Asynchronous motors - is induction the new future?

[Allex]

Source: http://electrotransport.ru/ussr/index.php?topic=30259.msg674380#msg674380

Here is a interesting one. A very first Protoype for a asynchronous hub motor. He states that it is the first made in the world for ebikes.
Using a Curtis controller the bike propells to a stable 75Km/h at 48Volts.
No magnets so nothing to destroy(besides windings) when overheating. The developer says that the weigh is 18kg and the motor is rated at 20kW nominal and 200Nm, I dont know how much truth it is in that statement, but this kind of motors could be very interesting to traditional ones we use today.
You should also have a smooth rotation without any cogging and drag, he talks about it at:
https://www.youtube.com/watch?v=cu5VBqep1sM&feature=youtu.be&t=985

Flame on!
[youtube]msTVUpMSEtk[/youtube]

 

[Nobuo]

This is very interesting. I've been thinking broadly why induction motors are not used yet on ebike applications. So, if these statements are right as their cost of manufacturing are really lower compared to BLDC motors, What would be the real challenge to make ebike induction motors?

- Efficiency:
BLDC: ~93%, inverter: ~97% (synchronous flyback or hysteretic control), NET=90%
Induction: ~91%: inverter: 97% (synchronous flyback or hysteretic control), NET=88%

- Wear/Service:
BLDC: Bearings (lifetime)
Induction: Bearings (lifetime)

- Specific cost (cost per kW), including inverter
BLDC: High - high power permanent magnets are very expensive
Induction: Moderate - inverters add cost, but motor is cheap

- Heat rejection
BLDC: Windings on stator make heat rejection straightforward. Magnets on rotor have low-moderate eddy current-induced heating
Induction: Windings on stator make stator heat rejection straightforward. Induced currents in rotor can require oil cooling in high power applications (in and out via shaft, not splashed).

- Torque/speed behavior
BLDC: Constant torque up to base speed, constant power up to max speed. Automotive applications are viable with a single ratio gearbox.
Induction: Constant torque up to base speed, constant power up to max speed. Automotive applications are viable with a single ratio gearbox. Can take 100s of ms for torque to build after application of current

- Miscellaneous:
BLDC: Magnet cost and assembly challenges (the magnets are VERY powerful) make BLDC motors viable for lower power applications (like the two Prius motor/generators). Regnerative braking comes essentially for free.
Induction: The motor is relatively cheap to make, and power electronics for automotive applications have come down in price significantly over the past 20 years. Regnerative braking comes essentially for free.

 

[johnrobholmes]

I would be more inclined to say that switched reluctance topology will be favored in the long run, when controllers are more mature. Torque density of PMDC but without magnets, seems like a winner.


Good to see an induction motor used on an ebike, can only be positive for the future.

 

[ridethelightning]

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[anpaza]

What you missed to say is that this type of motor has zero coasting resistance, even less than a geared hub motor.
If you run out of energy, you can use it like a regular bike minus the extra weight.
And at the same time you have regeneration AKA E-ABS! That's amusingly wonderful, if you think of it.
Another thing to say is that due to missing magnets you may overheat it more than a synchronous motor.
The only limit is the isolation of the wires, this is usually like 200 degrees Celsius.
vs around 100 degrees for magnets, after which they become useless pieces of metal.
So, overall, theoretically you may get better power density with an induction motor.
The motor above has been designed to theoretically handle 20 kW.
Yes, it's not a bike hub motor, the bike is used just for primary tests.
In the current tests the power is limited to about 2.5 kW.
See yourself what bike he uses, even 80 km/h is a serious risk.

And it's not the first motor, it's already the second prototype.
The first was a Magic Pie 2 with heavily reworked both stator and rotor:

Unfortunately, the Curtis controller couldn't handle it at low speeds (drops to default 50Hz while it should start from 0).
So serios traction begins at ~20 km/h.
The second prototype was made from scratch and with a different number of poles to better fit with Curtis.
Not to say it works ideal, but better than the first prototype.

Video with the development group leader (the man with glasses):
[youtube]http://www.youtube.com/watch?v=CfwYYU2nW78[/youtube]

Rushing through the streets (camera seems a bit dead):
[youtube]http://www.youtube.com/watch?v=UU4-za4oHbU[/youtube]

Now they're working on their own controller, at the same time they're working with Curtis to get better support in their controllers.
That's a really revolutionary thing, and it'll be quite hard to break the initial market inertia.
And, well, there's no difference if you call it a "inverter" or a "bike controller".
It's basically the same schematics, just different firmware.

 

[peters]

Interesting. But continuous torque vs. weight is the key factor for the hub motors, I would like to see developments targeting >100Nm in <5kg with whatever technology.

