4x less coasting resistance

[fechter]

If you just had magnets on rotor and stator, there would be cogging torque but no real loss once you get it spinning. I suppose some of the torque ripple would translate into noise and cause a small loss, but it would be very small compared to normal iron losses (which are already pretty small).

 

[Hummina Shadeeba]

I mean a loss in that I’m going 30mph and let off the throttle and when unpowered my potential energy seems to vanish as I only coast maybe 100ft instead of 300ft. Where did it go?

If ur saying this loss, as I describe it or even as u did with magnets on both sides, is less at higher speed why would that be? The inertia or momentum will roll through the indents and become unnoticeable to the tooth but still the resistance I imagine would be based only on how many revolutions occurred and the total magnet interactions as they’re at the speed of light.

Where does cogging torque’s lost potential energy go?

 

[Punx0r]

If I remember correctly, cogging has little or nothing to do with drag when the motor is unpowered (that is core losses responsible). I recall it coming up when a proprietary (Bionix?) 5 phase ebike system was being marketted as having lower drag because it had inherently lower cogging.

You could set your throttle so that 0% throttle actually feeds a handful of watts to the drive motors - just sufficient to approximately balance the core losses. This will allow the board to freewheel like the motor wasn't present. It wouldn't be sufficient power to move the board with a rider on it so you'll still come to a stop and stay still, but you'd need a "true stop" feature to stop the trucks turning when you picked up the board.

 

[fechter]

There is a certain amount of torque required to overcome the magnetic cogging, but it is essentially loss-less. All the force needed is returned as it goes from one cogging point to the next.

The drag you see with an unpowered motor is going to be mostly iron lossses, which increases with speed. If you had an ironless motor design, you could have near zero loss when unpowered. There would still be some eddy currents in the copper but with skinny wire you can get this down to near zero.

 

[Hummina Shadeeba]

I used to say the same as I’d read cogging torque was no net gain or loss but spinning a motor in-hand shows cogging torque is a big resistance even at the slowest speeds when iron losses are minimal and that resistance isn’t growing even linearly as speed increases. Increasing the teeth n poles in a pm motor will decrease cogging resistance n increase iron losses so a motor w just two poles will have more resistance at low speeds yet have minimal iron/switch losses


A simple test would be comparing the unpowered coast distance when letting off the throttle at 10 vs 30 miles per hour. Maybe at the higher speed iron losses would decrease the speed faster but still

 

[Voltron]

The cogging feel of spinning a motor shaft by hand is deceptive. You feel the resistance as it builds to hopping towards the next magnet, but it's easy to miss the force it would take to slowly ease it to the next neutral point, instead of just letting it jump there. They balance each other out.

On a bumpy surface, I could def see hitting a bump at the same time as the torque ripple was in sync with it could slow you down more, but on a smooth surface not so much.

 

[Hummina Shadeeba]

How can people say cogging torque balances out when it can be seen the the minimal spin that results in relation to effort that went into the initial spin of a motor by hand? With a few pole motor you could spin to 1000rpm and even with the enertia of the heavy rotor and ...woooosh...back down to not moving in moments compared to if no magnets in there

 

[flat tire]

Read your own thread. Luke's post on first page. Iron core losses and hysteresis.

 

[Hummina Shadeeba]

I did read that but did u read my understanding of how it seems there’s more to it? If cogging torque were only iron losses it would be almost nonexistent at slowest speeds but that’s not the case and it would also increase at least linearly w speed which I don’t think it’s doing. Also cogging torque decreases with more poles generally while iron losses increase.

 

[Toorbough ULL-Zeveigh]

yeah there's got to be more to it or there wouldn't be so many papers out there on trying to reduce it if it really was just zero sum.
as a general rule of thumb, most techniques to reduce cogging are a compromise that come with some trade-off.
you give up a little of something that you can live with(out) in order to get smoother torque delivery that may be more critical or useful.
for example as i'm sure you know, skewed teeth come at a price of on efficiency hit & is why lives4fizz absolutely detests skewing.

Please dont skew the rotor teeth....

It will only help with the torque ripple at the cost of efficiency, and mimimaly effect the noise.

