Q&A with Tesla’s lead motor engineer (Full Interview)

Tesla Motor Engineer Q&A

The principal motor engineer at Tesla describes why modeling and optimization is so vital to its design process.

Creating a start-of-the-art electric vehicle requires a deep understanding of all the components. More importantly, it requires a continual process of analysis and optimization of the components to push the limits of driving range, efficiency, performance and cost reduction. The internal combustion engine has had the benefit of millions of man-hours in engineering analysis and refinement over the past century, while the collective engineering effort of the EV industry has just begun.

It comes as no surprise that Tesla, the EV trailblazer, spends a considerable amount of resources on internal R&D to develop better parts for EVs, and that its testing facilities and engineering talent are at the forefront of the industry.

As Tesla’s Principal Motor Designer, Konstantinos Laskaris is responsible for the electromechanical design and optimization of the company’s existing and future traction motors. Before joining Tesla, Laskaris earned a PhD from the National Technical University of Athens, Greece. There he combined advanced methodologies and developed algorithms for motor geometry optimization.

Charged recently chatted with Tesla’s motor guru to learn more about the process the company uses to continually evaluate and optimize motor design choices.

Charged: In general, how are electric motors inherently better for traction applications than combustion engines?

Laskaris: When you simply compare any other high-end conventional car to a Tesla, you see a tremendous difference. This is because of the technology.

As for the motor, specifically, there is a huge efficiency advantage, and it is extremely quiet and vibration-free, with very high power density and instantaneous direct response to inputs. All these characteristics of electric motors give an unparalleled performance advantage.

Tesla-Motor-Engineer-QA PQ2

This is why it was so important for Tesla, as a company, to break the stereotype that’s been out there for years. People needed to see that performance, efficiency and range can coexist in an EV. The dual-motor powertrain Model S is the fastest sedan that has ever been mass-produced. The total motor power exceeds 700 hp, and it spins as fast as 18,000 rpm – speeds that we previously only found in Formula 1 racing vehicles.

You could say that the electric motor is magic from the perspective that it awakens the soulless car.

Tesla Motor Engineer Q&A2Electric motor cutaway on display at Tesla headquarters in 2013 (Windell Oskay – CC BY 2.0)

Charged: When Tesla decides to change a parameter of its vehicles – like increase the peak battery current or add towing capacity – what does that mean for your motor design team? Do you have an iterative design process? 

Laskaris: At our factory in Fremont, we manufacture practically every aspect of the car in-house. We have a motor winding and manufacturing facility, so we can optimize every aspect of our motor manufacturing and control the quality of the product. Also, we can implement changes in production very fast, we’re a very agile company from that perspective.

Tesla Motor Engineer Q&A3

We can generate motor geometries and analyze them with finite element analysis very quickly. We have a big computer cluster with more than 500 core processors that run finite elements – a typical personal computer has two cores, maybe four. That means you can create many virtual models in parallel and do a very large number of calculations. Basically it allows us to solve the loss and efficiency maps very fast and see – according to any metrics that we created – how good any motor design is for an application we’re designing for.

Charged: There seems to be an endless array of electric motor topologies, architectures and configurations. How do you begin to evaluate and compare all of the possible options?

Laskaris: Understanding exactly what you want a motor to do is the number-one thing for optimizing. You need to know the exact constraints – precisely what you’re optimizing for. Once you know that, you can use advanced computer models to evaluate everything with the same objectives. This gives you a panoramic view of how each motor technology will perform. Then you go and pick the best.

With vehicle design, in general, there is always a blending of desires and limitations. These parameters are related to performance, energy consumption, body design, quality, and costs. All of these metrics are competing with each other in a way. Ideally, you want them to coexist, but given cost constraints, there need to be some compromises. The electric car has additional challenges in that battery energy utilization is a very important consideration.

Everyone will have a different perception of what tradeoffs should be made. How much driving range are you willing to trade for faster acceleration, for example? Once these parameters are set, you can begin to evaluate options and optimize.

Charged: You have a background in creating the algorithms that allow computers to simulate how a motor will function in the real world. How do these simulations translate into better vehicles?

Laskaris: The mathematical modeling technology, or methodology, that you use is very important and has tremendous impact on the success of electric vehicles. When I say “modeling,” I mean to understand the mathematical principles behind a system and then create software tools that will represent accurately how it will act in the real word.

