Science, Physics, Math, & Myth

[Miles]

I'd love to see Death Bike's Motor in this database...

You will, once Luke's weighed it.

 

[speedmd]

You will not be able to do that with normal plastic wire insulation. 2/0 is good for 2300 amps, 3/0-2700 amps for ten seconds before they evaporate in a poof of smoke going by the chart. You will need some high temp wire insulation or go bus style with some heat sinking.

That fusing current chart seems to be on the extremely conservative side of things. My own test experience (which I can repeat on video if you like) shows cables not fusing nearly so rapidly at those current levels. I think it's worth looking into the test process for collecting the fusing currents, my hunch is maybe that the test it conducted while the cable is already at maximum temperatures or something.

Good point Luke. Most data I have seen are with steady DC current and no air movement which may be worst case for melt down. Pulsed current, and free air numbers would potentially be significantly higher /different. For reliability a bus bar type with some type of heat sink / conductor may be best at these levels.

 

[Punx0r]

This is shifting the topic back a bit, but I'd just like to reconcile something for the sake of my own understanding.

In the thread titled "PM Motor theory - formulae etc." (http://endless-sphere.com/forums/viewtopic.php?f=30&t=16376) Miles states:

Maximum (theoretical) power output occurs at half of no-load speed

.

Bearing in mind the post is from 2010, is this just out of date, or applicable to a brushed motor on a non-current limited supply?

Otherwise it implies (to my mind) that a fast-wind motor cannot produce the same power as a slower wind at a given speed?

 

[liveforphysics]

This is shifting the topic back a bit, but I'd just like to reconcile something for the sake of my own understanding.

In the thread titled "PM Motor theory - formulae etc." (http://endless-sphere.com/forums/viewtopic.php?f=30&t=16376) Miles states:

Maximum (theoretical) power output occurs at half of no-load speed

.

Bearing in mind the post is from 2010, is this just out of date, or applicable to a brushed motor on a non-current limited supply?

Otherwise it implies (to my mind) that a fast-wind motor cannot produce the same power as a slower wind at a given speed?

It's for a non-obvious reason that peak power occurs at half no-load RPM. I will give you a big clue, you don't change the drive voltage, you just add load until the RPM drops to half.

It's the same reason why peak power from a battery is when you're sagging clear down to half no-load voltage, or why a motor at peak power output is at 50% efficiency. In either situation, if you draw an additional amp, you decrease your output.

 

[John in CR]

I call that maximum possible power, which to me only drag racers should concern themselves with, because heat is so high (equal to power out). It only occurs where the stator is at full saturation, where added amps add no more torque. Peak power of our systems OTOH occurs at higher than 50% of no-load rpm and higher than 50% efficiency, because we limit the current. eg On the dyno, Hillsofvalp limited current into his ventilated HubMonster to 410A (despite my repeated recommendations not to run such high current), and his peak power at the wheel with voltage sagging to about 74V was 20.8kw on 30.3kw input for an efficiency of 68.6%. I run much lower current, 245A peak from the battery, for less heat stress, and have little doubt that my bike hits peak power in the mid to upper 80's in terms of efficiency. I recall Luck saying that Zero tunes their motors to achieve at least 90% efficiency through most of the operating range, which would obviously include peak power. With their big motor and the need to tune the bikes for use by the general public, it makes perfect sense to me that they would tune them so conservatively and eliminate heat failures.

 

[Punx0r]

Thanks, I think I get it now.

Of course, you change the applied voltage (like with the current) for a high Vs. low speed wind, so No Load Speed can be kept constant and both motors make the same power at the same speeds, assuming equal copper fill.

To make more power we either spin faster and use gear reduction, or keep speed the same and increase torque by growing radius and increasing tooth count? The limitation in both cases being core losses from increased commutation frequency?

Sometimes this stuff feels like joining the dots of previously isolated nuggets of knowledge...

 

[liveforphysics]

To make more power we either spin faster and use gear reduction, or keep speed the same and increase torque by growing radius and increasing tooth count? The limitation in both cases being core losses from increased commutation frequency?

The goal is to make get magnets passing by stator teeth in a way that ends up making the desired force at the wheel. Let's look at 2 examples.

If you had a smaller diameter motor, you could spin it quickly and run it through say 10:1 gearing to end up with the torque you desired.

