Which Controller (edit: hub motor) for my daughter's wheelchair?

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Re: Which Controller for my daughter's wheelchair?

Post by SlowCo » Dec 16, 2017 1:24 pm

buk___ why are you calculating with 16" wheel size and at 5km/h? Glyn has 20" rims with 2.25" tires. Most likely the diameter is >21" so the circumference will be around 21 x 25,4 = 534mm = 0.534m x 3.14(pi)= 1,67m. Although 5km/h is walking speed on a level terrain the actual average speed looking at the photo's of the path that needs to be climbed seems to be more around 1 or 2km/h and even less on the really steep parts.

Glyn you write on your webpage that it is a 9 mile return trip to and from the top. So 4.5 miles = 7,2km of climbing. You calculated it would take 6 hours to reach the top. That means 7,2km / 6h = 1,2km/h on average. So some parts will be even slower. If you take that 1,2km/h it means that the direct drive motors you've ordered will be turning (1200m/h = 20m/min / 1,67m/rev =) 11,97rpm :shock:
So a 1:16 Q128 will only be turning 191,5 rpm. That will be so low in the efficiency curve of either motor that likely 80% or more of the electrical energy put in will be turned into waste heat. So if you need to put in the 1500W that the motors are rated for then they would generate 1200W = 1,2kW of heat each. And that times 6 :shock:

And are you really planning on letting your daughter descent those steep parts on 6 bicycle disk brakes all the way down? :|
That will be a workshop with a "steep" learning curve in down hill biking... :shock:

The more I think this through the more I dislike the idea. Especially as you don't seem to be very knowledgeable in this area (neither am I BTW) so it looks like this might end in tears. Please rethink this challenge and don't risk your daughters health or worse with what seems a noble cause.

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Re: Which Controller for my daughter's wheelchair?

Post by Glyn » Dec 16, 2017 2:07 pm

SlowCo wrote:
Dec 16, 2017 1:24 pm
buk___ why are you calculating with 16" wheel size and at 5km/h? Glyn has 20" rims with 2.25" tires. Most likely the diameter is >21" so the circumference will be around 21 x 25,4 = 534mm = 0.534m x 3.14(pi)= 1,67m. Although 5km/h is walking speed on a level terrain the actual average speed looking at the photo's of the path that needs to be climbed seems to be more around 1 or 2km/h and even less on the really steep parts.
My average walking speed in the mountains is about 4/5kph. If the wheelchair could do 5kph then I'd be happy. Long parts of the path are relatively flat and with easy ground it's just that, as you've seen in the photos, there are some sections which require the ability to overcome large/steep obstacles. My 6 hours is a generous, off-the-top-of-my-head without referring to my calcs, time with plenty to spare for faffing and taking breaks.
SlowCo wrote:
Dec 16, 2017 1:24 pm
Glyn you write on your webpage that it is a 9 mile return trip to and from the top. So 4.5 miles = 7,2km of climbing.
That's correct, and I really appreciate you taking the time to have a look. Thank you.
SlowCo wrote:
Dec 16, 2017 1:24 pm
You calculated it would take 6 hours to reach the top. That means 7,2km / 6h = 1,2km/h on average. So some parts will be even slower. If you take that 1,2km/h it means that the direct drive motors you've ordered will be turning (1200m/h = 20m/min / 1,67m/rev =) 11,97rpm :shock:
That's not correct. 6 hours, as stated above, was a throw away comment. It would be nice if it could climb continuously for 6 hours though.
Total distance (to the top) is about 7.2km with a total height gain of about 1km so the average gradient is 7.7 degrees or 13.361%. Obviously in parts it will be steeper though.
SlowCo wrote:
Dec 16, 2017 1:24 pm
So a 1:16 Q128 will only be turning 191,5 rpm. That will be so low in the efficiency curve of either motor that likely 80% or more of the electrical energy put in will be turned into waste heat.
Lets say with inflated tyres the wheels have a diameter of 22".
That gives a circumference of 69" or 176cm.
To travel 1km that would require (100000 / 176) = 568 revolutions.
To travel 4km would require (568*4) = 2,272 revolutions per hour.
Or, (2,272 / 60) = 38 revolutions per minute at the wheel.
With a 1:16 reduction ratio, that's 608rpm at the motor.