Tesla Motor Engineer Q&A4Tesla’s electric motor rotor in 2007 (Tinou Bao – CC BY 2.0)

Accurate motor modeling is so important because through it we can evaluate a hypothetical motor before we produce it – the losses, performance capabilities, torque ripple, thermal management, and anything that we are interested in to classify how good or bad is a motor. And, in this way, we avoid unnecessary prototypes and unpleasant surprises.

Beyond that, through good motor modeling, we can achieve the best optimization – which means we can achieve exotic performance without the use of exotic materials and exotic manufacturing methods.

Optimization is the art of being able to navigate through different motor candidates to see what is good and what is bad, and by how much. As you begin to do optimization, you realize that without good modeling, it is meaningless. This is because the process would be based on a bad representation of the motor, and in the end the motor wouldn’t be truly optimal.

Charged: Could you give us an example of some tradeoffs that you would use modeling to optimize for?

Laskaris: Yes. A large portion of the time people spend driving is in low-torque highway situations. However, there are a lot of motors that offer great 0-60 MPH performance but are very inefficient in the low-torque highway-speed regions. So the question is, can I have everything – both high efficiency and high performance? The answer, unfortunately, is no. But you can make intelligent choices between things that are competing with each other.

Tesla Motor Engineer Q&A5

This is the beauty of optimization. You can pick among all the options to get the best motor for the constraints. If we model everything properly, you can find the motor with the high performance 0-60 MPH constraint and the best possible highway efficiency.

Another example is the overall motor efficiency versus its cost. There are cases where making a motor in more expensive ways could potentially increase efficiency and buy off multiple times the cost difference by saving money on the battery, or other aspects of the car. So, if you are able to model the motor efficiency and costs accurately, you can plot it against battery cost savings. Now you can see that the optimal motor for total cost minimization is often different from the cheapest motor.

These all come together to form the characteristic metrics of the car that you want to build. It’s a general approach of how we start from parameter design and end up with ultimate configuration.

Charged: At what point do you perform physical prototype tests to verify the results from your virtual models?

Laskaris: We do many verification tests before there is even a prototype for a specific application. They are what we call characterization experiments. And they allow us to get a known correlation point and to see if the isolated modeling tools are in sync with reality. So it’s a back-to-back comparison between what the model predicts and what is actually measured. It might not even resemble a motor, it might just be a piece of iron spinning, for example.

We then, of course, build and test full prototypes as well.

 

This article originally appeared in Charged Issue 21 – September/October 2015. Subscribe now.

  • Wade

    I’ve never read or heard why Tesla chose induction motors over permanent magnet motors. I am not a motor expert by any means, but I would love to know what the pros and cons are. Obviously, in the age of VFD’s, both types are going to work well. It’s more about the Drive than the Motor.

    • Dent Arthur Dent

      It could well have been because of future supply-chain considerations.

      There was a piece on NPR a few months ago about how wind-power turbines rely most easily on permanent magnet generators (for the ease of construction and efficiences associated with its use), and that the building materials of permanent magnets are rare earth materials that, while relatively easy to obtain presently, are a finite resource that may soon become, well, rare. Same thing with permanent magnet motors.

      This report says as much, and even speculates on the possible reasoning for Tesla’s motor design, as regards Induction vs. Permanent Magnet motors (see Section 2.1.1. line 141):

      http://energy.gov/sites/prod/files/2015/02/f19/QTR%20Ch8%20-%20Critical%20Materials%20TA%20Feb-13-2015.pdf

      • Wade

        Thanks that makes sense. Overall, size isn’t that big of an issue given the Tesla form factor. And the technical challenges are managed by the control/drive, which it seems Tesla has worked through long ago. And since Tesla is winding it’s own motors, they don’t need rare earth metals like you mentioned. I suspected that could be the reason, but hadn’t read it anywhere.

    • Mike

      Induction motors are a bit less efficient than permanent magnet motors because you use part of the electric current to induce a magnetic field in the rotor. With permanent magnet motor the magnetic field is there for ‘free’ so to speak. There are couple of downsides to PM motors. They require a few pounds of expensive neodymium magnets (which can also be hard to get depending on political climates). A permanent magnet is also susceptible to demagnetization – If you reach a particular combination of temperature and magnetic flux through the magnet you can partially or completely demagnetize the rotor of a PM motor. And this isn’t just a theoretical concern. I have partially or completely demagnetized a number of PM rotors in design validation and durability testing.