 

[anpaza]

Well, if the second prototype will really deliver 20 kW that's more than a kW per kilogram (the prototype weights about 17kg).
Compare to e.g. CroMo, which delivers about 10 kW weighting 11kg.
High torque at low speeds is another question not yet confirmed (because Kelly doesn't work quite well yet with it).
Right now the motor has been sent to a laboratory for tests. Will see what the results will be.

 

[Deafcat]

so does this need a variable frequency drive (or vector drive) to achieve full power at any RPM including starting from zero? I've got a lot more experience with induction (3 phase) motors than BLDC and I would assume that is necessary to full-rev-range performance...

I would love to run this btw.

 

[major]

so does this need a variable frequency drive (or vector drive) to achieve full power at any RPM including starting from zero? I've got a lot more experience with induction (3 phase) motors than BLDC and I would assume that is necessary to full-rev-range performance...

You can not achieve full power at any RPM including zero. At zero, that would require infinite torque and stuff would break. But for vehicle propulsion, induction motors can deliver full torque at stall when using vector control with speed feedback. Sensorless schemes can almost work but fall short of the true torque control needed for smooth, safe riding with full torque at zero RPM.

 

[Deafcat]

full power within reason, of course. What I mean to say is "lots of power across the full rev range" like you get with conventional 3-phase vector drives. Been doing a bit of research on FOC theory over the past bit, has anyone experimented with or built a Switched Reluctance motor with FOC in regards to an ebike drive? no magnets at all and a lack of conductors in half the motor is pretty cool. RPM and power range seems to be relevant to ebikes with SRMs, cost is potentially low for the drive, good weight, consistent torque achievable with FOC (sensor or sensorless configuration).

 

[anpaza]

We theoretically discussed SRM hub motors on our forum, but came to conclusion that this type of electric motors will provide way smaller specific power (watts per kg). That's because the rotor in SRMs is not magnetic, it's just a piece of iron, so it does not produce any force per se. That's in contrast to synchronous motors and asynchronous motors, where both stator and rotor are magnets.
Maybe we're wrong, but that's what we came to in the end. If somebody has other info, it would be interesting to hear.

 

[nutnspecial]

This is very intriquing- i don't think I've been aware of induction/ permanent-magnetless motors at all.
Looks like alot of strengths with almost no weakness- perhaps just controller tech?
Kudos to the pioneers- hey what's the forum?

 

[johnrobholmes]

We theoretically discussed SRM hub motors on our forum, but came to conclusion that this type of electric motors will provide way smaller specific power (watts per kg). That's because the rotor in SRMs is not magnetic, it's just a piece of iron, so it does not produce any force per se. That's in contrast to synchronous motors and asynchronous motors, where both stator and rotor are magnets.
Maybe we're wrong, but that's what we came to in the end. If somebody has other info, it would be interesting to hear.

SR can have the torque and power density of PM motors. As far as motor design is concerned, it would make a great replacement for PM. Driving them well is another challenge.

 

[anpaza]

SR can have the torque and power density of PM motors. As far as motor design is concerned, it would make a great replacement for PM. Driving them well is another challenge.

Driving them should be pretty easy as they are synchronous motors and you can get it working even without vector control.
Perhaps even existing ebike controllers may drive them, just install some type of position sensors (halls won't do because there are no magnets), three would be enough.
But I can't see how you can make them same power density as in PMSMs.
In PMSMs driving force is created by the interaction between the magnetic field of the stator and MF of the rotor. You have two types of interaction: attraction and repulsion.
In SRMs only the stator creates a magnetic field, and rotor just follows it. So only attraction works here, half of what we have in PMSMs. So it creates twice less torque. Yes, you can overheat SRMs more than PMSMs, so you gain some power density here, but anyways I doubt you can double it.

In asynchronous motors we have both attraction and repulsion, but we have to spend a little energy to induce current in the squirrel-cage.
So, an async motor will be a few percent less effective than direct-drive PMSMs.
On the other hand, on geared PMSMs we lose same few percents in the gears.
From user point of view, an async motor is closer to geared PMSMs, it is even better as it has absolutely zero coasting resistance and at the same time it has recuperation.

 

[Deafcat]

It's a bit more involved than just "reluctance" vs "attraction + repulsion". SRMs have much more power compared to volume than induction motors, due to the differences in construction... SRM is also lighter, cooler running, and lower cost.

additionally, torque at stall in an SRM is 100%, with no heat being produced in the rotor whatsoever. temperature rise only occurs in the stator, meaning it's easy to cool. the stator windings are very concentrated and do not overlap in any way, so heat-related insulation failure/phase to phase shorting isn't possible and it would take a lot more heat to damage an SRM motor than an induction motor (though in my experience, good induction motors are extremely reliable in their own right).

as for positioning, both hall effect sensors and external encoders work in SRMs, you do not need a permanent magnet for an HES (only a magnetic field clear enough to produce accurate orientation signals, and a controller capable of working with it). With a field oriented control you can operate SRMs accurately without any position sensors whatsoever.