According to the BMSB website, the Q128 motors have a max rpm of 201. Can anybody confirm if this is before or after reduction? Presumably it's after in which case the max rpm before reduction is (201*16) = 3,216rpm.

I don't understand the significance of this but what you're suggesting SlowCo is that if the motors aren't turning at their max rated speed then they'll overheat. Is this correct?
SlowCo wrote:
Dec 16, 2017 1:24 pm
And are you really planning on letting your daughter descent those steep parts on 6 bicycle disk brakes all the way down? :|
Absolutely! :mrgreen:

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Re: Which Controller for my daughter's wheelchair?

Post by Addy » Dec 16, 2017 2:23 pm

Glyn wrote:
Dec 16, 2017 2:07 pm
if the motors aren't turning at their max rated speed then they'll overheat. Is this correct?
Take a look at this simulator tool:
http://www.ebikes.ca/tools/simulator.html

You can experiment with different motors, but they all show the same pattern in their characteristics. You can see that the motor's efficiency is not good at lower speeds. The motor's mechanical output power increases with speed, up to a limit. At low speeds, the motor cannot put out much mechanical power, even if you are pushing a lot of current through it. Like others have mentioned, if your motor is always operating in this region, it will be dissipating a lot of heat. This is also a problem because you may be travelling slowly and the motor won't get as much air cooling compared to an ebike application.

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Glyn » Dec 16, 2017 2:46 pm

Thanks Addy. I've been referred to that tool before but this time around I'm actually beginning to understand it :)
I see what you mean about the motors running with reduced efficiency at low speeds.

Unless I'm mistaken though, they all have the highest torque at low speed right? Is this not an advantage? ...with appropriate cooling?

To summarise the main parts so far then (please amend where appropriate):
  • Each brushless motor requires its own controller.
  • Some brushless controllers are capable of driving the motors in forwards and reverse directions.
  • There are some cheap controllers on the market worth looking at, but they will only drive the motors in one direction.
  • Hub motors aren't the best solution for this project.
  • For want of a better solution though, geared hub motors would be preferred to direct drive motors.
  • The largest reduction ratio available on the market (in a hub) is 16:1, and this can be found in the BMSB Q128H.
  • Even with this reduction though, the motors would run hot.
Is that right?

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by SlowCo » Dec 16, 2017 2:51 pm

Glyn wrote:
Dec 16, 2017 2:46 pm
Is that right?
Yes

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Chalo » Dec 16, 2017 3:46 pm

I'm not sure how hot you can make the motors run, if you're spreading the load across six of them. Just use low current controllers to limit the maximum heat and torque load applied to any one of them. That will keep down the weight, size, and cost of controllers, and it will ease up the maximum power requirement for the battery. Each one of these motors is adequate for a single person on a full sized bike in normal riding conditions, so six on smaller diameter wheels should be plenty for extreme conditions.

The only problem with Q128 or any other geared hub motor is that they can't drive in reverse. For your application, that seems like a deal breaker.
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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Glyn » Dec 16, 2017 4:09 pm

Chalo wrote:
Dec 16, 2017 3:46 pm
Just use low current controllers to limit the maximum heat and torque load applied to any one of them.
I've tried that before and the controller (which had overcurrent and thermal protection) went up in flames! https://www.youtube.com/watch?v=uOqwVHEkTu0&t=62s

I believe that was a faulty controller though and after I returned it to the manfucaturer I received a full refund.

Won't that defeat the purpose of having larger motors though? Or, wait a moment, I think I'm about to have a revelation here... :) Looking at the simulation tool, am I right in thinking that the motors would only draw full current at top speed? Up until now, I've always thought that if you apply full throttle for example, then they would draw full current, even if they weren't able to turn at all?
Chalo wrote:
Dec 16, 2017 3:46 pm
That will keep down the weight, size, and cost of controllers, and it will ease up the maximum power requirement for the battery. Each one of these motors is adequate for a single person on a full sized bike in normal riding conditions, so six on smaller diameter wheels should be plenty for extreme conditions.
...but they're still likely to burn out right?
Chalo wrote:
Dec 16, 2017 3:46 pm
The only problem with Q128 or any other geared hub motor is that they can't drive in reverse. For your application, that seems like a deal breaker.
Are you sure about that? I don't mean to question your advice, clearly you know more about this than I do, but if this is the case it will have important implications so I just want to be sure.
I understand (thanks to the ES wiki) that some hub motors have two speeds and to engage the gear for the optional second speed the motors have to turn turn in reverse.
Looking at the internal mechanism of a hub motor which is geared for the purpose of reduction though, I can't see any reason why it wouldn't work in both directions:
Image

