# Reluctance Synchronous Rotor Design



## RIPPERTON (Jan 26, 2010)

Jim Hendershot.
Its about the structural design of the rotor.
The RSM will succeed the PMSM as Neodymium supplies dwindle.

https://www.youtube.com/watch?v=XuwV5dSvjh4

https://www.youtube.com/watch?v=bxWy2WwC3qg

https://www.youtube.com/watch?v=Aa-QY5T0_8w

https://www.youtube.com/watch?v=armSa5MllsQ


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## dcb (Dec 5, 2009)

Yup, I posted his links a while ago, fascinating stuff. The shortened magnetic paths are handy.

I think the complexities of RSM (a switched reluctance rotor designed to work with an induction motor stator and inverter topology) will eventually give way to SRM with asymmetrical inverters and better rotor integrity and higher speeds, mono-polar excitation, less end windings, hopefully with shortened magnetic paths and minimal current/magnetic reversals, better salience ratios, etc.

But RSM (esp the stamped rotor lam, despite web leakage) seems a potential intermediate for induction.

Of course, it is just stuff I see on the net and try to make sense out of. I would be tempted to get some rotor lams laser cut though after watching all that. Though I'm leaning towards SRM lams at the moment and 6 extra diodes in the inverter.


http://eam.tugraz.at/fileadmin/user...nt_Chapter_Meeting_2013/IEEE-Chapter-Graz.pdf


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## RIPPERTON (Jan 26, 2010)

Awesome PDF dcb page 54 is interesting how the carriers were epoxied together then the outer slots machined
whats even more baffling to me is how the RSM does regen or generator duties if theres no current in the rotor.
Im onto this in axial gap format, maybe I wont have to buy new magnets for the R1 after all.


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## dcb (Dec 5, 2009)

I like Jims enthusiasm, but from what I can tell RSM falls short of SRM in every performance aspect. The main benefits seem to be reusing an existing induction stator, and inverter topology (though losses there for non-unipolar switching and still need custom control logic). But the rotor AND stator are both horribly complicated by comparison to SRM. The SRM rotor is built like a wood chipper and can do way more rpm with ease with less inertia. And the reduction in copper on the end turns should be obvious.

Jim does seem focused on RSM here (maybe it is like forcing kids to learn COBOL or something, lots of machines out there that just need an upgrade) http://my.ece.ucsb.edu/York/Bobsclass/134/Handouts/Reluctance Synchronous Motors Hendershot.pdf

But he has a few SRM patents, including this one for short magnetic paths (percent fill is affected though). If you can get past the inverter and stator changes, it looks real promising since you are into custom rotors already  (don't know about axial flux).










Basically the bottom switch does gross commutating and the top switch does current hysterisis (bottom provides "freewheel" path for pwm-off), both off to dump the stored magnetic energy back in the bus (before the negative torque angle). Braking would be basically a mirror image about the negative torque point I guess.


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## kennybobby (Aug 10, 2012)

These sure look like stepper motors to me. Make maximum torque when not moving, lots of cogging torque, instability at high rpm. Trying to make a motor without using a magnet, either induced or permanent, on the rotor.

These seem to be a half-step away from the free energy motors--just remove or route the back emf and they produce more than they consume.

It would be interesting to see how it responds on a dyno.


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## dcb (Dec 5, 2009)

It is still pretty bleeding edge, ironically, since reluctance motors have been around for ever, but the switching devices havent. Here is a 2014 patent from oak ridge, I kind of assume rotor flux is contained by using complimentary coil currents (so it doesn't try to reach across the rotor). I assume the housing structure is aluminum, so fairly simple assembly with the right casting.

http://www.google.com/patents/US20140021809

I don't really know how SRM will pan out, *looks* more efficient at partial load than bldc and acim, and full continuous torque at 0 rpm, plus excellent high speed operation claimed (no back emf, sounds familiar). Tons of research papers on it but few comparative dyno graphs.

The noise concerns seem like a 4th order concern though, not even worried about it till I see how it performs. And even then it might be a "feature"...


