Motor comparison spreadsheet

[crossbreak]

hey Punx0r, what_'s wrong with that example? I know formulas look better if written in LaTex, but that was too much effort for this tiny simple example. i will sum up the formulas again like i did a few pages ago so people can play around with them, and yes i will use LaTex for that, of course.

Some pages back we thought about how to figure out a reasonable nominal speed for a motor. My argumentation: for best efficiency, copper loss equals Iron loss (fact). For best machine exploitation, hysteresis loss shall equal eddy current loss (assumption). Both Fe losses are about equally hard to fight. Eddy current loss by thinner lamination, Hysteresis loss by more sophisticated machine design, better machining/smaller air-gap or more expensive material.



Experience shows that this may be a meaningful. Maybe the 25%/25% are not the whole truth, but at least it determines a nominal rpm. It can simply be calced by dividing Hysteresis torque constant by the Eddy current torque constant. No load currents must be measured quite precisely to get a reasonable number by this. Note the almost 2 times higher rpm of the 40mm SAW compared to the 20mm SAW. How can the 40mm one have so little eddy current loss as it is double the stack compared to the 20mm one?

the cyclone motor simply stays behind it's possibilities. As it's an 8 pole inrunner, it can run a lot faster than the 3500rpm cyclone proposes.

What do you think?

 

[speedmd]

Why would you wind a large diameter the same way you wind a small diameter motor? Is not the sample meaningless doing so? Eddy current losses would be very different also given the vast difference in rpm in the 4x diameter example.

 

[crossbreak]

the large one gains lower kV just by
1.) connecting coil groups in series. A motor will have 1/4th of the kV of a motor using the same wind (like 4 turns per coil for example) with just a single coil group.. Rm rises accordingly by a factor of 4
2.) the fact that the big one has 4 times the radius. Lever (same as air gap radius) rises by a factor of 4, which lowers kV by another factor of 4. Rm stays unchanged

these are the reasons you may build a hub motor with 4 turns per coil that works reasonably well in the rpm range you need directly at the wheel as well as a much smaller motor with gear reduction that has also 4 turns per coil and runs at the same battery voltage

Eddy current losses would be very different also given the vast difference in rpm in the 4x diameter example.

eddy current loss does not depend on the numbers of turns you wind. It's depended on
1.) (lamination thickness)²
2.) (e-rpm)²
3.) lamination's electrical conductivity

to stay with Lukes' example:
small motor = 16 times the e-rpm of the small motor direct drive, as it has 1:16 reduction drive
big Direct drive motor = 4 times the e-rpm of the small direct drive

so we see that e-rpm is not the same for both which again makes it no apples-to apples comparison. But you see here that small motors depend more on thin lamination, as eddy currents get much more critical at higher e-rpm. as it rises with e-rpm²

 

[Arlo1]

the large one gains lower kV just by
1.) connecting coil groups in series. A motor will have 1/4th of the kV of a motor using the same wind (like 4 turns per coil for example) with just a single coil group.. Rm rises accordingly by a factor of 4
2.) the fact that the big one has 4 times the radius. Lever (same as air gap radius) rises by a factor of 4, which lowers kV by another factor of 4. Rm stays unchanged

these are the reasons you may build a hub motor with 4 turns per coil that works reasonably well in the rpm range you need directly at the wheel as well as a much smaller motor with gear reduction that has also 4 turns per coil and runs at the same battery voltage

Eddy current losses would be very different also given the vast difference in rpm in the 4x diameter example.

eddy current loss does not depend on the numbers of turns you wind. It's depended on
1.) (lamination thickness)²
2.) (e-rpm)²
3.) lamination's electrical conductivity

to stay with Lukes' example:
small motor = 16 times the e-rpm of the small motor direct drive, as it has 1:16 reduction drive
big Direct drive motor = 4 times the e-rpm of the small direct drive

so we see that e-rpm is not the same for both which again makes it no apples-to apples comparison. But you see here that small motors depend more on thin lamination, as eddy currents get much more critical at higher e-rpm. as it rises with e-rpm²

You would simply rewind the big motor with less turns and increase the KV while lowing the resistance. And you do not need to keep the same magnet polls you can decrease them to lower the E-rpm Remember you need to look at this from an engineer stand point when EVERY thing in the system can be changed to make it work better.
If your example was true then tesla should replace their water mellon sized motor that weighs 111 lbs with a 10lb motor that is the size of an apple and spin it faster with a further gear reduction.

 

[bunya]

Doesn't Tesla already have a large reduction between the motor and the diff to allow the motor to spin faster and increase the power density?