Alternatively, you could mount the magnets on a ring with more radius (something like ~3.16 x larger radius), and now they move faster past the teeth and develop increased torque as a result of applying the torque on a longer lever-arm.

In otherwords, having a big diameter hub motor accomplishes the torque increasing effects without having additional system losses, complexity and failure modes of gearing vs having a tiny motor spinning quickly. The reason we aren't all rocking massive diameter hubmotors is only because someone hasn't made a lightweight, high efficiency, and economical massive diameter hubmotor yet. From a performance perspective, it has the potential to be the lightest and most efficient and highest continuous power. Something like tiny 2-3mm teeth wound with all half-turns (just a tiny rectangular copper bar being woven through them to wind) and a ring of tiny magnets, but on a radius close to rim size. Existing controller tech likely wouldn't power it very effectively due to the ultra high commutation frequency, but that doesn't mean it's not possible to do, just hasn't been done yet.

 

[John in CR]

....Alternatively, you could mount the magnets on a ring with more radius (something like ~3.16 x larger radius), and now they move faster past the teeth and develop increased torque as a result of applying the torque on a longer lever-arm.

In otherwords, having a big diameter hub motor accomplishes the torque increasing effects without having additional system losses, complexity and failure modes of gearing vs having a tiny motor spinning quickly. The reason we aren't all rocking massive diameter hubmotors is only because someone hasn't made a lightweight, high efficiency, and economical massive diameter hubmotor yet. From a performance perspective, it has the potential to be the lightest and most efficient and highest continuous power. Something like tiny 2-3mm teeth wound with all half-turns (just a tiny rectangular copper bar being woven through them to wind) and a ring of tiny magnets, but on a radius close to rim size. Existing controller tech likely wouldn't power it very effectively due to the ultra high commutation frequency, but that doesn't mean it's not possible to do, just hasn't been done yet.

To get around the commutation frequency problem is it possible to wind a number of consecutive teeth in the same direction of turn on the same phase, and then put a number of consecutive magnetics with the same pole facing inward to make it run like a giant low slot and pole count motor without needed the heavy core? The other alternative would be to use a lot of copper and wind the very high slot and pole count motor as a bunch of 3 phase motors and run it with a bunch of RC controllers. Then sensorless would have good startup, you'd have high frequency commutation covered, and it would be simple to reject the heat....Maybe use electrically isolated aluminum "spokes" for those end winding connections instead of copper, which would do double duty as both the stator support structure and part of the electrical system. Tensioned spokes like a bike wheel could be very light.

 

[liveforphysics]

Maybe someone with more motor knowledge can help me out, but I think if you decreased the pole count by putting a bunch of the magnets oriented the same way together on the rotor, it would substantially impact the performance. That's not to say it still might be better performance than any other alternative.

 

[flathill]

I don't think the controller is the problem

eddy current loss, abnormal loss, and hysteresis loss are all directly proportional to the magnetic field frequency, a.k.a. erpm which is proportional to the pole count divided by 2 (a.k.a pole pairs)

for large wheel rim motor you may need amorphous metal to keep efficiency up (and beat an equivalent geared system)

or you could just use smaller wheels

or any size wheel with a totally silent unipolar/homopolar rim motor with true DC direct drive...

 

[flathill]

[youtube]WPQJHMT0AQ0[/youtube]

 

[Alan B]

Analysis of typical Radial Flux Iron-Core Motor having variable winds

Concerns & Declarations:

1. Claimant posits motors of the same manufacture/series having the same load and speed and wheel diameter will use the same power regardless of the number of winds.
2. Effects of Controllers, Batteries, Cabling leading up to the Motor connectors are to be ignored; let us not introduce distraction. Presume these items are of top-flight quality and can deliver ideal power always. The claim is about the Motor, and nothing else.
   ...
   Further analyses will be forwarded as free-time allows over this holiday.
   Happy thanksgiving in advance, KF

1. 
   
   Sad to note that Alan's last post on ES was earlier in this very thread. Hope we didn't lose him over this discussion. Great progress has been made here, at some cost.

 

[johnrobholmes]

No, you could not reduce commutation frequency using repeated magnets while keeping the advantages of single conductor coils and short flux paths.

If you halve the commutation frequency for a revolution, you have doubled the flux path distance and require twice the back iron to compensate. The matching coils would have to span this farther "pole pitch", there are many ways it could be done. Multiple teeth can share a polarity but the connecting iron behind must compensate for the span, and now there is more stator insulation per unit of copper and slot area vs a shorter span or single larger tooth that covers full pole pitch.