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Chalo » Dec 16, 2017 4:26 pm

All geared hub motors I'm aware of have a one-way roller clutch in them to allow drag-free coasting. If the gearbox is otherwise able to transmit torque in both directions, you'd have to weld up the clutch or replace it with a solid sleeve of the same dimensions, to permit the motor to drive in both directions.
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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Glyn » Dec 16, 2017 4:28 pm

Chalo wrote:
Dec 16, 2017 4:26 pm
All geared hub motors I'm aware of have a one-way roller clutch in them to allow drag-free coasting. If the gearbox is otherwise able to transmit torque in both directions, you'd have to weld up the clutch or replace it with a solid sleeve of the same dimensions, to permit the motor to drive in both directions.
That makes sense. Presumably that means they're not regenerative when free-wheeling then?

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Chalo » Dec 16, 2017 4:30 pm

Glyn wrote:
Dec 16, 2017 4:28 pm
That makes sense. Does that mean they're not regenerative when free-wheeling then?
As furnished, they can't regenerate because they can't back drive the motor. With modification to disable the one-way clutch, they would be able to regenerate and electrically brake within their torque limits.
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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Glyn » Dec 16, 2017 4:33 pm

Cool. Thank you :)

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by amberwolf » Dec 16, 2017 4:51 pm

Some thoughts for alternatives to let you do what you need to; there's a few sections to the post, so its' kind of a lot to read--sorry:



As others have pointed out, http://www.ebikes.ca/tools/simulator.html is a great tool to learn how different things affect a system under different conditions, and how changing parts of that system (voltage, current limits, which motor is used, etc) fix some problems and create others.

The thing that you need for your project is not power, it is torque. Takes power to make torque *but* the power itself is irrelevant, because you can waste lots of power without making sufficient torque, and that just makes lots of heat and drains your battery pack.

(in trying to create more torque for climbing, you turn up the throttle, which ups the voltage on the motor, which increases current. Usually this would make the motor spin faster, but if you have a motor meant to spin at 20-25MPH because of it's winding, and you're only able to push it to 2-3MPH, most of the power is just going into heating the windings, not moving the motor, because nromally the motor's higher speed that it is wound for would cause a back-voltage (BEMF) to be created, negating most of the voltage being applied, which then negates most of the current being applied, so the motor doesn't overheat. If the motor was wound for the slower speed, then the BEMF would be able to help prevent this while letting the motor operate as needed at the lower speed).


So what you want, if you have to use hubmotors, is ones that have windings specifically done for the very low speeds you are after. Wheelchair and wheelbarrow motors are probably the only ones already made this way, and the only company I know that advertises making those is Goldenmotor. (there are probably others, though). If you go to https://goldenmotor.com/ and click on Hubmotors on the left, then on the right scroll down a little more than halfway, you get to the motors already built to move at those speeds. Unfortunately they are typically for low power as well, so may not work for you. There probably is no existing hubmotor to do exactly what you want.


But you could have existing motors rewound for the speed you are after. You'll want motors with as wide a stator as you can get, at as large a diameter at the magnet ring/stator interface as you can get, because those two things themselves directly contribute to (or take away from at smaller diameters) the torque-making ability of a motor.

Because of your requirement that a single motor be able to support the chair's movements, It might require finding a large-diameter-stator motor, then taking *two* of the motors apart and joining their stators side by side (it's been done before, see Farfle's motor thread) and then winding it to create the necessary speed.

You'll need to work out how much torque you actually need to move the chair up the worst parts of the slope (the sharp rises of climbing over boulder edges, when it also has the worst traction) at the speeds it has to go. I don't know the specific math on that, but it's straightforward.


You'd have to work out what winding you would need to achieve the speed you're after at the voltage you'll be running. If the motors you will be starting with are already in the simulator, or you already have them in hand, you can figure out what that will be by experimenting / in the simulator to see what speed they run at full throttle (for max current and thus torque), then seeing what winding they already have, and using the motor winding math (which I don't know but is in a lot of places on ES and elsewhere, like RCgroups.com) to determine the number of turns of wire you need for the torque and speed you're after. AFAICR you need more turns of wire per stator tooth for lower speed.