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## dcb (Dec 5, 2009)

did find a bit about production SRMs in the 30 to 350hp continuous range from US motors:
http://www.usmotors.com/Our-Products/Switched-Reluctance/1800RPM-BaseRange.aspx

here's a datasheet on the 30hp jobbie with 250% overload controller:
http://www.usmotors.com/~/media/USM...e/Datasheets/SRM256TN-180_SRC3-000.ashx?la=en

cast iron frame @ 330lbs, and under 3k rpm limit, so obviously not really pushing it, but far less than a cast iron 800lb 30hp induction motor all the same. (their 350hp motor weighs 700 lbs less than a random 350 induction motor, and doesn't need 4000 volts).

they "look" like they outperform induction on all performance accounts:
http://www.nidec-motor.com/~/media/...erature/Brochures/Industrial_SR_Brochure.ashx

Supposedly comparable to bldc below rated rpm, and much better above. Haven't found a dyno yet though.

I assume most of this applies to RSM as well, except the compromises made for reusing an acim stator/inverter.

I imagine the extra controller costs are probably the limiting factor in more broad adoption vs the cheaper motor manufacturing costs, but the solid state costs are more susceptible to downward pressure than motor costs (and a diy-er can get switches for cheap enough in surplus).

Note, the 30hp looks like ~80% efficient, inverter and probably 3 phase 480v pfc and all, in this rather industrial example. 34A @rated hp, .97 displacement factor, from the wall. So off to a good start anyway. It says the inverter dissapates ~2hp of heat, which bumps the remainder up to %85, and has 3 x 230uH external reactors to account for too (probably not an issue in battery operated mode).

edit: here is coulomb asking about an RSM SRM in 2009 used in a brusa:
http://forums.aeva.asn.au/forums/the-brusa-hsm61712-hybrid-motor_topic1245.html, it seems to hold constant power pretty well to 11000 rpm compared to their acim in the same housing (and make quite a bit more torque).

















http://www.metricmind.com/wp-content/uploads/2013/10/BRUSA_DB_EN_HSM1.pdf
3.5kw/kg? While all the sparks are nice, need to get something like this on the drag strip, or something

edit! doh, coulomb thinks it has some magnetism in the rotor, perhaps not a "true" RSM.

They have a listing with no magnets (rare earth?) though , 3kw/kg, and it looks like it can handle a lot of abuse.
http://www.metricmind.com/products/brusa-ssm1-6-17-12/


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## RIPPERTON (Jan 26, 2010)

So can a Reluctance Synchronous 4 pole motor run a trapezoidal Kelly controller with a hall sensor ?


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## dcb (Dec 5, 2009)

I wish I knew for sure, been looking mostly at srm, and they are pretty adamant about having an asymmetrical bridge and 6 leads so the field collapses quickly before the negative torque region (and doing something with the energy besides making more heat).

I'll give it a maybe though  Maybe someone knows for sure what is up with RSM control, gobs of "research" stuff in google of course. https://www.google.com/#q="Reluctance+Synchronous+Rotor"+control

it sounds like it starts and runs like an induction, but can briefly accelerate and "snap" into synchronous mode when conditions are right (and if the controller doesn't freak out at 0 slip, i.e an open loop vsd). I imagine with a bldc style of control it would just always be in sync, but might as well look at SRM as you don't have an acim stator. I haven't seen much on axial flux SRMs FYI, if you are trying to reuse your pancake stator, will give it a little thought though. Do you have any pics or specs of the motor you have in mind?

edit: digging through some of your stuff, wow! (though this is more of a pmac or acim type stator AFAICT than what I think of for bldc or srm, and I have no clue as of yet to RSMify this rotor without radial lams). If you got into power electronics you would be dangerous


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## RIPPERTON (Jan 26, 2010)

So as far as RSM is concerned the axial gap concept is not usable for 2 reasons.
A: there is no need to use both sides of the magnet
B: its totally impossible to design the rotor laminations.

Next query, would an RSM have the same power to weight ratio as a PMSM
as the stator cannot push the rotor it can only pull it.


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## dcb (Dec 5, 2009)

Again, just reading the virtual tea leaves, not really speaking with confidence here, only brevity.

With axial on a racetrack, my guess is you are better off with the magnets, unless those magnets are really really expensive.

I think RSM is out of the picture there, and I can't think of a SRM configuration that is simple to manufacturer for axial flux that would be robust. And RSM seems more an ACIM efficiency stop-gap, performance is worse than srm or bldc.

That isn't to say you couldn't make a radial flux SRM in the same size package though.