Nothing's for free!

 

[liveforphysics]

I've got good news for you my friend, it's being done and works.

http://www.thingap.com/high-power-density/

http://www.jobymotors.com/public/views/pages/jm2s.php
They have higher power versions with larger radii that scaled up beautifully as well.

I think the next step is to slip making turns on the tooth and just weave a solid bar through a stator insulated with one of the many high temperture industrial high toughened temperature ceramic coatings (applied at ~0.2mm). This alone would be a rough copper fill penalty to give such a small slot, but we far more than make up for it by using that ceramic insulated slot to feed a non-insulated slot filling solid copper bar extrusion (custom die's for small aluminum or copper extrusions cost ~$500) through there. Die-cut sheets of Mica with whatever little finger tabs were needed to insulate the current-transfer runs (because they aren't quite end-"turns", it's has only a 1/2 turn winding per tooth). Call me lazy with my winding, but if I can get ~>80% copper fill factor (better than possible with any round wire based solution), for way less winding effort (because a half-turn is just half the labor of taking even 1 turn per tooth in winding), I'm going to take it. It's convenient that there would be so many half turns it would still end up having some inductance and likely a reasonable 12-24V winding is possible. Maybe one designed to take advantage of the ID provided by a 29" wheel! How handy to get some advantage rather than penalty for having a large diameter wheel when direct drive.

 

[liveforphysics]

MIca commutator die-cut and pre-bent pieces are available in 0.2mm thickness, and offer 10kV of dielectric strength to >600degC. The punch tool to get a batch of tiny die cut pieces, even from fast SF-bay area venders is like $300. The material costs would be insubstantial per motor, and once they had the die we could tell them to just add it to there available parts inventory so anyone could buy them. We could do the same sort of thing with the copper or aluminum slot extrusion. If we picked a common tooth geometry and simply scaled up and down the number of repeating teeth/magnet sets as the wheel diameters grew, it would make it very economical for DIY'ing custom larger or smaller diameter lightweight torque dense high efficiency direct drive motors.

http://aximmica.com/products/mica-commutators/

The lamination strip will be thin and flexible enough to use cheap tiny roll-stamps tooling and do high-speed stator coils (Being a continuous strip has no effect on eddy losses because the current isn't having any net changes in that axis). They make automatic machines that do this process with much heavier and wider stuff than what we will be using. Might be sub $5k to tool up for the stator strips to get wound into stators of the width of your choice if we find the right shop.

I've even got a factory-direct connection for custom magnets I stumbled across cycling through China in an area rich (or cursed) in 'rare' earth.

I've got more time now to get quotes for things, I will send a few emails to local vendors I've got contacts with to see if we can ball park doing a proper enthusiast level hubmotor. If you're Miles you get it in 4mm wide and make the worlds most elegant lightweight beautifully perfectly functional art bike with it, if you're JohninCR you get it in 125mm width and smoke superbikes, if you're Arlo you go 24" width and go for messing up the world of ICE drag racing.

These kinds of motors are possible, and I bet this new generation of controllers like Leboski's and Justin's Grinfinion will be able to run them.

They have many structural rigidity challenges due to being just a light weight ring of thin flexible iron, with strong magnetic forces tugging on it, and the rotor will require being something very stiff, like a deep dish triple wall rim or something (conveniently we have a ton of profiles already available to pick something). Maybe 3D printed adaptors that the rotor iron gets pressed into, and then the adaptor gets pressed in and epoxied to the rim or something. I've got access to a 24"x24"x24" gigabot printer a friend got to print rotor/rim adaptors.

DIY leads industry. There is room to improve today, in motor mass, torque, and continuous power, and it's possible to do all of them, and possibly possible to do all of them and make a cheaper motor with DIY possible tooling costs.

 

[johnrobholmes]

Joby splits magnets to reduce the eddy currents from within, the magnet pole pitch still allows for full pitch winding around a single tooth. That's what generally happens when a motor is designed from scratch. Thin gap motors are pretty rad too, the back iron moves with the magnets so steel losses are really low.

I'm with you on single or half turn coils (wouldn't it only be a half turn if the phase starts and stops on opposite sides of stator?) being an ultimate design, although the labor saved in winding seems to be offset by the trouble to get all the coils connected in my limited experience.