Regarding using RC controllers / heli controllers, those are generally designed for free-spinning propeller motor control, and are not suitable for controlling extremely low speed motors under heavy load at that speed. Most likely they will blow up; if they don't they may have trouble controlling the motors at the low RPMs you're needing.







Another way around the problems, as has been suggested, is to use geared motors (preferably non-hub for power-dissipation and size/durability reasons). Use a motor that can spin at high RPM to keep the motor itself smaller for the same power output, then use a high-reduction gearbox for converting that speed into torque. It's not as efficient as a direct drive (DD) hub would be, but it's a lot easier to find stuff that would directly do what you want.

There's geared hubs but these have power dissipation limits; if they get too hot inside, they either kill their hall sensors or damage their plastic gears or both, and if you can get around that eventually the limit is the windings themselves.

The other issue, as pointed out, is most of them have a one-way clutch so they can only spin the wheel in one direction and have no ability to electrically brake.


Or you can use motors that don't mount in the wheel itself, but are separate with their own gearbox and output shaft to drive the wheel (either directly or indirectly).

Most of these don't have a directional limitation, and are designed to drive the wheel both directions, and could also probably electrically brake.


The easiest source for these is old powerchairs (powered wheelchairs), which have a pair of them with axle mounts already on them, for directly driving wheels--often 14" size, but sometimes smaller or larger. Get three sets that are the same so they'll all be setup for the same RPM at the same voltage. I see chairs like this at goodwill and yard sales every few months here in Phoenix, just in my local area, so I'm sure you can locate some in your area. They often go for $100-$200, and mostly all taht's wrong is the SLA batteries are dead, maybe worn out tires/seats/etc. Motors I've seen are always working (but if they're not it's probably either a wire or a brush).

There are a number of types and sizes, and some of htem are over 600-700w rated at 24v--but that's usually a lot more torque at that power and wheel diameter than the DD hubmotors, and that is a *continous* rating (they can take a much higher momentary power, and if you add cooling (like a fan sucking air thru the motor) you could probably double the continous power.

Usually these are 24v systems. Because you don't want to run them fast, you don't want to run them any higher voltage, either. Usually they are meant for around 4-8MPH; you can check the specs for the chair they come off of (or just test it before you take it apart). If you find some that are made for a faster speed than you want, then use smaller wheels/tires on them--they'll run at slower speeds *and* higher torque that way. ;) Or if you have to use them at a higher voltage, get motors meant to drive larger wheels and then use proportionally smaller ones to get the intended speed out of them at the higher voltage.



They are also usually brushed motors, which means you *can* use one controller per side, and series (or parallel) the motors, simplifying control systems.

If you series them, then you need to use a battery three times the voltage the motors are rated for, so each one will get the right voltage to give it the right speed.

If you parallel them, you need to use a battery the same voltage they are rated for, and a controller rated for at least three times the current you need at maximum, to supply them all with power when they need it at the same time.



The downside is these are generally significantly heavier than the DD hubmotors for the same power, but they are much more suitable for the slow speeds ahd high torque you are after. They're also less efficient than brushless, but again--I think in the right configuration they'll do what you want, though you may have to actively cool them if they're used way above their ratings for the whole climb.

See my old CrazyBike2 thread (and http://electricle.blogspot.com posts) about these motors, where I abused them at higher power and voltage levels than intended (and did damage one from overheating). At one point I had a chain jam from the frame twisting from the torque these things put out, and before the chain broke, it wrapped around the sprocket and destroyed that, and pulled the rear wheel out of position after destryoing the axle and the rim and spokes.



If I were to build a rock crawling wheelchair, my first iteration of it would use the highest-power (physically largest) powerchair brushed motors w/gearbox that I could find, driving wheels of a diameter that would get me the best speed / torque / object-climbing compromise I could make.









Another thought about the whole system, is that regardless of what kind of motor system, you will want to figure out how much power it's going to take to do the whole climb. How many watts, over how much time.

You can estimate this with a simple slope vs power calculator, of which there should be a bunch on the web (probably are links to some in the calculator threads of ES).