One of the biggest advantages I've seen for SRM applications to come, is that they do not suffer field weakening at higher RPMs compared to induction and permanent magnet motors, both of which end up fighting against the back EMF of either their permanent magnetic fields, or residual fields in the induction windings. SRMs are capable of operating into 100,000 RPM ranges, limited solely by their bearings and to some extent by control capabilities.

efficiency wise, induction motors *need* vector or VFD to reach 90%+ efficiency range, without it they have a fairly narrow sweetspot. SRMs achieve 90% efficiency with greater ease, and with the right control they are theoretically the best... although it's been theorized that combining permanent magnets into an optimized SRM could yield the very highest efficiency possible. It's also worth noting that induction motors with VFDs will consume more power than SRMs simply due to switching losses and PWMs... to the extent that an induction motor may consume twice as much power in DC inversion compared to an equivalent SRM.

as for recuperation (or regeneration), you can generate power from an SRM if the controller knows how to do it. specific programming would be required to take advantage of this, since the stator windings must be charged in order to get generated surplus current back out while the rotor is pushing back against the field in generator mode.

My conclusion is this: while permanent magnet BLDC motors currently possess slightly greater torque density than SRMs, I suspect that SRM-PM configurations will eventually take the cake for applications like electric vehicles (and bikes). I suspect that conventional induction motors do not offer any tangible advantages in efficiency or overall performance for ebikes, even with the latest in controls, but I could be wrong and only further experiments by the community will tell for sure. I do have a preference for induction motors as I've been building equipment with them for many years, but my scientific and practical gut feeling tells me that SRM technology is where the gains will be coming from, along with controls to maximize those gains.



edit, for application-specific design considerations: it's important to take into consideration for the application of ebike DD hubs, that an SRM design would actually be more complicated than in a stationary motor installation, due to the fact that the rotor and stator are effectively inside out (the inside is stationary, the outside is rotating). this is relatively easy to build with an induction design, but I've never seen it done with SRM configuration... now I want to see somebody build an inside out DD-SRM motor! however for the purpose of mid-drive arrangement, or a planetary geared hub, no problem to use existing SRM.

edit, for considerations of factors affecting future performance of above-mentioned electric motors:
-improvements in material science is a leading factor in past and present performance gains in all these electric drives, there is still a ton of room for improvement here, examples include reduction of back EMF with diamagnetic composites/laminates in proximity to permanent magnets or windings to effectively tweak their fields for higher RPM or efficiency, cooling improvements or temperature range of magnets, etc.
-improvements in the geometry and ferrous structure layout of the rotor in SRMs will definitely yield greater results yet, currently they are fairly rudimentary from what I've seen and fairly "young" in terms of overall refinement in construction. I did read a very cool paper about advanced simulations in SRM rotor design with FOC controls, yielding some promising results with improved rotor geometry... definitely looks like improvements are coming down the line in this regard.

 

[Punx0r]

So what's the catch with SRM to explain their lack of use? Is it the complexity of the control side?

 

[Deafcat]

SRM are relatively new, compared to induction motors which have been in use and steadily improving for a hundred years or so. Permanent magnetic motors (both AC and DC) have also developed steadily due to the ease of design and function within a constant magnetic field (less challenges with consistent torque and power). Most high precision motors have traditionally relied on electromechanical design features rather than control features for performance (such is the case for servo and stepper motors), and historically all the existing motor technologies currently employed in the industry have sufficed for all the various applications, but high efficiency levels have never been an innate requirement. with battery-powered EVs, it becomes a primary concern... which is creating a huge push for improved electric drive motors (amoungst other technologies like batteries and controllers to name a few).

SRMs have some existing applications but they have definitely been overlooked in favour of existing conventionally-proven solutions. Adoption into the automotive industry in particular would be a huge leap, or into industrial machinery, but both fields have traditionally made use of older technology until recent changes in design philosophy surrounding energy efficiency. This seems to have piqued renewed interest in SRM technology, which has never had a real incentive to develop further in the past, far as I can see. Looks like we will be seeing a lot more of SRM tech in mainstream applications soon, and I would not be surprised if they take off in the EV industry soon as well.

One of the most excellent applications I've seen recently in one of my "home field" industries (machine tools) with SRM tech is Okumas PREX motor in their new CNC Lathes, this is a super high efficiency, direct drive SRM motor powering both the turret and live tooling, and the main spindle itself (again, direct drive, no belts or gearing). You can find some good articles summarizing the performance and technical improvements (including how they achieved the SRM improvements), here's one in particular:

http://industrialpr.net/%E2%80%98secret%E2%80%99-motor-drives-lathe-spindles-as-well-as-turrets-and-live-tools/

some hot figures from that article:

"On the new Okuma LB3000 lathe, the PREX spindle motor provides an output of 30 kW and develops a torque of up to 700 Nm at 3,000 rpm. The larger motor on the LB4000 offers 37 kW/1,200 Nm/3,000 rpm.

