Pushing a 24V Hub Motor to 30km/h on Lead-Acid batteries. Is Field Weakening our only hope?

[mvrocket]

Hello! We are a highschool student EV team.
Our school has seized our workshop and we have almost no budget left. However, we want to beat our rival school (who have a respectable aluminum welded frame) in the next Eco-Run race.
The workshop and budget are currently under negotiation so I think we can somehow make it work, but to save money as much as possible, we plan to use the hub motors we bought two years ago.

Crazy rules:
* Distance: 5.5km (550m x 10 laps)
* Stopping every lap for a driver change (9 times total!)
* Battery: Only two 12V lead-acid batteries (24V 3Ah)

Current strategy (is this correct?):
* Supercapacitors: To get through the 9 accelerations without draining the lead-acid batteries.
* Field weakening (flipsky VESC 4.20): To push the hub motor to 30km/h (at the cost of efficiency).
* Pit stops: Aiming for driver changes under 6 seconds.
* Motor: One 250W hub motor (rated for 24km/h)

Question:
Is "hub motor + field weakening" a suicide mission? Should we switch to a chain drive (mid-drive) instead of a hub motor, even if we have no money?

Are there any tricks to reduce hub motor consumption on a twisty course?

Thanks!

Addendum: The winning team crossed the finish line in 13 minutes and 3 seconds.

 

[glennb]

Aikema manufacture a 250w hub wound for 30km/h in 26" wheel at 24v. One of the AKM-85 or AKM-100 variants. Could be more efficient than field weakening or a mid-drive.

 

[Nix Liou]

Hello! We are a highschool student EV team.
Our school has seized our workshop and we have almost no budget left. However, we want to beat our rival school (who have a respectable aluminum welded frame) in the next Eco-Run race.
The workshop and budget are currently under negotiation so I think we can somehow make it work, but to save money as much as possible, we plan to use the hub motors we bought two years ago.

Crazy rules:
* Distance: 5.5km (550m x 10 laps)
* Stopping every lap for a driver change (9 times total!)
* Battery: Only two 12V lead-acid batteries (24V 3Ah)

Current strategy (is this correct?):
* Supercapacitors: To get through the 9 accelerations without draining the lead-acid batteries.
* Field weakening (flipsky VESC 4.20): To push the hub motor to 30km/h (at the cost of efficiency).
* Pit stops: Aiming for driver changes under 6 seconds.
* Motor: One 250W hub motor (rated for 24km/h)

Question:
Is "hub motor + field weakening" a suicide mission? Should we switch to a chain drive (mid-drive) instead of a hub motor, even if we have no money?

Are there any tricks to reduce hub motor consumption on a twisty course?

Thanks!

Addendum: The winning team crossed the finish line in 13 minutes and 3 seconds.

I'm not an expert on field weakening, but my understanding is that it's pretty inefficient as you say, and that maybe a mid drive would be better just to allow for gearing- I mean, requiring a lot of acceleration seems like the ideal use case for a mid drive with a derailleur setup so you can start in a low gear and get more torque.

If your battery is limited to 24v 3ah of lear acid, I'm not sure you have much extra energy for field weakening...

 

[amberwolf]

* Supercapacitors: To get through the 9 accelerations without draining the lead-acid batteries.

You should read up some of the threads here about SC's. They are bigger and heavier than any battery of comparable capability. You cannot get back all the energy you put into them (only a tiny fraction), because of the narrow voltage range your battery powered system operates in. You could design and build a complicated DC-DC that charges and discharges the SCs thru their whole voltage range and converts that to / from the battery voltage range on the system bus, and add it's weight and cost to that of the SCs on the bike.

If they are your only option to do what you want...then I guess you're stuck with them and the complications of using them efficiently (or simply using htem inefficiently and losing the energy that you can't access in them, which will decrease your range by that much).

* Field weakening (flipsky VESC 4.20): To push the hub motor to 30km/h (at the cost of efficiency).

Is "hub motor + field weakening" a suicide mission? Should we switch to a chain drive (mid-drive) instead of a hub motor, even if we have no money?

Use the hubmotor *as* a middrive.... Lots of threads about that you can skim for ideas.


