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PM or Induction

Discussion in 'Tech Talk' started by Samiam, Apr 9, 2016.

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  1. Samiam

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    Does anybody know if the Model 3's new motor technology is induction or some type of synchronous PM motor.
     
  2. Badback

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    From a brief gimps in one of the Tesla factory videos (can't remember which one but it was long) I would conclude that it is a a 3 phase induction motor. This would allow the inverter to reverse the motor and eliminate the need for a transmission.
     
  3. Samiam

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    It's definitely an AC motor. Tesla has just been using an induction motor, which is typically a little less efficient than a synchronous motor. It would be interesting if they made a change.
     
  4. Badback

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    Samiam: A synchronous motor would be a bad choice for an EV. You can find some of the reasons here:

    http://electricalquestionsguide.blogspot.com/2012/11/synchronous-motors-advantages.html

    They cost more and they are heavier. When coupled to an inverter, a simple induction motor is the most advantageous.
    Because induction motors can slip, relative to the supplied frequency, they can run at a speed different than the load. This is important for starting from a stop. A synchronous motor must be started before a load is applied because it cannot supply torque at rest. An induction motor can be forced to higher than base torque by supplying more than rated volts/Hz. If you try this with a synchronous motor you risk demagnetization of the rotor magnets. There is more, but I won't bore you with the rest.
     
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    • Samiam

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      Actually, induction motors have great benefits but a lot of EVs and hybrids use synchronous IPM motors. All modern EVs have variable frequency drives so self starting doesn't really factor in much. Modern controllers can go into field weakening with an IPM motor by applying Id current. Early surface permanent magnet machines were highly efficient at light loads but not so great in field weakening. Modern IPM machines are more of a hybrid between a surface PM machine and a synchronous reluctance machine. They operate at higher efficiency than induction across much of the operating range. The big benefit of induction motors is elimination of magnet cost and there is no back EMF in a fault. There are some trade offs where cost is favorable for an IPM when the efficiency saves more battery cost than added magnet cost. Researchers are now working on higher reluctance machines with ferrite magnets (the new Chevy Volt generator has a ferrite generator). It would be really interesting if Tesla was considering alternate machines like this.
       
    • Badback

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      Perhaps this link will help:



      Ultimate efficiency is only important at or near full load. Most of the time the motor is required to output a tiny fraction of it's maximum.

      Efficiency differences at light loads are very small.

      I am sure that the Tesla engineers are well aware of what the competition has and will be doing, and have made there choices accordingly.
       
    • garsh

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      With the release of the EPA Certification Data for the Model 3, it looks like Tesla went with a Permanent Magnet motor for this car.

      Of course, EPA documents always have transcription errors, so we can't trust this new definitively by itself. However, there is other evidence that supports the conclusion that Tesla is using PM motors for the 3:

      In an interview from April 2017, Konstantinos Laskaris, Tesla’s Chief Motor Design Engineer, started talking about PM motors when asked about trends in motor costs.
       
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      • Badback

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        Please keep in mind that it is a PM AC motor, not a PM brush-less DC motor.
         
      • garsh

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      • Badback

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        #10 Badback, Aug 18, 2017
        Last edited: Aug 19, 2017
        Not quite, DC brushless are typically outrunners.

        edit:
        Sorry, I got interrupted by my OTHER last night.

        Tesla's motor is three-phase, a DC brushless is not.

        It would happily run from a three phase source without an inverter just like any salient pole or induction motor.
        DC brushless needs an H bridge electronic commutator.

        But, surely you knew this.
         
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        • swampgator

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        • dahollen

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          Here's some interesting Model 3 motor speculation from Otmar Ebenhoech, designer of the Zilla motor controller and all around great guy:
           
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          • JWardell

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            Great discussion here, thanks for posting. Makes me think it could also make sense for Tesla to add a small induction motor to the front in the dual motor cars.
             
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            • John

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              Yeah, I like the first part of the video—good discussion of how inductance and PM motors work, for instance—but the speculation at the end, I dunno. I've never heard of a good field-weakening strategy for permanent magnets to avoid eddy losses. The only strategy I've heard of is "moving the magnet farther away," but that's impractical to build in a powerful, reliable, low cost motor. Seems like designers have turned from trying to weaken the magnetic field to trying to find a wonder material for the armature that reduces eddy losses. For instance: a superconductor with very high magnetic permeability.
               
            • 2170pwr

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              So if i understand him correctly, he is suggestion that the Model 3 will have an Interior Permanent Magnet Motor so that torque will be provided by both permanent magnet torque and reluctance torque. good description of this here: (https://www.motioncontrolonline.org...er-Traction-Motor-Applications/content_id/789)
               
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              • mt.west.ev

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                You guys are way over my head.
                Let us all know the "good news" when everything is known and understood.
                 
              • garsh

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                No worries. It's not that one type of motor is better than the other. There are some minor tradeoffs (size, weight, cost, efficiency), but it's not a huge difference.
                 
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                • SSonnentag

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                  Since this is a motor-related thread I'll ask my question here. Is it harmful or bad practice to back your EV out of a parking space and then shift into Drive while still rolling backwards? Does the motor care that it is still spinning one way when power is applied to spin it in the opposite direction?
                   
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                  • 2170pwr

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                    no. shouldn't matter.
                     
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                    • Badback

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                      No, it absolutely does not matter.
                       
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