Wild guess (probably wrong), just for the sake of showing the calculation: Say it takes 3000w average over the whole climb, and it takes four hours to make the climb.

That means 3kW * 4h = 12kWh.

That would mean at minimum you'd need a 12kWh battery pack, which is huge.

I have an almost 2.5kWh EIG NMC pack on the trike that weighs around 35lbs+, and is the size of a small stack of books.


Another issue is that the battery has to be able to support the continuous current draw of the whole system, as well as the max peak draw.

Let's say it is 3kW continuous, and 10kW peak, just for some random numbers to calculate with.

If you use a "48v" pack, that makes it 3000W / 48V = 62.5A continous. 10000W / 48V = 208.3A peak.

My EIG pack could handle both of those (it could actually sustain just under the peak draw continously), but not all batteries can do that. Some of them would need to have more pack capacity than will actually be used, just so there are enough cells in parallel to handle the current draw. Most of them probably wont' have an issue with it, but you'll want to size the pack with this in mind, as well as how long the pack needs to run the system for (range).

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Buk___ » Dec 16, 2017 4:54 pm

SlowCo wrote:
Dec 16, 2017 2:51 pm
Glyn wrote:
Dec 16, 2017 2:46 pm
Is that right?
Yes
I think you are wrong. Heat is a product of amps. Amps is a product of load and throttle position. With 4 motors, the load/motor is 1/4, therefore the amps required per motor, even if not running at peak efficiency, is much less.

You also seem to think that the motor will run at a steady state from bottom to top, whereas the reality is likely to be small bursts of speed, followed by coasting to stop.

But hopefully, one of the 2WD experts will kick in here; because I cannot see how to configure the simulator to adequately represent the Q128H; but the simulations I have run show that 2 motors at least halved the heat produced.

I'll bow out now; but I commend Glyn to seek further council.

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by amberwolf » Dec 16, 2017 4:56 pm

Regarding the low-current controllers, you can also get ones capable of higher current (so you know they can more than handle the load), at least twice the current you will need, and then cut one of the shunts inside, which will halve the current the controller will then provide, without straining the controller.

(many (but not all) ebike controllers use several shunts in parallel. a common but not universal theme is about one shunt for every 15-20A of controller capability. )

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Re: Which Controller for my daughter's wheelchair?

Post by Buk___ » Dec 16, 2017 5:01 pm

SlowCo wrote:
Dec 16, 2017 1:24 pm
buk___ why are you calculating with 16" wheel size and at 5km/h? Glyn has 20" rims with 2.25" tires.
I didn't see anywhere that was what he had, just what he was thinking about.

5kph because that what people tend to walk at. Up hill on difficult terrain, you rarely walk at a steady state speed, but rather move forward in short bursts and then pause; burst then pause; and I believe that's how the wheelchair would move also. 5kph for a couple of meters then stop adjust trajectory; burst, stop adjust.

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Chalo » Dec 16, 2017 5:57 pm

amberwolf wrote:
Dec 16, 2017 4:51 pm
So what you want, if you have to use hubmotors, is ones that have windings specifically done for the very low speeds you are after. Wheelchair and wheelbarrow motors are probably the only ones already made this way,
By undervolting a motor, and lacing it into a smaller than intended wheel, you can accomplish the same effect without rebuilding the motor or sourcing a specialty item. When you run a hub designed for 48V and a 26" or 700c wheel at 36V in a 20" wheel, the full speed at the perimeter of the wheel is only a little more than half what was designed.
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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Chalo » Dec 16, 2017 6:02 pm

Here's a couple of questions you might already have answered elsewhere, but which might fill in some of the blanks:

Is this vehicle going to have mechanical steering or skid steering?

Is this vehicle going to be mechanically suspended or not?

The answers to both these questions have a bearing on wheel diameter and available traction, which affect the choices of motor.
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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by SlowCo » Dec 16, 2017 6:26 pm

Buk___ wrote:
Dec 16, 2017 4:54 pm
I think you are wrong. Heat is a product of amps. Amps is a product of load and throttle position. With 4 motors, the load/motor is 1/4, therefore the amps required per motor, even if not running at peak efficiency, is much less.

