4x less coasting resistance

[Hummina Shadeeba]

The cogging torque resistance greatly varies with the motor design and can sometimes be substantial. I can coast roughly 3x further with a regular wheel vs a pm hub motor. That’s a lot of lost momentum


I’m hoping u or anyone else can go beyond saying that lost energy goes to heat and explain how it could be transferred specifically. How would a magnetic resistance produce heat if not through eddies? Isn’t it possible to have the loss only a mechanical resistance and not a loss to heat? Analogous to drag from a bearing but without a friction that produces heat. Magnetic friction.

And also trying to confirm cogging torque would be 2/3 greater when coasting than powered where it would be showing as torque ripple

 

[sn0wchyld]

The cogging torque resistance greatly varies with the motor design and can sometimes be substantial. I can coast roughly 3x further with a regular wheel vs a pm hub motor. That’s a lot of lost momentum


I’m hoping u or anyone else can go beyond saying that lost energy goes to heat and explain how it could be transferred specifically. How would a magnetic resistance produce heat if not through eddies? Isn’t it possible to have the loss only a mechanical resistance and not a loss to heat? Analogous to drag from a bearing but without a friction that produces heat. Magnetic friction.

And also trying to confirm cogging torque would be 2/3 greater when coasting than powered where it would be showing as torque ripple

moving a magnet over iron or copper induces currents to flow within both. these moving electrons dont go anywhere, they just move around somewhat 'aimlessly'. the flow of current generates heat (as iron and copper have some electrical resistance). This is not however 'cogging torque' but eddy losses as you describe...

coging torque is purely the attraction of the magnet to the iron. there is a rapid switch between pulling toward the tooth behind a given magnet, and pulling toward the tooth in front of that magnet. this creates a rapid variation in braking/accelerating torque. its not in of itself lossy though - no heat is generated by this process, only the process of changing magnetic fields, as the pull backwards is roughly equal in magnitude and duration as it is to the pull forwards.

if for instance you had a solid block of iron in your stator, no windings, just a solid block... you'd have 0 cogging, as there's no variation in the distance between a large block of iron (as in a tooth) and the position of the magnets. The forces pulling the magnets toward a given tooth dont exist, because the 'tooth' (or solid iron core) is always the same distance from the magnets. You'd still have drag/eddy loss though (more so than in a conventional toothed core), because currents are still induced in the iron from the changing magnetic field. This is actually a way that trains brake - they induce current in a spinning piece of steel. The steel heats up, but there's no wear as there would be using friction via brake pads - just pure iron losses from it being forced to move through a magnetic field. Some use a disk for this, some actually use the rails themselves, energizing an electromagnet in close proximity to it. Its also how induction cook tops work - inducing currents in the pan to generate heat. its why some pans dont work on such cooktops, they arn't lossy enough.

That all said, reducing coging torque could infact help with coasting - as the rapid ossilations between the 'pull' and 'drag' wont necessarily be transferred to the road - they may well be absorbed in the drive line (stretching and loosening off on a belt/chain) or in the tires (ie 'wiggling' the tread back and forth) rather than actually transferring the torque to the road - ie generating heat in the drive line or tire rather than returning the 'potential' energy back to kinetic energy...

if you're still unsure remove the rotor from the stator, put it on an axle (that is well away from the rotor) and spin it up... you'll see it spins for a lot longer than with the stator in place. The losses (With stator in place) at low RPM are minimal, but so is the kinetic energy (kinetic energy is proportional to velocity ^2 - read up on kinetic batteries if you want further info on that) so bugger all stored kinetic energy requires bugger all drag to dissipate that energy and convert it to heat.

 

[Punx0r]

The cogging torque resistance greatly varies with the motor design and can sometimes be substantial. I can coast roughly 3x further with a regular wheel vs a pm hub motor. That’s a lot of lost momentum

A normal skateboard has very low friction on a smooth surface and it only takes a small increase in drag to substantially reduce the distance it will coast. The extra drag you're seeing is almost certainly from motor core losses.

How would a magnetic resistance produce heat if not through eddies? Isn’t it possible to have the loss only a mechanical resistance and not a loss to heat? Analogous to drag from a bearing but without a friction that produces heat. Magnetic friction.

A changing magnetic field in a conductor produces eddy currents, as you say. Simply magnetising a a piece of magnetic material results in hysteresis loss (also heat).

And also trying to confirm cogging torque would be 2/3 greater when coasting than powered

Why would this be?

Imagine having a steel block attached to a magent, with another, equally powerful, magnet opposite. You use your fingers to pull the block off the first magnet and towards the second, which when you cross the halfway point snaps it into place. Where is the loss?

 

[Hummina Shadeeba]

Cogging is for sure more than eddy currents as at the slowest speeds you can feel a large resistance. I wonder if u could even produce eddies at such low erpm.

if u were to spin a high cogging motor off the side of a driven motor while showing the current draw at different speeds could get to the bottom of it. I imagine the resistance would parallel rpm in a linear way vs exponentially as eddies would.

It doesn’t make sense that ct resistance is due to eddies as it would be increasing across the board with any increase in teeth and poles and the subsequent increase in passing of teeeth past magnets but that isn’t the case, the opposite is more so the case.

“Imagine having a steel block attached to a magent, with another, equally powerful, magnet opposite. You use your fingers to pull the block off the first magnet and towards the second, which when you cross the halfway point snaps it into place. Where is the loss?”

-The loss is there though as u can feel it. U can put a hard spin on the motor and it comes back to stationary very quickly. There’s no escaping this obvious loss