 

[crossbreak]

yep, telsa does not use hub motors, they use single-speed reduction inrunner bldc drives. that's my favorite. The motor spins at up to 14000rpm https://my.teslamotors.com/roadster/technology/motor
Gear reduction must be in the range of ~8. As an engineer, i'm sure it's a two stage reduction drive like the Nissan Leaf drive, but ATM i can't find an exact reference

Power density can be increased almost 1000% compared to a common DD-hubmotor this way, while maintaining superior efficiency over the drive cycle. Tesla claims outstanding 88% over the drive cycle. Maybe one reason Mercedes Benz simply buys those units instead of making their own. Also read about the Astro Matt Schumacher belt drives for reference https://www.electricbike.com/astro/

 

[speedmd]

Looks like double reduction. Photo has it at 9.73 to 1.

http://www.elioowners.com/threads/direct-drive-trans.5926/

 

[Arlo1]

yep, telsa does not use hub motors, they use single-speed reduction inrunner bldc drives. that's my favorite. The motor spins at up to 14000rpm https://my.teslamotors.com/roadster/technology/motor
Gear reduction must be in the range of ~8. As an engineer, i'm sure it's a two stage reduction drive like the Nissan Leaf drive, but ATM i can't find an exact reference

Power density can be increased almost 1000% compared to a common DD-hubmotor this way, while maintaining superior efficiency over the drive cycle. Tesla claims outstanding 88% over the drive cycle. Maybe one reason Mercedes Benz simply buys those units instead of making their own. Also read about the Astro Matt Schumacher belt drives for reference https://www.electricbike.com/astro/

why not make the motor spin 10x even smaller and spin it faster and just use a further reduction?

 

[liveforphysics]

A gearbox remains a device which converts shaft power and the energy stored in your battery into waste heat and friction as it chafes itself to eventual failure.

What is done today in the infancy EV's will be looked back upon by folks wondering WTF we were thinking.

If one finds themselves with an application where they want X amount of torque over a speed range (say 0-1,000rpm), it's possible to solve this many ways. We could start with something spinning at 100,000rpm and make an adequate gearbox. The end result would be starting with electricity, and converting it into high speed low torque shaft power output from a motor (which isn't what we wanted to end up with). Then we could take this input and devise a Rube Goldberg-esque series of levers to trade shaft speed for leverage, and in doing so add a number of series losses and added mechanical complexity and failure modes.

Alternatively, it's also possible to simply make the conversion from electricity to output shaft power using motor geometry that works over the speed range and torque desired, and add no further stages of loss or complexity.

I think if we look at RC as a predecessor, we already know where EV's are going. When brushless motors hit the RC plane world early, they were stupendously high RPM motors that all used a gear reduction to spin a prop. This worked acceptably, and RC planes flew well and when they crashed or got enough hours on them people replaced the gearboxes etc. Nobody was complaining at the time, as nobody knew of an alternative that was better. Today go to the RC flying park, every plane has a prop bolted to the rotor of a direct drive outrunner. Nobody uses a gearbox anymore, because motors became available with geometry that natively works (pancake outrunners) without needing to add the series loss and failure modes of a gearing stage. It took maybe 4-5 years for gearboxes to stop being used by say ~95% of performance RC flying machines, and now nobody would consider going back and what used to be done seems so pointless and silly to view with hindsight.

EV's will follow suit as they mature, it just takes enough $$$ gearbox failures happening and someone doing experiments with native high-torque electromagnetic typologies to see that it works.

ATB,
-Luke

 

[Hummina Shadeeba]

nice history lesson Luke. The summary in my mind is a quest and progression for more torque. to me the limitation is iron or electrical steel's ability to not saturate. And simply that a stiff enough structure to support multiple planes in the same pancake motor isn't here yet.

 

[speedmd]

Did not most of the early tesla gear boxes need to be replaced until they worked out the high torque spikes. Imagine this will be a maintenance issue long term unless this is significantly beefed up. Time will tell.

 

[crossbreak]

gear boxes are much older than electric motors or ICE. mankind evolved them to near perfection. this is why they are near silent and get near 100% efficiency today. they last millions of miles in trucks, where every penny counts. and yes, we will wonder in a few years why people invented hub motors. we will overcome this thing. at least in larger vehicles where they lack of sense. in smaller ones -that never climb hills- they will stay. I'm sure, since they can't be beaten in term of simplicity

 

[zener]

gear boxes are much older than electric motors or ICE. mankind evolved them to near perfection. this is why they are near silent and get near 100% efficiency today. they last millions of miles in trucks, where every penny counts. and yes, we will wonder in a few years why people invented hub motors. we will overcome this thing. at least in larger vehicles where they lack of sense. in smaller ones -that never climb hills- they will stay. I'm sure, since they can't be beaten in term of simplicity

Show us a link to a 99% or 98% or 97% gear box !

