Switched reluctance motors

MitchJi

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I’d love to get a better understanding of the implications of this motor. Elon said that it is switched reluctance motor. There are some controversy over that statement. How important is this?

https://cleantechnica.com/2018/03/11/tesla-model-3-motor-in-depth/
...".
You would examine all the different electric motor architectures that exist. In doing so, you would come across a design that’s actually older than Nikola Tesla’s 1892 invention. More than 50 years earlier, the reluctance machine had been 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.
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More iinformation

Understanding Reluctance machines:
Wiki article
https://en.wikipedia.org/wiki/Switched_reluctance_motor
MachineDesign.com article
http://www.machinedesign.com/motorsdrives/are-switched-reluctance-motors-you

Charged EV; A closer look at Switched Reluctance Motors
https://chargedevs.com/features/a-closer-look-at-switched-reluctance-motors/

Charged EV: A closer Look at Torque ripple
https://chargedevs.com/features/minimization-of-torque-ripple-is-important-for-motors-in-many-applications-because-it-is-one-of-the-main-causes-of-vibration

Adding PMs to reluctance motors
IEEE document
http://ieeexplore.ieee.org/document/6180757/

2011 white paper
https://vtechworks.lib.vt.edu/bitstream/handle/10919/77978/etd-01172011-105438_LOBO_NS_D_2011.pdf?sequence=1&isAllowed=y

Other:
Model 3 tear down by Ingineerix
https://www.youtube.com/watch?v=RGzwnPgow8U&lc=z225v32ywtayxbuuaacdp431w5dsak5cgpu3e315vydw03c010c.1519447508603632&feature=em-comments

History of the electric motor
https://www.eti.kit.edu/english/1376.php
 
Switched reluctance motors have no cogging when unpowered. That's a minor benefit, the big one for Tesla is the huge reduction in the volume of rare earth magnets, and also a very big reduction in the amount of copper per motor HP, compared to the original style of motor they are using.

They are "ramping up" their implementation by using switched reluctance on the front of the AWD versions of Tesla vehicles.

The SR motors are turning out to cost less too, and this is a vital part of their plan to make entry level Teslas more affordable.
 
spinningmagnets said:
Switched reluctance motors have no cogging when unpowered. That's a minor benefit, the big one for Tesla is the huge reduction in the volume of rare earth magnets, and also a very big reduction in the amount of copper per motor HP, compared to the original style of motor they are using.

They are "ramping up" their implementation by using switched reluctance on the front of the AWD versions of Tesla vehicles.

The SR motors are turning out to cost less too, and this is a vital part of their plan to make entry level Teslas more affordable.

SR is the top pick from an economic/business standpoint not an engineering one.
but people not in the know that look to others for guidance will think "well, if Tesla is doing it, it must be good."
just like they do with Tesla's sub-optimal engineering choice of 18650 which had more to do with not being held hostage by a single source supplier tanking the company (Fiskar, RAV4).
no RE means china can't jerk around their input cost.
as far as SR being a 'better' motor than PM that depends on the requirements, 'better for what?'
SR is not the be all end all holy grail that lukeforphysics seems to think.
 
Toorbough ULL-Zeveigh said:
SR is the top pick from an economic/business standpoint not an engineering one.
but people not in the know that look to others for guidance will think "well, if Tesla is doing it, it must be good."
just like they do with Tesla's sub-optimal engineering choice of 18650 which had more to do with not being held hostage by a single source supplier tanking the company (Fiskar, RAV4).
no RE means china can't jerk around their input cost.
as far as SR being a 'better' motor than PM that depends on the requirements, 'better for what?'
SR is not the be all end all holy grail that lukeforphysics seems to think.

Basically agree with this. An economic choice but Musk put a nice spin on it not to give customers the feeling its just a cheaper motor.

I did the math once on a SR motor, dont really see where the comment about bad cogging comes from and why magnets would be needed to solve it. You just need to take care that the inductance variation with rotor angle is sinusoidal. Only use that i can see for magnets is to keep more traditional sensorless controllers in sync for when phase current is 0 (when you release throttle). For 0 phase current a SR motor loses its backemf...
 
Economics is important.

And consider the range increase they got on the new MS-MX.

I thought that power per size and weight would be considered important here.

As for Tesla using small form factor cells they’ve learned to make packs at a very low additional,cost.

We’ll see what they announce or battery day. Elon said that they have a plan to get to 1-2 tWh of cell production year. That means that they have a plan to drastically increase their speed of production using less space and to drastically reduce the costs or their cells. Tune in in 2020 when they unveil their plans.
 
One more remark about SR motors,

For PMSM (what we have with the magnets),
Torque proportional with I, losses with I^2,
So at half of max torque, I=0.5*Imax, losses proportinal to 0.25*Imax^2

For SR,
Torque and losses both proportional to I^2
So at half of max torque, I=0.71*Imax, losses prop to 0.5*Imax^2

..... double the resistive losses for the SR at partial load....
 
by"Toorbough ULL-Zeveigh"
SR is the top pick from an economic/business standpoint not an engineering one.
but people not in the know that look to others for guidance will think "well, if Tesla is doing it, it must be good."

Since when does engineering ignore economics?

just like they do with Tesla's sub-optimal engineering choice of 18650 which had more to do with not being held hostage by a single source supplier tanking the company (Fiskar, RAV4).

What does better mean? Tesla has managed to make the largest capacity pack for the least money. Does it matter if they did that because they are working with a superior form factor or they managed to overcome the inherent problems with superior engineering? If they did not believe that the advantages outweighed the disadvantages they could have switched form factors instead of switching to 2170’s.
 
Toorbough ULL-Zeveigh said:
SR is not the be all end all holy grail that lukeforphysics seems to think.

Thanks for the link? Took me a while to check it because I thought that you were criticizing Luke.
 
side note on Tesla batteries

#1 - I think I read one of Maxwell's technologies was the "dry cell" (no solvent used). Meaning no solvent needed in the coating process which eliminates the drying line & capture/re-cycling of the solvent. This saves factory floor space. More production for the same floor space.

#2 - The 2170 or 21700 turns out to be more optimal for Lithium Ion cell vs 18650 - which predates Tesla and was (still is??) the most common cell form factor. Especially since Tesla started to use and make in Nevada.

Still unknown if anyone will figure out how to use supercapacitors in battery packs for increased performance.
(could bike sized packs also use??) (best price for bike sized pack 18650 or 21700?? we shall see)

Chemistries continually get tweaked. Cobalt use may well get eliminated by Tesla.

interesting times

side note: lower cost of Tesla motors could be volume & improved manufacturing techniques - not just raw material costs.
 
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