Switched Reluctance motors

[ggr]

I recently heard something that made me do a little research into Switched Reluctance motors. Here's an article. It sounds like they'd be perfect for something like electric cars, since they are slightly more efficient than AC induction, and can be cooled more easily. But I really don't know much about this area. Can anyone comment on their applicability or otherwise for Tesla?

 

[wart]

Another article here that goes into some pluses and minuses from an EV perspective. It does seem like we're likely to see someone put an SR motor in an EV before too much longer. I'm curious about the handheld Dyson vacuum that was mentioned in this article; specifically I wonder how loud it is, and if the noise is the reason Dyson didn't continue with it.

 

[rolosrevenge]

I'd be reluctant to switch to those motors.

 

[pbceng]

I work with SR motors as part of my job and it is true they do have a reputation for NVH issues. The problem is not specifically that they're noisy, but that they produce pure tone noise which is very noticeable and considerable design effort is require to eliminate the problem. Having said that it is quite feasible to produce a motor that is about the same at low speed and much quieter at high speed than an induction motor.

The losses in an SR machine tend to be concentrated in the stator which makes them easier to cool, also the rotor is lower inertia. Constant power speed range is typical twice that of an equivalent induction design and while the peak efficiency is lower, the high efficiency speed band is much wider.

Mechanical construction is simpler, but the achilles heel has always been in the electronics. Because the power is switched rapidly from one set of coils to the next about double the mean current flows through the electronics pack making it more expensive, and the competing requirements can be a lot higher.

Like everything there are advantages and disadvantages but in theory SR machines could offer a lot to an EV. Induction motors however are a commodity item these days and hence dirt cheap!

 

[pbceng]

I should have also added that they're DC motors - they don't require AC!

 

[jscholl]

I should have also added that they're DC motors - they don't require AC!

Does this mean that you could just drop the AC inverter, or would you likely need some sort of DC-DC converter between the battery and an SR motor?

 

[Ingineer]

They are not AC (do not require a full reversal) but most definitely do require inverters, they cannot run off pure DC. In fact, The semiconductor requirements are about the same as for BLDC or induction.

SR is ideal for an EV and can reduce the need for as much gear reduction as needed for AC induction. This could lead to more reliable drive units in Tesla's case.

 

[pbceng]

SR machines do require switching electronics but not inverters. They do also do work directly off DC - the electronics switch the DC across each of the coils in turn to attract or repel the rotor poles. There are several variants of electronics and winding patterns proposed or in use which have different cost/performance attributes but typically the electronics cost and the software are the biggest issues.

 

[okashira]

Inverter control will still be preferred. Switching DC will bring you back to a cheap BLDC motor and it would be noisy like a cheap hobby motor.
Field oriented control should still apply to SR motors and Ti is working on their solution.

I guess technically, you could even make a brushed SR motor. that would be interesting.

I dont fully understand the performance benefit of SR other then reduced core losses like an induction motor and no $$ magnets.
But I think a motor that combines permanent magnets and reluctance will offer better torque.

 

[kennybobby]

i've always equated SR motors to stepper motors, which make their max torque when not moving (all windings energized to hold position), and make little to no torque when actually moving from one position to the next.

Nearly useless for anything that requires torque and motion.

Lowest cost to produce but a sorry excuse for a motor... Look up the tethered satellite first mission for an example of a $B experiment failed by a crappy SR/stepper motor.

 

[TEG]

Green Car Congress: BMW’s hybrid motor design seeks to deliver high efficiency and power density with lower rare earth use

 

[ePower-AHP]

We have been working with SRM for electrical cars.
CIPED | Compact Intelligent Powerful Electric Drivetrain for EVs

Anders H Pedersen
ePower Technology
Denmark

 

[arnis]

So the biggest advantage for Ludicrous Tesla cars would be non-overheating rotor.

 

[Gerasimental]

Another article here that goes into some pluses and minuses from an EV perspective. It does seem like we're likely to see someone put an SR motor in an EV before too much longer. I'm curious about the handheld Dyson vacuum that was mentioned in this article; specifically I wonder how loud it is, and if the noise is the reason Dyson didn't continue with it.

Let's wait and see what Dyson put in their EV. I'm very excited for the first automotive SR motor. If the challenges of control and NVH can be overcome, they will most likely take over very quickly due to their low cost, high power density, and high efficiency (in the motor and the inverter). I suspect we will see them in agricultural and utility vehicles before passenger cars though.

 

[jkn]

Let's wait and see what Dyson put in their EV. I'm very excited for the first automotive SR motor. If the challenges of control and NVH can be overcome, they will most likely take over very quickly due to their low cost, high power density, and high efficiency (in the motor and the inverter). I suspect we will see them in agricultural and utility vehicles before passenger cars though.

I believe Toroidion uses SR motors (racing car with fast battery swap). Their prototype made lot of noise and has 4 direct drive motors. I haven't seen any new info lately. Hopefully next year.

 

[ratsbew]

Has it been confirmed that the Model 3 is using a switched reluctance motor? It seems like Tesla may have an engineering gem on their hands if they cracked this nut. It will allow the Semi to haul heavy loads efficiently and Roadster 2 to not overheat on the track....and that's ignoring the great efficiency the Model 3 is achieving.

 

[Jeff N]

Has it been confirmed that the Model 3 is using a switched reluctance motor? It seems like Tesla may have an engineering gem on their hands if they cracked this nut. It will allow the Semi to haul heavy loads efficiently and Roadster 2 to not overheat on the track....and that's ignoring the great efficiency the Model 3 is achieving.

Someone going by Ingineerix created a Reddit thread on his teardown of the Model 3 battery pack and related observations and stated that the Model 3 is using a switched-reluctance motor. It seems quite possible that this is the same @Ingineer who posted earlier on this thread 2-3 years ago.

Model 3 Teardown Videos (youtube) • r/teslamotors

and

Model 3 Factory Mode - Battery Pack Info Revealed • r/teslamotors

I believe Tesla has stated multiple times in EPA filings and in other public written statements that the Model 3 is a permanent magnet motor. Traditional switched-reluctance motors don’t use permanent magnets.

It’s possible that Ingineer was implying that the Model 3 uses something like the BMW i3 motor design which BMW refers to as Hybrid Synchronous. That BMW design is discussed in a GreenCarCongress article that TEG posted earlier in this thread. It is also discussed in this useful slide set from BMW:

http://hybridfordonscentrum.se/wp-content/uploads/2014/05/20140404_BMW.pdf

 

[Brando]

So the biggest advantage for Ludicrous Tesla cars would be non-overheating rotor.

less weight
more efficient
more reliable? limp home even when something breaks inside

Longer Range, perhaps higher top speed? slightly quicker.
oh, and less heat problems for the rotor.

 

[Brando]

Tesla Model 3 Motor — Everything I've Been Able To Learn About It (Welcome To The Machine) | CleanTechnica

 

[scaesare]

I believe Tesla has stated multiple times in EPA filings and in other public written statements that the Model 3 is a permanent magnet motor. Traditional switched-reluctance motors don’t use permanent magnets.

PMSRM designs have been around for a while now. The PM inserts in to the ferrous core improves the efficiency/performance.

The issue has largely been controlling of torque ripple and cogging. It's surmised that power electronics advances that Tesla has made may have achieved this.