Skewing the stator = Uber-fail.
...
If you skew a stator, you reduce torque ripple at the cost of ever being efficient or low harmonic.

and that's almost always the case that something is lost with whatever scheme is implemented.
so that linked paper in your OP unless i missed it i see no mention of any possible drawbacks, is it really a win-win?

https://discourse.odriverobotics.com/t/anti-cogging-feature/293

 

[billvon]

IIRC that's exactly what the old Tidalforce motors did - some odd number of salient poles on the stator. (14? I think there were 7 individually commutated phases.) These did not quite line up with the 16 magnets. There was almost no cogging.

from the engineer who built & designed it.

When you run off battery, You could not pedal the Tidalforce or E+. It was a killer on your knees.

Cogging != drag

 

[Hummina Shadeeba]

Low cogging yet bad on knees I don’t understand.


If it’s drag, a magnetic drag, where is the energy going exactly? Call it whatever u want but where is the energy going

 

[Toorbough ULL-Zeveigh]

+1??
TF is the only ebike i've owned & ridden from 2005 to date.
while the sine controller wuz a decade ahead any one else delivering silky smooth silent power, bike-like it aint when the battery runs out.
it's why i've done a ton of reading & have such deep abiding interest in this subject.

If I remember correctly, cogging has little or nothing to do with drag when the motor is unpowered (that is core losses responsible). I recall it coming up when a proprietary (Bionix?) 5 phase ebike system was being marketted as having lower drag because it had inherently lower cogging.

You could set your throttle so that 0% throttle actually feeds a handful of watts to the drive motors - just sufficient to approximately balance the core losses.

5phase is falco & i think you hit the nail as to what's going on in that they continue to call it zero-cogging. which as maketing hype goes doesn't bother me in the least as it does some on this sphere because technically it's correct.
perhaps to avoid similarly getting ripped a new one by spheroid gnitpikkerz our sphereless leader went with virtual electronic freewheeling.
i'm sure from day one every TF owner figured it out how to do it manually but it would be nice to reduce thumb RSI to have it done electronically.

 

[Voltron]

Low cogging yet bad on knees I don’t understand.


If it’s drag, a magnetic drag, where is the energy going exactly? Call it whatever u want but where is the energy going

The energy is going into heat... As the iron rotates thru the alternating magnet fields it's slowly getting hotter.

 

[Punx0r]

Sorry, I completely misremembered the company.

If it’s drag, a magnetic drag, where is the energy going exactly? Call it whatever u want but where is the energy going

Any energy lost as drag can only be converted to heat. So if you sealed up your motor in fantastic insulation and turned the shaft slowly, so that core, bearing and windage losses were negligable then you'd soon find out how much loss cogging creates. If you're right and cogging is responsible for the majority of the drag you feel on your board then your motor should get very hot quite quickly. You said cogging was a big resistance even when turned slowly by hand. If that force is all wasted and not returned, like a magnetic spring, then it'll be converted to a lot of heat.

 

[Hummina Shadeeba]

If cogging torque was eddies an aluminum stator would produce more resistance no? I’ve never seen that but think could get things clearer. An all iron stator w no slots spins verrrry freely.

Are heat n resistance in eddies in equal measure


All snark should be at the end of the post for organization and limit 1 ! per person per lifetime.

 

[Hummina Shadeeba]

Do pm motors with very low cogging have less current draw at the slowest rpm?

 

[Punx0r]

Cogging is simply trying to pull a magnet away from the lump of iron it is currently attracted to. Once the rotor rotates enough that the next magnet is closer, the force flips direction and the rotor is suddenly pulled in the same direction it was previously resisting turning.

Yes, you're alternately magnetising the stator as the rotor turns, and this is hysteresis, as you say, but that's a component of core loss, not cogging.

TBH the dominant drag for a person standing on a skateboard at 30mph is likely air resistance. That's going to really mess with your results if you're trying to equate coasting distance at different speeds to motor drag.

