Smallest Motor to Move a Person?

[Donkeyoaty]

Guys, i am really hoping you can help?
There seems to be a million and 1 factors determining what size of motor is required for various projects.
However my goal is to move a person (hence i'm starting at a kart forum)

So imagine a very lightweight wheelchair, if i were to put a motor & drive on either side (i want to do this as opposed to 1 motor in order to keep the weight balanced - don't ask!!)
Then what sized motors and gearboxes (if needed) would i need?

I know i'll need torque to move the dead weight of the person, but my top speed (say 12mph) will be nothing like you guys are trying to achieve in a kart.

I really need to go as small as possible.

What confuses me the most is how come an electric wheelchair motor is ridiculous in physical size, yet your kart motors are about the same size as an electric golf trolley and http://boostedboards.com can move a person up hills to 25mph using tiny 1000W brushless motors that are used in model aeroplanes?

Why can't i use the small brushless motors with speed controllers?
Surely if the person sat down on the board it would pull a person just as easily? Or is it more to do with the size of the wheels?
Hence the large motor needed to rotate the big wheels on a wheelchair?

If so how do you then calculate what motor size is required?
And how do you work out if then a gearbox is required? or in fact a drive with different sized pulleys?

Thank you

 

[bread]

http://www.instructables.com/id/Chibikart-Rapid-Prototyping-a-Subminiature-Electr/

 

[itsid]

think of it this way:
a cheap electric scooter for persons up to 80kg typically has a 120W motor;
an electric bicycle has 250W around here..

the cons of RC motors is this:
preamble: it's of course "possible" to use such thing (in a friction drive setup) and some expensive motors are nearly good enough even for a chain drive...
anyways
An rc motor especially brushless motors run at speed, not at torque..
so you need some ridicule gear ratio to get close to the rotation rate you'd want for your powered wheels to work.
The axle of the motor is flimsy.. too weak to not risk bending it when you apply a sudden load change (say you hit a curb)
all of that can nearly be cancelled out using it in a friction setup (hence the preamble )

But there's one other thing that needs to be considered:
RC manufacturers cheat the **** out of their products.
The 1000W motor you typically call 1000W brushless RC.. are nowhere NEAR 1000W
Yes, they can draw almost that much currents at their rated voltage (for a fraction of a second without burning in an instant)
but effectively (on the output shaft) you talk about maybe a 250W motor..
and that's what's the rating on a motor for electric vehicles state (power out, not In)

And again.. any electric motor needs to slip to produce torque..
long story short..
brushless motors are great for high speed low torque applications, but they're a pain in the behind to tickle some torque out of them..

The smaller the motor, the less room for powerfull magnets is left..
the less powerfull the magnets are, the less torque you have

Again, there are really chunky huge and powerfull RC motors very well up for the task of propelling a human (upwards of 3kW rc rc- rating);
but they typically cost close to six times what you'd have to spend for a 1000W my1020 brushed motor for electric vehicles..
(and they're not much lighter at all)

'sid

 

[Donkeyoaty]

think of it this way:
a cheap electric scooter for persons up to 80kg typically has a 120W motor;
an electric bicycle has 250W around here..

the cons of RC motors is this:
preamble: it's of course "possible" to use such thing (in a friction drive setup) and some expensive motors are nearly good enough even for a chain drive...
anyways
An rc motor especially brushless motors run at speed, not at torque..
so you need some ridicule gear ratio to get close to the rotation rate you'd want for your powered wheels to work.
The axle of the motor is flimsy.. too weak to not risk bending it when you apply a sudden load change (say you hit a curb)
all of that can nearly be cancelled out using it in a friction setup (hence the preamble )

But there's one other thing that needs to be considered:
RC manufacturers cheat the **** out of their products.
The 1000W motor you typically call 1000W brushless RC.. are nowhere NEAR 1000W
Yes, they can draw almost that much currents at their rated voltage (for a fraction of a second without burning in an instant)
but effectively (on the output shaft) you talk about maybe a 250W motor..
and that's what's the rating on a motor for electric vehicles state (power out, not In)

And again.. any electric motor needs to slip to produce torque..
long story short..
brushless motors are great for high speed low torque applications, but they're a pain in the behind to tickle some torque out of them..

The smaller the motor, the less room for powerfull magnets is left..
the less powerfull the magnets are, the less torque you have

Again, there are really chunky huge and powerfull RC motors very well up for the task of propelling a human (upwards of 3kW rc rc- rating);
but they typically cost close to six times what you'd have to spend for a 1000W my1020 brushed motor for electric vehicles..
(and they're not much lighter at all)

'sid

Thanks Sid,

The My1020 motors are what we'd suspected, sadly i calculated we may need 2 for my parameters above....... persons weight to 12mph turning a bike wheel.
The physical weight of 1 is a heavy 6kg so would really like to avoid having 2.

