Model S is 4,608 pounds given that it is a larger car to begin with. Could the difference in distance vs battery size be partially a result of this given that the Model S weights about 29% more? In fact, it makes me wonder if it is actually more efficient (though I know their motors are supposed to be less efficient) at the end of the day because it's not 29% behind in range.
     
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  4. mspohr

    mspohr Active Member

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  5. mspohr

    mspohr Active Member

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  6. Saghost

    Saghost Active Member

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    Apples and Oranges.

    Another data point for you: my 2016 Tesla Model X with two induction motors has the same EPA efficiency rating as my 2012 Volt with one permanent magnet motor did, despite being some 1300 pounds heavier, eight inches taller and seven inches wider.

    In theory PM motors are more efficient at peak power, but induction is often more efficient below that since it can vary the field strength. What matters is total package efficiency, and for their size and weight Tesla has always done very well - a trend I expect to continue.

    Permanent magnets cost money. The motors are more expensive - not what Tesla needs when they are trying to make a profit on the cheapest long range EV ever built.
     
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  7. No2DinosaurFuel

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    DC brushless is definitely more fragile and are less reliable. But from my research size for size, PMAC is definitely better. More power and torque.the only reason PMAC unfavorable to induction is the lack of magnets in induction motor which is cost saving.
     
  8. No2DinosaurFuel

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    I think PMAC is overall more efficient at any power. In fact induction motor is not efficient at low rpm speed.

    Yes I agree PMAC is more expensive compared to induction but if tesla want to squeeze all the range it can out of the battery they might be better off putting a smaller battery and more efficient drive as oppose to bigger battery and less efficient drive.

    I guess I can all dream...
     
  9. mspohr

    mspohr Active Member

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    To quote the great Lebowski: "That's, just, like, your opinion, man."
    The articles discuss why it's not a simple question to answer. Like all engineering, there are trade-offs.
     
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  10. No2DinosaurFuel

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    All my research so far have shown PMAC are more powerful and more torque at the same size. And it runs cooler. So I am not sure what you mean by wimpy. I am sure the motors in the bolt can handle quite a bit but they probably limit them. And about your 150 miles on 24KWh. I don't think anything can do that. I am talking about 238 miles at 60KWh. If you use the rear wheel drive s60, then that gets 210 miles out of the same 60KWh. Sure it is a heavier car. But the bolt is horribly non aerodynamic. Remember EPA is EPA. You are still losing 28 miles difference between the two vehicle and I am sure weight aero and front wheel drive vs rear wheel drive can explain all of it.
     
  11. No2DinosaurFuel

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    Of course there are trade offs. Like trading more battery for slight cheaper induction motors? Not a good trade off IMO. And It doesn't change the fact tesla seems to be using less efficient tech. If tesla's goals is to squeeze every bit of range out of their car they would choose the most efficient drive system. I mean they are trying their best to increase aero. If all it took was to start with efficient drive system and then later improve aero then all the better.

    And by the way it is not my opinion. It is scientific facts... Unless tesla is not publishing something about their drive system.
     
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  12. Saghost

    Saghost Active Member

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    There's a lot of speculation about numbers, but I think it's safe to assume that once the gigafactory is fully operational Tesla will have the least expensive batteries in the industry (I think they do now, too.)

    So from a cost optimization standpoint more battery probably makes more sense for them even if your assumptions are correct.

    Having said that, I'm pretty sure you're wrong about this. At low power settings and high rpm (the most common drive condition for an EV,) back EMF eats a lot of efficiency - the motor really wants to be a generator.

    An induction motor can reduce this loss somewhat by running at reduced field strength (adjusting the input voltage and current to get the power needed without building as much back EMF.)

    The PM motor can't do that because it gets the field from the magnets.

    Dual motors add more options - an induction motor with no field is just a spinning mass, so by using induction Tesla gains the ability to switch one motor out entirely with no EMF losses and operate the other one at a higher power point if that is more efficient (Tesla calls it torque sleep.)

    A PM car couldn't do that - or at least wouldn't gain as much from it.

    Finally, I'm thinking Tesla may go after the toad market with the 3 (and/or Y). By using an induction motor, in theory they can build the car to be flat towed safely.
     
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  13. ecarfan

    ecarfan Well-Known Member

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    Okay, got it the first time you said that, and the fourth time.

    Tesla is not using PMAC motors and there is no indication Tesla ever will.
     
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  14. No2DinosaurFuel

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    Well some people, not saying you but point to others, obviously have difficulty reading because they seem to think it is my opinion.
     