 

[Hummina Shadeeba]

Cogging is just pulling of a magnet from iron. Thinking about it like that it doesn’t seem a loss and just a mechanical resistance that would slightly increase the current needed to pull through. that would be the only source of heat produced by cogging i believe today. Copper loss

Do motors with less cogging have comparatively less current draw when powered at the slowest speed?

A pm 3 phase motor esc pushes current through two phases and the third is passively being used to sense voltage for position but I’ve heard that isn’t always the case and foc program will have all three phases in use making torque is that true?
If true and all teeth were driven then i imagine there would be no torque ripple due to cogging when motor powered, but otherwise and if not all teeth driven, and one phase always used to sense voltage, then passive unmagnetized teeth will be cogging as the motor is powered as well as unpowered as any unpowered teeth are cogging always

Who is going to disagree? I say cogging torque is similar to the bearing’s drag and rpm dependent and only resulting in copper loss as it’s overcome. Not eddies. As if it were bearing drag but with no heat produced from friction. ?

 

[Hummina Shadeeba]

Sorry, I completely misremembered the company.

If it’s drag, a magnetic drag, where is the energy going exactly? Call it whatever u want but where is the energy going

Any energy lost as drag can only be converted to heat. So if you sealed up your motor in fantastic insulation and turned the shaft slowly, so that core, bearing and windage losses were negligable then you'd soon find out how much loss cogging creates. If you're right and cogging is responsible for the majority of the drag you feel on your board then your motor should get very hot quite quickly. You said cogging was a big resistance even when turned slowly by hand. If that force is all wasted and not returned, like a magnetic spring, then it'll be converted to a lot of heat.

Heat from what? Not iron losses just a magnet pulling? I don’t see how that would produce heat other than as copper loss as the resistance Is overcome. How would just a magnet’s pull produce heat?

(I erased above the test intended to isolate iron losses from ct by comparing time taken to spin down from 30 and 10 mph on the table. Didn’t make sense. How u could identify the energy lost to ct from iron losses is the goal. How to get an accurate current draw at 60rpm? Is there a minimum speed at which eddies and hysteresis are produced so they could be completely eliminated to isolate ct?


Wouldnt the power needed to overcome ct at 60rpm be the same as at 60000rpm? Unpowered

When powering a three phase concentrated winding motor will 1/3 of the teeth always not be delivering torque or are all teeth driven in foc?

 

[Voltron]

Passing the magnets induces an electric current, that's where the heat is coming from. If it was just a stationary magnetic pull, it wouldn't be heating, it's the moving a conductor thru a magnetic field Faraday thing.

 

[Hummina Shadeeba]

what heat? heat from eddies and hysteresis but why cogging torque? what is cogging torque and how would it produce heat?



and how is it said the motor's moment of inertia fillers out ct at high speeds, sice the magnetic fields are going at light speed?

 

[Punx0r]

I was just saying that in general terms if your motor experiences drag from cogging then that loss will manifest as heat - so if your motor doesn't get hot then you can't have a significant loss.

 

[Hummina Shadeeba]

I understood that and wondering how heat would be produced other than copper loss overcoming the drag? Maybe everyone is in agreement that it’s just copper loss at this point though. In which case cogging torque would produce no heat when coasting and just be a loss of momentum

Not a significant loss maybe but one there when both unpowered and powered. I assume when powered maybe would drop to 1/3 of when unpowered as that would be what percent of teeth on the motor at any time are passively being sucked to magnets with that 1/3 of the motor being used for sensing back emf only no? Then cogging torque ripple includes 1/3 the torque of the cogging torque shown when unpowered?

 

[Punx0r]

Tbh I think you're trying to fit reality to your theory that cogging is a significant cause of drag/loss in an unpowered motor.

I suspect you are correct that there is a (tiny) extra copper loss, probably at low speed, in a powered motor to momentarily overcome the cogging torque, but at some speed rotor inertia will largely overcome the cogging torque.

You say about cogging torque not producing heat when coasting, just robbing momentum, but this cannot be. Any loss in momentium is a reduction in kinetic energy and this must be converted to another form of energy - and heat is it (ignoring a tiny fraction as vibration). To slow you down, there must be drag - a braking force. Brakes produce heat.