Increasing torque on a hub motor

[LeOgre]

Hello guys,

apologies in advance for probably choosing the wrong category for my questions.

I am playing around with hub motors and a Cheap Focer 2 for a 6x6 RC crawler/rover project. (I cobbled together a test bed for the wheels using old DC wind shield wiper motors, but I would like to use BLDC motors with VESC control in the future.)

My plan is to have a hub motor in each of the six wheels with 6 Cheap Focers as motor controllers. Problem is that my wheels have a diameter of 206 mm, so I need hub motors with loads of torque and low kV which are hard to come by. I found these hub motors for electric skate boards on ebay:

“70mm 150W 24V/36V brushless hub motor PU wheel for longboard”

This is all the available technical data of the motors:
Power: 150W
Rated Voltage: 24V/36V
No-load Speed: 1600 rpm
No-load Current: 0.3A
Maximum Current: 12A
Speed: 15-20KM/h
Diameter: Approx. 70mm/2.76inch
Height: Approx. 51.5mm/2.03inch

Using the drill method I figured the kV to be at about 66 RPM/V which corresponds to 1600 RPM @ 24 V.

I build a simple test rig to measure stall torque at various motor currents. The leaver attached to the motor has a length of 103 mm, so the measured force on the scales is the same force available on the outer diameter of my wheels (diameter 206 mm):

For the test I used the Cheap Focer 2 with VESC current loop control in steps of 0,5 A from 0,5 A to 11 A with the leaver pressing down on the scales. Hall sensors are in use.
The blue curve shows the force in grams, the orange curve shows the power, grey is the battery current. For example at a motor current of 7,5 A I got 40 W power and 1030 g of force while drawing 1,7 A from the battery.

So far, the motors are not too bad, however, with a nominal speed of 1600 RPM my rover would go at about 60 km/h with its 206 mm wheels which is dangerously fast, while the torque is quite a bit on the low side. I should prefer to have a top speed of 10 … 15 km/h and an increase in torque. Which finally leads me to my questions:

1. The motor has 20 poles i.e., 10 pole pairs, so presumably the max ERPM which I have to set up in VESC tool is 16000, is that correct?

2. If I run the motor at much lower ERPM than the nominal 16000 I will not lose torque, as torque depends on current and number of turns of the winding. Is that correct?

3. When using the ERPM control loop in VESC I can not get the motor started below about a 1000 ERPM, below it just clogs and stutters. 1000 ERPM are already about 4 km/h speed which is far too fast as starting speed for my rover. Are there any settings I could use to have much lower starting speed and low RPM control? The motor turns very nicely at the lowest RPMs when I slowly start to slow him down by hand until stall, so in theory the VESC is able to do lowest speed.

4. Can I increase the torque (and reduce speed) at a given motor current in any way except using a mechanical gear box or rewinding the motor to a lower kV? A gear box defeats me using hub motors in the first place and rewinding 6 motors is the last step of escalation I might consider.

Thank you very much for bearing with me, any advice is much appreciated,
Cheers Olaf

[ross_forp]

Re: 4 - You can increase the thrust per amp with smaller diameter tires.

[Flyboy]

1. Yes, eRPM = RPM * pole pairs
2. Torque is dependent on current primarily, electronically limiting the RPM will not reduce it
3. Low speed control without sensors can be possible with HFI or something, but i have no experience with that myself
4. You might be able to slightly increase the current capacity with statorade, but I’m afraid that your choices are gearing, rewinding, or hot motors

[tech.shit]

A gear ratio might just be what you need

[ross_forp]

@ross_forp

Re: 4 - You can increase the thrust per amp with smaller diameter tires.

…or more wheels…

[LeOgre]

Thats true, but my whole project is planned around those large tires.

[ross_forp]

2 hubs per wheel— 12 motors total

[LeOgre]

Thank you very much, it seems I need to have a look at rewinding the motors. Both gear boxes and hotter motors are not an option.
Regarding the low RPM control, I am using the hall sensors (they get recognised and work apparently fine) but for some reason starting is only possible above a certain ERPM.

[ross_forp]

Re-winding, you’ll get the same heat for the same torque

[b264]

@LeOgre

the max ERPM which I have to set up in VESC tool is 16000, is that correct?

The Maximum RPM in VESC Tool doesn’t do what you think it does. It does not prevent the motor from spinning faster. (only prevents power/brake while spinning faster)

Try setting Maximum Duty Cycle instead.

Limiting all programs to a set speed

Hello. I live in Norway, and here all boards that CAN go above 20 km/t is illegal. I have seen the vesc tool, and it looks pretty advanced. Is there a way to lock all your programs to 20 km/t in case i get pulled over? So if they try to switch programs it will still go only 20 km/t? Hope someone knows, or have a decent solution.

[LeOgre]

So rewinding can not achieve what a gear box does as in reducing speed and increasing torque at the same power and power losses?

[ross_forp]

Rewinding for more torque per amp will increase the resistance such that the heat per Newton meter stays the same.

Rewinding at the same copper fill won’t change the KM motor constant, which is the torque per the square root of the resistance heating.

Understanding the Motor Constant in DC Motor Sizing | RoboticsTomorrow

[ross_forp]

If you add more turns, the controller will run cooler for the same torque but the motor heating will stay the same.

[LeOgre]

Thank you, more reading to be done. And in all probability I need to find a larger motor with more room for copper…

[b264]

Probably best bet would be a mechanical reduction with gears or belts or chains.

[LeOgre]

I am afraid so… I wanted to avoid building gearboxes as it messes with my suspension arms

[ross_forp]

You could find a way to mount a motor similar to this beside the wheels for more torque:

Amazon.com : 48V1500W Electric MTB Bicycle E Bike Hub Motor Conversion kit MTX...

[hummieee]

1. Could get controllers that put out more amps

2. If u can get to the magnet wires of the motor (sometimes on cheaper motors the three motor leads are the same magnet wire) you could find the starts and ends of each strand and reterminate them to wye and drop the kv and amount amps needed for torque, assuming they’re terminated delta now which is very likely.

Choice 1 is easiest

[ross_forp]

Yes you may be able to briefly exceed the motor’s advertised current limit.

Another option is buy a few of these:

Amazon.com : EVERCROSS 10" Motor Replacement for H5 Electric Scooter : Sports...

[Blise518B]

Hoverboard motors would be ideal for this. They are made for low speed high torque.

https://www.aliexpress.us/item/2251832765344064.html?spm=a2g0o.productlist.0.0.205a19aaJfuqtA&algo_pvid=5dc4130e-adfb-4c9f-9176-3520569f79e0&algo_exp_id=5dc4130e-adfb-4c9f-9176-3520569f79e0-0&pdp_ext_f={“sku_id”%3A"66244859166"}&pdp_npi=2%40dis!EUR!39.38!33.86!!!!!%402100bddd16665798623437138ed3ba!66244859166!sea&curPageLogUid=LNu4m8QCZlfE

Depending on how heavy your vehicel is going to be 6 of the small hubs might be totaly fine. Even one of them is enough for low speed cruising for a human so 6 will likely be fine for something light even if the gearing is bad.

12A seems kind of low for a maximum rating I guess they should be fine with a short 20-30A burst to get them going.