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  15. trils0n

    trils0n 2013 P85

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    The interview @mspohr posted included this very discussion. Including increasing expense in motor with less batteries, so they already solved for that, with all their other performance and cost considerations.
     
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  16. No2DinosaurFuel

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    #16 No2DinosaurFuel, Oct 7, 2016
    Last edited: Oct 7, 2016
    I am not sure you understand back emf. All this does is prevent you from spinning faster. You need more voltage to spin faster. This doesn't mean you are less efficient. You are not pushing anymore current because you are limited by the Back EMF. Unless you are trying to do field weakening at the cost of efficiency. But you can solve this by gearing things down so you don't have to spin as fast. Induction motor is inherently less efficient. You have to drive 2x. Once to generate your field and once again to counter the one you just generated to generate the push force needed to spin the motor to put it in simple terms. This means more heat because more wires and more current. PM you already have the field so there is no need to generate the initial field.
     
  17. No2DinosaurFuel

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    #17 No2DinosaurFuel, Oct 7, 2016
    Last edited: Oct 7, 2016
    I think way back in the 2000s, Rare Earth Magnets were "rare" and hence expensive. Tesla probably didn't want to be at the whim of the chinese suppliers so they probably chose to use induction motors where they were not limited by how much rare earth they would need. But today, the rare earth magnets are much cheaper. So I am not sure how much "increasing expense in motor with less batteries" argument will hold up today. But then again maybe the price will go up again. But that can be said the same with the nickel/colbalt/lithium of lithium battery.

    Near term I definitely do not see tesla moving to PMAC. But long term, definitely after the Model 3, who knows.
     
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  18. trils0n

    trils0n 2013 P85

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    So do you think they stopped doing that type of analysis mentioned in the article after the 2000s? Even when they started making a new, smaller motor design for the D cars? Pretty sure they did the same type of analysis for their new motor design, to optimize cost, efficiency and performance of the motor, and system as a whole, so including batteries. That is the impression I got from the article, which was published in Jan 2016. They design lots of motors very quickly and can pretty easily model all these things before actually going to make prototypes or go to production. Seems pretty clear that PM motors have some factor that isn't desirable for Tesla's goals.
     
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  19. No2DinosaurFuel

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    Don't get me wrong if I was running tesla, I would not change the motor either. Not at this point. Not when they need to build 500K vehicles in the next year.

    To address the comment on about motor redesign, I am pretty sure it had to do with cost of redesign of the drive system. Sure you can quickly retro fit the motors and put a new controller. But they would have to test it for millions of miles. It is not something Tesla can afford right now. Look at what happened to the early cars with motor issues. Can tesla afford something like that again for another year or so? Also I don't think they did any revision of motor design for their D models. They wanted simple plugin and play setup. I think this is the sole reason they did not change the motor. Heck look at how they mounted the 2nd motor. They just placed it on top of the battery and remove the fronk space. They were set on keeping the big rear motor for non-AWD version as well as the AWD Performance version. It would be a major redesign and software nightmare to drive 2 different types of motor systems. Again it's cost and time argument. So I don't think I agree with the PM motor having some factor that isn't desirable to Tesla's goals. The only factor is they chose one design and now they have to stick it out.

    This is akin to what Google went through with the Java VM fiasco with Oracle. In the beginning they chose to use Java for android. Migrating everything at once to something else prove to be too big of a task even for the mighty Google. This would've have broken everything they built previously. But recently they have been moving slowly away from Java.

    I suspect Tesla, in the long term, slowly migrate away from induction motor.
     
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  20. techmaven

    techmaven Active Member

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    I don't think you actually read Walter Rippel's blog post.

    Also, there are a few other articles on the subject, but most are oriented towards low power, low speed applications. Here's one for a low end hybrid:

    http://www.parkermotion.com/whitepages/Comparing_AC_and_PM_motors.pdf

    At under 4,000 rpm, the PMAC was more efficient, but at over 4,000 rpm, the induction motor was more efficient. BTW, 4,000 rpm is all of 36.5 mph on a Model S. Note that this paper, like many, do not explore the performance regions where Tesla's motors often operate.

    ACIM, PMAC, PMDC motor efficiency and electric cars

    There's a reason why there you don't see any other EVs with big PMAC motors. Tesla's motors range from 193 kW to 350 kW.

    Here are some permanent magnet motors in EVs:
    Nissan Leaf: 80 kW
    BMW i8: 96 kW
    Chevy Spark EV: 105 kW
    BMW i3: 125 kW
    Chevy Bolt motor: 150 kW
     
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