You also seem to think that the motor will run at a steady state from bottom to top, whereas the reality is likely to be small bursts of speed, followed by coasting to stop.
You seem to not read all the info...
This is such a low speed application that a direct drive motor (like he already ordered) in a 20" rim will turn so slowly, even at 5km/h, that it will have very low efficiency. I don't know the numbers for the motors he ordered but let's be optimistic and say that it is around 30% efficient in this rpm range. That means that for every 100W you put into the motor only 30W is transformed into rotating it and 70W is used to heat the windings. As you probably have to put several hundreds of watts into each motor to climb the path for many hours a lot of heat will be generated.


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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Buk___ » Dec 17, 2017 7:48 am

SlowCo wrote:
Dec 16, 2017 6:26 pm
Buk___ wrote:
Dec 16, 2017 4:54 pm
I think you are wrong. Heat is a product of amps. Amps is a product of load and throttle position. With 4 motors, the load/motor is 1/4, therefore the amps required per motor, even if not running at peak efficiency, is much less.

You also seem to think that the motor will run at a steady state from bottom to top, whereas the reality is likely to be small bursts of speed, followed by coasting to stop.
You seem to not read all the info...
This is such a low speed application that a direct drive motor (like he already ordered) in a 20" rim will turn so slowly, even at 5km/h, that it will have very low efficiency. I don't know the numbers for the motors he ordered but let's be optimistic and say that it is around 30% efficient in this rpm range. That means that for every 100W you put into the motor only 30W is transformed into rotating it and 70W is used to heat the windings. As you probably have to put several hundreds of watts into each motor to climb the path for many hours a lot of heat will be generated.
He also said that he would either cancel those motors or sell them on having found out that direct drive motors were not right for his application. And whilst he does talk about 20" wheels; he does not say that he has already brought those, or is committed to them. All that happened before I joined in. You continuing to bang on about the unsuitability of that choice, long after he has accepted it is wrong, serves no one.

My suggestion was for 4x geared hubs and 16" wheels, and I offered math to support that choice. Based upon my observation that my Q128 can pull me (85kg) and my bike (~35kg) up a 10% slope at 5kph, using very little power draw (<100W) and no observable rise in motor temperature.

If you assume 200kg for the chair, but spread the load across 4 motors; recognise that you aren't looking at 6 hours of constant WOT, but rather short bursts of (say) 20% power with cool down time between; then the demand per motor is so low that even when running them at speeds where they are only 50% efficient, heat would not be a limiting factor.

However, without modification (to lock out the clutch) to enable regen, geared hubs in 16" wheels are probably not right for this application; but you banging on about the unsuitability of his original choices, which he has already accepted are not good, is just noise.

As irrelevant, out-of-date arguments between 'helpers' does nothing to help this worthy venture, I attempted to bow out. I do so again.

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by dogman dan » Dec 17, 2017 7:53 am

Very interesting read.

If you are going to use a hub motor, I think 4 of the smaller, slow wind, geared motors would do anything sane you'd like to try. ( at 700w each) Driving up the rock staircase though, I'd have to understand better why it has to be driven there. Sure, wanting to get there I understand. But maybe it does not have to be in a completely independent wheeled vehicle the last mile? I also question the decent. That truly is the harder part. 4 strong humans assisting on that rock staircase is not a crime. NOT carry it, just helping up when the step is too tall for a 20" wheel, and particularly,,, restraining and keeping it safe on the way down. 4 people, or two if that is all you need, and some leashes. If it cant climb it, believe me, the really tricky part is turning around to retreat. Finding out the hard way, that coming down will be in an end over end roll is not something you can let happen. You think I'm not serious but I am, one good bounce off the suspension and whee! On those kind of grades, endos happen.

A decent chair that can do almost anything less than that rock staircase will do. Up and down 20% grades will be no problem for it. That means typical logging roads in the mountains, and many quad motorcycle routes.

It's gonna be a blast for her!!! go for it with small hub motors, and let her romp on stuff that will let her run them at 10-15 kph. But take some of the load off of them when it's a long rock staircase. You can run them at 1 kph briefly, for a quarter mile or so of crazy steep, or just so rocky its darn uncomfortable to go faster. Re that,, you need really good 4 wheel suspension.

If you end up with the dd motors, you will end up with a thing able to go about 60 kph, so make sure your controllers can be limited in speed, but not amps. Controllers that can direct connect with a cycleanalyst would be good. Not sure if that means 4 CA's, but it might.

I see no need for 6 drive wheels, but idling wheels in the middle might be ok.

