Tesla’s top motor engineer talks about designing a permanent magnet machine for Model 3

Most mainstream EVs from major automakers have used some form of permanent magnet traction motor technology, with two high-profile exceptions: Tesla’s Model S and Model X both use induction motor technology. Internet engineering forums are full of compelling arguments for using both technologies in vehicles, as well as loads of speculation on what drove Tesla to favor induction machines from day one.

The best explanation I’ve read is one based on historical factors. By many accounts, the reason Tesla started developing an induction motor in the first place is because it inherited the design from AC Propulsion. The induction motor used in the Roadster actually had roots going all the way back to GM’s EV1 motor, which was designed by Alan Cocconi. Cocconi based it on existing AC induction motor specs. Tesla initially licensed the design from Cocconi’s company, AC Propulsion. However, Marc Tarpenning later said that Tesla had completely redesigned its induction machine “a year before we were in production…long before we were even into the engineering prototypes.” A lot has changed since those early days in terms of available R&D technology and material costs.

In any case, Tesla’s exclusive use of induction machines has been intriguing to motor experts and EV techies. So, it was particularly interesting in August 2017 when Model 3’s EPA certification application revealed a big change in the powertrain – the document states that Model 3 uses a 3-phase permanent magnet motor.

Tesla rarely offers official comment on technology decisions, so it can be hard to discern the exact engineering thought process. Luckily, I recently moderated a keynote panel discussion of EV tech experts at the Coil Winding, Insulation & Electrical Manufacturing Exhibition (CWIEME) in Chicago. The panel included Konstantinos Laskaris, Tesla’s Chief Motor Designer, so I had the chance to pry a few more details from the world-class motor engineer.

As usual when interviewing engineers from major automakers, Laskaris wasn’t at liberty to tell us all about the specific quantitative analysis that led to choosing a permanent magnet motor. However, he did offer some interesting insights into the engineering processes the company employed in the analysis.

The following is a transcript of our on-stage conversation, edited slightly for clarity.

Konstantinos Laskaris: It’s well known that permanent magnet machines have the benefit of pre-excitation from the magnets, and therefore you have some efficiency benefit for that. Induction machines have perfect flux regulation and therefore you can optimize your efficiency. Both make sense for variable-speed drive single-gear transmission as the drive units of the cars.

So, as you know, our Model 3 has a permanent magnet machine now. This is because for the specification of the performance and efficiency, the permanent magnet machine better solved our cost minimization function, and it was optimal for the range and performance target.

Quantitatively, the difference is what drives the future of the machine, and it’s a tradeoff between motor cost, range and battery cost that is determining which technology will be used in the future.

Laskaris later added: When you have a range target [for example], you can achieve it with battery size and with efficiency, so it’s in combination. When your equilibrium of cost changes, then it directly affects your motor design, so you justify efficiency in a more expensive battery. Your optimization is going to converge on a different motor, maybe a different motor technology. And that’s very interesting.

The EV technology expert panelists at the CWIEME Chicago event included, from left to right, Jaydip Das – Carpenter Technology Corporation, J. Rhett Mayor – DHX Machines, Konstantinos Laskaris – Tesla, Peter B. Littlewood – Argonne National Laboratory, Tom Prucha – Protean Electric, Matthew Doude – Mississippi State University, and Christian Ruoff – Charged Electric Vehicles Magazine.

If you combine these comments with those made by Laskaris during our 2016 on-stage interview at the CWIEME event in Berlin, you begin to see a clearer picture of the sophisticated process Tesla uses to evaluate different motor designs and optimize them for the specific desired parameters of each vehicle.

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.

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 constraint and the best possible highway efficiency.

To find answers for these tough engineering questions, Tesla uses sophisticated custom computer simulations that run custom algorithms designed in-house. Before joining Tesla, Laskaris earned a PhD writing the programs that allow computers to more accurately predict how different motor geometries will perform in the real world – a handy skill when you’re trying to evaluate countless motor topologies to find the best.

The next CWIEME event is in Berlin, June 19th to 21st, 2018. I’ll see you there.


