Ferrofluid and High Power Wheels

[InfiniteWheelie]

If you don’t know what ferrofluid is, it’s a magnetic fluid that bridges the gap between the stator and the rotor. This allows heat to make its way out of hub motors to enable higher power. This has been used in e-bike hub motors for years and it’s proven to work.

People often think batteries are the limiting factor for products, but in our case the available battery cells can far exceed what the manufactures are building. Today’s cells allow for power levels in the 20kw (continuous) range. To take advantage we need more powerful motors, which can be done by in part by using ferrofluid.

Another change that may help is using the rim itself as a giant heat sink. We already have this massive, rotating chunk of aluminum which happens to be an amazing heat conductor. This would involve placing the magnets directly on the rim (without any spokes), which allows the ferrofluid to transfer heat directly to the rim.

What do you think about using ferrofluid along with spoke-less motors to help achieve really high power?

Edited January 14 by InfiniteWheelie

[Panzer04]

These sorts of things have already been investigated in the ebike world - I don't see why it would be much different for EUCs. Look up "statorade" and "hubsinks" for the kinds of things I mean (or refer directly: https://ebikes.ca/product-info/grin-products/statorade.html )

They found that as little as 10ml ferrofluid/statorade roughly doubles thermal conductivity, so the long-term average power handling of an EUC motor would approximately double with it. This isn't quite the same as doubling power handling, since short-term fluctuations will still heat up the motor much faster, and you'll just need to wait less time for it cool down - but still pretty good. Combined with heatsinks you'd get much better cooling than normal, though, and for not much effort :).

Probably it's not been an issue thus far because most people just aren't pushing their wheels hard enough in ways to overheat them. I've done it recently, but only by going up very steep hills repeatedly, on a 16X (hardly the best case for motor cooling in any situation)

What I think would be cool is the possibility of full-on watercooling. Unlike ebike motors EUC hubs nowadays have huge hollows and bearings in the middle to route things through. It wouldn't be impossible to conceive of routing water lines directly to the copper coils and getting crazy amounts of cooling.. But I guess that's way overcomplicated and totally unnecessary XD

[mhpr262]

Axial flux motors could already be far more powerful - offering more power than 99.9% of EUC rider would ever want to use - without needing any cooling at all.

[InfiniteWheelie]

From the research I’ve done, it seems that hub motors in the 20kw (continuous) range aren’t possible without liquid cooling. While liquid cooling is technically possible, it’s not desirable since it adds significant cost, weight, and complexity. This makes motors the biggest bottleneck by far, unless you take something else into account: redundancy.

I think ideally, electric wheels should be 100% electrically redundant. This means 2 separate batteries, 2 separate controllers, and 2 separate sets of phases. It also means limiting the combined power output to 50%, so that during a sudden failure the remaining side is guaranteed to keep up.

Doing this leaves battery power in the 10kw range (about 2x our best motors currently). From what I gather that’s probably in the ballpark of what’s possible for a well designed, large, air-cooled, ferrofluid hub motor. It’s also much better for the batteries anyway, since cycle life and range would greatly suffer at their full continuous power rating.

So perhaps we could have our cake and eat it too, with 2x the power and full redundancy. The main performance hurdle seems to be the hub motor, since battery and controller tech is already widely available at these power levels (look at aftermarket Surrons).

Edited January 27 by InfiniteWheelie

[PourUC]

Redundancy is a waste in such a small form factor. Better to push for better reliability than doubling systems.

Even single prop planes don't have a redundant engine and prop because the form factor makes it too much hassle and defeats the purpose.

I think EUC's have greater potential heat dissipation than standard hub motors you will find in other applications.

On an EUC both side of the wheel are exposed to air. Keep the spokes, and use them as a radiator increasing their total surface area where possible. 

[InfiniteWheelie]

2 hours ago, PourUC said:

Redundancy is a waste in such a small form factor. Better to push for better reliability than doubling systems.

Even single prop planes don't have a redundant engine and prop because the form factor makes it too much hassle and defeats the purpose.

There's virtually no weight or space penalty... The battery is already naturally split (left an right), winding the motor as dual 3-phase doesn't require anything extra (except 3 more lead wires), and an extra controller is fairly light and small (plenty of room up top).

Also, since current battery tech is able to output over double what our air-cooled motors can handle, it's not even a waste of battery power. We are limited by the nature of air cooled hub motors, so why not use the excess battery power for full redundancy instead of leaving it untapped?

 

2 hours ago, PourUC said:

I think EUC's have greater potential heat dissipation than standard hub motors you will find in other applications.

On an EUC both side of the wheel are exposed to air. Keep the spokes, and use them as a radiator increasing their total surface area where possible. 

I don't think our motors have better heat dissipation than others. All air cooled hub motors suffer the same problem, which is transferring heat from the stator to the rotor (motor case). That's why ferrofluid works so well, because it creates a liquid bridge between the two.