I don't have a good handle on SRM overloading, you don't have to worry about demagnitizing anything though. The core just becomes more oversaturated. Also the core's are simple and can handle lots of revs, and the end turns are short. The next trick is keeping the magnetic paths short, compromised against the robustness/ease of manufacturer, and controller considerations and higher speed utilization.

Hendershot published a ACIM vs BLDC vs SRM comparison in 2001, and there has been a lot of research since I'm sure:
http://jimhendershot.com/Jim_Hendershot/Articles_files/acbrushlesssrmotorcomparison.pdf

But at the time the power density breakdown was (essentially same sized frame, continuous, optimized for 1800 rpm ish, probably wrong fan size):

```
type     ACIM     SRM      BLDC
hp/lb    0.10     0.20     0.26    
eff      %90      %92      %94
hp       5        9        14.5
```
Looking at the graphs the srm looks like it can make gobs of torque at the low end when overloaded.

Again, this is probably dated, and all motor types have seen various updates/optimizations along the way.

The differences in density/cost are less critical for larger vehicles of course.


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## dcb (Dec 5, 2009)

and of course, maybe I'm completely wrong about RSM 

Perhaps this is what Hendershot is seeing, the article sure looks like it is talking about a RSM, but calls it a Synchronous Reluctance Motor.
http://machinedesign.com/motorsdriv...gnet-reluctance-and-induction-motors-compared







(though it lists both images as SR motors?!?)

"From most efficient to least, here’s how the five motor technologies stack up: salient-pole PM, nonsalient PM, synchronous-reluctance, switched-reluctance and, last but not least, induction motors."

"Regarding power and torque density, as may be expected, the rankings mirror those for efficiency, with the PM designs having the highest torque density and induction designs the least."

So, blech... RSM would be a very attractive option, if it is true, making lams for testing is its own kind of headache though. 

https://www.google.com/#q=synchronous reluctance motor

some of the ABB SynRM motors are coming in above 93% efficient WITH inverter:
http://www.abb.com/abblibrary/downl...CsTDCsVVTwrHCsTD+mh4uP4EGfuTgJK8NY67u0zxHng==


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## Sonikaccord (Dec 17, 2012)

RIPPERTON said:


> So as far as RSM is concerned the axial gap concept is not usable for 2 reasons.
> A: there is no need to use both sides of the magnet
> B: its totally impossible to design the rotor laminations.
> 
> ...


A: I disagree, you still need both sides to complete the circuit. Have you ever seen a one-sided magnet? 

B: Not impossible.

You want the laminations to be parallel to the flux to minimize eddies.
You want to design flux barriers in the lamination so that the flux is aligned with the d-axis. (expensive as you would need multiple stamps to press out the designs.) THEN, you would need to stack them in the correct order to get the shape of the flux barriers right AND find a way to bond them together so that centrifugal force doesn't rip them apart esp in a racing application.

I've been reading a few motor design books lately, mostly about PMSM/BLDC motors. The RSM and SRM work exactly the same way except the torque mechanism used for the RSM/SRM is considered a parasitic loss in the PMSM world.

With that being said, you could use a singly, doubly, triply, etc. excited system where the torque component is equal to the sum of the current through the excited coils times the inductance as a function of rotor position. If you have a multiphase inverter and bang all your coils with max current at their optimal angles, you could have a 'very' torque dense motor. Silicon will determine the power density. The more flux/current you can apply to the rotor, the more torque...excluding saturation of course. I'm doing a thought experiment on that.

Permanent magnets provide greater flux per volume than soft steel, so SRM/RSM will never be as power dense as PMSMs. That's ok. While you are overdriving your magnetless rotor, you won't have to worry about coercivity.


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## RIPPERTON (Jan 26, 2010)

Well theres only 2 possibilities with lamination orientation for an axial gap RSM and that's stacking discs or spiral strip like the stators.
Spiral strip is impossible because there are pieces hanging in mid air and every single piece is a different shape so would need to be laser cut with overhanging joiners that would later be machined off after epoxy potting, just too hard, would never translate into production.


Stacking discs does not have the right laminar flow for the mag fields to follow, Im guessing.


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## dcb (Dec 5, 2009)

FWIW, I think you only have to bridge from one side to the other, a very short magnetic path across the gap between opposing coils, and let the back iron (stator) take it from there to the nearest pole.

No idea what it should look like, all things considered.


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