Lighter options
Would we have to go for 48V do you suspect? I imagine a 12V or 24V
set up would drain in no time? and perhaps not have enough power?
But these are much lighter.

The other thing is could we power 1 wheel using the my1020 for start off torque and have 2 smaller motors on the front wheels to kick in and help with speed?

Thanks kindly

 

[itsid]

what.. what's voltage to do with drainage? or power?

Anyways, take a look at my essence thread for some math...

but boiled down to the very basics:
it's completely unimportant (powerwise) if you draw 50 amps at 48V or 100Amps at 24Volts..
even more.. both motors are likely (if comparable motors that is) comparably efficient (ratio of power in to power out)
and require the same amount of batteries for the same mileage.

if the biggest my1020 (1000W constant power-out) is not powerfull enough, get a bigger motor;
but if you want to have a reliable setup .. stay away from RC motors completely!
those small light ones are NOT up for the task, they'll fail in very short order (a day and a half maybe)
maybe they cook in just a few minutes (neodymium magnets are very heat sensitive... and those motors are not designed to push lots of mass around, thus they'll heat up quickly)
and the ones that MIGHT be up for the task, are not exactly small, not light but most of all, they're not even cheap.

your choice of course..
But I cannot tell you to use them .. so I won't
If you want to test.. let us know how it turned out;
all info is good info.
But to test you'll be on your own I'm afraid.

'sid

PS if you want to spread the weight.. think about hubmotors for small scooters or wheelchairs and such.. maybe even a bicycle hub motor.. (wheelchair motors have single sided axles.. so that'd be my first guess )

 

[Donkeyoaty]

what.. what's voltage to do with drainage? or power?

Anyways, take a look at my essence thread for some math...

but boiled down to the very basics:
it's completely unimportant (powerwise) if you draw 50 amps at 48V or 100Amps at 24Volts..
even more.. both motors are likely (if comparable motors that is) comparably efficient (ratio of power in to power out)
and require the same amount of batteries for the same mileage.

if the biggest my1020 (1000W constant power-out) is not powerfull enough, get a bigger motor;
but if you want to have a reliable setup .. stay away from RC motors completely!
those small light ones are NOT up for the task, they'll fail in very short order (a day and a half maybe)
maybe they cook in just a few minutes (neodymium magnets are very heat sensitive... and those motors are not designed to push lots of mass around, thus they'll heat up quickly)
and the ones that MIGHT be up for the task, are not exactly small, not light but most of all, they're not even cheap.

your choice of course..
But I cannot tell you to use them .. so I won't
If you want to test.. let us know how it turned out;
all info is good info.
But to test you'll be on your own I'm afraid.

'sid

PS if you want to spread the weight.. think about hubmotors for small scooters or wheelchairs and such.. maybe even a bicycle hub motor.. (wheelchair motors have single sided axles.. so that'd be my first guess )

Sid, thank you, i think you have taught me more than all the reading online.

I have totally discounted RC motors, don't panic

So if you can help me one last time...

Focussing on the MY1020 or MY1016 motors which appear to be the way forward......

Can you tell me (knowing that i am completely new to this)

What different properties these motors will give me?
1) 48V Brushed 1000W Motor
2) 36V Brushed 250W Motor

My inclination is that the 36V Brushed motor (geared down or is it up?) will generate enough torque to rotate the wheel (as they are used in e bike conversions) and give us enough of a top speed (the max we want is 12mph)
If this is the case, it would be more beneficial to us as it is a considerably lighter motor than the MY1020 48V 1000W which i believe would give us a higher top speed? (Which we dont need)

But the my 2 parameters that i need to achieve .....
a) As big a "fuel" tank as possible i.e. the motor needs to draw the power to move the chair for as long as possible.
b) It doest need to go any faster than 12mph

To achieve that would you suggest the
MY1016 36V 250W with a 36V battery pack
or
MY1020 48V 1000W with a 48V battery pack

or is there a better solution.

Thanks in advance Sid.