I think electric gearing will outplay mechanical soon. Something like delta WYE automatic switching or fluxweeknin. Or something yet unknown.

 

[speedmd]

Gears look to be a discovery from nature. "three-millimeter long hopping insect known as Issus coleoptratus beat us to this invention"
Read more: http://www.smithsonianmag.com/science-nature/this-insect-has-the-only-mechanical-gears-ever-found-in-nature-6480908/#RYl3WsJaY1pc8orQ.99

 

[crossbreak]

I think electric gearing will outplay mechanical soon

it never has and it never will. 99% are even possible on belt drives. Sure not on the reduction we want, but still https://www.gates.com/~/media/files/gates/industrial/power-transmission/white-papers/energy-savings-from-synchronous-belt-drives.pdf
even 90% are in far far future for such small electric (or serial electric, non-gear) gear boxes. sorry

[..]Something like delta WYE automatic switching [..]

this just reduces stress on controller. not more, not less, just a little less switching loss that is not significant at all. sry

Or something yet unknown.

think that's most likely!! as well as alien tec

 

[liveforphysics]

Gearbox efficiency only looks good at near peak power levels.

At cruise power levels both chain drive and gear/gear look very lossy if the vehicle is also capable of power much higher than cruising loads.

 

[crossbreak]

yep, sadly that's true. as well as DD-Hub efficiency. Hysteresis loss kills it

We need both a new drive cycle efficiency and gear box drive cycle efficiency benchmark. anyone who need a bachelor/masters thesis

otherwise, we will never ever get this 88% drive cycle efficiency figure that Tesla claims. Another question would be: Do we really need it?

 

[speedmd]

SRM (switched reluctance motor) types would reduce hysteresis losses, would they not?

 

[crossbreak]

sadly not, they are very good for positioning/servo drives for 3D printers or mills, but have no advantage for us

Edit: Here is a reference, SRM may have excessive iron loss at high rpm http://www.motor-design.com/cmsAdmin/uploads/ecce_2010_hybridvehicles.pdf

 

[major]

yep, telsa does not use hub motors, they use single-speed reduction inrunner bldc drives. .....

Tesla uses Induction Motors, unless they have recently changed.

 

[Hummina Shadeeba]

Don't hysteresis go up sharply with flux density saturation...so if it's possible to better utilize the iron available In a non-geared motor it would be less of a problem.

 

[crossbreak]

indeed, you better avoid that

you can use better material to avoid this. you need more magnetic conductivity(as far as i know) like cobalt alloys that are very expensive. formula1 electirc motors have these materials in use like vacoflux50

or simply more material, so it does not saturate. I think you would rather use the more expensive material on a small motor with quite high reductiuon

 

[crossbreak]

yep, telsa does not use hub motors, they use single-speed reduction inrunner bldc drives. .....

Tesla uses Induction Motors, unless they have recently changed.

they have https://www.teslamotors.com/blog/induction-versus-dc-brushless-motors

 

[liveforphysics]

Tesla is still induction and never hasn't been. The motor has a stupid sharp slip% to torque curve, so its extremely low loss at cruising loads (aside from its gearbox loss which I would guess rivals the total losses of the motor where an EV spends most of its time.) Its also likely sub 2-3% slip at peak torque. The Teslas today are only performance limited by pack power, when the next gen of cells inevitably releases it will perhaps become a low 10s or even 9s car, while having improved range and cruise efficiency and high power capabilities.

http://www.motortrend.com/news/2015-tesla-model-s-p90d-ludicrous-upgrade-first-test-review/

Induction makes a lot of sense for any application that does street and track. For pure track or ultimate power to weight, PM still works better.

Short flux path SR (radial or transverse-flux) has the potential to be better than PM and induction for both street and track, but its non-trivial to control due to the huge swing in inductance making current control and low torque ripple difficult while still keeping a low pole count to minimize hysterysis. This can be solved by making a series of low pole count short flux path stators each with a radial offset, maybe 6 of them to let it be smooth and torquey still. If the user can tolerate that when making peak torque the motor has ripple, this motor is best. It can still be ~0% ripple at low and mid torque levels.

 

[crossbreak]

interesting. do you have a link/more explanation/formulas on that?

what do you think about my 25%/25% hyst/eddy-loss idea?