It's true that keeping the spokes (and adding radiator fins) should provide more continuous cooling compared to a spoke-less design. It just seems like a waste not to utilize the huge rotating aluminum mass of the rim somehow. Perhaps you could have the best of both with a spokes-less design, and small fins on the rim itself or the side covers.

Edited January 23 by InfiniteWheelie

[PourUC]

16 minutes ago, InfiniteWheelie said:

There's virtually no weight or space penalty... The battery is already naturally split (left an right), winding the motor as dual 3-phase doesn't require anything extra (except 3 more lead wires), and an extra controller is fairly light and small (plenty of room up top).

Also, since current battery tech is able to output over double what our air-cooled motors can handle, it's not even a waste of battery power. We are limited by the nature of air cooled hub motors, so why not use the excess battery power for full redundancy instead of leaving it untapped?

 

I don't think our motors have better heat dissipation than others. All air cooled hub motors suffer the same problem, which is transferring heat from the stator to the rotor (motor case). That's why ferrofluid works so well, because it creates a liquid bridge between the two.

It's true that keeping the spokes (and adding radiator fins) should provide more continuous cooling compared to a spoke-less design. It just seems like a waste not to utilize the huge rotating aluminum mass of the rim somehow. Perhaps you could have the best of both with a spokes-less design, and small fins on the rim itself or the side covers.

1) Extra controller is extra space. There isn't existing room for this.

2) Splitting the battery in two is in fact limiting the battery output by 2x. Maybe current motors are not uber-efficient, but they use more power than you are seemingly aware of. I can easily pull 70amps from the batteries. Splitting the amount of effective parallels in half would stop me from being able to do this. Whilst these bursts to 7kw + on current motors may not be sustained, we still need batteries capable of delivery these. In short, I can't run 2p without risking cutouts.

3) I am saying to use ferrofluid. I am countering the idea that the hub would need to be water cooler because the rim and spokes are more exposed on an EUC than they are in a car for example. With spokes you would still have the rim, just more surface area overall. Plus for 14" EUC's that would be quite a large hub motor.

[Panzer04]

11 hours ago, PourUC said:

Redundancy is a waste in such a small form factor. Better to push for better reliability than doubling systems.

Even single prop planes don't have a redundant engine and prop because the form factor makes it too much hassle and defeats the purpose.

I think EUC's have greater potential heat dissipation than standard hub motors you will find in other applications.

On an EUC both side of the wheel are exposed to air. Keep the spokes, and use them as a radiator increasing their total surface area where possible. 

I think I have to agree with this. Full redundancy is a fairly large ask for a failure mode that could be largely eliminated with good engineering (and the incentive of much reduced BOM cost makes it that most desirable outcome in any case - so long as the leftover resources are used to improve reliability).

10 hours ago, InfiniteWheelie said:

There's virtually no weight or space penalty... The battery is already naturally split (left an right), winding the motor as dual 3-phase doesn't require anything extra (except 3 more lead wires), and an extra controller is fairly light and small (plenty of room up top).

Also, since current battery tech is able to output over double what our air-cooled motors can handle, it's not even a waste of battery power. We are limited by the nature of air cooled hub motors, so why not use the excess battery power for full redundancy instead of leaving it untapped?

 

I don't think our motors have better heat dissipation than others. All air cooled hub motors suffer the same problem, which is transferring heat from the stator to the rotor (motor case). That's why ferrofluid works so well, because it creates a liquid bridge between the two.

It's true that keeping the spokes (and adding radiator fins) should provide more continuous cooling compared to a spoke-less design. It just seems like a waste not to utilize the huge rotating aluminum mass of the rim somehow. Perhaps you could have the best of both with a spokes-less design, and small fins on the rim itself or the side covers.

Cutouts due to controller failure are already nearly entirely relegated to some sort of engineering failure, not random component faults.

Batteries are likewise not likely to be a significant failure mode, at least not in the sense of a cutout 99% of the time. A battery failure is very unlikely to dump you immediately, as most EUCs (especially ones with bigger batteries) have enough packs in parallel that you'd have to be going very close to the limit, with 0 warning of failure. A good BMS that keeps track of battery health should be enough to inform you if something is going wrong.

Electric hub motors are also nearly entirely immune to failure, given their solid-state construction. The only real concern is heat, but a heat sensor easily negates that.

Yes, watercooling for EUC motor is probably a bit over the top XD. I do think ideas that improve the motor cooling on an EUC are interesting, because they open the door to using smaller, *lighter* motors. I think the big gains for a new EUC would be bringing the performance of a 40-45kg EUC down to 30-35. Weight makes a really big difference to feel, nimbleness and ease of use (easier to access power and force the EUC to do what you want). I think ferrofluid alone is a big improvement already over nothing, and presumably something like hubsinks wouldn't be difficult to add.