Most energy efficient powerchair?

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Most energy efficient powerchair?

Postby SamTexas » 16 Feb 2012, 19:57

Hello all,

This is my first post on this board. My name's Sam, I'm from Houston, TX (SamTexas). 53 years old, retired software engineer. My current interest/hobby is electric bicycles and planning to expand to other vehicle types (3 wheelers, wheelchairs, mobility scooters...)

Thankfully I am still fully functional and healthy so I have no need for wheelchairs at this moment. The reason I am interested in power wheelchair is because I just bought two used wheelchairs and upgraded them from lead-acid to laptop lithium battery for my handicapped brother-in-law.

That's when I learned about the HORRIBLE efficiency of power wheelchairs (more on that in another thread.) For now, I have one question for you all: What is the most energy efficient power wheelchair you have used so far? I would appreciate the actual measured figures on flat, smooth ground/terrain:

Wh/mile at xyz mph
or
Wh/km at xyz kph

I look foward to your replies. Thank you in advance.

SamTexas (Sam in Texas)
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Re: Most energy efficient powerchair?

Postby Burgerman » 16 Feb 2012, 20:56

You wont really get a figure. They are horribly inefficient though although SOME of this is the motor/gearbox.

Reason is mostly because of the way they steer. A scooter that goes where its wheels are aimed, typically has almost double the range of a powerchair. And the scooters range is quite predictable.

A powerchair has hugely varying demands.

On a level flat floor like a tennis court, airfield, its about 3/4s as good as a scooter. Theres some extra wasted power still though. Two brakes, less efficient gearbox + some power used staying in a straight line.

On ANY other non totally flat surface, or when turning, or when traveling along a transverse slope masses more power is used.

As an example if a rear drive powerchair has its brake manually lifted off, and is STATIONARY and held in this position by the joystick on a sideways slope, like the camber along a road edge it can take 20 amps or so. Take the power away and it will turn towards the gutter due to the forward c of g and the caster wheels.
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Re: Most energy efficient powerchair?

Postby ex-Gooserider » 16 Feb 2012, 23:03

Theory says, and there is some antectodal evidence that a "gearless brushless" type motor is more efficient just by getting rid of the gearbox, but aside from a few Invacare models, nobody seems to be using them at present in production chairs. The Invacare GB motor chairs reportedly do better on battery life, but do have reliability issues - the motors fail w/o warning and are VERY expensive to replace....

It is questionable as to whether or not the Invacare GB motors can be used with any controller other than the Invacare unit, which is subject to the usual issues of programming tool accessibility,

There are other (mostly Chinese) GB hub motors, but these tend to not work real well for chair applications, as they tend to not like the low speeds that chairs are used at, and many are bigger than will work with a chair that needs to keep the wheels under the seat in order to stay narrow...

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Re: Most energy efficient powerchair?

Postby rollingcowboy » 17 Feb 2012, 01:25

My take on the brushless is different than most of what you see hear.

I have had good luck with the Invacare brushless motors - very reliable and powerful.
They are very expensive - unless you find them used.
I have actually repaired the first set I had. As the brakes wear the dust/dirt causes slow or catching.
I took the motor apart cleaned it reassembled it and it was fine. The dealer I have talked to told me they only replace the motors - they had never repaired one. It is a challenge to re-assemble the motor due to the strong magnets.

There are some negatives - see my other current post.
I think the brushless motor only work with the brushless controller - for 1 thing the connections are different.
Plus the motors send feedback to the controllers.

If you get a programmer for the brushless then you can tune them to perform as you need.

Still they are not perfect - I was told, in Nov 11, by some rehab people that the GB motors have been dropped.
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Re: Most energy efficient powerchair?

Postby ex-Gooserider » 17 Feb 2012, 09:38

I don't have any personal experience with the GB motors, my comments were more or less a summary of what I've heard here and elsewhere. In terms of the alternative controllers, there are definite differences between the controllers for brushed and brushless motors, and they are definitely not interchangeable.

However BM is looking at using voltages other than the usual 24V, and the standard wheelchair controllers don't work at higher voltages. Therefore we are looking at using robotics controllers (especially the Roboteq units) which can handle higher voltages, and also have easier access to programming tools, among other advantages. Roboteq makes a controller for generic brushless motors, but it isn't clear whether the Invacare motors will work with anything other than the Invacare controller.

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Re: Most energy efficient powerchair?

Postby woodygb » 17 Feb 2012, 15:04

Sam,
This may not be exactly what you want ..but it may help.
Motor and gearbox efficiency is my guesstimate/calculation.
note:- Testing station has erroneusly used an old range formula using the 20 hr rate and assuming 100% discharge.
The attachment TEST1.jpg is no longer available
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Re: Most energy efficient powerchair?

Postby woodygb » 17 Feb 2012, 15:05

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Re: Most energy efficient powerchair?

Postby SamTexas » 17 Feb 2012, 17:54

Thank you woodygb (Woody in Great Britain?). That's exactly the information I'm looking for.
13.81A at 4.85mph. At 24V nominal that would be 331Wh energy consumed in one hour. So
68.3Wh/mile at 4.85mph. Almost exactly what I measured on my "3G Storm Series - Torque SP RWD", 70Wh/mile at 5.0mph.
Thanks again.
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Re: Most energy efficient powerchair?

Postby woodygb » 17 Feb 2012, 18:06

Yup
Woody in Great Britain.