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Glyn
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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Glyn » Dec 17, 2017 10:26 am

Chalo wrote:
Dec 16, 2017 4:26 pm
All geared hub motors I'm aware of have a one-way roller clutch in them to allow drag-free coasting. If the gearbox is otherwise able to transmit torque in both directions, you'd have to weld up the clutch or replace it with a solid sleeve of the same dimensions, to permit the motor to drive in both directions.
Looking at the clutch mechanism in this post on the GoldenMotor forum, it would appear to be quite an easy mod.

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Glyn » Dec 17, 2017 11:21 am

amberwolf wrote:
Dec 16, 2017 4:51 pm
(in trying to create more torque for climbing, you turn up the throttle, which ups the voltage on the motor, which increases current. Usually this would make the motor spin faster, but if you have a motor meant to spin at 20-25MPH because of it's winding, and you're only able to push it to 2-3MPH, most of the power is just going into heating the windings, not moving the motor, because normally the motor's higher speed that it is wound for would cause a back-voltage (BEMF) to be created, negating most of the voltage being applied, which then negates most of the current being applied, so the motor doesn't overheat. If the motor was wound for the slower speed, then the BEMF would be able to help prevent this while letting the motor operate as needed at the lower speed).
Firstly, thank you such much for taking the time to write this response.
Reading between the lines in your above statement, are you saying that the motors windings dictate it's optimal rpm? This would suggest that the windings can be modified for higher torque/lower rpm?
amberwolf wrote:
Dec 16, 2017 4:51 pm
So what you want, if you have to use hubmotors, is ones that have windings specifically done for the very low speeds you are after.
That partly answers my question :)
Having already purchased some motors (too late to cancel now - already been shipped) how easy would it be for a novice to modify the windings?
I've seen posts on this forum where people have requested custom windings from the manufacturers. Does anybody know how willing they are to do this? Presumably there are costs involved?
Would changing the windings be able to produce a hub motor which is suitable for my application?
amberwolf wrote:
Dec 16, 2017 4:51 pm
Wheelchair and wheelbarrow motors are probably the only ones already made this way, and the only company I know that advertises making those is Goldenmotor.
I've not been able to find anything suitable on the goldenmotor website. However, wheelbarrow motors(!!! :mrgreen: ) might work. Something like this 18" 48/60v 1200w wheelbarrow motor might work.
amberwolf wrote:
Dec 16, 2017 4:51 pm
But you could have existing motors rewound for the speed you are after. You'll want motors with as wide a stator as you can get, at as large a diameter at the magnet ring/stator interface as you can get, because those two things themselves directly contribute to (or take away from at smaller diameters) the torque-making ability of a motor.
Cool. That answers my above question. How easy is it do myself though, or, are there companies willing to do it?
I really am limited to stator width though. It must fit on a 100mm wide axle or less. If I went with V-brakes instead of disk brakes then that would allow a little extra room. Diameter is not an issue though.
amberwolf wrote:
Dec 16, 2017 4:51 pm
Because of your requirement that a single motor be able to support the chair's movements, It might require finding a large-diameter-stator motor, then taking *two* of the motors apart and joining their stators side by side (it's been done before, see Farfle's motor thread) and then winding it to create the necessary speed.
Not enough room.
amberwolf wrote:
Dec 16, 2017 4:51 pm
You'll need to work out how much torque you actually need to move the chair up the worst parts of the slope (the sharp rises of climbing over boulder edges, when it also has the worst traction) at the speeds it has to go. I don't know the specific math on that, but it's straightforward.
You'd have to work out what winding you would need to achieve the speed you're after at the voltage you'll be running. If the motors you will be starting with are already in the simulator, or you already have them in hand, you can figure out what that will be by experimenting / in the simulator to see what speed they run at full throttle (for max current and thus torque), then seeing what winding they already have, and using the motor winding math (which I don't know but is in a lot of places on ES and elsewhere, like RCgroups.com) to determine the number of turns of wire you need for the torque and speed you're after. AFAICR you need more turns of wire per stator tooth for lower speed.