      • Eco Logical

        Mike, PM motors are more efficient at low RPM when accelerating as you pointed out “due to rotor excitation”, but at higher RPM when cruising the core loss in the stator becomes significant “due to high frequency – high magnetic flux switching”. An induction motor actually has a wound field rotor so when less torque is required the excitation power and magnetic field strength is reduced therefore reducing the core loss in the stator (exponentially). The result is; induction motors are actually more efficient than PM motors when cruising (i.e. light load) at highway speed. One caveat, the Tesla X has a fixed ratio gearbox so motor RPM is directly proportional to speed. Hybrid cars typically have a transmission (CVT) that allows control of motor RPM vs speed. A PM motor in that situation can be more efficient at highway speeds since the motor RPM can be reduced through the transmission.

        • mhompg

          The bigger electric motor that is primarily used for propulsion in a Toyota Prius or Ford hybrid actually has a fixed ratio to vehicle speed. The smaller motor that is primarily used as a generator has a variable ratio to vehicle speed when the gas engine is running.

          On parallel hybrids from (older Honda hybrids), VW group, and Hyundai/Kia the electric motor is attached before a transmission and thus can have a changing ratio to the wheels.

          The GM full hybrids have a separate power-split mode that reduces the rpm of the large electric motor at highway speeds.

          • CalleQ

            I’m sorry but in the Prius the electric motor, the combustion engine and the wheels are interacting in a “planet gear” gear box. It does not have a fixed rpm as related to wheel rotation speed. https://en.m.wikipedia.org/wiki/Hybrid_Synergy_Drive

          • mhompg

            I said “the bigger electric motor that is primarily used for propulsion in a Toyota Prius or Ford hybrid actually has a fixed ratio to vehicle speed.” The big motor is known as MG2 in a Prius transaxle. My statement is correct.

            In the Wikipedia description that you link to it says: “One of the motor-generators, MG2, is connected to the output shaft,…” and “In Generation 1 and Generation 2 HSDs, MG2 is directly connected to the ring gear, that is, a 1:1 ratio….”. Under the “Prius Platform Generations” heading it says “the wheels are connected to the ring gear” and there is an accompanying illustration showing the ring gear of the power-split planetary gear connected directly to MG2 which is also connected directly to the wheels. Whenever the wheels turn, MG2 and the ring gear turn as well in a fixed gear ratio to each other (there is an additional gear reduction in the differential between the final output shaft and the wheels). In the later Prius generations they replaced the 1:1 gearing between mg2 and the ring and output shaft with a roughly 2.5:1 gear reduction so that mg2 spins faster than the ring and output shaft but it’s still a fixed gear ratio. MG2 always turns at a fixed ratio to the wheels.

      • neroden

        FWIW if you’re making small, light, low-power motors you can use cheaper magnets and avoid the neodymium. I think permanent-magnet motors will probably be popular for tiny motors.

        Wind turbines and other rotary electric generators (hydro, etc.) require very strong permanent magnets for their generators and are going to be eating up the neodymium magnet supply. Probably best to avoid neodymium magnets in other applications.

    • ecfl

      There’s no simple answer to this but generally speaking different types of well-designed machines compete pretty well. Three factors that are relevant are torque rating of motor, percentage of time at different torque/speed operating points, and of course production volume. As the torque rating of the motor goes up, then the fact that the induction machine requires current to create the field becomes less and less important because it becomes a smaller and smaller fraction of the total losses of the motor. This is partially why most power plant electric machines have wound fields. They are cheap to manufacture and the field losses are insignificant. The next factor is where in the operating range most of the time is spent and therefore where are most of the losses accumulated. Generally speaking a well designed PM will have better part load efficiency than a well-designed induction machine. Again, this is partially related to the field current requirement. So for utility-scale wind turbine applications in near constant wind speed areas (e.g. off-shore) which have cubic power laws virtually all the value is up near 100% operating speed and power, so induction machines and PM machines can compete pretty well, but if you are in a highly variable regime there might be good reason to favor a PM. But the last factor is production volume, and the materials in an induction machine are cheaper and there is much more history of automation in low cost high volume manufacturing so at scale induction machines can be cheaper especially if you have power density requirements where need to use very high energy product and expensive magnets. Also designing PM rotors at high rotor tip speeds gets trickier and for Tesla’s motor they want to operate at 18,000 rpm which somewhat higher speed than most of the commercialized hybrids on the road using IPM machines.