Woody as a login name is hardly unique around the WWW ...Hence the GB addition.
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Re: Most energy efficient powerchair?

Postby SamTexas » 18 Feb 2012, 01:08

Update: I finally found the major source of inefficiency on the "3G Storm Series - Torque SP RWD".

Unloaded power: Both driving wheels off the ground and moving straight forward at full speed (WOT).
225 watts at 7.0mph! Or 32.1W/mph. Simply unbelievable. In this unloaded test, the heavy weight of the wheelchair is removed from the equation, the caster wheels effect is also removed.

For comparison, same unloaded test: my 26" wheel with a 9C 2807 Direct Drive Brushless hub (on my electric bicycle) consumes 76 watts at 36mph or 2.1W/mph.
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Re: Most energy efficient powerchair?

Postby woodygb » 18 Feb 2012, 01:36

Not surprised....

I modelled the Jazzy as best I could @ the test speed and mass .. and came up with this.
test6.jpg
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Re: Most energy efficient powerchair?

Postby woodygb » 18 Feb 2012, 01:55

OK ...Here's the same spreadsheet results with zero mass on the drive wheels.
208 Watts consumed @ 4.8 mph or 3722 rpm....
test7.jpg
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FLAT out ...the most inefficient rpm ...the watts loss rises to 247.17.
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Re: Most energy efficient powerchair?

Postby woodygb » 18 Feb 2012, 02:15

What you have measured is the No Load current ... ( motor+gearbox amps pull with no load ) of around 5 amps per drive side....which is fairly standard.

10amps* 24 volts = 240 watts
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Re: Most energy efficient powerchair?

Postby JoeC » 21 Feb 2012, 20:26

There are two major sources (that I can think of) why the powerchair motors have poor efficiency when lightly loaded; that's to say that they require a lot of power to spin freely and not drive the chair.

The first source is the gearbox. Powerchair manufacturers need to provide a cheap, reasonably robust way of stepping down by ~20:1, and they often require a 90 degree turn in power transmission. Most of the chairs I've seen in the US have a helical/worm gear making the first stage of reduction, coupled with straight spur gears for the second reduction. I believe that the first stage of gearing has a lot of sliding friction, and is not very efficient at any level of loading. The UK sunrise F55 uses a single stage worm reduction that is apparently somewhat more efficient, at least at some levels of loading. Certain US Quickie chairs use other methods- the Quickie P200 used a two stage inline spur gear reduction, and in my experience was significantly more efficient than most chairs with right-angle gearboxes. The P222 used a single stage bevel reduction (Cylkro, as built by ASI Drives), and this chair seemed to work extremely well for having a 65 amp brushless drive.

The second source, I think, is in the motor itself. The condition of moving along lightly loaded isn't as common for powerchairs; much of the time they're starting or stopping, turning, or compensating for obstacles in their path. Efficiency near the top speed just isn't something that the manufacturers both to optimize. I'm sure there are plenty of eddy currents in the armature, as well as resistive losses. Powerchair motors often have resistances significantly higher than 0.1 ohms, and many are above 0.2 ohms. A much better motor of similar size and speed at a given voltage could have less than half the resistance.

Another thing that you may not have taken into account is the parking brake. These are always energize when the chair is driving, and they can draw from 0.5 to ~1 amps each, and the chair has one on each motor. These are built as a solenoid holding a sprung plate off of a friction disc. If something with (for example) a dog clutch were used, or something else with more complex mechanics, a much less power hungry solenoid could be used. I don't think it's worth pursuing, but if you want to have a good look at no-load current draw, it must be kept in mind.

Sam, if you're a retired software engineer, you really might have a lot to offer in terms of turning a Roboteq controller into something a little more "civilized" for wheelchair use. As far as we can tell, almost all of the useful safety and convenience features of mobility controllers can be emulated by writing some of their "micro basic" scripting and wiring a few external switches and buttons. If you have any interest, and your software experience matches that, I'm sure that a few people around the board would appreciate your input.
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Re: Most energy efficient powerchair?

Postby LROBBINS » 22 Feb 2012, 00:01

Just to add some data, here are the curves for Rachi's in-line motors.
MicroMotor PMNSt110_30_24V.jpg
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Note that these are essentially 5MPH (with 14" wheels) so have the peak efficiency band in the 100 to 120 RPM range where efficiency is >60%. Continuous mechanical output is rated at 300W, and the motor will stay at or below that above 105 RPM, so in "cruise" it actually runs pretty cool. This has an in-line concentric 18.7:1 gearcase, but the mfr doesn't reveal whether this is planetary+planetary or planetary+spur or spur+spur, though from the physical layouts of this and the 500W model I suspect that it is planetary+planetary on the 300W and planetary+spur on the 500W (for which I've not seen curves). Note also that this is a rather low RPM motor (ca. 2200 RPM) paired with a fairly modest gear ratio, which accounts for the fairly broad efficiency curve. One could probably gain a few percent peak efficiency, at the cost of a faster fall off away from peak, with a higher RPM motor and higher gear ratio; life is full of compromises.
Ciao,
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Re: Most energy efficient powerchair?

Postby popschief » 22 Feb 2012, 03:40

Gearing, no matter how you accomplish it eats power. I learned somewhere along the line that chain drives are very efficient and
I suspect the cogged belt is a close second although the new Harley system sickens me. A 90deg. setup fits well with John's and our narrow chair requirements but our super-bikes all used chain and are quite amazing. As far as compromises go - - - they suck. bp
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