Konstantinos Laskaris, Tesla’s Chief Motor Designer

SEE ALSO: (2015 Interview) Tesla’s motor guru says simulations are key to avoiding exotic materials and expensive manufacturing methods


This article originally appeared in Charged Issue 35 – January/February 2018 – Subscribe now.

  • Steve bakker

    Great reporting. Did Konstantinos by any chance elude to the PM motor as being a reluctance design? That’s the latest rumor.

    • Kurt Jenney

      PM motors and reluctance motors are completely different tech. A reluctance machine doesn’t use permanent magnets. Instead, it uses “magnetic reluctance”.

      • Karen

        Not an SRM; a PMSRM.

        • Kurt Jenney

          Interesting. I haven’t seen hybrid PM augmented SRMs before. 🙂

          • Karen

            New type of motor 🙂 I mean, there’s been hybrid PM/SRMs before (IPMs), but not like this.

          • Raphael Sturm

            AFAIK it is just an IPM with high reluctance torque, BMW actually uses one like that in the i3.

            Pretty efficient motors, that are cheaper to produce, since they have less permanent magnets, but still have more torque than the SRM.

            There have been some rumors that they would have put the PMs in the stator, instead of the rotor, but the window sticker clearly says IPM.

            Here is a nice little article on the 2012 patent application by BMW:

            But AFAIK BMW didn’t get the patent.

          • Karen

            Not according to Ingineerix, who asserts that it’s a PMSRM, which is not the same thing as an IPM.

            The Model 3 sticker also asserts that it has a 310 mile measured 5-cycle range, but that’s not true either. I guess we’ll find out more details when the first motor teardown occurs.

          • Raphael Sturm

            I guess so, but I really doubt Tesla used one. The efficiency and torque densities just aren’t there.

            Also, where does he say it’s an PMSRM? I watched his 3 videos and he even said BLDC (which would be wrong as well, pretty sure they at least emulate a sine wave.).

          • PieroB

            Apparently rare earth magnets placed in the stators to smooth things out is the purported difference.

          • Raphael Sturm

            That’s what some person has thought up, based on one paper some person has written.

            I really doubt Tesla would pack rare earth magnets in the stator. It makes manufacturing way more expensive and it would place the magnets in the hottest place of the motor and they don’t like heat.

            I am 99% sure the magnets are in the rotor, where they actively increase power density, keep relatively cool (this isn’t an induction motor) and can’t be broken, when the rotor and stator are married.

          • Raphael Sturm

            Edit: I read some of his reddit posts and apparently he first called it a SRM and then corrected himself by saying it has permanent magnets in the rotor.

            So he probably means a PMSynRM.

      • James Rowland

        It seems studies of switched reluctance motors with permanent magnet assistance have been in the literature for a few years. It’s all behind paywalls so I haven’t read any of it, but the abstracts mention improved efficiency and torque compared with pure switched reluctance designs.

        Tesla might be the first to use this approach in a production car’s traction motor.

  • Billy

    I’m keenly interested in the rumors of a hybrid switched reluctance motor. If true, Tesla may be years ahead of the competition on motor tech.

    • Raphael Sturm

      I think they use it, but BMW has used those in the i3 for years now.

      The benefit is simply having less permanent magnets in the rotor, since there is a not insignificant amount of additional reluctance torque. The downside is, that the motor can’t produce as much torque, but especially for a car I think an IPM with high reluctance torque is the best solution.

  • Eddie Wearing

    Do you guys mean synchronous reluctance rather than switched reluctance (called variable reluctance in the US)? I’m not sure that a PM – switched reluctance machine would be a good idea due the flux reversal.
    BMW (working with Brusa) and various others are using PM – synchrel machines, usually called PM assist or even ‘PMa-SynRM’. e.g US patent 2012/0274168A1.

  • Joseph Flynn

    How does Tesla’s PMSRM differ from QM Power’s PPMT which is also an SRM with magnets on the stator and is designed in many phases including 3 phase?

    • PieroB

      specially contrived rare earth magnets placed in the stators.

      • Joseph Flynn

        Explain ‘specially contrived rare earth magnets’

      • Joseph Flynn

        Specially contrived rare earth magnets describes many salient pole motors with stator magnets. A more specific answer would be necessary. (this assumes ‘contrived’ means arranged)

    • Joseph Flynn