You can use the ebikes.ca motor and trip simulators with your actual conditions / etc to find out what it will take to do the job you need to do, before you build anything or spend any money. It will take some practice and experimentation with it to understand it's output; there are instructions and explanations on the page itself, and there is a thread about the simulator around here somewhere with more info as well.

 

[Nix Liou]

You should read up some of the threads here about SC's. They are bigger and heavier than any battery of comparable capability. You cannot get back all the energy you put into them (only a tiny fraction), because of the narrow voltage range your battery powered system operates in. You could design and build a complicated DC-DC that charges and discharges the SCs thru their whole voltage range and converts that to / from the battery voltage range on the system bus, and add it's weight and cost to that of the SCs on the bike.

If they are your only option to do what you want...then I guess you're stuck with them and the complications of using them efficiently (or simply using htem inefficiently and losing the energy that you can't access in them, which will decrease your range by that much).



Use the hubmotor *as* a middrive.... Lots of threads about that you can skim for ideas.


You can use the ebikes.ca motor and trip simulators with your actual conditions / etc to find out what it will take to do the job you need to do, before you build anything or spend any money. It will take some practice and experimentation with it to understand it's output; there are instructions and explanations on the page itself, and there is a thread about the simulator around here somewhere with more info as well.

Sounds like they are strictly limited to the lead acid battery setup, so it might actually be viable to use super capacitors, as my understanding is that lead acid cells kind of suck at high power draw...
Would it even be too complicated to do the charge and discharge? seems like you would mostly need a buckboost converter of some sort.

 

[wturber]

Hello! We are a highschool student EV team.
Our school has seized our workshop and we have almost no budget left. However, we want to beat our rival school (who have a respectable aluminum welded frame) in the next Eco-Run race.
The workshop and budget are currently under negotiation so I think we can somehow make it work, but to save money as much as possible, we plan to use the hub motors we bought two years ago.

Crazy rules:
* Distance: 5.5km (550m x 10 laps)
* Stopping every lap for a driver change (9 times total!)
* Battery: Only two 12V lead-acid batteries (24V 3Ah)

Current strategy (is this correct?):
* Supercapacitors: To get through the 9 accelerations without draining the lead-acid batteries.
* Field weakening (flipsky VESC 4.20): To push the hub motor to 30km/h (at the cost of efficiency).
* Pit stops: Aiming for driver changes under 6 seconds.
* Motor: One 250W hub motor (rated for 24km/h)

Question:
Is "hub motor + field weakening" a suicide mission? Should we switch to a chain drive (mid-drive) instead of a hub motor, even if we have no money?

Are there any tricks to reduce hub motor consumption on a twisty course?

Thanks!

Addendum: The winning team crossed the finish line in 13 minutes and 3 seconds.

Do you have to use the hub motor as a hub motor? If not, then I'd rig it up Stoke Monkey style (where you use a hub motor to drive a chain that then drives the wheel) and drive it through a derailleur. The downside to that would be that you wouldn't get any regen for the nine stops. This would allow you to start in a low gear. OTOH, you could get regen back that by locking up the freewheel in the rear wheel and/or in the hub motor itself? Though running the drive chain backwards may not work well with the derailleur jockey wheels.

Can this be a two wheeler, three wheeler, four wheeler? Maybe a simpler scheme would be to have one hub motor on a smaller wheel and use that motor for starts and then switch to the larger wheel/motor combo once you have reached a higher speed. This would give you "gears" using a mechanically simple design. I'd use two separate systems with a separate throttle for the smaller and larger wheel. Regen from the larger wheel and let the smaller wheel freewheel.

If I did my math correctly, the winning team last time averaged about 15.7 MPH. Do you know what strategies they employed?

I would focus on doing the basics well and not get too wrapped up in some of the more esoteric electrical options. I would pay a LOT of attention to keeping weight low and having low drag. Maybe a recumbent tricycle design. Make sure you streamline wheels. Make sure you have a low front air dam. You'll want 10 lightweight people doing the driving. Maybe time to make friendly with some of the girls.

All ideas above just me thinking out loud. There are more knowledgeable people here who make poke some holes in these ideas.

