I am currently designing a next generation Power Wheelchair using the most up to date technology and materials.
However, I am currently finding it difficult to locate a brushless and gearless direct drive motor for the wheelchair. I have looked at motors such as the Mars brushless PMAC motor ( http://www.robotmarketplace.com/products/ETK-ETEKBL.html ) however this motor would require a gear reduction system which is not what I would like. To keep the wheelchair as maintenance free as possible, it is essential that the motor is gearless. The chair will be operating at 48V, which resulted in the Mars motor having an output of just under 3500 rpm, however I was recently told that the wheelchair motor should be turning at something about 150-250 rpm... which is a LOT slower than the actual motor output, hence the gear reduction needed.
I come here to ask if anybody on this forum knows of any motors that may be of any help to me.
The brushless hub motors that are used in electric bicycles such as ( http://www.elifebike.com/peng/iview.asp?KeyID=dtpic-2011-1G-K59E.3DNJU) offer a similiar rpm that is needed for the wheelchair, however I am not sure how i'd be able to implement them with wide flotation tyres such as these ( http://www.terraintyres.co.uk/user/products/large/Kenda%20Superturf%20K500.jpg ). So if anyone has any innovative ideas in that sense then by all means fire away.
Any help is greatly appreciated and I look forward to reading any replies.
The motor you linked to is a GEARED motor; the gearcase is within the hub rather than being external.
what's your problem with gears - they are the most sensible method to develop high torque at low speed.
The only reason that I would prefer it to be gearless is to keep the wheelchair as maintainance free and with as little moving parts as possible, and to keep the weight down as much as I can.
The reliability of these wheelchairs are essential to the user, and with gearboxes needing replaced every so often, removing this component in exchange with direct drive, will remove this worry from the wheelchair user.
What data are you using to evaluate the frequency of gearbox changes on wheelchairs. I understand your requirement for a low maintenance device, but you need to quantify the "real" problem and balance that against the complexity, reliability and cost of an alternative system.
Now for some numbers :
Wheel diameter say 0.5metres. Chair velocity 6kM/hr maximum
So wheel rotation speed is 6000 / ( 60 x 3.142 x 0.5 ) = 64 RPM
So you will be looking for a multiphase, multipole brushless motor with very high torque output. Technically this will require some 'special-build" motors complete with the necessary complex DC to AC drive systems.
In my opinion this requirement will far outweigh in both terms of complexity and cost any perceived disadvantage of using a 'conventional wheelchair motor/wheel arrangement with inbuilt epicyclic gearbox.
If you really want to develop a revolutionary wheelchair then look at designing an inexpensive light-weight but strong frame system that can be tailor fitted to the client and dismantled or folded to minimal size. Also look at battery technology to maximise durability with minimal weight.
Jackrae’s numbers would be for a rather atypical chair. More common would be 14" diameter tires (35.6cm) and 10 km/hr top speed, which works out to 150 RPM. Otherwise, however, he’s really giving you good advice.
Invacare had the corporate funds to build a gearless-brushless motor. The first generation was efficient, heavy and unreliable. The second generation was efficient, even heavier, and somewhat more reliable. The cost was enormous. Note too that not only do you lack torque multiplication of gearing for the motor, but the brake doesn’t have that multiplier either. The brake becomes large, heavy, current demanding (bye bye some of that efficiency) and, from many complaints, annoyingly noisy. Invacare even had an “in house” controller manufacturer - Dynamic - to design a controller for those motors, and they still had a product that didn’t exactly set the world on fire (except in their ad hype).
If you search, you should be able to find some pretty hefty, industrial quality, brushless motors with integral gearing that keeps them pretty compact. The largest even have forced cooling. They are not, however, cheap and finding controllers for brushless motors with adequate current capacity (say at least 100 Amps per motor) will not be easy, nor cheap.
Gearcase design is “mature” engineering. If one doesn’t cut corners, one can get something that will outlast the motor and, though not 100% efficient, can be as much as 90% efficient. If a pair of gearless motors adds 20kg to the weight of the chair, and its brakes draw 5 Amps instead of <1 Amp (as do those on geared motors), that extra bit of efficiency is being used to push more weight up hill and as waste heat in the brake solenoids. No other manufacturer has ever followed Invacare’s lead and I don’t think that this is entirely ascribable to poor engineering (though there’s plenty of that about too).
The Chinese are making lots of types of scooters.
Thanks for the replies again, all comments have been taken on board and I am now going to reconsider whether gearless motors are actually a step forward with this project or if it is just as good to have a gearing system in place.
My search for an efficient and high quality geared motor begins, however I will also continue looking into gearless motors in order to keep my options open.
From the replies on this thread however, it does seem that a geared motor would be the best option... Thank you for your input, it has really helped me understand this problem a lot more than I had before posting this thread!
Check out some of the BLDCs from the chinese firm "Golden Motors" - they do a variety of hub motors
from bikes to 4 wheel light electric vehicles.