      • mhompg

        I’m under the impression that PM motors are also generally have greater volumetric torque density which is why they are typically used inside of space-constrained packages like a Prius power-split transaxle.

    • tsport100

      The induction motor powertrain used by Tesla has it’s roots in the original GM EV1 from 25 years ago. Designed by Alan Cocconi while working at AeroVironment, the EV1 induction motor was based on well knows 400 Hz specs and when later combined with a copper rotor achieved torque density comparable to PM motors. Tesla Motors was founded in 2003 around an AC motor design licensed from Cocconi and all Tesla motors are evolution of this 400 Hz + copper rotor induction motor design.

      • Дима

        Is it 3-phase Y-shaped or Delta-shaped or some sort of combined winding?
        I’ve read that combined windings are more efficient.
        http://www.electromotors.info/sovmobm.html (in russian, google can translate) There are lots of stories and videos in russian about rewinding asynchronous motor which presumably gives it more torque and makes it more efficient.
        Can you comment on this, can it be true?

  • Wade

    I’ve never read or heard why Tesla chose induction motors over permanent magnet motors. I am not a motor expert by any means, but I would love to know what the pros and cons are. Obviously, in the age of VFD’s, both types are going to work well. It’s more about the Drive than the Motor.

    • Dent Arthur Dent

      It could well have been because of future supply-chain considerations.

      There was a piece on NPR a few months ago about how wind-power turbines rely most easily on permanent magnet generators (for the ease of construction and efficiences associated with its use), and that the building materials of permanent magnets are rare earth materials that, while relatively easy to obtain presently, are a finite resource that may soon become, well, rare. Same thing with permanent magnet motors.

      This report says as much, and even speculates on the possible reasoning for Tesla’s motor design, as regards Induction vs. Permanent Magnet motors (see Section 2.1.1. line 141):

      http://energy.gov/sites/prod/files/2015/02/f19/QTR%20Ch8%20-%20Critical%20Materials%20TA%20Feb-13-2015.pdf

      • Wade

        Thanks that makes sense. Overall, size isn’t that big of an issue given the Tesla form factor. And the technical challenges are managed by the control/drive, which it seems Tesla has worked through long ago. And since Tesla is winding it’s own motors, they don’t need rare earth metals like you mentioned. I suspected that could be the reason, but hadn’t read it anywhere.

  • http://mysmartelectricdrive.blogspot.ca/ Smart Electric

    Great article, excellent read for EV geeks (like me).

    • Hamalot

      Agree. Am learning so much from the comments too. Quality stuff here.

      • Arthur

        You’re all a bunch of master debaters.

  • Steve backer

    Got some real pros visiting here. I have a burning question regarding the Tesla Motors version of the induction motor. Too much back and forth to post. If you’d care to contact me at
    disqus at hottrainingmaterials com. Thanx!

  • Arthur

    Tesla employs only big fat liars.

    For example: “At our factory in Fremont, we manufacture practically every aspect of the car in-house.”

    What a big fat lie by the Greek. Over 50% of the Model S, by part content worth, is made overseas.

    The Greeks should be ashamed of this Konstant LaskaLiar

  • Willian Bueno

    Nice article! One thing that I would like to know more is the EV Mission Profile in a real world. Other company cars had enough time to collect millions of hours in customer vehicles to determine “How people drive”, an that was translated for example in homologation cycles.

    Off course, it is hard to define a common “way of driving” as people tend to drive in different ways (what is much easier on trucks as trucks drivers are always trying to have the best fuel consumption vs performance factor).

    Knowing mission profiles is a very important input for modeling as hinted on the article above.

    The question is: Because EVs have different autonomy, different acceleration, different behavior when idle (no engine running like Start stop cars), access to dedicate lanes in some states…Can we assume the same mission profile of a gas car for a EV?