As gears and belts are dispensed with, the switchover between speed ranges takes less than half a second, reducing unproductive idle time. When functioning as a C axis, the spindle is indexed with a repeatability of 0.005 degree.

A PREX motor continues to be used in the 12-station turret, offering 7.1 kW of servo power and generating 50 Nm of rotary tool torque, allowing high metal removal during prismatic machining parts of a cycle. Maximum tool rotational speed is 6,000 rpm."

I know for a fact the automotive industry is pursuing SRM tech to electrically actuate primary and auxiliary systems in similar fashion, the lack of permanent magnets and reduced conductor requirement for SRM makes it very attractive in the long run.


edit: turns out that Okuma PREX SRM does in fact have a few permanent magnets added into the rotor... I guess that really works after all! Looks like a great example of cutting-edge SRM design... I'd love to have a closer look inside one but they don't like it when I ask to take apart brand new machinery


edit again: further reading for renewable energy generator applications: http://wiki-cleantech.com/wind-energy/an-innovative-type-of-synchronous-electric-generator-without-rare-earth-materials-and-with-high-efficiency

 

[anpaza]

Well, if you add permanent magnets to SRM rotor, you just turn it into PMSM
The stators in permanent-magnet synchronous motors, asynchronous motors and switched reluctance motors is absolutely the same.

 

[Deafcat]

Well, if you add permanent magnets to SRM rotor, you just turn it into PMSM
The stators in permanent-magnet synchronous motors, asynchronous motors and switched reluctance motors is absolutely the same.

"absolutely the same",in that they contain a series of conductors... the stator winding arrangement varies considerably between these. The most obvious difference being independent (concentrated) windings versus overlapped (distributed) windings... you cannot build a reluctance motor inside the stator of an induction motor or a distributed winding IPM stator. SRMs are concentrated windings only, even with some magnets thrown in the mix. there are of course clear similarities between SPM and SRM stators... the difference being in how the rotor is constructed and functions.

 

[speedmd]

The SRM motor images I am finding look just like what we see in the BMW rotors in the factory video in the motor assembly thread. Explains the odd rotor design with few spaced magnets inserted in pockets. Interesting design.

 

[johnrobholmes]

My Mazak has an integrated DD spindle motor, and I bet it is also embedded magnet SR. No detent torque in rotation, and high torque density. I had asked a bunch of questions about it and none of the reps or repair men really know the nitty gritty of the drive design.

 

[Deafcat]

Hey totally! I just watched that video now,

https://www.youtube.com/watch?v=x3brfAEs_RY#t=307

that sure looks like SRM motor... wouldn't be surprised with BMW, as I've always been impressed with their tech advances and love working on their cars. compare the E46 generation factory video to Tesla's plant video nowadays and you can see so many similarities... BMW pioneered many of the automation production processes necessary to make truly excellent modern cars. (I drive and maintain a 99' E46 of my own and it's now sitting around 205K mileage with zero issues... joy to work on). They've also been kicking ass with EV tech in general and composite frame tech... kudos to them



edit: on closer inspection of the i3 motor video, the following is clear: 18 windings and 3 phase inputs of equal size on completion of the stator (plus a sensor connector), 12 rotor poles with comparatively small permanent magnets pressed in. This is consistent with an 18/12 3-phase SRM motor, of course with the extra awesomeness of magnets tossed in the mix. Cool!

 

[Deafcat]

My Mazak has an integrated DD spindle motor, and I bet it is also embedded magnet SR. No detent torque in rotation, and high torque density. I had asked a bunch of questions about it and none of the reps or repair men really know the nitty gritty of the drive design.

Wouldn't be surprised at all, although you'd think if they were using SRM drives they could charge a bit less for an Integrex spindle replacement LOL.

not as bad as Hermle... barely. Mazaks are good stuff

 

[johnrobholmes]

Yeah, I stuffed my subspindle not too long after getting it and it wasn't cheap to swap out. Got a good core swap value on my old one at least. I'm thinking a swiss lathe might have been smarter to learn production on, less mass when it crashes :lol:

 

[Deafcat]

Yeah, I stuffed my subspindle not too long after getting it and it wasn't cheap to swap out. Got a good core swap value on my old one at least. I'm thinking a swiss lathe might have been smarter to learn production on, less mass when it crashes :lol:

One thing I've definitely learned over the years... Why build aerospace and high tech, I should have gone into making spindles instead