Monday, March 12, 2018

Tesla Model 3 Motor — Everything I’ve Been Able to Learn About It - by Steve Bakker

Generic Switched Reluctance Motor (Credit: Click to Enlarge.
In a reluctance motor there’s only one magnetic field.  How can that be? ... When you move [a] magnet towards ... metal the magnet will of course pull the chunk of metal toward itself.  Now what if you were to fashion a rotor for an electric motor out of just a refined chunk of steel, but retain the existing electromagnets in the stator?  As the electromagnets are turned on and off in just the right sequence, they would coax the steel cylinder to turn. 

Congratulations, you just designed a reluctance motor!  And the fact that the electromagnets are switched on and off in sequence to spin the rotor (as with the induction motor), you have what is termed a Switched Reluctance Machine.

The reluctance machine was invented in 1838.  And it’s a surprisingly sweet design.  The reluctance machine is simple, it’s efficient, it’s compact in size.  And it’s inexpensive to produce.  Yet the reluctance motor sat on the shelf for over a century, suffering from a debilitating disease called Torque Ripple (due to the reluctance machine’s propensity to incur a phenomena know as cogging).  Torque ripple simply means that the power output of the reluctance motor fluctuates up and down.  Certainly not good for an EV.  When you put the pedal to the metal, you want a nice smooth acceleration ramp.

The reluctance machine was partially rescued by the same technology that made it possible to put the induction motor into an electric car — power electronics from Silicon Valley.  The reluctance motor is notoriously difficult to control (RPM, determination of rotor position, etc.), but modern inverters and control systems helped overcome that foible.  Still, the torque ripple issue remained a challenge even as the 21st century approached.  But in poking around, you start to notice some research on the topic taking place in the first decade of this century.  You come across a 2011 research paper claiming that the torque ripple issue has been addressed.  The researcher had embedded some small rare-earth magnets in the stator of a reluctance motor right along with the existing electromagnets.  In doing so, the torque curve had smoothed out.  As a bonus, the paper claimed to achieve a 30% boost in power output with the inclusion of the rare-earths.  Now there’s some first principles thinking.  Whoever first thought of lacing the stator with rare-earths has apparently come up with the greatest marriage since someone thought to sink a chocolate bar into a jar of peanut butter, producing the Reese’s Peanut Butter Cup.

Your thoughts coalesce.  With two of the major issues of the reluctance machine having been addressed, you take the plunge and start working with this design.  The first thing you are able to do is discard that expensive copper rotor in the legacy motor and replace it with a far cheaper ferrous metal rotor.  Probably steel.  And probably silicon steel.  You just saved a ton of money.  Next, although the rare-earths are expensive, they are going into the stator, not on the rotor as with a traditional permanent magnet motor, so you’re going to be augmenting the electromagnets with relatively smallish permanent magnets.  Your chosen design has some issues with acoustic noise, but you feel that it’s worthwhile to pursue this design because it’s the simplest and least expensive motor to build, yet highly efficient and powerful (especially with those rare-earths).  Good job!

So, the first puzzle piece in the theory that Tesla has put a switched reluctance motor in the Model 3 is the magnets.  We know they’re in there, and now we know that one of the latest breakthroughs in motor design is the inclusion of rare-earths in the stator of the reluctance machine.  This is huge.  It has brought the reluctance machine out of mothballs!

Another clue that the Model 3 motor is not using those rare-earths in a conventional permanent-magnet  motor design is that the car does not do regen all the way down to 0 miles per hour.  For example, the Bolt has a conventional 3-phase PM motor which allows it to do regen to 0 MPH.  I saw this for myself last year when I test drove a Bolt — you can stop without applying the brakes.  We’re calling this puzzle piece #2.

Here’s another one:  The “dealer” sticker on the Model 3 in the showrooms indicate a “Three phase, six pole, internal permanent magnet motor.”  The Tesla induction motor is a 4 pole design, as are many EV motors.  Why then a six pole motor?  This is a reference to the number of low-reluctance “lobes” on the rotor that are pulled on when the stator electromagnets are energized.  The lobes are termed salient (protruding).  The closer the spacing of the poles, the less time there is for torque to fall off.  It may be Tesla’s way to further smooth torque ripple.  That’s puzzle piece #3.

Puzzle piece #4 is that various engineering/motor design publications are starting to talk up the reluctance machine (see article links below).  And we are starting to see the reluctance design appear in EVs, such as the Prius.  Furthermore, UPS has announced that a switched reluctance machine will be used in a program to convert their fleet to electric power.  The company is claiming its implementation of the reluctance motor over other designs will reduce charging times and increase energy efficiency by up to 20% (the company is distancing itself from the use of rare-earth magnets, though).  And, in general, industry applications for updated reluctance motor designs are starting to pop.  For example, in a recent CleanTechnica article, Software Motor Company (SMC) is declaring that its new reluctant machine design — with what they are calling its own version of “secret sauce” — will save 50% on energy costs over the current induction motors in use at Walmart for HVAC, etc.

Finally, the motor in the Model 3 is indeed smaller than the Model S motor.  In a recent Jack Rickard EVTV video examining the Model 3, Jack asserted that the Model 3 motor is actually smaller than even the smaller front motor on the Model S.  Yet performance has not been overly compromised.  Some owners have reported 0–60 times as quick as 4.8 seconds in their Model 3.  That of course is due in part to the 1,000 lb of less weight than the S, but still let’s tentatively call this puzzle piece #5.

Further support of piece #5 comes from Rickard’s continuing comments while still under the car (Rickard, by the way, has gone as far down the Tesla drivetrain rabbit hole as anyone I’ve ever heard of).  Extrapolating from EPA documents, Jack is calling the Model 3’s “battery-to-wheels loss” as 6% more efficient than the Model S (89% of electrical energy is converted to forward motion, compared to 83% for the S).

With the breakthrough in reluctance machine design these past few years, we may be witnessing a sea change with regards to the powertrain for the electric vehicle market.  Given reports about the performance of the Model 3, the reported jump in miles per kWh that owners are reporting over prior Tesla models, along with our 5 easy puzzle pieces, it’s a reasonable bet that Tesla has perfected the reluctance machine and in doing so has pulled an engineering rabbit out of its hat.

Regardless of the exact motor design, Tesla has clearly hit it out of the park with the Model 3’s powertrain.  They gave their motor designer team, if not a blank check, a blank whiteboard, and the team came up with a design suitable not only for an affordable electric car, but for the upcoming Tesla Semi as well.

Note that Chief Designer Laskaris joined Tesla after the Model S had been developed.  His head must have been full of fresh ideas from when he earlier had co-founded a project to design and build an efficient electric car.  Like Straubel, Laskaris gravitated to Tesla already having a notion that the future was electric.

The 3’s motor design has also helped Tesla toward its stated goal of a 25% reduction in parts count by having the motor do double-duty as a heat source for warming the traction battery.  (Note:  Tesla has been so impressed with the talent coming out of the school Laskaris attended in Greece that the company has set up a small research center in the country.)

Although the use of a reluctance machine in the Model 3 has yet to be verified, with the sudden talk from so many quarters for so many applications of this motor tech, it’s hard to believe that Tesla would not have a front row seat to this event.  All in all, it is quite possible 2018 will go down as the year of the reluctance motor.

Read more at Tesla Model 3 Motor — Everything I’ve Been Able to Learn About It

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