 

[wturber]

Sounds like they are strictly limited to the lead acid battery setup, so it might actually be viable to use super capacitors, as my understanding is that lead acid cells kind of suck at high power draw...
Would it even be too complicated to do the charge and discharge? seems like you would mostly need a buckboost converter of some sort.

You could definitely use a buckboost converter to drive something this low powered and get a faster motor speed with higher voltage. You will lose some efficiency that way though. I ran 1000 watts through one for years commuting to work. They can fail in moisture rich environments. But if all this team needs is to go less than 4 miles, then no problem. Have a spare on hand, just in case. I have no idea if such a device would be smart to use for charging super capacitors though.

 

[mvrocket]

You should read up some of the threads here about SC's. They are bigger and heavier than any battery of comparable capability. You cannot get back all the energy you put into them (only a tiny fraction), because of the narrow voltage range your battery powered system operates in. You could design and build a complicated DC-DC that charges and discharges the SCs thru their whole voltage range and converts that to / from the battery voltage range on the system bus, and add it's weight and cost to that of the SCs on the bike.

If they are your only option to do what you want...then I guess you're stuck with them and the complications of using them efficiently (or simply using htem inefficiently and losing the energy that you can't access in them, which will decrease your range by that much).



Use the hubmotor *as* a middrive.... Lots of threads about that you can skim for ideas.


You can use the ebikes.ca motor and trip simulators with your actual conditions / etc to find out what it will take to do the job you need to do, before you build anything or spend any money. It will take some practice and experimentation with it to understand it's output; there are instructions and explanations on the page itself, and there is a thread about the simulator around here somewhere with more info as well.

Thank you for the reality check regarding Supercapacitors. I understand that without a complex DC-DC converter, we can only utilize a fraction of their capacity due to the voltage drop.

However, I'd like to ask about Regenerative Braking in our specific race format.
Since we must stop completely 9 times, we generate a lot of kinetic energy that would usually be wasted as heat (mechanical brakes), especially because our lead-acid batteries cannot accept high regen currents.

Our idea:
Use the hub motor's regen braking to dump that high current directly into the Supercapacitors during deceleration.
Even if the SCs are heavy, wouldn't capturing this "free energy" (which lead-acid batteries can't catch) and using it for the next acceleration make them worthwhile?

Or is the energy loss in the system (motor efficiency + controller losses) still too high to justify the extra weight of the SCs?

---

 

[mvrocket]

Do you have to use the hub motor as a hub motor? If not, then I'd rig it up Stoke Monkey style (where you use a hub motor to drive a chain that then drives the wheel) and drive it through a derailleur. The downside to that would be that you wouldn't get any regen for the nine stops. This would allow you to start in a low gear. OTOH, you could get regen back that by locking up the freewheel in the rear wheel and/or in the hub motor itself? Though running the drive chain backwards may not work well with the derailleur jockey wheels.

Can this be a two wheeler, three wheeler, four wheeler? Maybe a simpler scheme would be to have one hub motor on a smaller wheel and use that motor for starts and then switch to the larger wheel/motor combo once you have reached a higher speed. This would give you "gears" using a mechanically simple design. I'd use two separate systems with a separate throttle for the smaller and larger wheel. Regen from the larger wheel and let the smaller wheel freewheel.

If I did my math correctly, the winning team last time averaged about 15.7 MPH. Do you know what strategies they employed?

I would focus on doing the basics well and not get too wrapped up in some of the more esoteric electrical options. I would pay a LOT of attention to keeping weight low and having low drag. Maybe a recumbent tricycle design. Make sure you streamline wheels. Make sure you have a low front air dam. You'll want 10 lightweight people doing the driving. Maybe time to make friendly with some of the girls.

All ideas above just me thinking out loud. There are more knowledgeable people here who make poke some holes in these ideas.

Thanks for the solid advice! You are right, we shouldn't get lost in esoteric electronics and forget the basics like aerodynamics and weight.