 

[itsid]

well,
an electric motor draws the power it needs to go a certain speed (with a certain mass being moved along a certain tack..)
and just as with gas engines the two motors have a specified rpm range (I think both will turn up to 3000rpm)
so if both setups are geared the same, then both setups will have the same top speed.
if all you need to move that mass at that speed (again, please check my essence thread to do the math)
then the 1000W motor is of no real use and you're better off using the 250W version (because of it being lighter)
If you have any motor bigger than that (350, 500, 600, 750 or even 1000W)
it will do the exact same thing
(i.e. move the vehicle whilst providing 250W of mechanical power)
So apart from weight there will be no difference..

okay; that's not perfectly true..
all different configurations have slightly different power/efficiency plots so one might run at 84% another at 86% and so on..
if you want to go into such details.. use the one that has the highest efficiency at that load.
-but let's just ignore these for the sake of argument-

battery pack wise again, in an ideal world both motors need the exact same batter pack size to go the same distance.

let's just say both provde exactly 245W to move the wheelchair around
and run at 85% efficiency
so both will draw exactly 288Watts from the batteries
one takes 8 Amps at 36V the other one 6 Amps at 48V
Now let's just assume you want to use 12V 2Ah batteries
and have the wheelchair run for 4 hours
for the 36V setup:
you will need 32Amp hours and 36V (1152 Wh [288W * 4hrs = 36 Volt * 8 Amps * 4hrs])
- you serialize three batteries for one 36V 2Ah pack
- you put 16 such packs in parallel to have 32Ah
so you'll need 48 batteries

for the 48V setup:
you will need 24Amp hours and 48V (1152 Wh [288W * 4hrs = 48 Volt * 6 Amps * 4hrs])
- you serialize four batteries for one 48V 2Ah pack
- you put 12 such packs in parallel to have 24Ah
so you'll again need 48 batteries

see.. no difference at all.

if I'd be in charge.. I'd do some math to see what exact power I need.. then add ~20%
(to compensate for miscalculation and some unforseen stuff)
get the motor that has that powerrating (or the next bigger one if costs allow that)
if I have a choice of picking that motor in 36V or 48V (like the 1000W version is available in both configurations)
I'd take a look at the batteries I have access to.. (dimension, weight, capacity)
then configure the electric layout of these batteries and play around with that.
whichever seems to be niccer will get picked,
if they're identical.. the next thing to consider is the motor controller (price differences perhaps...)
all things being equal I'd take the 36V version for no other reason than squared six

'sid

 

[Donkeyoaty]

well,
an electric motor draws the power it needs to go a certain speed (with a certain mass being moved along a certain tack..)
and just as with gas engines the two motors have a specified rpm range (I think both will turn up to 3000rpm)
so if both setups are geared the same, then both setups will have the same top speed.
if all you need to move that mass at that speed (again, please check my essence thread to do the math)
then the 1000W motor is of no real use and you're better off using the 250W version (because of it being lighter)
If you have any motor bigger than that (350, 500, 600, 750 or even 1000W)
it will do the exact same thing
(i.e. move the vehicle whilst providing 250W of mechanical power)
So apart from weight there will be no difference..

okay; that's not perfectly true..
all different configurations have slightly different power/efficiency plots so one might run at 84% another at 86% and so on..
if you want to go into such details.. use the one that has the highest efficiency at that load.
-but let's just ignore these for the sake of argument-

battery pack wise again, in an ideal world both motors need the exact same batter pack size to go the same distance.

let's just say both provde exactly 245W to move the wheelchair around
and run at 85% efficiency
so both will draw exactly 288Watts from the batteries
one takes 8 Amps at 36V the other one 6 Amps at 48V
Now let's just assume you want to use 12V 2Ah batteries
and have the wheelchair run for 4 hours
for the 36V setup:
you will need 32Amp hours and 36V (1152 Wh [288W * 4hrs = 36 Volt * 8 Amps * 4hrs])
- you serialize three batteries for one 36V 2Ah pack
- you put 16 such packs in parallel to have 32Ah
so you'll need 48 batteries

for the 48V setup:
you will need 24Amp hours and 48V (1152 Wh [288W * 4hrs = 48 Volt * 6 Amps * 4hrs])
- you serialize four batteries for one 48V 2Ah pack
- you put 12 such packs in parallel to have 24Ah
so you'll again need 48 batteries

see.. no difference at all.

if I'd be in charge.. I'd do some math to see what exact power I need.. then add ~20%
(to compensate for miscalculation and some unforseen stuff)
get the motor that has that powerrating (or the next bigger one if costs allow that)
if I have a choice of picking that motor in 36V or 48V (like the 1000W version is available in both configurations)
I'd take a look at the batteries I have access to.. (dimension, weight, capacity)
then configure the electric layout of these batteries and play around with that.
whichever seems to be niccer will get picked,
if they're identical.. the next thing to consider is the motor controller (price differences perhaps...)
all things being equal I'd take the 36V version for no other reason than squared six

'sid

Sid,

You sir are a legend.

Thank you once again for taking the time to share your considerable knowledge. You've been incredibly helpful.

I'm going to work on your suggestions as we speak

 

[itsid]

you're welcome, care to give that thanks-button a click?

'sid