How much torque can a BLDC hub motor produce at low RPM

[InjectioN]

Hi guys,

I'm currently developing a selb balancing unicycle as a project of mine. I've been looking at this motor http://www.conhismotor.com/ProductShow.asp?id=94 to do the job. I've done the maths and the power requirements is well above spec, but I'm having issues trying to make sure the torque will be acceptable at low RPM. I've worked out that at walking speed (2.24m/s) my unicycles wheel will do 84RPM. I'm struggling to find a value of torque at this speed. I'm concerned that I may need to buy a geared hub motor instead of this direct drive one.

Has anyone had any experience with hub motors at low RPM? I know that torque is inversely proportional to speed, so I'm assuming at lower RPM the torque should be higher? Can someone confirm?

Thanks very much!

 

[amberwolf]

How much torque can a BLDC hub motor produce at low RPM

Entirely depends on which motor, all else being equal between different ones.

Go to http://ebikes.ca/simulator and play with the different motors and other things, you'll see what I mean.

You might also search on unicycle here on ES to find JustinLE's builds.

 

[Punx0r]

Torque is proportional only to current. The relationship with speed is simply that as it increased BEMF also increases, which means you require greater voltage to achieve a steady current. If your voltage supply is limited then current and torque gradually reduce. Realistically, considering your low-speed requirements, even with a modest battery voltage you'd be able to generate as much torque as you want until the motor saturates or over-heats.

If you can find the Kt (torque constant) for the motor you're looking at you're set. These motors are known as 9-continent (9C) clones. Of the four-letter designation the first two is the stator width (28mm) and the last two the number of turns in the winding e.g. 07. The winding will affect the Kt but not the ability of the motor to generate torque as the different windings tend to have the same total amount of copper (plus or minus the odd strand), some winds just mean stuffing more or less current into the motor to achieve the same torque.

Anyway, the charts on the page you link to suggests Kt is approximately 1Nm per Amp.

The 1000W version of this motor would appear to saturate somewhere around 100A phase current. There also appears to be no difference between the 500/750 & 1000W versions other than the sticker, so a 9C clone is pretty much a 9C clone.

 

[Jonathan in Hiram]

A geared motor won't work for a self balancing rig without modifying it to lock up the freewheeling clutch, it only has thrust in one direction unmodified.

The specs on the motor you linked to says max torque of 24 NM, the Kt of that motor is very close to 1 NM per Amp.

Bear in mind that as wheel diameter is decreased the thrust for each unit of torque (current) is increased, with the same motor at the same current a 29" wheel will only give about 2/3 of the thrust of a 20" wheel. Thrust is what's important for your application so a smaller diameter wheel should give you a more responsive setup.

 

[InjectioN]

The 1000W version of this motor would appear to saturate somewhere around 100A phase current. There also appears to be no difference between the 500/750 & 1000W versions other than the sticker, so a 9C clone is pretty much a 9C clone.

That's very interesting. I did read that most direct drive motors can happily run above spec and I did notice there doesn't seem to be much different in the other rated motors (other than price !)

Bear in mind that as wheel diameter is decreased the thrust for each unit of torque (current) is increased, with the same motor at the same current a 29" wheel will only give about 2/3 of the thrust of a 20" wheel. Thrust is what's important for your application so a smaller diameter wheel should give you a more responsive setup.

My unicycle wheel that I was going to use is 16inches in diameter I think. I used an online calculator to find out what spoke lengths I need if I use this hub motor, which I believe is 135mm? Here's a screen shot - http://imgur.com/yape0DJ

Would you guys say it's safe to assume I can use the motor I linked?

Thanks very much!

 

[Punx0r]

Your application would be pretty much unique on this forum, but it should be ok as long as it's a direct-drive unit. Have you considered the cast alloy motor-in-wheels Goldenmotor do? One might suit your size requirements and should save you some hassle building a wheel (big motor in small rim = nuisance to lace).

 

[Jonathan in Hiram]

You would need to radial spoke that motor to a 16" wheel, even 20" wheels are often radial spoked on that large a hub.

The motor itself should be fine, as PunxOr mentioned though a cast wheel/motor would eliminate the problem of spoking a rim.

 

[InjectioN]

Your application would be pretty much unique on this forum, but it should be ok as long as it's a direct-drive unit. Have you considered the cast alloy motor-in-wheels Goldenmotor do? One might suit your size requirements and should save you some hassle building a wheel (big motor in small rim = nuisance to lace).

You would need to radial spoke that motor to a 16" wheel, even 20" wheels are often radial spoked on that large a hub.

The motor itself should be fine, as PunxOr mentioned though a cast wheel/motor would eliminate the problem of spoking a rim.

I see what you mean, a casted wheel would be better, but being in the UK it's quite hard to source them and I can buy the motor I linked pretty easy on eBay. I think spoking it with 13.5cm spokes it a 2 cross pattern should fit? But my unicycle wheel is 16" so if I can find a UK supplier of a cast hub motor with a datasheet (so I can prove mathematically it meets spec) then I'll happily buy it.

Thanks so much for the help guys! I'll definatly post something on here about it when it's made for other's to use.

 

[Punx0r]

About three years ago there weren't many motors on ebay over 250W so I ordered a 1000W Goldenmotor kit from Devi Motion in Holland, who I believe are the European distributor. If you want the cast wheel and can't find on in the UK you might consider buying one from them:

http://www.devi-motion.com/webshop/bldc-hub-motor/p-2/D1000620--mw16b-16-%22-motor-wheel-rear-48v-1000w.html

IIRC it was about 30-Euros for delivery.