Ok. Thank you. I'll come back to this once I've done some more research.
amberwolf wrote:
Dec 16, 2017 4:51 pm
Another way around the problems, as has been suggested, is to use geared motors (preferably non-hub for power-dissipation and size/durability reasons). Use a motor that can spin at high RPM to keep the motor itself smaller for the same power output, then use a high-reduction gearbox for converting that speed into torque. It's not as efficient as a direct drive (DD) hub would be, but it's a lot easier to find stuff that would directly do what you want.
I guess the primary reasons for using a hub motors are space saving (fits within a conventional fork) and simplicity. Buying a hub motor means I don't have to fabricate complicated mechanisms for transferring power from the motors to the wheels.
If I can't find a hub motor to suit then this sounds like the next best option though.
amberwolf wrote:
Dec 16, 2017 4:51 pm
Or you can use motors that don't mount in the wheel itself, but are separate with their own gearbox and output shaft to drive the wheel (either directly or indirectly).
Most of these don't have a directional limitation, and are designed to drive the wheel both directions, and could also probably electrically brake.
My thoughts about fabrication apply here. For motors with an output shaft to drive the wheel directly, these seem to be intended for much smaller caster-type wheels. If I attached the 20" wheels I'd lose a lot of torque.
amberwolf wrote:
Dec 16, 2017 4:51 pm
The easiest source for these is old powerchairs (powered wheelchairs), which have a pair of them with axle mounts already on them, for directly driving wheels--often 14" size, but sometimes smaller or larger.
I'll keep a look out.
amberwolf wrote:
Dec 16, 2017 4:51 pm
If you series them, then you need to use a battery three times the voltage the motors are rated for, so each one will get the right voltage to give it the right speed.
Of course. Doh! Thanks for pointing that out.
amberwolf wrote:
Dec 16, 2017 4:51 pm
You may have to actively cool them if they're used way above their ratings for the whole climb.
How is this achieved? fans? liquid cooling?
amberwolf wrote:
Dec 16, 2017 4:51 pm
If I were to build a rock crawling wheelchair, my first iteration of it would use the highest-power (physically largest) powerchair brushed motors w/gearbox that I could find, driving wheels of a diameter that would get me the best speed / torque / object-climbing compromise I could make.
That sounds like sound advice.
amberwolf wrote:
Dec 16, 2017 4:51 pm
That would mean at minimum you'd need a 12kWh battery pack, which is huge.
At present, there is enough room in the design to achieve 6.72kwh.
amberwolf wrote:
Dec 16, 2017 4:51 pm
Another issue is that the battery has to be able to support the continuous current draw of the whole system, as well as the max peak draw.
Let's say it is 3kW continuous, and 10kW peak, just for some random numbers to calculate with.
If you use a "48v" pack, that makes it 3000W / 48V = 62.5A continous. 10000W / 48V = 208.3A peak.
I was going to save this for another thread but seeing as (much to my delight) you brought it up....
So, let's say I was using 15ah 3.2v Headway LiFePo4 cells. I'd need to wire 15 cells in series to achieve 15ah at 48v.
Each cell has a Max Discharge Current (Continuous) of 5C. Wired in series, am I correct in thinking that they have a combined discharge rate of 5C, not 5C per cell?
Also, am I correct in thinking at the max continuous discharge current then for this 48v pack is (15ah * 5C) = 75a?
If this is the case, then two packs which contained the above 15 cells would give me max continuous discharge current of 150a at 48v. Correct?

Again Amberwolf, a huge thank you for your post. Lots of useful information and lots to think about.

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Glyn » Dec 17, 2017 11:25 am

Chalo wrote:
Dec 16, 2017 5:57 pm
By undervolting a motor, and lacing it into a smaller than intended wheel, you can accomplish the same effect without rebuilding the motor or sourcing a specialty item. When you run a hub designed for 48V and a 26" or 700c wheel at 36V in a 20" wheel, the full speed at the perimeter of the wheel is only a little more than half what was designed.
That's useful to know. Presumably then less heat is produced. However, I also presume it would produce less torque or have I not understood how torque is achieved?

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Re: Which Controller (edit: hub motor) for my daughter's wheelchair?

Post by Glyn » Dec 17, 2017 11:27 am

Chalo wrote:
Dec 16, 2017 6:02 pm
Is this vehicle going to have mechanical steering or skid steering?
Skid. Tank style.
Chalo wrote:
Dec 16, 2017 6:02 pm
Is this vehicle going to be mechanically suspended or not?
Only a sprung seat.

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