To answer your questions:

1. Do we have to use it as a hub motor?
Technically, the rules allow us to mount it to the frame (Stoke Monkey style).
However, since our budget is currently near zero, we can't afford the chains, sprockets, and machining required to convert it to a mid-drive system.
So, I have a zero-budget idea I wanted to ask you about:
What if we rewire the hub motor internals from Wye (Star) to Delta configuration?
I read that this increases the Kv by 1.73x. Since our motor is currently capped at 24km/h (Wye), Delta wiring could theoretically push it to ~41km/h without using inefficient Field Weakening.
Do you think this "Delta Rewire" is a viable strategy for an efficiency race? We are worried about low-end torque, but we hope our Supercapacitors can handle the launch. Unlike the mid-drive with a freewheel, this method keeps the regenerative braking capability for our Supercaps.

2. Vehicle Type?
The rules say 3 wheels or more. We are planning a 3-wheeler to save weight.

3. Winning Team's Strategy?
Here is the info on the champion team (13min 03sec):

• Weight: Extremely light at around 16kg (likely including motors).
• Motors: They used two separate Outrunner motors (one for high-torque acceleration, one for efficient cruising).
• Riding Position: A kneeling style (low profile, like a motorcycle rider tucking in).

Their 16kg dual-outrunner machine is a beast. We can't build a new lightweight dual-motor system with our budget.
That's why we want to bet on "One Hub Motor (Delta mod) + Supercapacitors". We hope to beat their complex setup with a simpler, high-voltage burst strategy.

Does this sound like a fighting chance?

---

 

[wturber]

Rewiring an electric motor seems complicated and bound for trouble to me. Anything non-standard troubles me. Can you get a controller easily and cheaply for a Delta wound motor? What specific hub motors do you have? Are they direct drive or internal gear types?

I'm assuming you have supercapacitors because they are left over from last year? If not, how do you have a budget for them? I don't have any experience using them. Maybe someone else here can weigh in on them.

It sounds to me like your competition took an approach much like my second idea of running two different sized wheels with your hub motor. A smaller diameter wheel gives you motor lower torque but a limited top speed. A larger diameter wheel gives your motor higher speed with more limited low speed torque. With a tricycle, you could have a smaller front wheel and a larger back wheel. Relacing hub motors into different rim sizes seems pretty low budget to me and I'm biased that direction because that was my "simplest" notion and because something similar seems to have worked for your competition.

 

[amberwolf]

However, I'd like to ask about Regenerative Braking in our specific race format.
Since we must stop completely 9 times, we generate a lot of kinetic energy that would usually be wasted as heat (mechanical brakes), especially because our lead-acid batteries cannot accept high regen currents.

What is the max current they can charge at?

What is the max regen current of your controller?

Many SLA can take a momentary peak current as high as their max peak discharge current. While they won't likely absorb all the energy (peukert effect), they will *probably* usefully absorb more than the SCs would, *and* you wouldn't be carrying around the weight of the SCs, their wiring, and their containment structure, attachment to the trike, etc.

Which specific SCs do you have? What are their specific capabilties?

 

[amberwolf]

What if we rewire the hub motor internals from Wye (Star) to Delta configuration?
I read that this increases the Kv by 1.73x. Since our motor is currently capped at 24km/h (Wye), Delta wiring could theoretically push it to ~41km/h without using inefficient Field Weakening.
Do you think this "Delta Rewire" is a viable strategy for an efficiency race? We are worried about low-end torque, but we hope our Supercapacitors can handle the launch. Unlike the mid-drive with a freewheel, this method keeps the regenerative braking capability for our Supercaps.

Regarding the delta mod, it may have higher RPM, but it will have proportionally lower torque, so you will accelerate more slowly. Depending on how long it takes you to accelerate up to the faster speed, it might actually slow you down overall.

 

[wturber]

Or is the energy loss in the system (motor efficiency + controller losses) still too high to justify the extra weight of the SCs?

---

You'd have to know and run the numbers to get an good answer. Doing that would require more technical data and engineering knowledge than I have for sure. But considering that it is commonly reported that regen only gives about 5% increase in range for day-to-day ebikers, by bet is that it would not justify the extra weight. Also, how do you buy the super capacitors if you have zero budget? Also does your school have a metal or auto shop that could be used to help you out? And if budget is a problem, are you allowed to seek sponsors? My gut tells me that you are trying to "outclever" your competition rather than learn from their success.