 

[johnrobholmes]

I still want to make the motor Mlles modeled. Got behind in fun stuff here lately.

 

[liveforphysics]

I'm with you on single or half turn coils (wouldn't it only be a half turn if the phase starts and stops on opposite sides of stator?) being an ultimate design, although the labor saved in winding seems to be offset by the trouble to get all the coils connected in my limited experience.

Believe it or not, a winding scheme is possible that requires only 3 continuous pieces of wire taking an alternating weaving course through the teeth. If delta terminating, no connections are even needed, if wye, a single crimp joint.

 

[Punx0r]

Great info guys

I've been a believer in the idea of lightweight, large diameter direct-drive motors for a while, thanks to the convincing arguments presented on this forum. It was just looking at Mile's spreadsheet, where the specific-Km of the little RC motors was so much greater, even allowing for the weight and efficiency penalty of a gear reduction made me think there was something wrong with the direct drive motors by comparison.

I find it interesting that most (all?) commercial electric cars use fixed gear reductions upto ~10:1. I guess the complexity and efficiency penalty is minor compared to designing a suitable direct-drive motor.

 

[Miles]

I've added another column to show eddy torque at the target rpm, separately from the total drag torque. I thought it might help to show how the hysteresis torque to eddy torque ratio changes with speed. What do you think?

 

[actionobject]

i love you guys! you blow my mind on a daily basis THANK YOU!

 

[teslanv]

• Presume –
• Same Motor, except for winds <- the only physical difference.
• Winds use the same AWG; you said same same same same… therefore everything is the same!

• 
  No, the copper fill is what stays the same, nobody is talking about just unwinding copper off a stator and then comparing it's performance... Talk about pointless Trolling to spread misunderstandings...

• 
  
  For the record, this is exactly the point where KF and John/Luke diverge.
  
  KF is pushing the concept that in his theoretical model, the winds are NOT the same copper fill, but rather a common total cross-sectional area, where higher number of turns will result in proportionally more copper fill than a lower number of turns.
  
  Kingfish's statement is 100% valid, if you accept his parameters.
  
  This argument is not about math or engineering, but about the acceptance of the Copper Fill parameter. Luke and John outright refuse to accept this different copper fill parameter because it is not common practice. However if one wants to truly understand torque as it relates to number of winds, this is the most basic of concepts.

 

[Miles]

However if one wants to truly understand torque as it relates to number of winds, this is the most basic of concepts.

I don't think we need FEM to resolve this one

 

[Arlo1]

• Presume –
• Same Motor, except for winds <- the only physical difference.
• Winds use the same AWG; you said same same same same… therefore everything is the same!

• 
  No, the copper fill is what stays the same, nobody is talking about just unwinding copper off a stator and then comparing it's performance... Talk about pointless Trolling to spread misunderstandings...

• 
  
  For the record, this is exactly the point where KF and John/Luke diverge.
  
  KF is pushing the concept that in his theoretical model, the winds are NOT the same copper fill, but rather a common total cross-sectional area, where higher number of turns will result in proportionally more copper fill than a lower number of turns.
  
  Kingfish's statement is 100% valid, if you accept his parameters.
  
  This argument is not about math or engineering, but about the acceptance of the Copper Fill parameter. Luke and John outright refuse to accept this different copper fill parameter because it is not common practice. However if one wants to truly understand torque as it relates to number of winds, this is the most basic of concepts.

• 
  No because the copper fill stays the same you will have say 8 turns with 4 strands or 8strands with 4 turns all gives you 100% the same copper fil . A motor winding person would not do it any other way the goal with every slot is always to fit as much copper as possible once you find the # of total strands that can fit through a slot you will always use that number its just a matter of how many are parallel and how many are in series (series is # of turns . No matter the combo the number of prices of copper stays the same with the same gauge of copper.

 

[teslanv]

A motor winding person would not do it any other way the goal with every slot is always to fit as much copper as possible once you find the # of total strands that can fit through a slot you will always use that number its just a matter of how many are parallel and how many are in series (series is # of turns . No matter the combo the number of prices of copper stays the same with the same gauge of copper.

^^^ Yes, exactly. This is the copper fill parameter.
But an engineer still understands that a single wire wrapped 8 times around a stator tooth will provide twice as much torque (and have twice as much copper fill) than if that same wire were wrapped around the tooth only 4 times.